1065
Carndian Mineralogist
Vol. 30, pp. 1065-t075 (1992)
THEROLEOF FLUORINE
IN VESUVIANITE:
A CRYSTAL.STRUCTURE
STUDY
LEEA. GROAT
Deparxnent of Geological Sciences, University of British Columbia, Vancouver, British Columbia V6T IZ4
FRANKC. HAWTHORNE
Depanment
ManitobaR3T2N2
of Geological
Sciences,
Universiry
of Manitoba,Winnipeg,
T. SCOMERCIT
Division of Mineral Sciences,CanadianMuseumof Nature, Oftawa,Ontario KI P 6P4
ABSTRACT
The crysal stmcturesofthree samplesoffluorine-bearingvesuvianite
havebeenrefinedto R indicesof *3.5Voin the space
groupP4/nncusingsingle-crystalMoKcrX-ray intensitydata.The samecrystalsweresubsequently
analyzedby wavelength-dispenion electron-microprobe
analysis.Fluorinesubstitutesfor (OH) at boththe OH andO( | 0) sitesin thestructure.At theOH site,
positionaldisorder;changes
thelossofthe OH-O(7)
thereis nodiscernable
requirements
accompanying
in thelocalbond-valence
hydrogenbondare satisfiedby slightchangesinvolvingthe coordinatingcations.Bond lengthsfrom O(7) to he Z(3) position
remainconstant,
in thecrystal.Fluorinealsosubstitutes
buttheO(7)-X(3)distancecontracts
with increasing
F-for-OHsubstitution
for (OH) at theO(10)position.In thiscase,thereis significantpositionaldisorderinvolvingboththeanionsat theO(10)position
with observedvariationsin bond
azd the cationsat the coordinating/(3) position.Correlationof possiblelocal arrangements
Iengthsandchemistryallow a plausiblelocalmodelto beconstructed
for this substitution.
Keywords:vesuvianite,
crystal-structure
refinement,electron-microprobeanalysis,fluorine,hydroxyl.
SOMMAIRE
La structurecristallinedetrois6chantillons
dev6suvianite
comenant
du fluor a 6t6affin6ejusqu'i un r6siduR d'environ3.57o
MoKo.
dansle groupespatialP4lnnclnousavonsutilis6desdonn6es
surcristalunique(rayonnement
de diffractionX obtenues
d'6nergie.Le fluor remplace
Lesm€mescristauxont ensuite6t6analys6s
6lectronique
endispersion
aumoyend'unemicrosonde
les
le (OH) sur lessitesOH et O(10)de la structure.Surle siteOH, il ne semblepasy avoirded6sordredepositiondiscemable;
entreOH et O(10)sont
changements
danslesexigeances
desvalencesde liaisonslocalesduesi Ia pertede la liaisonhydrogbne
satisfaitspardeschangements
Les longueursdesliaisonsO(7YZ(3)
subtilsimpliquantlescationsdansla sphbredecoordinence.
restentconstantes,
du OH parle F. l€ fluor
maisla distanceO(7)-X(3)diminueh mesurequ'augmente
le degrdderemplacement
remplacel'hydroxyledansle siteO(l 0). [,a prdsence
du F causeun ddsordredepositionimportantdesanionset descationsdans
de
le site (3) encoordinence
aveclesvariationsen longueurs
aveccesanions.Unecorr6lationdesagencement
locauxpossibles
liaisonset encompositionmdnenti un modbleplausiblepourexpliquercettesubstitution.
(Traduitpar la R6daction)
Mots-cl6s:v6suvianite,
6lectronique,
fluor, hydroxyle.
affinement,structurecristalline,analysee la microsonde
INTRODUcnoN
Grcat et al. (1992) reported results of a detailed
chemicalexaminationof a largenumberof vesuvianite
samplesfrom a wide rangeof localities.They showed
that the chemicalformula of vesuvianitecanbe written
asX DYBZ BTs-5O6sl,[e,
in whichX = Ca,NA REE,Pb2*,
sb3+;r=Al, Mg, Fe3*,Fe2*,Ti4, Mn,Cu,zn;Z=Si;T
=B:.W = OH, F, o2-. Theyalsoshowedthattwo of the
in vesuvianite
are
mostimportantchemicalsubstitutions
Mg + OH = Al + 02- andB + Mg=2H + Al, both of
which involvehydrogen.The variableH contentis thus
a very importantaspectof thechemistryof vesuvianite.
Fluorineis a commonconstituentin vesuvianite;it
reaches
upto -3.2wL7o(-4.2a.p.f.u.,atomsperformula
unit).The H atomsin vesuvianiteareinvolvedin fairly
configurations(Yoshiasa&
stronghydrogen-bonding
Matsumoto1986,Groat et al. 1992),and the simple
1066
TIIE CANADIAN MINERALOGIST
substitutionof F for (OHf is thus expectedto causea After the normal intensity data had been collected,
significant perturbationon local stereochemistry.This psi-scandatawerecollectedfor anempiricalabsorptionbeingthecase,thesubstitution
of significantamountsof correctionassumingan ellipsoidalshapeof thecrystals
F for (OH)- may havean importanteffect on the range thatwasrefinedaspartof thecalculation.As thecrystals
of otherchemicalsubstitutions
in vesuvianite.
In addi had beengroundto "spheres",the psi-scancorrection
tion, the presenceor absenceof F has a spectacular producedvery low.Rindicesfor the azimuthaldata;the
effect on the infrared absorptionspectraof vesuvianite correctionswere subsequentlyapplied to the normal
(work in progress),the interpretationof which requires intensitydata.A reflectionwasconsidered
asobserved
a detailedknowledgeofthe local stereochemical
effecrs if its intensityexceedsfour timesbackgroundbasedon
of the F -for-(OH)- substiturion.In orderto shedsome countingstatistics.
light on thesetwo points, we here report on structure
refinementsof three crystalsof F-bearingvesuvianite Structurerefinement
andcomparethemwith the structureof low-F vesuvianite.
Scatteringfactorsfor neutralatomsweretakenfrom
theInternational
TablesforX-ray Crystallography,
VoL
ExpsntN4nNTeL
ly (1974). All calculationswere done with the
SHELXTL system.R indicesareof the usualform and
The vesuvianitesamplesexaminedhere were pre- areexpressed
aspercentages.
viously characterizedby Groat et aL (1992), and the
The first point to considerin the refinementof
samplenumbersusedherearethe sameasin that study. vesuvianitestructuresis the mostappropriate
choiceof
spacegroup.The vesuvianitestructure-type
can have
Collectionof X-ray data
symmetryP4/nnc,However,mostcrystalsof vesuvianite showevidenceof smalldepartures
from thissymmeCrystals were ground into spheresusing a sphere try, both in observeddiffraction-intensities
that violate
grinderto minimizedifferentialabsorption,andattached the symmetrycharacteristics
of P4/nnc,and in optical
to a glassrod with "5 minute" epoxy.The crystalswere propertiesindicativeof biaxial symmetry(Grcatet al.
mountedon a Nicolet rtirz automatedfour-circle dif- 1993).For the threesamplesof vesuvianiteexamined
fractometer,andthe settinganglesfor 25 automatically here,thefollowinggeneralproperties
(i)
wereobserved:
aligned reflectionswere usedto refine the orientation only a small numberof very weak reflectionsviolate
matrix and the (tetragonallyconstrained)cell dimen- P4/nncsymmetry(9, l0 and 1l for V4, Y27 andY33,
sions;the latter are reportedin Table l, togetherwith respectively).In addition, the largest intensity of a
otherinformationpertinentto datacollectionandstruc- violating reflectionobservedhas F/o = 0.3 for each
ture refinement.Intensity datawere collectedover two crystal.(ii) measured
2V values(Tablel) aresmall,and
asymmetricunits (assuming4/mmm Laue symmetry) showonly minordeviationsfrom tetragonalsymmetry.
out to a maximum20 value of 60o accordineto the
As a result of theseobservations,
the spacegroup
experimentalprocedureof HawthorneA Croai(tSSS). P4/nnc wasusedfor the structurerefinementof F-bearing vesuvianite.Startingparameters
werethoseof the
F-freevesuvianiteY12 (Groat.Hawthorne& Ercit. in
prep.),andthe nomenclature
of sitesis that of Groater
al. (1992).Thestructures
refinedrapidlyto convergence
!A3I^E 1. PIiYSICAL DA1A AND INFORUA1ION Or REIINE}!S!fI FOR TflNEE
CRYSTAIJ OF !.BEARINE VESuVIANITB
for an isotropicdisplacement
model.Conversionto an
anisotropicform ofthe displacement
factorsandleastv27
v33
squaresrefinementof all variablesresultedin R indices
, (f)
L5.522<L)
r.s.558(1)
15.563(2)
15.s38(2)
of approximately3.4Eo. At thisstage,the f( I ) andO( I 0)
c (A)
11.802(2) r.1.805(1)
11.806(2)
11.786(2)
v (Ar)
2843,3(7) 28s7.4(5)
atomswerefoundto be veryanisotropic,
andthedegree
2859.5<7)
2845.6(6)
Stec6 group
P4/nc
P4/tuc
P4/mc
P4/tuc
of anisotropyvaried from sampleto sample.Consez4
444
quently,theseatomswere"split" into half-atomshaving
F (a.p.f,.u.)'
