American Mineralogist, Volume 70, pages 106-123,I9E5
Iligh resolution solid-statesodium-23,aluminum-27, and silicon-29nuclear magnetic
resonancespectroscopicreconnaissance
of alkali and plagioclasefeldspars
R. JeMes Ktnrpernrcr
Department of Geology
University of lllinois at Urbana-Champaign
l30l West Green Street. Urbana. Illinois 61801
Ronnnr A. KrNsev. KeneN ANN SuIrn
School of Chemical Sciences
University of lllinois at Urbana-Champaign
505 South Mathews Avenue, Urbana, Illinois 61801
Donelo M. Hr,NoBnsoN
Department of Geology
University of lllinois at Urbana-Champaign
1301 West Green Street. Urbana. Illinois 61801
eNo Enrc OlorrBI-o
School of Chemical Sciences
University of Illinois at Urbana-Champaign
505 South Mathews Avenue. Urbana, Illinois 61801
Abstract
We presentin thispaperhigh-resolution
samplespinningsodiumsolid-state
magic-angle
-27, andsilicon-29nuclearmagneticresonance
(NMR) spectraof a varietyof
23,aluminum
plagioclaseand alkali feldsparsand feldsparglasses.The resultsshowthat MASS NMR
spectracontaina greatdeal of informationaboutfeldsparstructures.In particular,we
observethe following.(l) AUSiorder/disorder
andexsolutioneffectsin alkalifeldsparsare
readily observable.(2) To our presentlevel of understanding
the publishedstructure
refinements
of anorthiteare not consistentwith the NMR of spectraof anorthite.(3) The
plagioclases
spectraofthe intermediate
in termsofe- and/-plagioclases,
areinterpretable
with much of the signalapparentlycomingfrom strainedsitesbetweenalbiteJikeand
(4)Thenearest-neighbor
anorthite-likevolumesin the e-plagioclases.
atomicarrangements
in albite and anorthiteglassesare very similar to thosein the corresponding
crystals,
althoughthe rangeof cation-oxygendistancesis largerin the glasses.(5) Thereare no
peaksin the rangeof 0 ppm that would indicateoctahedrallycoordinated
aluminum-27
aluminumin eitherglass.
Introduction
We present in this paper high-resolution solid-state
sodium-23,aluminum-27,and silicon-29nuclear magnetic
resonance(NMR) spectroscopic results for a variety of
plagioclaseand alkali feldsparsand feldspar glasses.The
objective of this work is to explore the applicability of
high-resolutionsolid-stateNMR to the examinationof the
structuresof complex silicate materials, such as feldspars
and feldspar glasses.The emphasisin this paper is on the
102-0r06$02.00
m03-004)vE5/0
spectroscopicaspects of this problem' Future work will
examine the mineralogical and petrological applications
in more detail. Both magic-anglesamplespinning(MASS)
and variable-angle sample spinning (VASS) techniques
have been used.
The samplesdiscussedhere include low albite, microline perthite, cryptopenhite, sanidine, a range of plagioclase compositions, and albite and anorthite glasses.The
feldspar samplesare listed in Table 1.
In this paper we briefly summarizefeldspar nomencla-
106
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
Table l. Silicon-29 NMR chemical shifts of feldspars and
feldspar glasses
ruTERIAL
LOCALIN
Alblte
(Ab99)
&elta,
AIblte
Class
(Abr00)
Synthetlc
Sanldin€
(or60Ab38An2)
Cyonlng
CHEMI6L
SHTF1
Vlrglnla
-96.:
-104.4
-98
-97
- I OI
-93
-96.0
Mlcrocllne
Perthlte
COffiNTS
l2n
T20
Tlo
poorly
separated
-l0l,8
-106,0
Ab T2tr
T2b
T2o + Ab T2o
Tln
Ab Tlo
-96-o
-98.0
- 103,5
Ab
Ab
Or
Ab
CryptoperthlEe
( Spencer R)
( or43,4b,53,k4)
Nomay)
01lgocla6e
( An23 )
Bakersvllle,
North
Carollna
-47
-93.2
- 9 7. 2
- 105.3
Labradotlte
(Ans2 )
Sydney Ene6,
kbrado!
-83
-88
-91
- 101
-107
Eytomlte
(An70)
Crystal
kyl
Mlnnesota
-84
-89
T2E
Do+0rT2
Tl
TID
Shoulder
(An Fak)
Ab peak
Corlelatlon
to
- 101
(h92)
Pacaye Volceno
GuatgMla
-83.0
-84.6
-89.2
Synthetic
-42.9
-84.8
-89'6
Synrherlc
-87
(Anr00)
Anorthlte
(hroo)
Glass
Flrst
tro peaks
poorly
reeolved
Cortelatlon
to
publlshed
Etructure
poor
reflnerentB
ture and structure and previous NMR work on feldspars,
review the theory of MASS NMR (especially as it pertains to the study of quadrupolar nuclei), and present and
discussour results.Theseresultsare, we believe,only a
beginning of the application of MASS NMR to feldspar
mineralogy, and we present our conclusions with the
recognition that they are in someinstancestentative. It is
clear from the work we have done, however, that MASS
NMR spectraof alkali and plagioclasefeldsparscontain a
great deal of information about Al/Si orderidisorder in the
tetrahedral sites, about chemical bonding in the sodium,
aluminum and silicon sites, and also, we believe, about
distortion of these sites due to exsolution. Our results
indicate that solid-state MASS and VASS NMR. when
combined with X-ray and electron difraction and transmission electron microscopy (TEM) can provide new
insights into the structures of a wide range of mineral
phases.
Previous work
Feldspar structure and nomenclature
Feldsparsare the most abundantminerals in the earth's
crust and have received corresponding attention from
mineralogistsand petrologists. Excellent reviews of feld-
t07
spar mineralogy are given in the compendia of Smith
(1974) and Ribbe et al. (1983).
For the benefit of those unfamiliar with the feldspars,
we summarize their main features (following Smith, 1974
and Ribbe et al., 1983), state some of our assumptions,
and define some of our nomenclature and symbolism.
l. Feldspar structures are based on a framework of cornerlinked SiO+ and AlOa tetrahedra. The ratio of aluminum to
silicon rangesfrom l:3 to l:1. We use the notation Q4(nAl)to
describe the silicon sites, where the superscript indicates the
four bridging oxygen atoms and the integer n (0 to 4) denotesthe
number of aluminum (AlOo) tetrahedra to which a silicon (SiO+)
tetrahedronis connected.
We assume,as is customary in dealingwith tetrahedralsilicon
and aluminum occupancies,that the "aluminum avoidancerule"
(Loewenstein, 1954; Goldsmith and Laves, 1955)holds for all
feldspars.That is, no aluminum tetrahedron is linked to another
aluminum tetrahedron,only to four silicon tetrahedra.A// aluminum sites, therefore, should be A4 (4Si). We will show, however,
that for our synthetic anorthite this is not completely true.
2. Feldsparframeworks have inherent monoclinic or pseudomonoclinic symmetry. Because AlO4 tetrahedra are slightly
larger than SiO4 tetrahedra, some patterns of AllSi distribution
give monoclinic frameworks, whereasothers causeslight distortions and triclinic symmetry. The pattern of tetrahedral site
occupancy leads to the important concept of AVSi tetrahedral
site order/disorder.Other things being equal, high AVSi disorder
promotes monoclinic frameworks, whereas increasing order
promotes triclinic frameworks having increasing cell obliquity
(indicated by the magnitude ofthe departure ofthe B-axis from
B*).
The large, irregular cages in the tetrahedral framework are
occupiedby potassium,sodium, or calcium ions. When occupied
by the larger potassium ions, the cages are stretched and the
framework tends to retain its monoclinic symmetry if it has a
proper AUSi distribution. Ifthe AUSi pattern is not appropriate,
the symmetry is triclinic. When occupied by the smaller, sodium
or calcium ions, the cages crumple a little and distort the
framework slightly so that the symmetry is triclinic, regardlessof
the AVSi order/disorder (except for the highest temperature
sodium feldspar, monalbite).
3. Feldsparcompositionsare conveniently describedin terms
of percentagesof the endmembercomponentsKAlSi3Os(Orthoclase, Or), NaAlSi3Or (Albite, Ab), and CaAl2Si2Os(Anorthite,
An). The rock-forming feldspars occur in two series, the alkali
feldspars(Or-Ab) and plagioclases(ALAn).
4. The principal potassic feldspars include, from low to high
temperatureforms, microcline (triclinic, Cl), orthoclase or low
sanidine (macroscopicallymonoclinic; monoclinic by polarized
light microscopy but likely to be triclinic (low obliquity) by X-ray
and electron diffraction), and high sanidine (monoclinic, C2im).
The principal sodic feldspars are low (temperature) albite and
high albite, both triclinic, Cl; high albite has the lower cell
obliquity. High sanidineand high albite form a continuous solid
solution seriesdespite their different symmetries.
5. In the triclinic alkali feldspars(microcline, low albite, high
albite) there are four sets of similar but not identical tetrahedral
sites conventionally labeled Tlo, Tlm, T2o, T2m. At high
temperatures, the aluminum and silicon ions are randomly
distributed over these sites. If a high temperature feldspar is
quenched, this disorder is retained. In the lowest temperature
KIRKPATRICK ET AL,: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
forms, maximum low albite and maximum microcline, AVSi
order is, ideally, complete,with all the aluminum in the Tlo sites
and all the silicon in the other three sites.
6. In the monoclinic alkali feldspars there are two sets of
similar but not identical tetrahedral sites conventionally labeled
Tl and T2. High sanidineideally has one-quarterof an aluminum
and three-quaxtersof a silicon in each site on the average
(complete Al/Si disorder). Orthoclase (low sanidine) is more
ordered, with equal numbers of aluminum and silicon atoms
randomly occupying the Tl sites and silicon only occupying the
T2 sites.
7. Non-endmember, single phase, high temperature alkali
feldsparsexsolve into intergrowths called perthites when cooled
slowly enough. Cryptoperthite is perthite in which the intergrowth occurs on a submicroscopic scale. In the finer cryptoperthites, the lamellaeof the potassium and sodium-rich phases
are thought to maintain coherency across the interface causing
strain of the sites near the interface. In the coarser perthites,
microperthites and macroperthites,the boundariesbetween the
lamellae are noncoherent, and there probably is no significant
straining of the sites near the interfaces.
