American Mineralogist, Volume63, pages461465, 1978
Virgilite: a newlithium aluminumsilicatemineralfrom the Macusaniglass,Peru
BnveNM. FnsNcs
7408 WyndaleLqne,CheuyChase,Maryland 20015
Prrnn A. Jrzrr ,rNoDnNrrl E. AppuueN
Departmentof Mineral Sciences,NationalMuseumof Natural History
SmithsonianInstitution, Washington,D. C. 20560
Abstract
Virgiliteis hexagonal,
P6"22or P6a22;
a = 5.132,c : 5.454A;n : 1.520,uniaxial(-). Cell
contentof the crystalsstudiedis Lio..,(Si2.3rAlo..oFeo.orPo.o,)ou.
virgilite is found in volcanic
glassof unusualcompositionfrom Macusani,Peru.It is the only naturally-occurring
representativeof the solid-solutionseriesbetweenB-quartz(Qz) and LiAlsiro. (Sp) with a stuffed
p-quartzstructure.The compositionof our materialcorrespondstoSpu,Qzrr.
Introduction
National Museumof Natural History, Smithsonian
The clearpale-greenglassfound as streamcobbles Institution,Washington,D. C. underMeteoriteColnear Macusani, Peru (latitude l4oS, longitude lectionnumberNMNH 2143-17.
70"30'W)hasbeena geological
curiosityfor overhalf
Morphology and occurrence
a century(Linck, 1926;Preuss,1935;Heide, 1936;
Martin and de Sitter-Koomans,
1955;Barneset al.,
Virgilite occursas colorlesseuhedralcrystalsup to
1970).Its peculiarities
are unusualclarity,high alu- 50 pm across(Fig. I ), usuallyprominenthexagonal
mina content, and the presenceof euhedralmicro- bipyramidsmodifiedby a hexagonalprism.Sucheuphenocrysts
of andalusiteand smallerneedles
of sil- hedralcrystalsare rare in the 30 sampleswhich we
limanite(Linck, 1926;Frenchand Meyer, 1970).We examined;but irregular rosette-shaped
fibrous overreportherethe identification
of a new mineralwhich growthsof virgilite are commonlyobservedon the
occursas a rare to commonphasein the Macusani crystalsof quartz(Figs.2, 3), K-feldspar,biotite,and
glass.The mineral has a stuffed B-quartzstructure spinelin the glass.The virgilitecrystalsusuallyoccur
and is theonly knownnaturally-occurring
memberof in parallel bandsresemblingflow structure,suggestthe B-quartzsolid-solutionseriesbetweenLiAlSirO6 ing that virgiliteis a primary phasewhich crystallized
and SiOr. The name uirgilite is proposed for the whiletheglasswasmolten.Threeof thespecimens
we
LiAlSirOsend-member.
In accordance
with custom- examinedcontain up to l0 percentby volume of
ary mineralogicalpractice,this name should be re- euhedralvirgilite;one samplecontainsoval partlystrictedto membersof the solid-solution
serieswith resorbedvirgilitecrystals.
more than 50 mole percentLiAlSirO..
Macusaniglasshasa graniticcompositionwith an
The name uirgilite is given in honor of professor unusuallyhigh contentof Al, F, Li and B. Chemical
Virgil E. Barnesof the Bureauof EconomicGeology, analyses
(Martin and de Sitter-Koomans,
1955;Elliot
Universityof Texas,Austin,Texas.It recognizes
his and Moss, 1965;Barneset al., 1970)give(in weight
pioneeringwork in the study of tektites,impactites, percent):SiO, 70-73, AlzOs16-17,FeO l-2, MgO
and othernaturalglasses,
aswell ashiscollectionand 0.1-0.2,CaO l-2, Na2O 3-4, KrO 3-6. Elliot and
investigationof the Macusanimaterial which was Moss (1965)report in additionB2Os0.4, LirO 0.8,
usedin our study.The mineraland namehavebeen and F 1.4.Lessthan I weightpercentHrO hasbeen
approvedby the Commissionon New Mineralsand found in the analyzedsamples(Barneset al., 1970).
Mineral Namesof the InternationalMineralogical Preliminaryanalyses
madeduring the courseof our
Association.Holotype materialis depositedat the study show some apparentdiscrepancies
with these
-OMX
m,03
I $02.