0.00
2.04
3.89
4.77
2V (')
15(2)
isotropicdisplacement
0
factorsconstrained
to be equal;
6(5)
Crystal alze
0,26 x 0.26 0.24 x O.24 0.24 x 0.24
positions
it
is
unlikely
that
the
split
have
equal
occupan(m)
x 0.26
x 0.26
x 0.24
cies,but this wasthe leastbiasedway in which to start
Redlatlo!
llofo
UoNo
lltolc
Itonocblomcot
Craphlte
oraphr.r€
this phaseofthe refinementprocedure.The refinements
Craphl.te
.R (el@tha1)
*
1.8
1.8
1.8
rapidly
convergedto R indicesidenticalor marginally
ToraL no. lrl
2108
2110
2!O3
Iessthanthosefor thesingle-atom
model.However,the
No. of lPl obs
t949
L959
1918
X (obser€d)
3
3.4
"
3.2
split-atommodelgreatlyfacilitatesthe interpretationof
3.5
(obs6sed)
rR
I
3.1
3.3
3.7
local disorderas a functionof bulk and sitechemistry.
P - :(l Fl - lrl),/:l ri
Final
R indicesfor all refinements
aregivenin Table 1.
3
,
2
l
o
'
wR l'u(
Fl
F j > ' z/ > e F
w
| / d2(F')
Thecrystalsall showeda smallnumberof reflections
'Awelage ol oL€ctron
Elcroprobe
resuLts
ostensiblyviolatingP4/nncsymmetry;thesereflections
1067
T]{E ROLEOF FLUORINEIN VESUVIANITE
were observedto be very weak, and we did not check
them for double-diffraction effects. To examine the
possibilityof channelorderingft'.e.,splittingof X(4) and
pairsof siteswith differential
Y(l) into nonequivalent
cationorderl,we alsorefinedthe stnrcturesofV4 and
V33 in the spacegroupP4/n (cl Giusepetti& Mazzi
1983,Fitzgeraldet al. 1986,Allen & Burnham1992).
Therewereno significantimprovements
inRindicesand
no discernableorderingof cationsover nonequivalent
pairsof sites;the observeddisplacement-anisotropy
at
the li(l) and O(10) positionwas still observedwhen
tfiese sites were split into nonequivalentpairs by the
reduction in symmetry from P4/nnc to P4/n. T\ese
pointsindicatethatthe long-rangestructureofF-bearing
vesuvianiteis bestdescribedwith P4/nncsymmetty,and
thatthe split-sitebehavioris dueto localorder.
x ( 3)
x
ueq
x(4)
v
uaq
y(1A)
t
z
teq
v(18)
t
'ueq
zl2)
x
t
ueq
z(3)
x
t
u6q
x(L)
x
v
2
uoq
3/4
v4
3/4
L/4
61(2)
71(3)
66(3)
-0.18089(4) -0.18033(4) -0.18110(5)
0.04u0(4)
0.04030(4) 0.04051(5)
0 . 8 7 0 9 9 ( 5 ) 0 . 8 7 1 3 4 ( 5 ) 0,87L27<6)
62(L)
62<2)
70(2)
-0.18097(6)
0.04035(5)
0.87121(7)
60(2)
-0.08314(4) -0.08302(4) -0.08262(5)
-0.15084(4) -0.rs077(4) -0.15057(5)
0 . 3 6 4 8 2 ( 5 ) 0 . 3 6 4 6 6 ( 6 ) 0.36466(6)
72(r)
75(2)
80(2)
-0.08293(6)
-0.15062(6)
O.36465(7)
1t(2)
n/4
L/4
r/4
't6(2)
3/4
L/4
7/4
109(3)
3/4
L/4
L/4
102(3)
3/4
L/4
L/4
103(3)
x
t
lr€q
-0.18919(3)
0.04379(3)
0.37990(4)
79(r)
-0.18912(3) -0.18893(3)
0.04380(3)
0.04375(3)
O,37967(4)
0.379s1(5)
82(1)
85(1)
-0.18908(4)
O.04425(4)
0.37937(6)
74(2)
0
0
0
72(3)
0
0
0
70(3)
- 0 . 0 4 9 2 3 ( 1 1-)0 . 0 4 9 1 9 ( 1 2 ) - 0 . 0 4 8 4 0 ( 1 4 )
0 . 2 2 1 8 0 ( 1 1 )0 . 2 2 L 7 L ( L X ) 0.22778(74)
0 . 0 7 5 9 6 ( 1 4 )0 . 0 7 5 5 9( 1 5 ) 0 . 0 7 5 8 3 ( 1 9 )
77(4)
8 2( 5 )
74<6)
-0 .06227 (L0)
0.10658(10)
o.47O22(L2)
84(4)
- 0 . 0 5 1 7 8 ( 1 0 -o.06L79<r2)
)
-0.06142(14)
0 . 1 0 6 1 9 ( 1 0 )0 . 1 0 6 0 0 ( 1 2 ) 0 . 1 0 6 3 1 ( 1 4 )
o.47002(L4' 0 . 4 7 0 4 0( 1 5 ) 0.47040(18)
80(5)
8 5( 5 )
75(5)
-0.17019(9)
0.01500(10)
0.17833(12)
99(4)
- 0 . 1 7 0 3 3 ( 1 0-)0 . 1 6 9 8 1 ( 1 2 ) - 0 . 1 7 0 2 5 ( 1 4 )
0 . 0 1 4 8 3 ( 1 1 )0 . 0 l s 5 3 ( 1 2 ) 0 . 0 1 5 1 6 ( 1 5 )
0 . 1 7 8 7 6 ( 1 5 )0 . 1 7 8 6 5 ( 1 6 ) o.L79O9(20)
113(5)
9e(6)
107(s)
0.88r.08(11)
-0.27075(10)
0.06013(13)
132(4)
0.88171(16)
0 . 8 8 1 8 1 ( 1 3 )o.88244(14)
- 0 . 2 7 1 3 s ( 1 r .-)0 . 2 7 1 3 3 ( 1 3 ) - 0 . 2 7 1 8 2 ( 1 5 )
0 . 0 5 9 5 4 ( 1 5 )0 . 0 5 9 2 3 ( 1 7 ) 0 . 0 5 7 8 3 ( 1 9 )
155(6)
138(7)
1 5 6( s )
Yeq
0.05579(10)
0.17308(10)
0.32L92<14)
115(4)
0 . 0 5 6 0 2 ( 1 1 )0 . 0 5 6 0 4 ( 1 2 ) 0 . 0 5 6 1 8 ( 1 5 )
0.17355(15)
0 . 1 7 4 3(01 1 ) 0.r7422(X2)
0 . 3 2 2 1 8 ( 1 6 )0 . 3 2 1 9 8 ( 1 7 ) 0 . 3 2 U 7 ( 2 1 )
1 1 0( 5 )
113(6)
1 0 2( 5 )
o(8)
x
7
z
ueq
" 0 . 0 6 0 9 8( 1 0 )
-0.0910r.(9)
0.06618(13)
88(4)
" 0 . 0 6 0 7 8 ( r 0 )- 0 . 0 6 0 6 4 ( 1 2 -) 0 . 0 6 0 6 0 ( 1 4 )
-0.09047(10)- 0 . 0 9 0 1 5 ( 1 2 -) 0 . 0 9 0 4 s ( 1 4 )
0 . 0 6 6 8 3 ( 1 4 )0 . 0 6 6 4 1 ( 1 6 )0 . 0 6 6 3 7 ( 1 9 )
93(s)
84(6)
84(4)
o(9)
x
v
2
Iteq
-0.14454(10)
-0.14454(10)
L/4
L02<4)
- 0 . 1 4 4 2 6 ( 1 1-)0 . 1 4 3 9 3 ( 1 2 -) 0 . 1 4 4 5 7 ( 1 4 )
-0. 14426(1r.)-0.L4393(L2)-0.144s7(14)