8. The plagioclasesare even more complex. The albite endmembers have been described. The calcic endmembersare Panorthite (triclinic, Pl) and /-anorthite (triclinic, 1l), which occur
below and above 250'C, respectively. On cooling, the 1 to P
transition is not quenchable.P- and /-anorthite have doubled c
cell edge lengths of about 144 comparedto the 7A of the other
feldspars.Even the P-anorthite structure has a high degreeof1pseudosymmetry
(Ribbein Ribbe, 1983).
P-anorthite has 16 sets of similar but not identical tetrahedral
sites. Eight are occupied by silicon and eight by aluminum. 1anorthite has 8 sets, four occupied by silicon and four by
aluminum. Both kinds of anorthite ideally have complete Al/Si
site order. Each silicon tetrahedron is connected to four aluminum tetrahedra, Qo(4Al), and vice versa, Aa(4Si).
9. Standard compositional ranges of the plagioclasesof intermediate compositions are labeled by the names oligoclase,
Anro-ro,andesine,Arro-so, labradorite, Anso-ro,and bytownite,
AnTs-e6(see Table l). These names refer to bulk compositions,
not phases.The intermediate plagioclasesare all triclinic.
At the highest temperaturesthere is extensive solid solution
between the Ab and An components. At lower temperatures,
intermediate composition plagioclasesexist as exceedingly fine
intergrowths of albite-like and anorthite-like domains on several
scales. The complexity of these plagioclasesarises from the
incompatibilitiesof the AVSi site orderingsin albite and anorthite
and from the fact that subsolidusatomic rearrangementprocesses are so sluggish.
We shall acceptthe e-plagioclasemodel as describedby Ribbe
and by Smith in Ribbe (1983).The intergrowths in plagioclases
occur on at least two scales. The finer type, e-plagioclase,is
thought to consist of a modulated structure composedof coherent albite- and anorthite-like domains of the order of a few tens
of A across. e-plagioclaseseems to be restricted to the range
Anzo-ro.
A second category of intergrowth occurs on the micrometer
scale and is associatedwith the so-calledperisterite (-Anz-ro),
BQggild (-An+7-se) and Hiittenlocher (-An6s-q) gaps. Peristerite intergrowths are considered here to be composed of albite
(-An2) and an e-plagioclase(-An25). Bpggild ones are believed
to be composed of two e-plagioclases(-Anor and -56) and
Hiittenlocher intergrowths are thought to be composedof an eplagioclase(-Anoa) and /-anorthite (-Anso). This /-anorthite is a
type that appears to be body centered by diffraction, even at
room temperatures.It is consideredto have a composite structure made from domains of P-anorthite. Adjacent domains are
"out-of-step" to one another (antiphasedomains)with respectto
AVSi ordering, which, coupled with the earlier mentioned 1pseudosymmetryof each domain, causessystematicdifiraction
absences.Thesediffraction absencesgive an apparent1l symmetry for the averagestructure.
NMR of feldspars
Feldspars have been investigated by single crystal
sodium-23and aluminum-27wide-line NMR at low magnetic field strengths since the early 1960's. Brun et al.
(1960),Hafner etal. (1962),and Hafner and Laves (1963)
investigated alkali feldspars and were able to resolve
separatepeaks from different phases. Hafner and Hartmann (1964) determined electric quadrupole coupling
constantsof a variety of compositions. Staehli and Brinkmann(1974a,b)examinedthe aluminum-z7NMR of anorthite using wide-line methods and were able to separate
the eight aluminum sites, determine that the P to I
transition takes place between 220" and 250'C, and again
measure the values of the electric quadrupole coupling
constants. The quadrupole coupling constants of feldsparsand many other minerals have been summarizedby
Ghoseand Tsang(1973).
Silicon-29 chemical shifts obtained by high-resolution
MASS NMR have been presentedby Lippmaa et al.
Table 2. AverageSi-O bond distances,total cation-oxygen
bond strengths,and predictedsilicon-29chemicalshifts for
anorthite
St11cotr
Slte
Bond
S1-O
Strength
su (v.U.)
A
sEczsl-rsr
ksaw et al. (1962)
Tr(000)
rr (00r)
rr(M0)
Tl (rzt)
I.5I3
t.5 16
L608
r,626
8,058
8.083
8.036
7.990
-1.438
-I.471
-r.27 |
-1.266
r2(0zo)
T2(OzL)
T2(n00)
12(n01)
I .513
I .610
t.602
1.528
8.187
8,264
8 . I 5t
7.879
-1.441
-1.361
-t.346
-1.469
Tl(000)
rl ( o0r)
rl (Dzo)
Tl (nzl )
I .616
1.6t3
I .613
I .613
8. I44
8. I23
8 , 1 43
7 .926
-1.444
-t,243
-t.451
-1.268
12(020)
T2(021)
T2(D00)
T2(n0r)
I.6r7
r.608
I .515
I .614
8.r83
8.000
8.086
-l,446
klus
-t.442
( 1978)
Tr (000)
rr(001)
rr(Ezo)
rl(Mr)
I .618
I .613
t.6t4
I .513
8.146
8.r30
8.075
1 .939
-r.445
-1.458
-t.252
-r,213
T2( 020)
T2(ozt)
T2(oo0)
T3(Eot)
I .617
r .613
I .614
I .6I6
8.r29
8.143
8.009
8 . 0 6l
-r.460
-1.345
-1.445
KIRKPATRICK ET AL.: NI]CLEAR MAGNETIC RESONANCE OF FELDSPARS
(1980) for what they call anorthite, albite, sanidine,
orthoclase, and adularia. They presented spectra for
albite and sanidine, discussed structural interpretations,
and proposed peak assignments for the albite peaks.
Oldfield et al. (1983)have discussedthe effectsof iron
impurities on the aluminum-27 and silicon-29 MASS
NMR spectraof a variety of feldspars.K. A. Smith et al.
(19E3)havediscussedsilicon-29MASS NMR spectroscopy of a number of silicates, including some feldspars.
Recently,J. V. Smith et al. (1984)have presentedhigh
resolution silicon-29MASS NMR data for alkali feldspars
and discussed the peak assignments.Solid-state aluminum-27and silicon-29MASS NMR have probably found
their most important application to date in the study of
zeolitecatalystmaterials(e.g., Klinowski et al., 1982).
Experimental
methods
Spectrometers
The spectrapresentedhere were recorded using three "homebuilt" Fourier transform NMR spectrometersusing the methods
describedby Meadowset al. (1982)and Smirh et al. (1983).The
three spectrometersare based on 3.52, 8.45. and ll.7 Tesla
superconductingmagnetsand use Nicolet ll80 or 1280computers for data acquisition. All are equipped with Andrew-Beams
type magic-angle sample-spinningrotor assemblies (Andrew,
l97l), which can also be used to obtain static (non-spinning)
spectra.Chemical shifts are reported in ppm relative to 3M NaCl
for sodium-23,in ppm relative to lM Al(H2O)l+ for aluminum27, and in ppm relative to tetramethylsilanefor silicon-29.More
positive chemical shifts correspond to decreasedshielding.
Samplepreparation and characterization
All sampleswere ground to a fine powder using an agate(SiO2)
pestle and mortar and were tightly packed into Delrin rotors.
Most sampleswere pure minerals and were simply ground and
run. Pacayaanorthite was purified by hand picking and magnetic
separation.The Wyoming sanidinewas purified by hand picking
and shaking in water in a small mill to remove the soft matrix.
The anorthite glass was prepared from 99.D9Vo pure SiO2,
CaCO3,and Al2O3by melting for about one hour at 1600.C,three
times, with grinding in an agate mortar between meltings.
Syntheticanorthite crystals were grown by crystallizing the glass
at about 1500"Cfor about four hours and then quenching the
crystals. The albite glass was prepared by the Coming Glass
Technical Staffs Division of Corning Glass Company.
All sampleswere characterized by powder X-ray diffraction
and optical microscopy.
Theoretical background
A useful introduction to NMR spectroscopyis given in
the book by Farrar and Becker (1971). Magic-angle
sample-spinning(MASS) in general,and silicon-29MASS
NMR spectroscopyin particular, have been discussedby
Lippmaaet al. (1980)and by Smith et al. (19E3).We note
here only that a given nuclide ordinarily has different
nuclear resonancefrequencies in different kinds of sites,
whether in diferent materials or in the same material.
even if the sites are as similar to each other as the
l(D
tetrahedral sites in feldspars. This is because they are
shielded from the imposed magnetic field by slightly
different electron distributions. Silicon-29 has a nuclear
spin I : ll2, and,as describedelsewhere(Andrew, l97l;
Smith et al., 1983),spinningof a powderedsampleon an
axis inclined at 54.7" to the applied magnetic field (the
"magic-angle") at kilohertz frequencies greatly narrows
the NMR peak breadth and increasesresolution. This is
due to an efective averaging of the nuclear dipolar and
chemical shift anisotropy interactions. Thus, nonequivalent silicon sites in an ordered crystal each give a single
nalrow peak. Peaks for glassesand disordered crystals
are generally broader, because a range of magnetically
nonequivalentsites is present.
Sodium-23and aluminum-27,unlike silicon-29,have
spins I : 312and 5/2 respectively and, therefore, exhibit
quadrupolarbehavior. Although MASS averagesthe first
order quadrupole interaction, for sites with large electric
field gradients (and thus large quadrupole interactions)
second-orderterms in the nuclear quadrupolar Hamiltonian, Hq, becomeimportant, and may causea significant
linebroadening.Such efects have been discussedin detail
by Abragam(1961),Kundla et al. (1981),Meadowset al.
(1982),Ganapathyet d. (1982),and Schrammand Oldfield (1982). We review here those aspects of theory
necessaryto understand the spectra presented.
A nucleusof spin I has (2I + l) different nuclear energy
levels when placed in a magnetic field, as shown for
silicon-29(l = ll2, two energylevels)and sodium-23(I =
312, 4 energy levels) in Figures lA and D. In NMR
spectroscopythe energies(or frequencies)of the transitions (1/2, -ll2), (112,312),etc. are measured.In the
absenceofany other magneticinteractions, only a single
resonancefrequency would be observed. However, in
real samples, there are a variety of interactions which
cause slight changesin these nuclear energy levels. For
silicon-29 the main efect is due to an orientation-dependent shielding of the main magnetic field at the nucleus.
This is called the chemical shift anisotropy (CSA) and
yields a broad resonancein powdered samples(Fig. 1B).
In MASS this interaction is effectively averagedby rapid
sample rotation in a sample rotor spun by an air-bearing
at frequencies,2,, typically from about 2,000to 6,000Hz.
The result is a sharp-line spectrum with a centerband
(CB, Fig. lC) at the samefrequencyas in the absenceof
the CSA, togetherwith a number of spinningsidebands
(SSB), spacedat the spinningfrequency(2.).