00
/ 78/ 0s06-046
461
462
FRENCH ET AL.: VIRGILITE
Fig. l. Euhedral virgilite crystal (40pm diameter) surrounded by
glass containing several other virgilite crystals. Plane polarized
light.
Fig. 3. Black-appearing anhedral quartz fragment overgrown by
fibrous virgilite in Macusani glass filled with small euhedral
virgilite crystals. This association of euhedral and fibrous virgilite
is rare. The quartz fragment is 400pm long. Partly crossed
polarizers.
publishedfigures.Additional work is in progressto
Anl Snue electronmicroprobe,with l5kV acceleraresolvethe differences.
Althoughthe Macusaniglasswasonceconsidered ting voltageand 0.02pA samplecurrent.Data were
to be a variety of tektite (Linck, 1926),subsequent correctedby the methodof Benceand Albee(1968).
studiesstronglyindicatea volcanicorigin (Martin Microprobe standardsused are: VG-568 (rhyolite
and de Sitter-Koomans,1955;French and Meyer, glass;J. Norberg,SmithsonianInstitution,personal
for Si, Fe, Na, and K; VG-2 (basal1970).The glasshas not yet beenobservedin out- communication)
1975)for Al; VG-A99 (basaltic
crop, but at the type locality it is associatedwith tic glass;Jarosewich,
volcanic ash flow glass;T.L. Wright,U.S.GeologicalSurvey,personal
outcropsof an andalusite-bearing
with a chemicalcompositionvirtually identicalto communication)for Mg, Ca, and Ti; Corning-D
for Cl; and
that of the glass.The age of this volcanicrock has glassfor P; Mn-ilmenitefor Mn; scapolite
beendeterminedto be about4 million vears(Barnes apatitefor F.
Virgilite crystals within a single glass specimen
et al., 1970).
showa verynarrowcompositionrangeand areprobChemistry
ably identicalwithin analyticalerror (TableI ). Only
overgrowths FeO shows significant variation between virgilite
Virgilite crystalsand rosette-shaped
of virgilite on other mineralswere analyzedon an from different samples.LirO in virgilite was determinedto be 5f0.5 weightpercentusinga laserspectrograph(Frenchand Meyer, 1970).
Fibrousvirgiliterosettes,overgrowingother mineral grains,are identicalin compositionto euhedral
virgilitesin the same specimen.In many samples
their composition
virgiliteis presentonly as rosettes;
rangesare very similarto thoseof the euhedralcrystals,with FeO showingthe largestvariation.
Fig. 2. Rosette-shapedfibrous overgrowth of virgilite on
euhedral quartz. Pseudo-euhedral
boundaries,as shown here,
frequentlyoccur at the glass-rosette
interface.Theseboundaries
are systematically
relatedto the orientationof the quartznucleus.
The long axisof the rosetteis l20pm. Partly crossedpolarizers.
Crystallography
Singlecrystalsof virgilitein hostglasswereinvestiof
gatedon the Buergerprecession
camera.Because
the smallsizeof thesecrystals,unfilteredMo radiaofup to 400hourswerenecessary.
tion and exposures
The resultsshow virgilite to have a B-quartz-type
structurewith spacegroup P6r22or P6n22(hexagocell dimensions
a : 5.13(2),
nal) and approximate
:
c 5.44(2)A.Thereis no evidencefor doublingof a
or c,
FRENCH ET AL.: VIRCILITE
Powderdiffractiondata (Table2) wereobtainedby
the Gandolfitechniquefrom glassfragmentscontaining severalsmallcrystals.The internalstandardwas
NBS StandardReferenceMaterial 640 silicon powder,a : 5.43088(4)4.
Intensitieswerevisuallyestimated with calibratedintensitystrips.The corrected
powder diffraction data were indexed and refined
with the iterativecomputer techniqueof Appleman
and Evans(1973),to yieldthe final cellparameters
a:
5.132(l),c : 5.454(l)A; cellvolumeV = 124.41(2)As.
Optical and physicalproperties
The small size and physicalincorporationof the
virgilitecrystalsin the host glassmakeit impractical
to separatethem for optical measurements.
In situ
measurements
in the glassshow that virgiliteis uniaxial (-) with an averageindex of refractionn =
1.520and birefringence
0.005-0.006.