L/4
r/4
L/4
114(5)
116(s)
107(6)
7
ueq
v
o (6 )
7
0(7)
o(r.0A):
7
teq
3/4
3/4
0.8661(3)
1 5 1( 7 )
0(108) x
t
:
teq
OHx
t
x(2)
0
0
0
65(2)
-0.04911(:.0)
0.22149(L0)
0.07643(L2)
83(4)
ueq
7
3/4
t/4
3/4
3/4
0.0436(3)
52(7)
- 0 . 1 1 6 7 7 ( 1 1-)0 . 1 1 6 8 2 ( 1 2 ) - 0 . 1 1 6 7 9 ( 1 4 )
0 . 1 5 9 6 7 ( 1 0 )0 . 1 5 9 8 4 ( 1 2 ) 0 . 1 s 9 9 7 ( 1 4 )
0.279U,(r4) 0 . 2 7 8 8 7 ( 1 5 ) 0.279r4(Lgt
e 7< 5 )
99(5)
90(5)
t
v
z
3/4
r/4
0000
60<2')
3/4
3/4
0.0392(3)
63(6)
-0.11732(10)
0.15964(10)
0.27828<L2)
89(4)
u6q
u6q
a
7
3/4
3/4
0.0333(3)
66(6)
- 0 . 2 1 9 9 7 ( 1 1-)0 . 2 1 9 9 0 ( 1 2 ) - 0 . 2 1 9 8 1 ( 1 5 )
0 . 1 7 2 6 2 ( r 0 ) o.L7274<L2' 0 . 1 7 3 1 1 ( 1 4 )
0.08s22(L4' 0 . 0 8 5 6 6 ( 1 6 ) 0.08541 (20)
87(4)
1 0 3( 5 )
98(6)
v
t6q
z<L)
3/4
x/4
0 .0747 (4)
52(7)
-0.219.15(10)
0.77269(9)
0.08546(13)
82(4)
Structurerefinement
t
3/4
3/4
0.0708(3)
63(6)
- 0 . 1 1 1 6 5 ( 4 ) -0 ,1L277(5)
0 . 1 2 0 5 8 ( 4 ) 0 . 1 2 1 0 2( 5 )
0.12695(5) 0.12651(6)
81(2)
74(2)
teq
2 -O4
.
3/4
3/4
( 3)
0.06s9
66(6)
-0.r1203(4)
0.12089(4)
0.12640(s)
6 7( 2 )
0(5)
0.00
3/4
3/4
0.0358(1)
109(3)
v
z
ueq
RESULTS
(a.p. f .u. )
3/4
3/4
0.L4147 (26)
L42(6)
ueq
Subsequentto the diffraction experiments,the crys- o(1)
tals usedfor the collectionof the X-ray intensitydata
were removed from their glass fibres, mounted in
Transoptic-, and polished for electron-microprobe o ( 2 )
analysis.Experimentaldetailsare given by Groatet al.
(1992).At leasttenpointswereanalyzedon eachcrystal
so as to get a representative
compositionof the whole
o(3)
crystalusedin the collectionof X-ray data.Fe2+/Fe3+
ratios and H2O contentsare taken from Groat et al.
(1992) and necessarilyrepresentvaluesfor the bulk
samplesratherthanthe crystalsusedin the X-ray work. o ( 4 )
2. ATOI.IIC PAIAUETERS lOR FLUORINE.BEARINO VESWIANITE
CRYS?AIJ
3/4
3/4
3/4
3/4
3/4
3/4
0 . 1 4 9 3 4 ( 1 5 )0 . 1 3 9 8 U 2 1 ) 0 . 1 4 0 2 5( 2 5 )
138(5)
170(6)
99(3)
0
0
0
72(2)
v (3 )
TA3IJ
-0.10062(4)
-0.77962(4)
0.887s3(6)
1ls(2)
u€q
Y<2) x
t
Electron-microprobe analysis
Final atomic parameters,
equivalentisotropic displacementfactorsand anisotropicdisplacementfactors
-0.10132(3) -0.10004(3) "0.10040(4)
-0.18123(3) -0.17951(3) -0.17965(4)
0.88698(4) 0.88558(5) 0.88627(s)
132(1)
131(2)
r69(1)
u€q
-0.11239(5)
0.12085(6)
0 . 1 2 6 7 5( 8 )
74<X)
3/4
3/4
0.8380(6)
L27(72)
3/4
3/4
0.8507(7)
rs2(14)
3/4
3/4
0.8552(10)
152(16)
3/4
3/4
0.8824(6)
r27 (L2)
3/,
3/4
0.8812(7)
1s2(14)
3/4
3/4
0.8764(10)
rs2(16)
-0.00480(10)- 0 . 0 0 4 6 5 ( 1 0-)0 . 0 0 4 1 8 ( 1 2-)0 . 0 0 3 9 8 ( 1 4 )
0 . 0 6 1 s 5 ( 1 0 ) o.06220(LO' 0 . 0 5 2 3 0 ( 1 1 )0 . 0 6 1 3 8 ( 1 4 )
0 . 1 3 6 3 7 ( 1 2 )0 . 1 3 5 7 1 ( 1 3 )0 . 1 3 5 7 0 ( 1 4 )0 . 1 3 6 1 2 ( 1 8 )
95(4)
58(4)
68(5)
59(6)
'Ueq - 1t.n x 104 (42)
1068
THE CANADIAN MINERALOGIST
aregivenin Tables2 and3,respectively.
Fortherelevant
atomicpositions,resultsaregivenforboththesingle-site
andthesplit-sitemodels.Tablesof stnrcturefactorsmay
be obtainedfrom the Depositoryof UnpublishedData,
CISTI, NationalResearch
Councilof Canada,Ottawa
Ontario KIA 0S2. Selectedinteratomicdistancesare
given in Table 4, where they are comparedwith
corresponding
valuesfrom a fluorine-freevesuvianite
(V12, takenfrom Groatet al., in prep.).Site-scattering
in termsof equivalentelectronsare given in Table 5;
TABLE4. sELscrED rN?BRATourcDrsrANcEs (A) rN F-BEARTNG
VESI'VIANIIECRYSTAII
IABI.E 3. ANISOTROPIC DISPIACE{ANT EACTORST'OR FLUORINE.BEARINC
VESWIANI?E CRYSTAIS
utt
u2
uss
urs
u9
63(3)
56(3)
114(3)
143(5)
10(2)
u3(2)
69(5)
61(4)
84(3)
r.2r.(8)
85(8)
87(8)
87(8)
101(8)
275(LO)
83(8)
7L(7)
139(7'
56(7)
OH
63(3)
66(3)
51(3)
103(5)
ee(2)
99(2)
69(5)
58(4)
7l(3)
70(8)
86(8)
70(7)
72(8)
142<9)
89(8)
109(8)
75(7)
739<7)
90(7)
s8(6)
0
6s(3)
7( 2 t
-2(2)
60(3)
81(4)
0
7 7( 2 )
3(2)
2
6(2)
184(3)
256(L2)
0
75(4)
3(4)
-4(2)
69(3)
72(8)
4<6)
e7(8)
0(6)
-5(6)
76(8)
80(8)
9(6)
-9(7)
81(8)
100(8)
2 7( 7 )
-7<7)
112(8)
10s(8)
14(6)
70(10)
6(7)
-11(6)
5 7( 7 ' )
-!.r,
-?,r,
75(4'
63(3)
117(3)
158(5)
75<2)
117(3)
104(7)
69(5)
84(4)
136(9)
90(9)
104(9)
e6(9)
106(9)
254(LL)
92<9)
7 7( 8 )
142(8)
7O(8)
75(4)
80(3)
58(3)
86(5)
104(2)
104(3)
104(7)
62(5')
81(4)
85(8)
105(9)
68(8)
73(9>
139(10)
116(10)
126(9)
92(9)
L42(8)
84(8)
z(r)
z<2)
z(9)
x(r.)