For quadrupolarnuclei (sodium-23,| = 312,aluminum27, | = 5 12),there are a number of transitions. In general,
the resonancesof these quadrupolar nuclei in powders
are broadened by the interaction of the nuclear electric
quadrupolemoment (eQ) with the electric field gradientat
the nucleus (eq). This interaction is describedin terms of
the quadrupole coupling constant, e2qQ/h. For sodium23, the first order quadrupole interaction leads to two
broad absorptionsdue to the satellite transitions (312,ll2;
-lD, -3/2) together with a relatively sharp central reso-
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
110
c
B
A
-roo
-roo
PPM
-200
-r50
PPM
E
D
ZEEMAN
-r50
-200
F
cB ( t/2,-t/21
QUADRUPOLAR
200
t50
loo
KHz
0
50
G
t,"'t""t""t""
r50
200
roo
KHz
0
50
H
Fig. 1. Energy level diagrams,static spectra,and MASS NMR spectraof a spin I = 1/2nuclide (A,B,C), a quadrupolarnuclide (spin
| : 312)with a small quadrupole coupling constant (D,E,F), and a quadrupolar nuclide with a large quadrupole coupling constant
(D,G,H). (A) Energy level diagramof spin I = l/2 nuclide showingZeemansplitting into ll2, and - l/2 spin statesand small additional
splitting due to chemical shift anisotropy (CSA). (B) Simulation of a static spectrumof a spin I : l/2 nuclide (silicon-29in Na2Si2O5)
(C) MASS NMR spectrum of spin I : l/2 nuclide (silicon-29)in Na2Si2O5showing the center band (CB, the true resonance)and
spinning sidebands(SSB) spacedat the spinning speed, zr. (D) Energy level diagramof a spin I : 3/2 quadrupolar nucleus showing
Zeernan splitting and a small additional splitting due to the quadrupolar interaction. (E) Simulation of a static spectrum of a
quadrupolarnuclide (I : 3/2 in this case)which has a small quadrupolecoupling constartt.(F) Simulationof a MASS spectrumof an I
= 3/2 nuclide showing the centerband(CB), which is the central (+ll2 + - l/2) transition and the spinningsidebands(SSB) mapping
out the shapeof the satellite spectra.(G) Simulation of the static spectrumof a quadrupolarnuclide with a large quadrupolarcoupling
constant(sodium-23in Na2MoO+).Only the central (+ 12+ -ll2)transition is shown. (H) Simulationof the VASS NMR spectrumof
sodium-23in Na2MoOashowing the narrowing possible at 75'. The MASS spectrum of this compound shows less narrowing.
nance(112,-ll2), as shown in Figure lE. In general,with
MASS, these interactions are again averaged, and a
sharpercentral line (112, -ll2) together with a number of
sidebandsdue to the satellite transitions are observed,
Figure lF. When the quadrupole coupling constant is
about 1 MHz in frequency units, only the central (l/2,
-ll2) resonanceis readily detected, since there is a large
residual broadening of the satellite lines, due to small
fluctuations around the magic-angledue to wobbling of
the sample rotor.
When the quadrupolecoupling constant is larger (great-
er than about 3 MHz), additional structure generally
appearson the central line due to the higher order effects
which arise when the quadrupole interaction is a significant fraction of the actual resonance frequency. For
example, for 23Na2MoOathe quadrupole coupling constant is approximately 2.6 MHz, and at all fields in the
range3.52to 11.7Tesla there is a so-calledsecond-order
broadening of the central transition, Figure lG. This
interaction has a different angular dependencethan that
causing the first order chemical shift or quadrupolar
broadenings. In order to reduce this interaction it is
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
necessaryto spin the sample at a different angle in the
magnetic field (variable-angle sample-spinning, VASS,
Ganapathyet al., 1982).
Thus, for silicon-29,or for quadrupolarnuclei in highly
symmetric environments (where electric field gradients
are small)-such as sodium-23 in NaCl, magic-angle
(54.7")sample-spinning
yieldsthe highestresolutionspectra, Figures lC and F. For quadrupolar nuclei in highly
asymmetric sites, such as sodium-23in Na2MoO4, variable-anglespinning, at 75" in this case, yields the narrowest line width, Figure lH. Both techniquesare usedhere.
As discussedin detail elsewhere (Meadows et al.,
1982),the second-orderquadrupole interaction is a fielddependent interaction, and results in different apparent
chemical shifts for different applied magnetic freld
strengths(Abragam, 1961).As a result, there may be
significant discrepanciesin ihe chemical shifts of a quadrupolar nucleus when results obtained at different magnetic field strengthsare compared.All data for sodium-23
and aluminum-27 presented here were obtained at the
three different magnetic field strengthsto help determine
the true chemical shifts. Because silicon-29 has spin
I : ll2, its chemical shifts are not field-dependent.
For the quadrupolarnuclei it is often useful to simulate
(calculate)the spectrausing the methods of Ganapathyet
al. (1982).The parametersneededfor the simulation are
the quadrupole coupling constant (e2qQ/h),the asymmetry parameterofthe electric field gradient tensor (4), and
the spinning angle (0). The asymmetry parameter is a
measureof the deviation of the electric field gradient at
the nucleus from cylindrical symmetry (Abragam, 196l).
Such simulations are used here in conjunction with published quadrupole coupling constants and asymmetry
pa"rametersto determine isotropic chemical shifts, and to
look for contributions from exsolved phases.
For the silicon-29NMR spectrawe report the chemical
shiftsof narrow peaksto +0.1 ppm, but subscriptthe last
place becauseaccuracy and reproducibility from laboratory to laboratory is probably only about 0.3 ppm. This is
primarily due to magnet drift over the relatively long
times (up to 12 hours) needed to collect some silicon-29
spectra. For broader silicon-29 peaks and shoulders we
report the shifts to only -r I ppm.
For the quadrupolarnuclei we report the valuesofmost
of the singularities or peak maxima to + 1 ppm. This is
becauseeven at ll.7 Tesla the peaksare at least 10 ppm
wide. We report some of the narrowest singularities to
-+0.1ppm with the last place
subscripted.
t'si
-96'.
-92'"
llt
8,45 T MASS
-,oc,,A
ALBITE
,rr,1/t/\"t"fdli
. -l f - 100
-L10
-
LOU
-qo
iJJ
-ror
-160
D
\
---f-]
-tlo
-too
-160
-+o
- 10c
- 160
E
CRYPTOPERTHITE
l--T-----_l
-+o
IUU
. IOU
PPM
Fig. 2. 8.45 Tesla 2eSi NMR spectra (corresponding to a
resonancefrequency of71.5 MHz) ofnatural alkali feldsparsand
a synthetic albite glass. (A) Albite (Amelia, Virginia), 2.4 kHz
MASS, 3l scansat 600 sec recycle time, 8.6 kHz spectralwidth,
20 prsec90'pulse excitation, 4096data points, zero-filledto E192,
25 Hz linebroadening due to exponential multiplication. (B)
Microcline perthite (Delaware Co., Pennsylvania), 2.3 kHz
MASS, 3000scansat l0 sec recycle time,8.6 kHz spectralwidth,
2.6 psec 23' pulse excitation, 4096 data points, zero-filled to
8192,5 Hz linebroadeningdue to exponential multiplication. (C)
Synthetic albite glass, 2.4 kHz MASS, 87 scans at 600 sec
recycle time, 8.6 kHz spectral width, 30 psec 90" pulse
excitation, 2048 data points, zero-filled to 8192, 100 Hz
linebroadeningdue to exponential multiplication. (D) Sanidine
(Wyoming), 2.4 kHz MASS, 2000 scans at l0 sec recycle time,
8.6 kHz spectral width, 2.6 p,sec23" pulse excitation, 4096data
points, zero-filled to 8192, 25 Hz linebroadening due to
exponentialmultiplication. (E) Cryptoperthite ("Fredriksvarn",
Norway),2.7 kHz MASS,65 scansat 6ff) sec recycle time, 8.6
kHz spectral width,22 psec 90" pulse excitation, 204E data
points, zero-filled to 8192, 25 Hz linebroadening due to
exponential multiplication.
Results and discussion
Silicon-29
We present in Figures 2 and 3 the silicon-29 MASS
NMR spectra of some representativefeldspars and feldspar glassesobtained at a magnetic field strength of 8.45
Tesla. Table I summarizesthe chemical shifts obtained
from these spectra, together with sample locality and
composition.
The spectra of all the crystalline samples contain
considerable structure arising from silicon sites with
different electronic environments. The spectra of albite
and anorthite glass are both broad, single peaks with
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
t12
ttsi
-rlo
9,45 T MASS
- tou
-L+O
- 1uU
-160
-{o
- lUU
-+0
-lJJ
-82't
- LOU
- tou
I
monrxrreI
GLASS i
SYNTHETIC
ANORTHITE
I
I
I
;
I
fiA*"-;
-t+0
-100
PPIY
-160
r--
-rto
-f-100
PPIY
-160
Fig. 3. 8.45 Tesla 2eSiMASS NMR spectra(correspondingto
a resonancefrequency of 71.5 MHz) of natural and synthetic
plagioclase feldspars and a synthetic glass. (A) Oligoclase
(Bakersviile,North Carolina),2.7 kHz MASS, E2 scansat 300
sec recycle time, 8.6 kHz spectral width, 30 psec 90" pulse
excitation, 2048 data points, zero-filled to 8192, 25 Hz
linebroadening due to exponential multiplication. (B)
Labradorite (Sydney Mines, Labrador), 2.4 kHz MASS, 52
scansat 600 sec recycle time, 8.6 kHz spectral width, 9 psec 90'
pulse excitation, 4096 data points, zero-filled to 8192,75 Hz
linebroadeningdue to exponential multiplication. (C) Bytownite
(CrystalBay, Minnesota),2.3kHz MASS, 313 scansat 8.6 sec
recycle time, l0 kHz spectral width, 2.3 psec 23' pulse
excitation, 4096 data points, zero-filled to 8192, 25 Hz
linebroadeningdue to exponential multiplication. (D) Anorthite
(PacayaVolcano, Guatamala),2.7 kHz MASS,64 scansat 600
sec recycle time, 8.6 kHz spectral width, 22 psec 90' pulse
excitation, 204E data points, zero-filled to 8192, 25 Hz
linebroadeningdue to exponential multiplication. (E) Synthetic
anorthite, 2.5 kH.z MASS, 3l scansat 6C)0sec recycle time, 8.6
kHz spectral width, 25 trr,sec90o pulse excitation, 2048 data
points, zero-filled to 8192, 25 Hz linebroadening due to
exponentialmultiplication. (F) Synthetic anorthite glass,2.4 kHz
MASS, 9l scansat 600secrecycletime, 8.6 kHz spectralwidth,
25 g,sec90'pulse excitation, 2048data points, zero-filled to 8192,
100Hz linebroadeningdue to exponential multiplication.
essentially no structure. Because silicon-29 has spin I =
ll2 and does not suffer from quadrupolar broadening,
these broad, structureless peaks imply a continuum of
electronic environments around the silicon sites. We first
discuss the alkali feldspars whose spectra are shown in
Figure 2.