The GladstoneDale relationshipgivesa calculatedmean refractive
463
Table2. VirgiliteX-ray powderdiffractiondata
h k 1
d(calc.)
d(ob6)
I/Io
00
01
10
o2
II
4.445
3.445
2.566
2.124
2.122
4.442
3.44r
2.567
a 1aa
13
100
6
00
01
L2
0 03
2 o2
2.222
2,O58
r. 859
1.818
r.723
2.224
2.O59
1.870
4
25
L.721
2
I
2
2
I
3
03
IO
11
13
00
1.683
r,580
l,606
1.483
r.482
1.605
1 t,aa
9
I2
01
03
o4
o2
1.430
1.430
t.407
1.3034
1. 3018
1.408
9
20
2T
13
10
r.2831
r.2490
1.2338
r.232A
1.2040
t.2024
11
candolfl
canera.
h k I
40 1
10 5
t1
4
22 3
40 2
1.0888
1.0592
1.0586
1,0483
t.o290
3I
r, 0203
1.0197
1.0038
1,0032
r, 0023
1.2833
3
1 '11A
2
1 'n1r
3
11
30
32
d(obs-)
I/Io
L.l23L
1.0584
20
0.9791
( 2 4 celculated llnes omltted)
3l 5
0.8169
j
^ al^e
33 2
0.4762
2
2l 5
3
f I
0
47 4
)t
1
0.8028
0.7994
0.7985
0.7983
0.7903
0.7898
1.3017
114.59 m
L,t62l
1.1610
1,1484
r.1233
1.1112
2
2
d(calc.)
4
2
3
2
0
dianete!,
cu/Nl
ladlatlon,
0,7903
sl
2
standard,
P67a22; a = 5,132, c = 5.454A.
Table I Chemical analysesof virgilite
Sample
2I43-L7
Sanple
2L43-3
wE7"
Range
vt%
Range
sio2
77.29
77.06-77.52
76.63
76.33-76.88
A12O3
L6.57
L6.40-16.75
16.L4
1 5 . 9 9 - 1 6. 3 9
FeO
0.64
0.59-0.68
1.50
1.40-1.65
Lizo*
4.93
P2O5
O.57
(Table l). Micontents: Lio.61(Siz.s?Ale.6sFeq.o2Po.o1)Oe
q
0,54-0.50
0.58
TtOz
tr
ltgo
tr
ET
I'tIO
tr
tr
CaO
tr
NazO
tr
ET
K2O
tr
tr
c1
0.0
100.0
99.85
Atom/6
si
A1
Fe
Li
P
Oxygens
2.37
0.60
0.02
0.61
0.01
ala
0.54
0.04
o.62
0.01
Tetrahedral
3 .0 0
catlons
(Si, A1, P, Fe)
3.0r
(+) charges
1 2 .0 0
*Deternined
by laser
and Meyer, 1970);
t2.o
spectrograph
on 2L43-3
obtalned by difference
(8. P. Henderson,
measurements
Smithcrohardness
are681
sonianInstitution,personalcommunication)
VickersHardnessNumberminimum.722VHN maximum. For comparison,quartz parallelwith the c
to c I130 VHN.
uis is 981 VHN, perpendicular
Synthesis,structureand stability
ET
0.0
Total
0. 53-0.62
ET
Ftr
index i = 1.523,basedon the measuredchemical
composition(TableI ), the calculateddensity,and the
constantsof Mandarino(1976).
It hasnot beenpossibleto measurethe densityof
virgilite.A densityof 2.46g cm-s is calculatedfrom
the refined cell dimensions(Table 2) and the cell
(French
ot 2L43-L7.
memberof
Virgiliteis identicalto an intermediate
Munoz (1969)along
the "B-quartz"""synthesizedby
the join LiAlSirO6 ("spodumene")-SiO,(quartz).
Munoz' data indicatethat continuoussolid solution
exists above l0 kbar between the compositions
all with a B-quartz(or stuffed
and SpoQztgo,
Sp,ooQzo
B-quartz)structure.The compositionof the natural
virgilite from Macusaniis approximatelySp.'Qzsr.
The cell dimensionsof this materialagreealmost
exactlywith thoseobtainedby Munoz from synthetic
materialof compositionSp6eQ4e.