x(2)
x(3)
x(4)
Y(2)
Y(3)
o(1)
o(2)
0(3)
0(4)
0(5)
o(5)
o(7)
o(8)
o(e)
oH
68(5)
55(4)
9e(4)
170(7)
63(3)
93(3)
11r_(8)
5 7( 6 )
77<4)
129(11)
e6(r0)
97(11)
75(10)
86(11)
202(13)
70(10)
67(10)
120(9)
58(r.0)
68(5)
50(4)
sr(4)
7 7( 6 )
90(3)
84(3)
1U(8)
64(6)
72<4)
75(r0)
88(11)
59(10)
68(10)
r23(11)
103(12)
152(!2)
79(10)
L20(9)
85(10)
61(4)
0
6e(3)
5(3)
67(3)
1(2)
65(4)
0
77(2)
4(2)
1"74(3) "20<2)
272(L4)
0
86(4)
s(4)
-e(3)
7 7< 3 )
er(8)
3(7)
101(9)
10(7)
78(8)
0(7)
88(9)
11(7)
139(10) 98(9)
725(9'
32(8)
112(9)
4(8)
114(9)
10(7)
70(11)
4(8)
-10(6)
51(7)
?..
- &. i
10a
z(2)-o(2)b
v12
927
0.00
2.04
1.639(3)
1.637(2'
L,640(2)
1.631(2)
1.639
L.637
1.640
1.631
L.646<2)
7.64X<2)
r.678(2)
r.61L(2)
r.646<2) L.641<2)
L.644(2) 1.640(2)
1.687(2) L,679(2)
L,6t"6(2) L,6r6(2)
4.77
z(2)-o(7)b
-49(2)
16<'l
1.645(2)
L,643(2)
r.676(2)
7.677(2)
26(2)
qQ)-o>
t.645
L,645
1.648
7-644
2(4)
6<4,
L.634(2)
I.605(2)
L.620<2)
1.660(L)
r.634(2)
1.603(2)
L,621(2)
1.658(1)
L.637(2)
7,606(2)
7.623(2>
r..659(1)
1.636(3)
1.608(3)
7.622(3)
r.659(2)
1.630
7.629
1.631
1.631
2.333(2)
2.5L4(2)
2.335(2)
2.528(2)
2.331,(2) 2,327(2)
2.527(2) 2.522(2,
<r(1) -o>
2.424
2.432
2.429
2.425
:(2) -o(1)f
x(2) -o<2)
:(2) "o(3)b
.r(2)-0(4)
r(2) -o(5)
r(2) -o(5)f
r(2)-o(6)o
:(2) "o(8)d
2.489<2>
2.433(2)
2.375<2)
2.443(2)
2.439(2)
2.332(2)
2.9r3(2>
2.3L8<2)
2.488(2)
2.434(2)
2.374(2)
2.451-<2>
2.432(2)
2.337(2)
2.977(2)
2.324<2)
2.492(2)
2.436(2)
2.372(2'
2.451(2t
2.431,<2)
2.345(2)
2.9rO(2)
2.326(2)
2.487(2)
2.t27(2)
2,372(2)
2.453(2)
2.422<2>
2.336\2)
2.929<3)
2,326<2)
<r(2)-D 18l
2.468
2.470
2.470
2.469
<r(2)-0> !71
,J,04
,A06
,JtOB
,JrO3
x ( 3 )- o ( 3 ) J
x ( 3 )- 0 ( 6 ) p
x(3)-o(6)r
x ( 3 )- o ( 7 ) l
. r ( 3 )- o ( 7 ) m
r(3) -o(7)b
x(3)"o(8)k
)r(3)-0(104)q
.x(3)-o(108)q
x(3) -onj
2.455(2)
2 . 4 8 1( 2 )
3.010(2)
2 . 5 6 9( 2 )
2.49r(2)
2.37L(2)
2.613(2)
2.556<L)
2.456(2)
2.5t8(2)
3.022(2)
2.547<2)
2,492(2)
2.363(2)
2.627(2)
2.63eQ)
2.578<2)
2 , 4 5 9( 2 )
2,459(2)
2.505(2)
3.015(2)
2.555(2)
2.495(2)
2.366<5)
2.6L7(2)
2 , 6 0 7( I >
2.574<L')
2.460(2'
2.444(2)
2.482(2)
2,978(2)
2.556(3)
2.509(2)
2.36t(2)
2.599(2)
2.594(2)
2,569(2>
2.469(2)
-i<'>
'i,lj
.tii)i
itl>, z(3)-o(5)d
_ri)ii
z(3)-o(6)e
47(7)
40(7)
) , 7( 6 )
7( 6 )
-24(s)
-7 (6)
4(3\
:;[:i ;i:i il3]:3[3]"
L4(7)
26(8>
6(7)
2 7( 6 )
-6(7)
-2(6>
0
-2(3)
2(3)
0
-5(2)
-4L(2)
0
6(4)
e(3)
-2(7)
-27(7)
-8(7)
-10(7)
-7(8)
32(8)
15(7)
28(7)
-4(8)
-6(6)
0
-3(2)
5( 3 )
0
7( 2 )
22(2)
0
1(4)
0(3)
s(7)
-5(7)
0(7)
-6<7>
r.0(7)
46(8)
18(7)
1s(7)
16(11)
20(7)
v33
60(7)
66(4)
62(4)
65(5)
72(3'
170(3)
171(15)
90(6)
76<4')
94(10)
109(11)
68(10)
85(11)
91(t1)
110(11)
114(!r)
105(11)
83(13)
38(9)
-O>
1(2i rizi |iil.|ilii
v4
z(I)
z<2)
z(3)
x(1)
x(2)
x(3)
x(4)
v(2)
Y(3)
o(L)
o(2)
o(3)
0(4)
o(5)
0(6)
0(7)
0(8)
0(e)
oH
z(L) -o(1)a
4(L)
!27
z(r)
z(2)
z(3)
x(L)
x(2)
x( 3)
x(4)
Y( 2 )
Y(3)
o(1)
o(2)
o(3)
o(4)
o(5)
0(6)
o(7)
0(8)
0(9)
F (e.p.f.u.)
000
-4(3)
5(3)
1(3)
1(3)
000
-5(2)
7(2)
-20(3)
-35(3)
000
-1(5)
7(5)
-r.(3)
8(3)
2L(9)
7(9)
-3(9)
-39(9)
0(8)
13(8)
r7(8)
8(8)
-r5(e)
1e(e)
30(9)
30(10)
23(10) 13(9)
13(8)
22(8)
-4(10)
4(10)
-4(8)
-11(8)
4(3)
-3(3)
e(2)
13(2)
8(5)
-6(3)
8(9)
"r-8(8)
7(8)
3(8)
3e(e)
32<9)
19(9)
1(8)
-38(13)
-13(8)
-o>
x(1)-0(1)a
x(1)'o(2)a
x4
x4
\L/2
x L/2
2.498(2)
<r(3)-o> [9]
2.56L
2.565
<x(3)-o> [8]
2.505
2.508
2.506
2.500
:(4)-o(6)f
r ( 4 ) - o ( e )f
2.3L4(2)
2.602(2)
2.279(2)
2.669(3)
2.279<2)
2.669(2)
2.294(2)
2.674(X)
<r(4) -o>
2.458
2.474
2.474
2.4A4
v(1A)-o(6) f
Y(1A)-0(LoB)s
Y(LA)-o(10A)s
2.O80(2)
2,004(4)
-
2.077(2)
2.166(7)
2 . 6 9 L <)7
2.O77(2)
2.L66(8)
2.691(8)
2.093<2)
2.238(9)
2.598(9.)