The MASS NMR of low albite (Fie. 2,{) was first
examined by Lippmaa et al. (1980) and has also been
discussedby K. A. Smith et al. (1983)and J. V. Smith et
al. (1984).Becauselow albite has three particularly well
resolved peaks, it has become a standard material in
many NMR laboratories. The assignmentsof the three
peaks to the three silicon sites in albite (Harlow and
Brown, 1980)listed in Table I are those of Lippmaa et al.
(1980),basedon qualitativearguments,and elaboratedby
K. A. Smith et al. (1983),basedon a procedurethat uses
cation-oxygenbond strength.The peak at -92.2 ppm is
assignedto T2m, which has two aluminum next-nearest
neighbors and two silicon next-nearest neighbors, Qa
(2Al), and those at -96.3 and -104.7 ppm to T2o and
are the
Tlm, which are Q4(tlt) sites.Theseassignments
same as those of J. V. Smith et al. (1984)basedon the
mean secant of the Si-O-T angle.
The silicon-29 NMR spectrum of microcline perthite
(Fig. 28) exhibits five resolved peaks. The sample is a
typical microcline perthite from a pegmatite. The microcline phase has a 13l-l3l powder X-ray peak separation
of -0.7E" (CuKa) and is a maximum microcline.Our site
assignmentsare based on the NMR measurementsof
feldsparsby J. V. Smith et al. (1984),
cation-exchanged
which are diferent from those of K. A. Smith et al. (1983)
based on the structure refinement of Brown and Bailey
(196/.).The new assignmentsof J. V. Smith et al. give the
sameorder of peaksas for low albite. We assignthe peaks
at -96.0, -98.6 and -101.8 ppm in Figure 28 to T2m,
T2o and Tlm, respectively,of maximummicrocline.The
small peak at -93, the added intensity under the peaksat
-96.0 and -98.6 ppm and the small peak at -106.0 ppm
belong to the small amount of intergrown low albite in the
perthites.
The spectraof low albite and microcline perthite clearly demonstrate the high degree of Si/Al tetrahedral site
ordering that has been deduced for these minerals by Xray difraction. The fact that there is no recognizable
silicon-29signalfor the Tlo site in either spectrum is also
consistent with the accepted models for these minerals
which propose that Tlo is occupied entirely by aluminum.
The spectrum of cryptoperthite in Figure 2E has some
similarities to those of albite and microcline perthite.
There are four resolved peaks, but the backgroundunder
these peaks is higher than for the other two minerals.
This cryptoperthite comes from "Fredriksvarn" (now
Stavern) in the well known Larvik region in southern
Norway. Many of the cryptoperthites from this region are
moonstones.Our sampleis the much-studied"Spencer
R" (Spencer, 1937).The cryptoperthites from the Larvik
region are highly sodic, vary from place to place and are
likely to be zoned compositionally and texturally (Spencer,1937; Oftedahl, 1948;Muir and Smith, 1956;Smith
and Muir, 1958).SpencerR has a bulk composition (mole
percent) of about Or+rAbsrAnr (Spencer, 1937). The
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
aggregateperthite of our sample has monoclinic optics.
Albite exsolution lamellae, though very fine, are clearly
visible under the polarizing microscope, even at low
magnifications. Nonetheless, transmission electron micrographsof SpencerR (e.g. Bollmannand Nissen, 1968)
show albite lamellaeless than 1000Aacross.
Stewart and Wright (1974)report the potassic phaseof
SpencerR to be monoclinic (orthoclase)by powder X-ray
difraction and to have a composition (mole percent) of
Ors2. They also report it to have the second largest
amount of "strain" (Aa) of any potassic feldspar for
which they have data.
It is to be noted that even the alkali feldsparsfrom the
Larvik area have significant calcium contents ranging
from around Ana to at least Anle (Spencer, 1937; Oftedahl, 1948;Muir and Smith, 1956;Smith and Muir, 1958).
Ribbe (1983, p. 243), for example, reports that the exsolved plagioclaselamellaein cryptoperthite from Larvik
(presumed by us to be similar to Spencer R) have a
compositionin the rangeAnrs_ra(oligoclase).In line with
the bulk composition Ora3Ab53Ana
for Spencer R, transmissionelectron micrographsof this and similar perthites
from the Larvik area show the cross-sectionalarea ofthe
plagioclaselamellae to exceed slightly that of the orthoclase.
The NMR spectrum (Fig. 2E) of the cryptoperthite is
dominatedby sodic plagioclasepeaks at -92.3, -96.0
and -103.5 ppm becauseofthe high degreeofexsolution
and the high proportion ofsodic feldspar. Thesepeaksare
almost identical to the ones for low albite in Figure 2,4,
and 28, despite the probably higher An content and
higher temperature state of the plagioclase lamellae in
SpencerR.
Only two peaks attributable to the potassic feldspar
phaseare apparent:the added height ofthe peak at -96.0
and the small peak at -98.9 ppm.
In monoclinic alkali feldspars(orthoclaseand sanidine)
thereare two setsof tetrahedralsites,Tl andT2.In ideal
orthoclase, the Tl sites are thought to be randomly
populated by aluminum and silicon ions; the T2 sites are
thought to contain only Si ions. Such a phase should
exhibit two silicon-29NMR peaks, the one corresponding
to T2 having about twice the area ofthe one corresponding to Tl. In ideal sanidine,on the other hand, the Tl and
T2 sites are both thought to be randomly populated by
one-quarter aluminum and three-quarters silicon, on the
average. In this case, one expects two NMR peaks of
about equal area.
Unfortunately, the unavoidable interference by the
plagioclase peak at -96.0 ppm makes it diffcult to
estimatethe relative areasof the two peaksfor orthoclase
at -96.0 and -98.9 ppm. We infer that the low peak at
-98.9 is from the Tl site becausesuch a site is more
likely than a T2 site to resemble the Tlm site of microcline having a peak at -101.8 ppm. The addedheight of
the peak at -96.0 ppm, therefore,is assignedto the T2
site.This is closeto the peak at -98.6 ppm (Fie. 28) from
the T2o site of microcline.
ll3
By qualitative comparisonof the -92.3, -96.0, and
-103.5 ppm peaksof SpencerR (Fig. 2E) with those for
low albitein Figure 2A,we believethat the componentof
the peak at -96.0 ppm (Fig. 2E) contriburedby the T2
site of orthoclase is appreciably larger than the peak at
-98.9 ppm contributedby the Tl site. This is consistent
with the characterization of the potassic phase of the
cryptoperthite as orthoclase.
We chosethis cryptoperthite for study partly becauseit
is thought to represent a potassic feldspar having a large
number of strained sites in the coherent boundary regions
betweenlamellae(Stewart and Wright, 1974).Weexpected to observeconsiderablybroadenedNMR peaksarising
from ranges of distorted silicon sites in these boundary
regions. The individual peaks in the cryptoperthite spectrum are just as sharp and narrow as those for low albite
and microcline perthite. The unresolved background in
the cryptoperthite spectrum, however, is much larger.
We interpret this background as signal arising from
strained sites in coherent boundary regions. The signalin
the narrow peaks is coming from the unstrained volumes
away from the lamellae boundaries.
The silicon-29 NMR spectrum of sanidine (Fig. 2D)
consists of a broad doublet. This feldspar (Or6sAb36An2)
occurs as clear, glassy, I to 5 mm phenocrystsin rhyolite
from Wyoming. Our interpretation of the sanidine spectrum is the sameas for the orthoclasein the cryptoperthite. The two overlapping peaks represent the two sets of
tetrahedral sites, Tl and T2, which are the only two
present in monoclinic alkali feldspars. We infer that the
peak at - 101ppm for sanidinecorrespondsto the peak at
-98.9 ppm for orthoclaseand arisesfrom the Tl site, and
that the peak at -97 ppm in sanidine correspondsto the
peak at -96.0 ppm for orthoclase and arises from T2.
Again, this is consistent with the low albite and microcline data, for which T2m and T20 are less shielded(less
negative)than Tlm.
The peaks for sanidine are much broader than any of
those discussed so far. Because silicon-29 has spin
| = l/2, there is no broadeningfrom quadrupolarinteractions. Moreover, there should be no peak broadening
from field inhomogeneity because our magnetic field
inhomogeneity is less than I ppm. Consequently, we
interpret the peak broadeningto arise from AVSi disorder
in the occupancy ofthe two tetrahedral sites. The apparently equal areas under the two overlapping peaks indicates an approximately equal distribution of silicon and
aluminum between the Tl and T2 sites, as expectedfor a
high sanidine.Becauseofthis disorder,for eachofthese
sitesthere are probably Q4(0Al), Q4(lAl), Q4(2Al), and
Q4(3Al) sitesfor both Tl and T2. Becauseof the overlapping of the peaks from each of these eight sites, and
becauseof KA.{a disorder, they cannot be resolved. The
wings in Figure 2D in the range of 105 and 88 ppm are
probably due to Q410Al) and Q4(3Al) sites respectively.
A significantconcentration of Q4(4Al) sites would not be
expectedfor a phase with a SiiAl ratio of 3/1 (Klinowski
et al., 1982).
ll4
KIRKPATRICK ET AL,: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
We consider now the silicon-29 spectra of our plagioclases (Fig. 3). We have already dealt with the most
commonNa endmember,low albite.We considerfirst the
calcium end-member,anorthite, and then the intermediate plagioclases.
The spectrum (Fig. 3E) of the synthetic anorthite has
three well resolved peaks: at -82.9, -84.8 and -89.6
ppm and a small peak at about 95 ppm.
The spectrum of the natural anorthite (Ansz), Figure
3D, is essentially identical to that of the synthetic anorthite, except that the peaksat -83.0 and -84.6 ppm are
not as well resolved in the natural sample. The intensity
in the range of -95 ppm may be due to Q4(3Al) sites,
although the signal intensity is at about the noise level.