Syntheticend-membervirgilite with composition
Sproowas designated"B-quartz," (LiAISLO6)" by
Munoz (1969).Li and Peacor(1968)call this phase
refinementwas
"LiAlSi2O6III". A crystal-structure
performedon LiAISLO6III by Li (1969).He found
FRENCH ET AL.: VIRGILITE
464
that the compound had a stuffed B-quartzstructure
with a completelydisordered(Al,Si) distributionin
the single tetrahedralsite. Li is also in tetrahedral
coordination,with the Li tetrahedronsharingedges
with the (Si,Al) tetrahedron.The singleLi atom in
the unit cell is disorderedover an equipointwith
multiplicity3, so that statisticallyeachLi site containsonly l/3 of a Li atom.By contrast,B-eucryptite
(LiAlSiO4) has a stuffed B-quartz structurewith 2
typesof superstructurereflections,indicatinga doubling of both the a and c axes.These are due to
orderingboth of (Al,Si)andof Li (Pillarsand Peacor,
1973).
Both our naturalvirgiliteand the materialsynthesizedby Munoz (1969)were checkedcarefullyby
single-crystal
techniquesand no evidencefor any superstructurereflectionswasobserved.This procedure
wasnecessary
because,
as pointedout by Munoz,the
powder diffraction patternsof p-eucryptitq" and 0quartz""are essentially
identical.Thus there is no
questionthat virgilite has the stuffeddisorderedBquartzstructuredescribedby Li (1969)for LiAlSirOu
III.
Virgilite has not been identifiedin a seriesof socalled "stuffed B-quartz" glass-ceramics
manufactured by Corning Glass (Beall et al., 1967),even
though the compositionsof someof theseproducts
are virtually identicalto that of the Macusaniglass.
Beall et al. repoft the formation of B-spodumene
solid solutionsand cristobalite
in specimens
of glassceramiccrystallizedat 700to 900'C at I atmosphere.
The small size of the crystalsmakesidentification
difficult, and virgilite could also be presentin these
syntheticmaterials.
Discussion
Virgilite in the Macusaniglassrepresentsthe first
naturaloccurrenceof a mineralwith a B-quartzstructure in the LiAlSirO'-SiO, solid-solutionseries.It is
noteworthythat the mineral occursin peraluminous
glass,coexisting
with both andalusite
and sillimanite.
The P-T conditionsof formationof virgiliteare not
yet known. Although Munoz (1969)suggested
that
"B-quartz""" similar to virgilite would probably be
expectedto form in natureonly at pressures
above.l0
kbar, it seemsunlikelythat virgilite in the volcanic
Macusaniglassformed in this range.One possible
explanationis that sluggishreactionratesin the system LirO-AlrOs-SiO2necessitatehigh P and 7" to
promote reaction in laboratory experimentswith
pure materials.In the naturalMacusaniglass,however,reactionrates may havebeenincreasedby the
presence
of Fe,alkalis,volatiles,etc.,thuspromoting
the formation of virgilite at lower pressures
within its
stabilityfield.
An equallyprobableexplanationis that the virgilite crystallizedas a metastablephase,well outsideof
its stability field. Favorableinitial structurein the
glassmight promotethis type of crystallization.
The
naturaloccurrenceof virgilitedoesnot helpgreatlyin
establishing
the stability field of the phasesin the
LirO-Al2Os-SiO,
system.
Origin
The origin of the Macusaniglassitselfis a problem
further study. The glassshowsmany
that deserves
chemicalsimilaritiesto more commongraniticpegmatites,and its compositionis virtuallyidenticalto a
minimum-melting
compositionin the granitesystem
at P(HzO) : 3-4 kbar (Luth et al., 1964).Available
field data are consistentwith the idea that both the
glass and the associatedash-flow depositsresulted
from the explosiveeruption of a volatile-richperaluminousgraniticmelt. It is possiblethat the original melt was formed by the deep melting of aluthusproducinga meltwith Li and
minoussediments,
high enoughto stabilizea lithium
Al concentrations
phase,virgilite,on the liquidus.
aluminosilicate
Acknowledgments
We wish to thank J. L. Munoz and D. R. Peacor for pointing out
the possible metastable crystallization of virgilite.
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FRENCH ET AL,: VIRGILITE
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