<r(lA)-D
2.065
2.095
2.095
2.L22
[5]
2.55!
1069
THE ROLEOF FLUORINElN VESUVIANITE
x4
r(u)-0(6)f
r(18) -o(108)r
-o(10A)&
Y(18)
2.099(2)
r..781(8)
2.306(8)
2.099<2)
1.781(8)
2.306(8)
1.865(9)
2,225<9)
<v(t B) - o>
2.035
2.035
2.O54
1.940(2)
1.864(2)
1.867(1)
L.e44(2)
1.871(2)
L,813(2)
r.s4r(2)
L.864(2)
r.873<2)
Y(2)-o(4)h
r ( 2 )- 0 ( 8 )
Y(2)-oH
x2
x2
x2
r.948(2)
1.871(2)
r..874(1)
2.r0L<2)
1.8es 1.seo 1.8e6 t,-
<v(2) -o>
TABLE 6. AVERAGECOMPOSITIONOF VESWIANITE SAUPLES
slo2 wc.t
36.27
37.05
36.52
ALzq
Ttoz
MgO
18.31
0,76
L.46
L6,2L
2.93
L.72
L5.87
0.80
t-.65
*3
Fa203
v ( 3 )- o ( 1 )
r(3) -0(2)
v ( 3 )- o ( 3 )
r(3) -o(4)h
r(3) -0(5)
r(3) -0F
1.909(2)
r.893(2)
7.934(2)
2.058(2)
L.973(2)
r.906(2)
1.919(2) r.s33(2)
1.8e8(2) 1.8es(2)
1.951(2)
I.947(2)
2,064(2)
2.068(2)
1.972(2' r.978(2)
r.e24(2) L.899<2)
r..913(2)
r.ageizi
r.949(2)
2.063(2)
7.965(2)
t.924(2)
<v(3) -o>
L.946
1.955
1.954
L.952
x(4) -:(4)d
x(4) -r(1A)
x(4) -r(18)
2.X77(4> 2.6L6(5)
L.340<2) 0.86s(4)
I.25O(4)
2.604(6)
0.813(5)
1.187(4)
2.572<6)
0.780(6)
1.147(5)
0.373(s)
0.367(6)
v(1A)-v(18)
-
0(l-0A)-0(10A)t
o(10A)-0(108)
o(10A)-o(108)r
-
-x(3)f
x4
o(1.0A)
o(10A)-r(1A)k
o ( 1 0 A-)y ( t B ) k
o(108)..r(3)f x4
o (r0B)-r(1A)!
0 ( 1 0 B-)v ( 1 8 ) k
Equl.val€nc
positlons:
0.385(5)
9"o
Na2o
InzOa
BzOa
so."
i
cl
Itzol
Itzoi
l/2-!-1,
I/2-zt
fr l+r.
I'y,
u:
y, t/2-.t
lt t/2-x-t,
). r,t,
I
x. l+z: a: i, ttZ*y.t,
t/2+zl
-lrr,
I/2-ztp,
yi l+z; qt -l+t,
IOTAL
Bs
se
r
oHa
x
,z
assignment
of specificcation-occupancies
alsorequires
consideration
of thechemicalcomposition.
Chemicalcomposition
The resultsof chemicalanalysisof the crystalsare
givenin Table6. Unit formulaewererecalculated
on the
basisof50 cationsandwereassigned
to specificgroups
of positionsasrerommended
by Groatet al. (1992).
TABI.E 5. IEEINED SITE.@C1'PANCIES' IN F-EEARINC
vEsucl"rNlTs
v(1)
r(3)
?:9i
3:31
i.32
L.22
L.76
37.54
r-7.33
0. 66
1.7s
0.73
3. 1 6
o.37
35.42
0.t0
36.49
0.00
0 .0 0
0.00
0.00
0.00
0.01
34.79
0 .2 1
0.00
2.67
L.57
0.04
0.00
0.00
0.01
2.48
0.08
l-.70
L.57
ttjr-
tt tt
0 .5 9
99.81
1. 0 6
101.79
1.31-
99.L2
9 8 .1 6
98.75
100.48
o-F, cl
2,0'16(13) 2.378(16) 2.487(24)
0.525(9) 0.360(11) 0.2s00.7)
si(f
2.60r(9) 2.738(11) 2.730<77)
AI.3+
2.556(1) 2.638(2) 2.607(t) 2.594(2)
TL4+
2.OO4(4) 2,69I(8)
2.598<9) 2.587(L3)
2 . 3 O 6 ( a ) 2 . 2 2 5 ( 9 ) 2 . 2 2 0 ( L 3 ) tlgz+
l,lnz+
2.578(r) 2.s74(1) 2.569(7)
Fe2{
2.L66(8) 2.238(9)
2.337(13)
Fe3+
1.781(8) 1.865(9) 1.970(13)
Ca2t
Na+
a: 1ar. y, rt b: y,
i. lr2+za
c, l/24y-1,
h3{
i. t..z: d: y. x. I/2-zt
e, l-x.
g: r, y,:-l:
l/2+z-1,
t
t,;.
l-2: k: t. y,l tzt It l/2-y-1.
l/2-x.
\,
l/2-t.zt
o: y,-l+r.
-1.!,
z, tr y. x, l/2-z+1.
v21
!L2
0 .2 1
r..30
0 .1 7
tf.
lJ
0.00
0.00
3.10
0.00
1.63
1.30
L7.75
t 8.38
18.10
L8.27
1 0 .5 6
0 .2 8
t-.07
0. 0 3
0.54
0.65
9. 4 8
t .06
L.27
0.00
t-. t-1
9.86
0 .3 0
L.22
0 .1 4
L.23
0.46
9.94
o.24
L.27
0 .3 0
L.29
0. t-4
19.13
0.00
0.00
t 8.49
0 .2 0
0.00
18.65
0.04
0.00
L8.47
0.09
0 .0 0
0 .0 0
0.08
0.00
0. 0 0
0.00
0 .0 0
0.00
0. 0 1
8.73
t9.L3
L3.L2
L7.75
2.O4
0 .1 4
5.20
t-8.70
L2.92
18.38
3 .8 9
0 .0 7
5.62
t 8.70
13 . 2 0
18.10
0.00
2.ZA
18.s6
L3,L7
L8.27
fMeasured, +Cal-sul-atedfor charge bal-ance.
V33 lncludes 0.15 wt.t Beo. The nwber of analyses
Ged ln tha averagLngare (ln brackets) V12 (6), V28
(5), v4 (8), v33 (2).
DISCUSSION
In the structureof vesuvianite,
the hydroxylgroups
aregenerallyinvolvedin fairly stronghydrogenbonding. As a consequence,
the substitutionof F for (OH)mustinvolvesignificantlocalchanges
in stereochemistry in orderto satisfythebond-valence
requirements
of
thoseanionsthat actedas hydrogen-bond
acceptorsin
F-freevesuvianite.
TheOH environmentin F-free vesuvianite
11.43
t4.34
'Occtrymi.ee
glvsa la totu
Nrsber - ao. of, elactr@
LL.97
14.33
of U.A.l.
at al!e).
L2.64
14.38
(liee
Aeonts
In thevesuvianite
structure,therearetwo anionsites
that canbe occupiedby (OHf andtF" the OH siteand
the O(10) site.For the convenience
of this discussion,
r070
TTIECANADIAN MINERALOGIST
(c)
(b)
D
I
x.
\/.-,
ORIGINAT Y(2}
z
I
ly
X
Ftc. l. Theanionsitesthatcanbeoccupiedby OH- or F in thevesuvianite
structure,togetherwith theneighboringstructure:(a)
theOH site;(b) the O(I 0) site;hydrogenbondsareshownby dashedlines.
the local environmentof thesesites is illustratedin
Figurel.