Anorthite at room temperature ("P-anorthite") has
eight nonequivalentSi sites (Megaw et al., 1962;Wainwright and Starkey, l97l; Staehli and Brinkmann,
l974a,b;Kalus, 1978).The peaksin the anorthitespectra
can be assignedusing a correlation between the secantof
the mean Si-O-Al angle for each site and the silicon-29
chemical shifts, similar to those for the silica polymorphs
and alkali feldsparsdevelopedby J. V. Smith and Blackwell (1983)and J. V. Smith et al. (1984).This correlation
gives three clusters of points using all three available
structure refinements (Megaw et al., 1962; Wainwright
and Starkey, 1967; and Kalus, 1978), while the bond
strength sum and mean Si-O bond distance correlations
of K. A. Smith et al. (1983)and Higgins and Woessner
(1982)do not. The only discrepancyis the reversal of
Tl(ooi) and Tl(mzo) in the Wainwright and Starkey
structure, probably due to a long Al-O distance for their
refinement. The variations in the bond strength sum and
bond distanceare small(0.2 V.U. and 0.005A,respectively), and the predicted values are in the range of Q4(4Al)
sites(Lippmaaet al., 1981;K. A. Smith et al. (1983).
. The small peak for the synthetic anorthite at about -95
ppm is probably due to Q4(3Al) sites, which indicate a
lack of perfect AVSi order in this rapidly crystallized
sample.The poorer resolution of the peaks in the natural
anorthite (AnqJ is probably due to the albite component.
Many of the peaks of the spectra of the intermediate
plagioclasesin Figures 3,4,,38, and 3C can be assignedto
silicon sites similar to those in albite or anorthite. For
none of thesesamples,however, can all the peaksof both
albite and anorthite be recognized. Consequently, a simple model involving only domains of albite and anorthite
structures and compositions cannot account for the observedspectra.We can, however, account for the spectra
in terms of the earlier-mentionedlow albite, e-plagioclase
and /-anorthite structures. We attribute much of the
signalin each spectrum to sites in strained volumes in eplagioclase.
The spectrum of oligoclase in Figure 3A, has many
similarities to that of albite (Fig. 2A). The principal
differencesare that the peaks at -93.2, -97.2, and
-105.3 ppm for oligoclaseare broader,overlapconsiderably more, and have diferent relative heights, and that
there is a small peak at -87 ppm. Becausethe chemical
shifts are identical to those of albite, we assignthe peaks
at -93.2, -97.2, -105.3 ppm to the T2m, T2o and Tlm
sitesof albite.We believethe low peak at -87 ppm arises
from Q4(4Al) sites like those in anorthite.W. -H. Yang
(1984personal communication) has found a very similar
spectrum for an An23plagioclase.
It is possible to interpret the oligoclase spectrum in
terms of a single e-plagioclaseof composition An23, a
composition between the peristerite and Bpggild gaps.
Mclaren (1974)gives transmissionelectron micrographs
of an oligoclase from the same locality as ours which
show cross-hatchedlamellae having a spacing of about
1004. This is about the right order of magnitudefor the
spacings(204) that Smith (1974)says are to be expected
for the modulated structure of Ab-rich and An-rich domains in this compositional range.
We believe that the peak broadening and overlap and
the extra intensity in the -90 to -100 ppm range come
from a range of silicon site environments in strained
regions between the Ab-rich and An-rich domains in the
modulated structure. The fact that the cbll volume of
albite (660A3)is less than the comparablehalf-cell volume
(6704) of anorthite suggeststhat the bond angles are
distorted and that the silicon tetrahedra of the albite
domains may be slightly stretched whereas those of the
anorthite domains may be slightly compressed in the
strained volumes around the coherent boundaries. The
longer Si-O bonds in the outer parts of the albite domains
are related to decreased shielding and less negative
chemical shifts. The shorter Si-O bonds in the anorthite
are related to increased shielding and more negative
chemical shifts. There are probable changes in T-O-T
bond angles also. The spectrum for this composition,
therefore, is albiteJike becausethe "albite" domains are
larger. Becauseof this the ratio of strained to unstrained
volumes of the albiteJike material is relatively low.
Becausethe volume of An-rich domains is small, all or
most of this material is strained. This accounts for the
lack of anorthite signalin the -83 ppm range. In addition,
there could be a significant number of Q4(3Al) sites along
the boundaries between the domains contributing signal
in the range -90 to -95 ppm.
The small peak at about -108 may be due to Q4(0Al)
sites in the boundaries between domains or to quartz
impurity, which was not detected optically or by X-ray.
The spectrum of labradorite (Fig. 3B) shows rather illdefinedpeaksor shouldersat -94, -101, and -107 ppm
which may be assignableto Q4(zat) and Q4(lAl) albitelike sites, but more signal than the oligoclase at less
negative chemical shifts, a poorly resolved peak at -88
ppm, and a shoulder at -83 ppm. The significant amount
of signalnear -83 ppm indicates that the most deshielded
Q4(4Al) anorthitelike sites are present in this sample.
This labradorite (AnsJ falls in the B/ggild compositional gap, and probably consists of two e-plagioclases.It
seemslikely that the albite-like peaks arise mostly from
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
Q4(lAl) and Q4(2Al) sites in albite-rich volumesin the
more albite-rich e-plagioclase.As for the oligoclase, the
extrapeak height in the -93 to -Ifi) ppm rangeprobably
comes from sites in the strained regions around the
marginsof the albite volumes. If the calcium-rich phasein
this material is also an e-plagioclase,somecontribution to
the albite-like peaks must also come from albite volumes
in this material. The intensity at less negative chemical
shifts (the peak at -88 ppm and the shoulder at -83 ppm)
comes from Q4(4Al) anorthite-like sites, which occur in
both e-plagioclases.As for the oligoclase, some signal in
the central part of the spectrum could be due to e4(3Al)
sites at the margins of albite and anorthite-rich volumes.
The relatively shielded value of the peak at -107 probably indicates some contribution from e4(OAt) sites.
The bytownite spectrum (Fie. 3C) shows poorly resolvedpeaksat -84 and -89 ppm assignableto e4(4Al)
anorthitelike sites, shoulders at -95 and -l0l ppm
assignableto Q4(2Al) and e4(lAl) albitelike sites, and
little signal in the range of - 105ppm, although there is a
small shoulder that may be real.
This bytownite (An7s) probably consists of (volumes
of) I-plagioclase and e-plagioclase. The e4(4Al) sites,
which we believeto causethe peaksat -84 and -89 ppm,
are in both materials.The Q4(2Al) and e4(lAl) siresare
in only the e-plagioclase.It seems likely that no - 105
ppm peak is present(or it is very small) becauseessentially all of the albite volume is strained, causing all of the
silicon sites in the albite to becomedeshielded.The peak
at -84 ppm is probably due to overlap of the -82.9 and
-84.8 peak ofpure anorthite.
The silicon-29NMR spectraof the albite and anorthite
glasses(Figs. 2C and 3F) consist of singlebroad peaks,
with no apparent structure. They are each approximately
12 to 15 ppm broader than the total peak breadth of the
correspondingcrystalline phases.The peak for albite has
a maximum at -98 ppm and covers the rangefrom about
-85 ppm to -ll5 ppm. The peak for anorthite
has a
maximum at -87 ppm and covers the range from about
-72 to - 105ppm.
Assuming, as discussedbelow, that all the aluminum in
these glasses is in tetrahedral coordination, both must
have a fully polymerized framework structure. This is
because the ratio of tetrahedral cations (Si + AI) to
oxygen is l/2. All the silicon sites must, therefore, be
Qa(nAl).One questionis what valuesdoesn have.W. -H.
Yang (1984,personal communication) has examined the
silicon-29MASS NMR behavior of analbite produced by
heating Amelia albite at 1073'C for 120 days and
found that detectable amounts of e4(3Al), e4(2Al),
Q4(lAl), and Q4(0Al) are present. The signal in rhis
spectrum covers the range from -85 to -115 ppm.
Becausethe spectrum of albite glassextends to even less
shieldedvalues, it seemslikely that detectableamountsof
all five possible types of silicon sires et4Al) through
Qo(Oetl are present in albite glass.
For anorthite glass the average silicon site must be
Q4(4Al). Becauseof the rangeof sites in albite glass,it
seemslikely that a range of sites are present in anorthite
also.This is consistentwith the presenceof Q4(3Al) sites
in crystalline anorthite. Becausethe amount of signal in
the silicon-29 spectrum of anorthite glass at values more
shielded (more negative) than - 100 ppm is small, the
concentration of Qa(0Al) sites must be quite small. The
presenceof a significant amount of signal at values less
shielded (less negative) than -80 ppm must be due to
distortedQ4(4Al) sites. Someof thesemay be causedby
the presenceofAl in the secondtetrahedral coordination
shell. In perfectly ordered anorthite all ofthese would be
occupied by Si, but in AVSi disordered material some of
these must be occupied by Al.
Aluminum-27
Figures 4 and 5 show the ll.7 Tesla aluminum-27
MASS NMR spectra of our samples. Table 3 lists the
AI II,7T MASS
57
INE
MICROCL
PERTHITE
I
Ioc
oor5o
c
Fig. 4. ll.7 Tesla 27AlMASS NMR spectra(correspondingto
a resonance frequency of 130.3 MHz) of alkali feldspars and
albite glass. (A) Albite (Amelia, Virginia), 3.5 kHz MASS, l0
scansat 25 sec recycle time, 15 kHz spectral width, 3 trr,sec
90'
pulse excitation, 8192data points, 10 Hz linebroadeningdue to
exponential multiplication. (B) Microcline perthite (Delaware
Co, Pennsylvania),3.5 kHz MASS, l0 scansat 25 sec recycle
time, 15 kHz spectralwidth, 3.5 psec 90'pulse excitation,8192
data points, l0 Hz linebroadening due to exponential
multiplication. (C) Albite glass (synthetic), 5 kHz MASS, 200
scans at I sec recycle time, 15 kHz spectral width, 3 psec 90'
pulse excitation, Sl92 data points, 50 Hz linebroadeningdue to
exponential multiplication. (D) Sanidine (Wyoming), 3.2 kHz
MASS, 158scansat 25 sec recycletime, 15kHz spectralwidth,
3.5 ;r.sec 90o pulse excitation, 4O96 data points, 50 Hz
linebroadening due to exponential multiplication. (E)
Cryptoperthite("Fredriksvarn", Norway), 3.2kHz MASS, 76
scans at 5 sec recycle time, 15 kHz spectral width, 4 psec 90'
pulse excitation, 4096 data points, 50 Hz linebroadeningdue to
exponential multiplication.
116
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
t ' At
at 59.3ppm and a full-width at half-heightof about 8 ppm.
We assignthis peak to the Tlo site.