EachOH site is coordinatedto cationsat the X(3),
Y(2)and(3) sites;thesethreesitesform theapicesof a
trianglesubparallel
to (001), with theOH siteoccupying
a "pyramidal"position above.Where the OH site is
occupiedby (OHf, the H(l) atomlies aboverhedonor
oxygensuchthatthe O(donor)- H bondis at an angle
of-30o to thec axis.Bond-valence
(Yoshiasa
arguments
& Matsumoto1986.Groat 1988)showthat thereis a
stronghydrogenbondbetweenH( 1) andO(7); theZ(2)
andX(3) cationstlat coordinateO(7) conribute only
-1.7 v.u. in F-freevesuvianite,
andthe hydrogenbond
from H(l) is necessary
for bond-valence
satisfactionat
&e O(7) anion.
TheO(10)anionlieson thefour-foldaxisthatpasses
downthechannel(s)
throughthestructure(Fig. lb), and
is coordinatedby one (1) and four X(3) cationsin a
squarepyramidalarrangement.
Exclusiveof F, O(10)
must be occupiedby some OH, and it is generally
considered
that both OH- and02- occupythis site(see
summaiyof currentandpreviousviewsin Groatet al,
1992).The neutron-diffractionstructurerefinementof
I-ageret al. (1989)locatedthe H(2) site;it is asymetri-
cally positionedbenveentwo O(10)siteson the4-fold
axis.In the crystalthey examined,the H(2) positionis
only partly(0.85)occupiedandis assumed
to be locally
disorderedaboutthe centerof symmeuJat (114,ll4,
1/4).Presumably
when H(1) is close(-1 A; to one
O(10) oxygen,it forms a stronghydrogenbondto the
neighboringO(10)approximately1.8A away.
TheF -for-(OH)- substitutionat OH
Inspectionof Tables2 and3 showsthatthereplacementof (OH)- by F acrossthis seriesdoesnot involve
positionaldisorderaboutOH or O(7).
any discernable
The isotropicdisplacement
factorsat both theseposi
tionsdo not showanysignificantincreasewith increasing F contentof the crystals.Similarly, there is no
increasein the anisotropyof the displacement
factors
acrosstheseries.Henceanylocaladjustments
thatoccur
as a result of the F-for-(OH)- substitutionmust take
placeat the cationscoordinatingOH or O(7) (or both).
Inspectionof Table4 allowsusto identifythosefeatures
that arespecificallyrelatedto theF-for-(OH)- substitution. Comparisonof the Z sites(occupiedonly by Si)
acrossthe seriesshowsthattheir coordinationgeometry
1071
TTIEROLEOF FLUORINEIN VESUVIANITE
is virtually unaffected,despitethefact thatH( I ) strongly half-occupied.The bond-valencestructuresof these
of (OH)- three arrangementsare shown in Figure 4. We focus
hydrogen-bonds
to O(7) andthatreplacement
incidentto
bond-valence
Similarly,thereis specificallyon the aggregate
by fts mustdisturbthis arrangement.
from
parameters
of bothO(10)anions;numericalvaluesarecalculated
no changein theanisotropicdisplacement
for Vl2
Z(3) with changingF content.This indicatesthat the the observeddistancesand site-occupancies
curvesof Brown(l 981),
deficiencyat O(7),causedby removalof (core)usingthebond-valence
bond-valence
conribution,mustbe compensated whereasfractionalvaluesare calculatedby Pauling's
the hydrogen-bond
incident
from one or secondrule (Pauling1960).Thebond-valences
by an increasein incidentbond-valence
shouldbe
moreof theothercoordinatingcations.The O(7) anion to bothO(10)sitesin thethreearrangements
is bondedto Z(2) andthreeX(3) cations,Inspectionof equal to the sum of the formal valencesof the simple
Table4 showsthat thereis significantvariationin the anionsat thetwo O(10)positions;if thesesimpleanions
O(7)-X(3) bond lengthsacrossthe series(Fig. 2), an areoxygen[i.e.,both O(10) positionsareoccupiedby
aggregateshorteningcorrelatedwith increasedF--for- OH- or O2-1,the sum of the incidentbond-valences
(including those from the H) should be 4 v.u. The
OH- substitution.
calculationsfor the three arangementsare shown in
Figure4. EachO(10)oxygen(labeledO in thefigure)is
in vesuvianite
Channe
I configurations
bondedto four equivalentX(3) cations(primarilyCa),
of 0.17x 4 v.u.
possible
andreceivesan incidentbond-valence
channel
arangelet
examine
the
First
us
mentsin fluorine-freevesuvianite(Fig. 3). Important from thesecations(anda Paulingbondstrengthof 218).
constraints
on thepossiblearangementsaretheexperi- The O(10)anioncanalsobe bondedto the y(l) cation,
contributionof 0.5I v'u.
mentalobsevationsthatthe (l) andX(4) sitesalways with an averagebond-valence
215for divalentoccuof
bond
strength
Pauling
a
O(
l0)
is
fully
whereas
the
site
be
half-occupied,
seemto
[and
occupied.This givestwo possibilities:(i) therearetwo pancyof y(l)1. The H atomat H(l) forms two bonds
of 1.0v.u. The aggrebond-valence
distinctlocal arrangements
flabeled(l) and (2) in Fig. with an aggregate
incidentto bothO( I 0) anionsfor the
3l that occurin equalamounts;(ii) thereis one local gatebond-valence
(Fig.4) are3.70,2.68
and3.19v.u.,
arrangement
fiabeled(3) in Fig. 3] thatis polarandcan threearrangements
occur in two different orientations.It is perhapsof respectively.There is a problem here: the incident
arefar too low for both O(10)positions
significance
thatthereis noobviousreasonwhy arrange- bond-valences
ments( 1) and(2) shouldoccurin equalproportions[thus to be occupiedby oxygen (for which the incident
givingriseto half-occupied
K I ) andK4) sitesl,whereas bond-valenceshould be 4.0 v.u'). In a fluorine-free
(3) constrains(l) and X(4) eachto be vesuvianite,there seemsto be no way around this
arrangement
x(3)€c4i
----tl2.570
x(3)-oC4m
2.560
x(3)-pf4b
-----F-----l--
€
o 2.550
0)
o
(6
ct
i-
E e.soo
.9
z.ago
E
o
c)
F
2.370
F (a.p.f.u.)
Frc.2. Variationin O(7FX(3)bondlengthsasa functionof F contenlin vesuvianite.
1072
THE CANADIAN
ORDERED ORDERED
POLAR
Y(1)
x(4)
x(4)
Y(1)
o(10)
H
o00)
Y(l)
x(4)
x(4)
Y(l)
o(10)
H
o(10)
Y(1)
x(4)
x(4)
Y(1)
o00)
H
o(10)
Y(1)
x(4)
x(4)
Y(1)
MINERALOGIST
/n
\7
Y(1)
X(3)-\ i-l/X(3)
=chatge = 3 - o r 1 -
xtsl-li--.-x(sl
H(1)
x(4)
-_.--'
i Y(1)i
io11o)i
rHi
l o o oi t
l-ML
x(3)-_\A-l/x(3)
"l',
o,]t
x(4)
i
lo(10)
iHl
te!ei
@
i*tl
i o ( 1 0 r)
x(4)
Y(1)
o(10)
Y0)
o(10)
x(3)-_\^_//x(3)
'l+
!o =
x(3)-?)x(3)
X(e)-.-_ ;-zX(3)
lni
Lol!)-i
xtal
H0)
x(3)
Y(1)
x4x2 + 1.0= 3.19v.u.
:= O.5lrl + O.21
tp=2t5x1+2tgx4x2+ |=3.4
H
x(4)
t----:
x(3)-/it'\x(3)
Y(1)
Y0)
o(10)
x(3)-/"---x(3)
Y(1)
H0)
x(3)
: = - + 0 . 2 x14 x 2 + 1 . 0 = 2 . 6 8 v . u .
2P=-+Agx4\2+1=3.0
H
o(10)
Y(1)
H
o00)
x(4)
x(4)
var
Ftc. 3. Possible arrangementsin the channel in fluorine-free
vesuvianite. There are two distinct arrangements,labeled
"ordered" and "ordered polar". In the ordered configurafion, there are two local ,urangements,labeled I and 2; in
the ordered polar arrangement, there is one local arangemenL labeled 3, that may point along lc. The four X(3)
cations bonded to each O( l0) anion are omitted for clarity;
their positions may be seenin Figure 4.
problem.We emphasize
this very stronglybecause
it is
oneof the majorstumblingblocksin understanding
the
vesuvianite
structure.