160
B
A
/\.o
Figure 6 shows observed and simulated aluminum-27
l,
LABRADORITEI r,
,
OLIGoCLASE
and static and MASS NMR spectra for albite at 11.7,
r'l'
8.45, and 3.5 Tesla. The great narrowing of the spectra
\,./\
I t\----..f'-'
with increasingmagnetic field strength demonstratesthat
\n
d
--'^-j
the peak shapeis dominatedby the second-orderquadrupole interaction. The resolution increasesas the squareof
the field strength (Meadows et al., 1982).The simulations
in Figure 6 are in very good agreementwith the experimental spectra. These simulations assume one kind of
aluminum tetrahedral site with a quadrupole coupling
constant of 3.29 MHz and an asymmetry parameter of
0.62. Thesevalues are basedupon and in good agreement
with the earlier single crystal NMR measurementsfor
albite by Hafner and Hartman (1964). The simulations
yield an isotropicchemicalshift of 62.710.3ppm. This is
in the range of chemical shifts expected for tetrahedral
50
toc
0
;c
lOC
PPlil frm
AlCl3
aluminum(Mtiller et al., l98l).
PPtl 116 AlCl3
The spectrum of microcline perthite (Fig. 48) consists
to
Fig. 5. I 1.7Tesla27AlMASS NMR spectra(corresponding
of a doublet having maxima at 54.3 and 57.3 ppm. Single
a resonancefrequency of 130.3MHz) of natural and synthetic
crystal aluminum-27NMR studies by Hafner and Hartplagioclase feldspars and anorthite glass. (A) Oligoclase mann (1964)have shown that microcline has one type of
(Bakersville,North Carolina),36kH2 MASS, 36 scansat l0 sec
aluminum with a quadrupole coupling constant of 3.27
recycletime 15 kHz spectralwidth, 5 psec 90'pulse excitation,
MHz and an asymmetryparameterof 0.21. Figures 7C
4096 data points, 50 Hz linebroadening due to exponential
and E show the observed aluminum MASS NMR spec(Sydney
(B)
3.3
Mines, Labrador),
multiplication.
Labradorite
kHz MASS, 1065scansat 5 sec recycle time, 15 kHz spectral trum of microcline perthite and a simulation assumingthe
Hafner and Hartmann values. The agreementis not very
width, 4 psec 90' pulse excitation, 4096 data points, 50 Hz
good. There is additional intensity in the experimental
linebroadeningdue to exponential multiplication. (C) Bytownite
(Crystal Bay, Minnesota), 3.4 k}lz MASS, 549 scans at 5 sec
spectrum around 60 ppm. We believe that this is due to
recycletime, 15kHz spectralwidth, 4 p.sec90'pulse excitation, aluminum in the exsolved albite. Figure 7D shows a two
lt,7T MA S S
4096 data points, 50 Hz linebroadening due to exponential
multiplication. (D) Anorthite (PacayaVolcano, Guatamala),3.3
shifts of feldspars and
kHz MASS, 500 scans at 5 sec recycle time, 15 kHz spectral Table 3. Aluminum-27 NMR chemical
glasses
feldspar
width, 4 pcsec90'pulse excitation, 4096 data points, 50 Hz
linebroadeningdue to exponential multiplication. (E) Synthetic
lp"mutl
FIILD
anorthite,5.1 kHz MASS, 1300scansat 10 sec recycletime, 15 UTERIAL
CHEMICAL
STRENGTH
( TESI,A)
STIIFT
kHz spectralwidth, 4 psec 90'pulse excitation, 4096data points,
-24,2,ZO,t3
50 Hz linebroadening due to exponential multiplication. (F)
(Ab99)
AIbtte
53,56
a.45
Syntheticanorthiteglass,4.9 kHz MASS, 1889scansat I sec
59
recycletime, 15kHz spectralwidth, 4 psec 90'pulse excitation,
(AblOO)
3,5
Class
Albtte
4096 data points, 50 Hz linebroadening due to exponential
54
I .45
55
II.7
multiplication.
Sanldine
observed chemical shifts. Aluminum-Z7 has spin | : 512
and, therefore, the aluminum-27NMR spectraare affected by peak broadening and frequency shifts due to
quadrupoleinteractions as discussedabove. This quadrupolar broadening prevents us from obtaining as much
detailed structural information as with silicon-29. AII of
the observed aluminum-27chemical shifts are consistent
with aluminum in four-fold coordination (Miiller et al.,
1981;Fyfe, et al.,1982).
We consider first the aluminum-27NMR spectraof the
alkali feldspars(Fig. 4). The spectrumfor low albite (Fig.
4A) consistsof a singleasymmetricpeak with a maximum
( -0160)
TI.7
8.45
58
50,55
54,57
ilicrocllne
Perthlte
Cryptoperth
ite
1t .7
58
0ligoclase
(An23)
It.7
60
lI.7
60
Bytocntte
(An70)
lI.7
6 2, 5 6
Anorthite
(An92)
Ll.7
61,55
Anorthtte
(Anl00)
8.45
tL.7
45
58
6 2, 5 5
8,45
11.7
60
Labradortle
Anorthite
'lFor
(An52)
Class
quadrrpolar
Value at 11.7
55
(Anlo0)
nuclel
Tesla ls
fleld
strength.
changes cith
shift
appareot
chemlcal
chentcal
shift.
tle
true lsocropic
essenttally
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
'hr
aLBrtE
mss
--r[.
20a
o
a
-zao
Fig. 6. 27Al static and MASS NMR specrra of albite along
with the MASS NMR computersimularionsat I1.7, g.45and, 3.5
Tesla.(A) ll.7 Tesla, static, 30 scansat 25 sec recycletime, 7g
kHz spectralwidth, 3 psec 90" pulse excitation, 4096data points,
50 Hz linebroadeningdue to exponentialmultiplication. (B) I 1.7
Tesla,4.0 kHz MASS, l0 scansat 25 sec recycle time, jg kHz
spectral width, 3 psec 90opulse excitation, 8196data points, l0
Hz linebroadeningdue to exponential multiplication. (C) ll.7
Tesla, MASS simulation, 65 Hz linebroadening.(D) g.45 Tesla,
static, 50 scansat 25 sec recycle time, 56 kHz spectral width, 3
psec 90opulse excitation, 4096data points, 50 Hz linebroadening
due to exponential multiplication. (E) 8.45 Tesla, 4 kHz MASS,
50 scansat 25 sec recycle time, 56 kHz spectralwidth, 3 !.sec 90o
pulse excitation, 8196data points, l0 Hz linebroadeningdue to
exponentialmultiplication. (F) 8.45Tesla, MASS simulation. 150
Hz linebroadening. (G) 3.5 Tesla, static, 300 scans at 5 sec
recycle time, 24 kHz spectral width, 3 psec 90. pulse excitation,
{D6 data points, 100 Hz linebroadening due to exponential
multiplication. (H) 3.5 Tesla, 4 kHz MASS, 300 scans at 5 sec
recycle time, 24 kHz spectral width, 3 psec 90opulse excitation,
zOlX data points, 100 Hz linebroadening due to exponential
multiplication. O) 3.5 Tesla, MASS simulation, 100 Hz
linebroadening.
component simulation assuming 75Vo microcline and25Vo
albite. Figures 7A and B show the observed albite spectrum and a simulation at ll.7 T for comparison. The
agreement between the two component simulations and
the observed spectmm is excellent. The simulation (Fig.
7E) yields an isotropic chemical shift for microcline of
58.5+0.5 ppm. As for the albite, the aluminum-27 NMR
data for microcline clearly indicate that essentially all the
aluminum is in one tetrahedral site.
The spectrum for the cryptoperthite (Spencer R, Fig.
4E) consists of a single symmetric peak having a maximum at 57.9 ppm. As mentioned earlier, the bulk composition of this sample is about Ora6: there is a dominant
oligoclase and a less abundant orthoclase. The quadrupolar broadening masks the splitting of the peaks from the
Al sites in the orthoclase and oligoclase.
The spectrum of sanidine (Fig. aD) is also a single
symmetric peak, but is slightly broader than those of the
other alkali feldspars (12 ppm wide at half-height). The
|7
maximum is at 58.5ppm, intermediatebetweenlow albite
and cryptoperthite. Single crystal NMR studies of sanidines (Brun et al., 1960)have not yielded discrete rotational patterns for different types of aluminum. The high
resolution aluminum-27 MASS NMR lineshape of sanidine (Fig. 4D) agrees with this and is consistent with a
range of quadrupole coupling constants, asymmetry parameters and/or isotropic chemical shifts. The Ho field
dependenceofthe lineshape(data not shown) shows that
the average quadrupole coupling constant is about 3
MHz, becausethe location and breadth of the centerband
is similar to those of albite and microcline.
We consider now the aluminum-27 spectra of the
plagioclases.The spectrum of synthetic anorthite (Fig.
5E) is more complicated that those of the alkali feldspars.
It consistsofa peak at 61.5ppm and a broad shoulderat
about 55 ppm, and is strongly asymmetric.
Anorthite at room temperaturehas eight slightly differ-
'ht
n,7T MASS
SPECTRUM
SIMULATION
'io
PPM
Fig. 7. ll.7 Tesla, 27Al MASS spectra and computer
simulationsof albite and microcline perthite. (A) Albite, 4.0 kHz
MASS, l0 scansat 25 sec recycle time, 4 kHz spectral width, 3
g,sec90" pulse excitation, E196data points, l0 Hz linebroadening
due to exponential multiplication. (B) Albite, computer
simulation using quadrupolecoupling constant of 3.29 MHz and
asymmetry parameter of. 0.62, 65 Hz linebroadening. (C)
Microcline perthite, 4.0 kHz MASS, 20 scans at 15 sec recycle
time, 4 kHz spectralwidth, 3 g,sec90opulse excitation, El96 data
points, l0 Hz linebroadeningdue to exponential multiplication.
(D) Microcline perthite computer simulation using sum of albite
simulation (B) and a second component having a 3.22 MHz
quadrupolecoupling constant and asymmetry parameterof 0.21,
75 Hz linebroadening due to exponential multiplication. (E)
Computer simulation using 3.22 MHz quadrupole coupling
constant and asymmetry parameter of 0.21 and 75 Hz
linebroadeningdue to exponential multiplication.
llE
KIRKPATRICK ET AL.: NIJCLEAR MAGNETIC RESONANCE OF FELDSPARS
ent aluminumtetrahedralsites.Singlecrystal NMR studies yield eight different electric field gradient tensorswith
quadrupolecoupling constantsrangingfrom 2.66 to 8.42
MHz (Staehli and Brinktnann, l974a,b). The broadened
peak in the MASS spectrum is due to overlap of the
signalsfrom the different Al sites.
The aluminum-27spectraof the four natural, intermediate plagioclases(Figs. 5A, 58, 5C, and 5D) all have peak
maximabetween60.2 and 61.5 ppm. The peaksbecome
broader and more like those of the synthetic anorthite
with increasing An-content. Even for the Pacaya anorthite, the large peak and shoulder are less resolved than
for the synthetic anorthite. The quadrupole broadening
preventsresolutionof the many closely spacedpeaks'
The aluminum-27NMR spectra of albite and anorthite
glass (Figs. 4C and 5F) consist of broad, structureless
peaks (about 18 and'28 ppm wide, respectively)with
maxima at 55.0 and 59.6 ppm, respectively.Quantification of the total aluminum content in albite glassusing the
heightof the free-inductiondecayyields about75 percent
of the expected intensity. Once again we interpret these
spectra as indicating that the aluminum sites in the glass
have a wide range of quadrupole coupling constants,
asymmetry parameters, and/or isotropic chemical shifts.
The anorthite glass spectrum is skewed to less positive
chemicalshifts, with a very broad underlying component.
Quantification of the aluminum indicates that only about
50Voof the expected intensity is observed. As for the
albite glass, we interpret this low intensity as indicating
that the aluminum in the glass is in a wide range of
electronic environments, many of which have large quadrupole coupling constants.
Little is known about the effects of Al-O bond distances or angles, total cation-oxygen bond strength, or
other structural parameters on aluminum-27 chemical
shifts and quadrupole coupling constants, so it is not
possible at the present time to give a more detailed
analysisof our resultsin structuralterms.It seemslikely,
however,that at least part ofthe peak broadeningis due
to a wide range of electric field gradients around aluminum in the glasses.There is no signalin the chemical shift
range near 0 ppm, which would indicate octahedrallycoordinated aluminum, although becausewe do not observe the full intensity expectedon the basis of aluminum
content, we cannot rule out the presence of highly
distorted, six-coordinated sites. X-ray radial distribution
studies(Taylor and Brown, 1979)and Raman spectroscopy (Mysen, et al., 1982)do, however, suggestthat 6coordinated aluminum is unlikely to be present in these
glasses.
'uNo
n,7T MASS
M I C R O C LNI E
PERTHITE
-50
Fig. 8. 23NaMASS NMR spectra of natural alkali feldspars
and synthetic albite glass at ll.7 Tesla (coresponding to a
resonance frequency of 132.3 MHz)' (A) Albite (Amelia,
Virginia),4.0kHz MASS, 30scansat 25 secrecycletime' 15kHz
spectral width, 4 (psec 90opulse excitation, 8192data points, l0
Hz linebroadening due to exponential multiplication. (B)
Microcline perthite (Delaware Co., Pennsylvania), 4.0 kHz
MASS, 100scansat 25 secrecycletime, 15kHz spectralwidth, 3
90opulse excitation, 8192data points, l0 Hz linebroadening
;.r.sec
due to exponential multiplication. (C) Albite glass (synthetic)'
4.7 kHz MASS, 1ffi3 scansat I secrecycletime, 15kHz spectral
width, 4 psec 90'pulse excitation, 4096 data points, 150 Hz
linebroadeningdue to exponential multiplication. (D) Sanidine
(Wyoming),4 kHz MASS, 159 scansat 5 sec recycle time, 15
kHz spectralwidth, 3.5 psec 90'pulse excitation' 4096 data
points, 50 Hz linebroadeningdue to exponential multiplication.
(E) Cryptoperthite ("Fredriksvarn", Norway), 4 kHz MASS'
708scansat 5 sec recycle time, 15kHz spectralwidth, 4 psec 90o
pulse excitation,4096 data points, 50 Hz linebroadeningdue to
exponentialmultiPlication.
has a smallernuclear spin than aluminum-27,for the same
quadrupole coupling constant, sodium has a larger second-order quadrupole breadth and hence broader lines
(Meadowset al., 1982).
The sodium-23 spectrum of low albite (Fig. 8A) con-21.9 ppm'
Sodium-23
sists of a doublet with peaks at -13.1 and
two sodium
presence
of
We show in Figures 8 and 9 the ll.7 T sodium-23 The split peak is not due to the
quadruorder
second
the
by
is
caused
MASS NMR spectra for all of our sodium-containing sites in albite, but
by
partially
averaged
only
which
is
pole
interaction,
samples.Table 4lists the chemical shifts at the singularilow
of
studies
NMR
sodium-23
=
crystal
Single
MASS.
like
aluminum-27
I
312
and
ties. Sodium-23 has spin
suffersfrom quadrupolarbroadening.Becausesodium-23 albite have yielded a quadrupolecoupling constantof 2'62
KIRKPATRICK ET AL.: NT]CLEAR MAGNETIC RES0NAN1E oF FELDSPARS
tuNo
n
PPMfrom NoCl
n,7T MASS
-cn
o
_En
PPM frodt NoCt
Fig. 9. 23NaMASS NMR spectraof natural plagioclase
feldsparsat IL7 Tesla(corresponding
to a resonance
frequency
of 132.3
MHz).(A) Oligoclase
(Bakersville,
NorthCarolina).
3.6
kHz MASS,ll8 scansat l0 secrecycletime, 15kHz spectral
width, 5 psec 90' pulse excitation,4096data points, 50 Hz
linebroadeningdue to exponentialmultiplication.(B)
Labradorite(SydneyMines, Labrador),3.3 kHz MASS, 725
scansat 5 secrecycletime, 15 kHz spectralwidth, 4 psec90.
pulseexcitation,4(D6datapoints,50 Hz linebroadening
dueto
exponentialmultiplication.(C) Bytownite(CrystalBay,
Minnesota),
3.4kHzMASS,314scansat 5 secrecycletime,15
kHz spectralwidth,4 ,rsec90opulseexcitation,4096datapoints,
50 Hz linebroadening
due to exponentialmultiplication.(D)
Anorthite(Pacaya
Volcano,Guatamala),
3.4kHz MASS,1000
scansat 5 secrecycletime, 15kHz spectralwidth, 4 psec90.
pulseexcitation,21096
datapoints,100Hz linebroadening
dueto
exponential
multiplication.
MHz and an asymmetryparameterof 0.25 (Hafner and
Hartmann, 1964).The static and MASS lineshapesas a
function of magneticfield strength(Fig. l0) are consistent
with the lineshape being dominated by the quadrupole
interaction.The 3.52T MASS spectrumis very complex,
containing many spinning sidebands,due to the fact that
the spinning speedis much less than the spectralbreadth,
when expressedin frequencyunits.
We have obtainedan excellentsimulationof the I1.7 T
MASS spectrum of albite (Fie. il) assuminga single
sodium site with a quadrupole coupling constant of 2.5g
MHz and an asymmetryparameterof 0.25, and yielding
an isotropic chemical shift of -6.8-{-0.3 ppm, in good
agreement with the value of -7.3 ppm obtained by
Lippmaaet al. (1980).The agreementbetweenthe experimental and simulated spectra clearly indicates that there
is only one sodium site in albite.
The values of the quadrupole coupling constant and
asymmetry parameter of sodium-23 in albite are confirmed by variable angle sample-spinning(VASS) NMR,
ll9
Figure I I, which generatesdifferent lineshapesdepending
on the spinning angle, when the secondorder quadrupole
interactiondominates(Oldfieldet al., 1983;Ganapathy
et
al.,1982). As expected, better averagingis obtained near
40o or 75". Note that at 75' the first order spinning
sidebandsare absent (Ganapathyet al., 1982).
The sodium-23spectrum of microcline perthite at ll.7
T (Fig. EB) showspeaksat -13.1, -21.9 and -26.5 ppm.
From the Ho magnetic field dependenceof the lineshape
(data not shown) there appearto be two types of sodium,
one in exsolvedlow albite (-13.1 and -21.9 ppm) and
one in microcline(-26.5 ppm). The -26.5 ppm line is not
appreciably shifted in going from 8.45 T to ll.7 T,
suggestingthat the sodium site in microcline has a smaller
electric field gradient and, hence, is in a more symmetric
environment than in albite. Spectra of microcline obtained at ll.7 T simulatewell with an albite component
and an additional llVo contibution from a line with a
quadrupolecoupling constant of about 0.5 MHz, this
additional intensity being from sodium in the microcline.
The low quadrupole coupling constant is in agreement
with the small magnetic field dependenceof the chemical
shift. However, because equally good spectral simulations can be obtained using two powder patterns having
e2qQ/hof about 3 MHz, further work is neededin order to
substantiatethis very tentative assignment.
The sodium-23 MASS NMR spectrum of the cryptoperthite (Fig. 8E) shows a doublet due to the secondorder quadrupoleinteraction. These peaks are very similar to those of albite, except that they are less well
resolved. This spectrum is in agreementwith the silicon29 and aluminum-Z7data, which indicate that some Si/Al
ordering has occurred in this material, but that neither is
as complete as in low albite or microcline perthite.
Table 4. Sodium-23NMR chemical shifts of feldspars and
feldsparglasses
FrdLD
STRENGTH
( tEsLA)
STERIAL
(Ab99)
Albtte
AlblEe
p"anettt
frE{I(;AL
SqISl
clasB
sanrdtne
Ilcrocltne
(Abroo)
rt.7
-I1.1
-21.9
rl.7
-13
MMNTS
Quad(upole
Spttt
Qudrupore
Spllr
Quadrupole
Spllr
-21
( "or60)
-Il
Perrhtre
-26
-r3
-22
Crtptoperthtre
olt8oclase
Labradortt€
(An23)
(An52)
lI.7
-15
11.7
-I6
-14
Bytohlte
(An70)
ll.7
Ano(Ehlte
(e92)
lr.7
rFor
quadrupolar
nuclet
apparedr cheolcat
shlrr
chanS€s di!h
fteld
srreErh.
peak le oot spllt
by second-order
quadrupolar
effecrsr
value ar tt.7
Tesla
e€e€nttally
th€ t3orroplc
chetrtcal
shtft.
If
ts
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
r20
ttNo
ALBTTE
STATIC
il,7 7
8,457
of sites with different quadrupole coupling constants,
asymmetry parameters, and/or isotropic chemical shifts'
In the plagioclases, much of this is probably due to
distortion of the sodium sites near the margins of albite
and anorthite volumes. The relatively large spinning
sidebandsin these spectra, like the aluminum-27 NMR
spectra of these same samples(Fig. 5), are probably due
to the presence of magnetic impurities (Oldfield et al.'