Previousinvestigators
(Codaetal,
1970,Allen 1985)havediscussed
theOH occupancy
of
O(10),but did so only on a qualitativelevel,wherethis
problemis not apparent.
This situationis greatlyrelievedin fluorine-bearing
vesuvianite.Replacement
of the (hydrogen-bond)
acceptor anion by F removesthis charge problem in
ilrangements(2) and (3) (Fig.4), but still givessomewhat of a problemwith arrangement
(l). It is perhaps
significantthatarangement(3) alsohasgoodbond-valencearangements
aroundeachO(I0) position,in view
of thefollowingpoints:
(i) Arrangement(3) requiresequaloccupancyof the
I(l) and X(4) positions,whereasthe combinationof
(l) and(2) doesnot.
rurangements
/;\
Q/
Y(1)
x(3)-.\A-l/x(3)
z=c =z
xts)-/i--\xtsl
X(3).-
H(1)
I -zX(3)
\/\z
X(g)..-Y(1)
\.-X(3)
x(3)
H(1)
> = 0.51x2 + 0.21x4x2+ 1.0= 3.70v.u.
' : p = 2 1 5 x 2+ E B x 4 x 2 + 1 = 3 . 8 0
Ftc. 4. Bond-valence structures in the three possible channel
arrangementsof Figure 3. The bond-valence (and Pauling
bond-strength) sums arc the aggregatebond-valencesincidentto both O( I 0) anions in each arrangement.
(ii) Arrangement
(3) cangive completeorderwithin a
specificchannel,andpolardisorderbetween(or within)
channelswillleadto P4/wrcsymmetry;it is notablethat
thestructures
of fluorine-bearing
vesuvianite
refinewell
in thatspacegroup.
(iii) Mostsamples
of vesuvianite
haveminortosignificantfluorine,suggestingthat the vesuvianitesfucture
hasan affinity for fluorine.
Thesepoints suggestthat arrangement(3) will be
preferredin fluorine-bearing
vesuvianite.
However,we
arestill left with theproblemof localcharge-balance
in
fl uorine-freevesuvianite.
TheF-for-(OH)- substitutionat O(l0)
Now let us examinethe bond-valence
constraints
on
thepossiblelocalconfigurations
involvingthissubstitu-
t073
T'IIE ROLEOF FLUORINEIN VESUVIANITE
(a)
o(10)
H
o(10)
(b)
o
I
H
(c)
(d)
o
!
I
H
!
ooFF
Ftc. 5. Possible local configurations of the channel involving the replacementof OH- by F
at O(10).
tion. Firstly, Table 4 showstwo distinctX(3)-O(10)
distances.It seemsreasonableto assisnthe shorter
length!2.57 A) to a Ca-F bond and thJlonger length
(2.62A) to a Ca-O bond,with the resultingbond-valencesof0.158and0.181v.u.,respectively;
thusF- at
O(10)receives
0.158x 4 =O.63v.u.,and02- at O(10)
receives0.I 8 I x 4 = 0.72 v.u.fromthecoordinating
X(3)
cations.
Thepossiblecombinations
of 02- andF surrounding
a singleH(l) positionareshownin Figure5; (a) is the
standardarrangementwith no F, (b), (c) and (d) have
compositionsOF, OIIF and Fr, respectively.For configuration(b),theaggtegate
bond-valence
sumsincident
x4+ 0.5= 1.13v.u.)and[0.l8lx
toFand02-are[0.158
4 + 0.5 = 1.22v.u.l, respectively,assumingthat both
(l) sitesareoccupied.The sum at the 02associated
anionis fartoosmall,andhenceconfiguration(b) cannot
occur. For configuration (c), the analogousbond-valencesumsfor F and02- are [0.158x 4 + 0.5 + 0.3 =
1.43or 0.93 v.u. if the (1) site adjacentto the F is
vacantland [0.181x 4 + 0.7 + 0.5 = 1.92 v.u.],
respectively. This arrangementonly has acceptable
bond-valencesumsaroundF and 02- if oneof the two
(l) sitesis vacant;this is only compatible
associated
with the orderedpolar channelconfiguration (Fig. 3).
For configuration(d) (Fig. 5), the bond-valencesum
aroundeachF is [0.158x 4 + 0.5= 1.13v.u.];asboth
(1) sitesmustbe occupiedfor this (acceptassociated
able) bond-valencearrangement,configuration(d) can
only fit into configurationl.(Fig. 3) of the ordered
channelarrangement.
by F. If configuration(d) occurs,againO(10)cannotbe
morethanhalfoccupiedbyFbecausethisconfiguration
(l) (Fig.
is only compatiblewith channelarrangement
4)" which must combinein a l:l ratio with channel
arrangement(2) in orderto satisfythe observedchemistry of the y( I ) andX( ) sites.As channelanangements
(1) and(3) cannotoccurtogether[theobservedY(l) and
arenot compatiblewith thisl, the F
X(4) occupancies
occupancyof the O(10) position can never exceed
one-half;thusa maximumof I F a.p.f.u.canoccurat
Notethat this is in direct
the O(10) sitein vesuvianite.
contradictionto the proposalof Ohkawaet al. (1992)
refined
thattheO(10)sitein two samplesof vesuvianite
by themis completelyoccupiedby F.
Configuration(c) doesnot changethe OH- occupancyof theO(10)site(whichremainsat 0.5),whereas
configuration(d) reducesit to 0.25.
Disorderat thechannelpositions
Unlike the OH position,thereis strongvariationin
the magnitudeof the equivalentisotropicdisplacement
F contentof thecrystals.
factorat O(10)with increasing
This variation is positively correlatedwith an increase
in anisotropyof displacement,indicatingthattheF -for(OH)- substitution produces significant disorder at
O(10).The O(10)anionis bondedto one Y(l) andfour
coefncients
X(3) cations.The anisotropicdisplacement
anionsatO(10)
indicatethatthe
oftheinitial refinements
are positionally disordered along the 4 axis of the
TheO( l0) displacestructure.
channelin thevesuvianite
mentsareparallelto theO(10)-y(l) bond,andthis will
significantlyaffectthedetailsofthis interaction.InspecConstraintson O(I 0) site-occupancies
tion of Table 2 showsthat the isotropic displacement
with
factor at the Y(l) positionsignificantlyincreases
These arrangementshave implications concerning increasingF contentof the crystal.Accompanyingthis
thebulk chemistryof the O(10) site.If configuration(c) change is a very strong increase in the degree of
(Fig. 5) occurs,O( I 0) cannotbemorethanhalf occupied anisotropy of this displacement.The direction of the
r074
THE CANADIAN MINERALOCIST
maximumdisplacement
attheY(I ) positionis parallelto
the maximumdisplacement
of the anion at the O(10)
position; this suggeststhat these displacementsare
locally coupledand relate to the variation in local
bond-valence
requirements
with theprogressive
lossof
hydrogen-bonding
that accompanies
the F--for-(OHf
substitution.
The final split-sitemodelsarean effort to
get some indication of the geometry of the local
configurations.
However,it shouldbe emphasized
that
we do not havesufficientresolutionto deriveaccurate
occupancies
for thesplit sites;asa consequence
of this,
thelocalbond-lengthsderivedareprobablylessaccurate
thantheirprecisionwouldsuggest.
There are two O(10) anionsfairly close together
(2.4-2.8A) alongthe four-fold axis,resultingin three
differenttypesof substitutionthatcouldoccur(Fig. 5).
F could replacean (OHf group (Fig. 5b), with the
adjacentO(10)remainingasan02- anion.Fluorinealso
couldreplacean 02- anionthat wasthe acceptorof the
hydrogenbond(Fig.5c); in this case,rheF alsowould
be the acceptorof the hydrogenbondunlesstherealso
is a rearrangement
of the H atom. In addition,both
substitutionscould occur simultaneously
at the local
scale(Fig.5d).