1983).
The sodium-23NMR spectrum of albite glass (Fig' 8C)
is very broad and featureless,againindicating sites with a
range of quadrupole coupling constants, asymmetry parameters. and/or chemical shifts.
SummarY and conclusions
The foregoing results demonstrate that MASS-VASS
NMR spectroscopy complements well the other main,
reasonablydirect methods of investigatingthe structures
of feldspars-single-crystal X-ray and neutron diffraction,
,T
electron diffraction, transmission electron microscopy
.t
5C3
PPMt?6 NoCl
(TEM) and single crystal NMR spectroscopy' Each of
these methods has its strengths and its weaknesses'The
Fig. 10. 23Nastaticand MASS NMR spectraof albiteat 11.7,
particular strength of NMR spectroscopyis that in many
8.45 and 3.5 Tesla. (A) 11.7 Tesla, static,30 scansat 25 sec
recycle time, 132kHz spectral width, 3 psec 90'pulse excitation'
4096 data points, 50 Hz linebroadening due to exponential
multiplication.(B) 1l .7 Tesla,4.0 kHz MASS, 30 scansat 25 sec
recycle time, l3zkHz spectralwidth, 3 psec 90" pulseexcitation,
El96 data points, l0 Hz linebroadening due to exponential
multiplication. (C) 8.45 Tesla, static, 100scansat 25 sec recycle
time, 95 kHz spectralwidth, 3.5 psec 90'pulse excitation'4096
data points, 50 Hz linebroadening due to exponential
multiplication.(D) E.45Tesla, 4.8 kHz MASS, 100scansat 25
sec recycle time, 95 kHz spectral width, 3 psec 90' pulse
excitation, El92 data points, 50 Hz linebroadening due to
exponentialmultiplication. (E) 3.5 Tesla, static, 1000scansat 25
sec recycle time, 39 kHz spectral width, 3.5 psec 90' pulse
excitation, 40!)6 data points, 100 Hz linebroadening due to
exponentialmultiplication.(F) 3.5 Tesla, 3'5 kHz MASS' 1000
scansat 15 sec recycle time, 39 kHz spectral width, 3 psec 90o
due to
pulseexcitation,4096data points, 100Hz linebroadening
exponential multiplication.
The sodium-23 MASS NMR spectrum of sanidine (Fig.
8D) consists of a single broad peak at -21'2 ppm, which
is slightly broader than those of albite and microcline
perthite. The lack of a well resolved quadrupolar splitting
for sanidine suggests to us, once again, that Al/Si disorder
results in a range of sodium sites with different quadrupole coupling constants, asymmetry parameters' and/or
chemical shifts. The presence of a split sodium-23 peak in
alkali feldspars appears to be a good indicator of AUSi
ordering.
The sodium-23 MASS NMR spectra of the intermediate
plagioclases (Fig. 9A-9D) all show single, relatively sharp
-13.0 to -15.9
centerband resonances in the range of
ppm, that are similar in shape and breadth to the sanidine
spectrum. As for sanidine, this probably indicates a range
ttNo
8,45 T VASS
SIMULATION
SPECTRUM
A
750
D
STATIC
loJ
J.,"
-1JJ
anele),(C) 40', (D) static'
rco
orrll
-1oc
KIRKPATRICK ET AL.: NLICLEAR MAGNETIC RES2NANCE oF FELDSPARS
instancesit providesdirect evidenceabout the electronic
environmentof individual kinds of atomic sites,even for
those ditrering as little from one another as the Tlm, T2m
and T2o silicon sites in the triclinic alkali feldspars.
Moreover, MASS-VASS NMR allows direct and easy
comparison of sites in crystalline and noncrystalline
materials.MASS-VASS NMR spectroscopyuses easily
obtained powder samples whereas single-crystal NMR
spectroscopy requires difficult-to-obtain, large, high_
quality singlecrystals. The major limitations on MASSVASS NMR samplesare that they must be relatively free
of paramagneticimpurities and that at least about l-10 mg
for sodium-23and aluminum-27 andabout 100-500mg for
silicon-29must be available.
Although MASS-VASS NMR spectrometerscurrently
are quite expensiveand not numerous,this is likely to
changefor the better in the near future.
We now summarizefor each type of material we have
examinedthe resultspresentedabove.Again, we empha_
size that while our interpretations are consistent with the
available knowledge of and ideas about feldspar structure, they must at this time be consideredtentative.Our
interpretationsof the intermediateplagioclasespectraare
the most uncertain.
Alkali feldspars
Albite. Becauseit has an orderedand well-understood
structure, the NMR spectra of low albite are the least
ambiguousto interpret. The silicon-29NMR spectrum
hasthreewell-resolvedpeaksof nearly the sameintensity
due to the three different silicon sites.The aluminum-27
NMR spectrum is a single relatively narrow peak, due to
the one tetrahedralaluminumsite. Both the silicon-29and
aluminum-27data clearly support the high degreeof Al/Si
tetrahedral site ordering deduced by X-ray diffraction.
The sodium-23spectrum consists of a sharp, well-resolveddoublet due to the second-orderquadrupolarinteraction, indicativeof a rather asymmetricsodiumsite. All
of theseconclusionsare consistentwith the well-known
structure of albite.
Microcline perthite. The spectra of this sample reflect
the presence of both microcline and low albite. The
silicon-29NMR spectrumcontains three peaks which are
assignable
to the three silicon sitesin microcline,and two
small peaks due to the small amount of intergrown albite.
The third albite peak apparently underlies two of the
microcline peaks. The aluminum-27 NMR spectrum of
the perthite consists of a quadrupole split doublet from
the microcline,and extra intensity due to the albite. The
sodium-23NMR spectrumconsistsof a quadrupolarsplit
doublet,due to low albite, and a secondcomponentdue
to sodium in the microcline. The sodium site in the
microclineis tentativelythoughtto be in an envlronment
more symmetric than that in albite.
Cryptoperthite. The NMR spectraof the cryptoperthite
(SpencerR) are consistentwith partiat AVSi ordering and
separationinto potassiumand sodium-richvolumes.The
12l
silicon-29NMR spectrum is relatively well separatedinto
a number of peaks. Three of these can be assignedto
albiteJike sites and to Tl and T2 sites in orthoclase (low
sanidine).The resolutionofthe peaksis, however,not as
clear as for the microcline perthite. There is only one
aluminum-27 resonance, again indicating that neither
phase has a composition approachingan end member.
The sodium-23 NMR spectrum shows a somewhat
smoothedout doublet due to quadrupolar splitting, again
suggestive of at least enough Al/Si ordering to cause
distortion of the sodium site. There is no sodium-23
resonancespecifically assignableto the orthoclase.
Sanidine. Because sanidine is highly disordered,its
spectra are also relatively easy to interpret. The two
broad, poorly split silicon-29 NMR peaks are due to a
rangeof silicon sites causedby AUSi disorderin the Tl
and T2 sites and to Or-Ab solid solution. These peaks
cover the chemical shift range of the albite spectrum and
indicate a continuum of electron distributions in e4(0Al),
Q4(1Al), Q4(2Al), and e4(3Al) sires. The aluminum-27
NMR spectrumconsistsof a singlepeak which is broader
than that of albite, also suggestinga range of aluminum
environmentsin Tl and T2. The sodium-23resonanceis
not split by quadrupolar effects, suggestinga range of
sodium environments and probably a more symmetric
envrronment.
Plagioclases
Anorthite. The silicon-29NMR spectrumof synthetic
anorthiteconsistsof three Q4t+et)peaksthat are assignable to the eight silicon sitesin anorthite plus a small peak
that we assign to Q4(3AD sites. The silicon-29NMR
spectrum of the natural anorthite (Ansr, a phenocryst
from Pacayavolcano(Guatamala),is similarbut lesswell
resolved.The aluminum-27NMR spectrumof both anorthites are broad, with one well resolved peak, and a
shoulderat lesspositive (more shielded)chemicalshifts.
MASS NMR is apparently not able to resolve the eight
different aluminum sites. The sodium-23 spectrum of
natural anorthite is discussedbelow along with those of
the other intermediate plagioclases.
Intermediateplagioclases.The spectraof the intermediate plagioclasesseem to be consistentwith structural
models involving e- and /-plagioclaseswith e4(4Al) anorthite-like sires,e4(lAl) and e4(2Al) albite-likesites, and
distortedequivalentsofall threein strainedregionswhere
the albite-likeand anorthite-likevolumes meet. e4(3Al)
sitesmay also be presentat the boundaries.
The silicon-29NMR spectra of the oligoclase(An3)
and labradorite (An5) exhibit three peaks that can be
readily assignedro Q4(1Al) and e4(2Al) sites in albite.
The peaksdo not all have the sameheight, as they do in
low-albite. We have attributed the extra height in the -93
and - lfi) ppm range to deshieldingof the albite-like sites
and increasedshielding of the anorthite-like sites caused
by coherencystrain of the silicon sites in e-plagioclase
and, perhaps, to Q4(3AD sites at the margins. In the
t22
KIRKPATRICK ET AL.: NUCLEAR MAGNETIC RESONANCE OF FELDSPARS
bytownite (Anro) the most shielded silicon peak is not
presentor is very small,perhapssuggesting
that all ofthe
albite-like volumes have strained silicon sites. The natural anorthite (Anqz)has no peaks attributable to albite-like
sites.
Peaks or shoulders in the silicon-29 NMR spectra
which are attributable to anorthite-like sites increase in
intensity with increasingAn-content. For the oligoclase,
there is only one shoulder at a relatively shielded chemical shift (-88 ppm), which we interpret as arising from
strained Qo(4Al) anorthite-like sites. The peaks assignable to Qo(4Al) sites are larger in labradorite. We have
attributed these peaks to anorthite-like sites in e-plagioclases.The peaksattributable to anorthite-like sites in the
bytownite are very large. We attribute these peaks primarily to sites in /-anorthite, although some of the signal
must come from sites in the e-plagioclase.As noted
above, the silicon-29NMR spectrum of the natural anorthite is similar to the synthetic anorthite, but less well
resolved.
The aluminum-27 NMR spectra of the intermediate
plagioclasesconsist of broad peaksthat becomemore like
that of anorthite with increasing An content.
The sodium-23NMR spectrashow single,unsplit peaks
of about the same breadth as in sanidine.Becausethe
silicon-29NMR peaksindicate significant Al/Si order, we
tentatively attribute the non-split sodium-23resonances
to variable amounts of strain of the sodium sites, which
increases the range of electronic environments around
sodium.
Facility. E. O. is a USPHS Research Career Development
Awardee, 1979-1984.
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