Thesplit-siterefinements
produceseveraldisordered
positionsin the channel,andit is now necessary
to try
and derivethe most reasonable
local atomicarrangements.Firstlet us considerthelocalcoordinationofthe
KIA) andY(lB) sites.As indicatedin Table4, thereare
four calculatedf(lA"B)-O(l0A,B) approaches,
only
two of whicharepossible.
possibilities
Thesealternative
TA31A 7. POSSIBIT I.OCAL COOIDII{ATIONS AND IOND-VALENCES' AROUM
TnE y(L) SITES IN SOI(B!-BEARIN6 VES.IJVIANITE.SAI.IP1,:BS
Auang€dont
y(1A) -0(6)
v(1A) -o(10B)
<v(1A) -O>
x4
(1)
2.077 A
2.1,56
0.407 v.u
0.323
2 . 0 9 3A
2.238
0 . 3 9 0v . u
o.270
2.095
r.951
2.r22
1.830
r(18) -o(6)
y(r.B) -o(10A)
2.099
2.306
0.384
0.229
2.101
2.225
o.342
o.279
<r(18) -O>
2.)-40
t.765
2.126
1.807
Arren8en€nC
(2)
v(1A) -o(6)
r(1A) -o(108)
2.077
2.697
0.407
0.098
2.09X
2.59A
0.390
0.119
<r(1A) -o>
2.200
L.726
2.),94
1.679
y ( 1 8 )- 0 ( 6 )
r(18) -o(10A)
2.099
L,78L
0.384
0.949
2.101
1.865
o.382
o.736
<v( lB) -0>
2.035
2.485
2.054
2.264
TA31.E 8. BONDLENCTHS(A) AND BOIID.VAIJNCES'ABOI]T IIIE O(1OA) AND
OT VESWIANI?B
O(1OB) POSITIONS IN SELECTBDSTRUCTT'RBS
v12
o ( 1 0 A ) - x ( 3 )x 4
o(10A)-v(18)
2.555
2.OO4
S@
0(108)-x(3) x4
0(108)-r(1A)
2.556
2.OO4
Su
'Calcqlat€d
lith
l{-O
v33
o.207
o.495
2.638 0.r74
2.306 0 . 2 2 9
L.324
0.925
1.043
o.207
0.496
2 . 5 7 8 o.L97
2 . L 6 6 0,329
2.569 0.201
2,238 o . 2 7 0
t.324
1.111
L.074
bond"val€nce
cu&€s
of
Blom
2.594
2.225
0.191
o.279
(1981)
arelistedin Table7, togetherwiththelocalbond-valence
arrangementsin each case"The values are only very
approximatehereasthe site occupancieswereassumed
to be 0.5,andthebond-valence
curvesusedarevalidfor
Fe2*-O only. Nevertheless,
arrangement(l) is much
morereasonablethanarrangement(2) for both V27 and
V33,bothwith regardsto observedstereochemistry
and
the calculatedbond-valences.
Thus we may conclude
tharI( I A) bondsto o( I 0B) andz( I B) bondsto o( 10A).
Next, we will examinethe bond-valencestructure
around both of the O(10) positions (Table 8). In
fluorine-freeVl2, thebond-valence
sumto O(10)from
the bondedcationsis '1.324v.u.:this leaves0.676v.u.
to be suppliedby the associatedhydrogenatom,which
then hydrogen-bonds
to the adjacentO(10) anion.In
bothcrystalsof F-bearingvesuvianite,
thebond-valence
sums to O(l0A) and O(10B) are -1.0 v.u. when
calculated
for 02- at bothO(10)positions.Whateverthe
constituent
anions,thisindicatesmuchweakerhydrogen
bonding than in F-free vesuvianite;this finding is
consistent
with thelossof a low-frequency
(-3200cm-l)
band in the infrared absorptionspectraof F-bearing
vesuvianite(Groat,Hawthome,Rossmanand Ercit" in
prep.)
Fluorine avoidanceby Fd*
It shouldbenotedthatincorporation
ofFatthe O(10)
siteyia channelarrangement(3) (Fig. a) meansthatFe2+
at the (3) site shouldneverbond to F, only to OH-.
Although this seemsto be dictatedby the satisfactionof
local bond-valences
that specificallyinvolve the presenceor absence
ofhydrogenbonding,theobservation
is
in line with the well-known'Tluorine-avoidance"
rule
(Rosenberg
& Foit 1977).
ACKNoWLEDGEMEI{TS
'Calculat€d
rlth
U-O bond-valeice
cun€s
of Blom
(1981).
The authors are indebted to the Roval Ontario
MuseumandDr.P.Cernf forthesamples
of vesuvianite,
without which the work could not have been done.
Financialsupportwasprovidedby the NaturalSciences
THE ROLEOF FLUORINEIN VESUVIANITE
ru5
and EngineeringResearchCouncil of Canadain the -,
HawrsoRNE,
F.C.& ERcrr,T.S.( 1992):Thechemistry
of vesuvianite.
Can.M ineral. 30. I 9-48.
form of OperatingGrantsto thefirst andsecondauthors
and an InfrastructureGrantto the secondauthor.
& PurNrs,A. (1993):Thesymmetryof
vesuvianite.
Can.Mineral.31,(in press).
REFERENCES
At-t-pN,F.M. (1985): Structural and Chemical Variations in
Vesuvianite.Ph.D.disserration,
HarvardUniv.,Cambridge,
Massachussetts.
HAwTHoRNE,
F.C.& Gnoer,L.A. (1985):Thecrystalstructure
of wroewolfeite,a mineralwith [Cua(OH)6(SO4)(H2O)]
sheets.
An. Mineral.70. 1050-1055.
INTERNATIONAL
TABLES
FoRX-RAY CRysrAl-I-ocnepsy.VoL.
IV (1974):KynochPress.Birmingham.England.
& Bunnuau,C.W.( 1992):A comprehensive
structuremodel for vesuvianite:symmetryvariationsand crystal
LAGER,
G.A., Xrc, Q., Ross,F.K., RossueN,c.R., ARMBRUSgrowth.Can.Mineral.30, l-18.
rER,T., RoTELLA,
F.J. & ScHULrz,A.J. (1989):Crystal
P4lnncvesuvianitefrom Tanzania,Africa.
structure
of
a
(1981):
BRowN,I.D.
The bond-valence
merhod:an empirical
Geol.Soc.Am' Abstr. Programs21, A120.
approachto chemicalstnrctureand bonding./r Structure
andBondingin CrystalsII (M. O'Keeffe& A. Navrotsky,
OHrewn, M., Yosuresa,A, & TarcNo, S. (1992):Crystal
eds.).AcademicPress,New York.
chemistryof vesuvianite:site preferences
of square-pyramidalcoordinated
sites.Am.M ineral. 77. 945-953.
Cona,A., Delle Grusra,A., Iserrr,G. &Mtzzt, F. (1970):
On the crystalstructureof vesuvianite.Atti Accad. Sci.
PauuNc,L. (1960):TheNatureofthe ChemicalBond(3rded.).
Torino105,63-M.
ComellUnivenity hess, Ithaca,New York.
-
FrrzcrneLo,S., RHerNcor-o,
A.L. & LEAVENs,
P.B. (1986):
Crystalstructureof a non-P4/nnc
vesuvianite
from Asbestos,Quebec.
Am.Mineral.7l, 1483-1488.
P.E.& Forr,F.F.,Jn.( I 977):Fe2+-Favoidance
RosENBERG,
in
silicates.Geochim.Cosmochim.
Acta 41.345-346.
Grusrrrrm, G. &MAzzt, F. (1983):The crystalstructureof a
vesuvianitewith P4ln symmetry.TschermaksMineral.
Petrogr. Mi tt. 31.217-288.
YosHresa,A. & Mersurraoro,
T. ( 1986):Thecrystalsrucure
from Nakatatsumine:reinvestigation
of vesuvianite
of the
cationsite-population
andof thehydroxylgroups.
Mineral.
J.13, t-t2.
Gnoer, L.A. (1988):The Crystal Chemistryof Vesuvianite. ReceivedJulv 22, 1991, revisedmanuscriptacceptedFePh.D.thesis,Univ. Manitoba,Winnipeg,Manitoba.
bruary 14, 1992.