Compressibility of sodalite and scapolite Rosnnr M. Hlznx ...

American Mineralogist, Volume 73, pagesI120-1122, 1988

Compressibility of sodalite and scapolite

Rosnnr M. Hlznx, ZacHanY D. Snanp

GeophysicalLaboratory, CarnegieInstitution of Washington,280l Upton Street,N.W., Washington,D.C.20008, U.S.A.

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Equation-of-stateparametersof sodalite (NaoAlrSi.O,rCl)and a meionitic scapolite(ap-
proximately CaoAluSiuOroCOhr)ave been calculatedfrom pressure-volumedata obtained
by single-crystalX-ray diffraction techniques.Linear compressibility of cubic sodalite is
6.4 x ll-a kbar-', correspondingto a bulk modulus of 0.52 + 0.08 Mbar (assumingK :
4). The pressureresponseof tetragonalscapoliteis almost isotropic with compressibilities
parallel and perpendicularto the c axis of3.5 x lO-aand 3.7 x l0-4 kbar-l, respectively;
the bulk modulus is 0.90 + 0.12 Mbar. Sodalite and scapolitecompressprincipally by
deformation of the Al-Si tetrahedral framework coupled with Na-O and Ca-O bond
compression,respectively.The greatercompressibility of sodalite compared to scapolite
is primarily a Coulombic effect,arising from the differencein bond strength of Na*-O
versusCa2*-O bonds.

INrnonucrroN both sodalite (N. Komada, E. F. Westrum, Jr., B. S.

Sodalite and scapolite are open framework silicatesthat Hemingway, M. Y. Zolotov, Y. V. Semenov,I. L. Kho-
contain halides or carbonatesof Na or Ca. NaCl is an dakovsky,and L. M. Anovitz, in prep.;Taylor, 1968)and
essentialcomponent of sodalite (NaoAlrSirO,rCl),where- scapolite(N. Komada, D.P.Moecher, E. F. Westrum,
ascoupledNaCl-CaCO. and NaSi-CaAl substitutionsre- Jr., B. S. Hemingway, Y. V. Semenov,and L L. Khoda-
late end members of the scapolite solid-solution series kovsky, in prep.; Graziani and Lucchesi, 1981).In this
between marialite (NaoAl,SinOroCl)and meionite study we have made compressibility measurementsfor
(CaoAluSiuOroCOrT).he unusual "stufled" open frame- thesetwo minerals to complete the thermodynamic da-
work structuresare of interestto crystalchernists,and the tabase,to searchfor possiblehigh-pressurephasetrans-
presenceof NaCl and CaCO, as essentialcomponentsin formations, and to evaluateinverserelationshipsof com-
thesesilicatesmakes them important for petrology and pressibility and expansivity for these open framework
structures.
geochemistry.

Sodalite exists as a primary phase in undersaturated ExpnnnrnNTAL DETAILS
syenitesand phonolites(e.g.,Deer et al., 1963).The ac-

tivity of NaCl in sodalite-bearingrocks can be calculated SampledescriptionS. odalitewascollectedfrom a pegmatitic

if the thermodynamic propertiesof sodaliteand sodalite- veinin a nephelinesyenitefrom Mont St.Hilaire,QuebecT. he
related phasesare known (Wellman, 1970; Heltrrich et sodalitecrystalsaremilky-whitedodecahedronusp to I cm in

a1.,1986).Thesedata could also be usedto evaluatethe diameterC. ompositionwasdeterminedusinganautomatedcA-
cooling histories of sodalite-bearingmeteorites (Gross- rvrnc,eql.ectronmicroprobeat the University of Michiganoper-
man and Steele,1976;Lumpkin, 1980)and to limit NaCl atingat l0-kV acceleratinvgoltagewith a defocusedbeamto
avoidvolatilizationof eitherNa or Cl. Thecompositionof the
activities in sodalite-freeundersaturatedrocks. sodaliteis (Na, nrIQo,)(Al nrFeaf4S,i5E)Or3ee(Cl,u(SOo)oor)(Helf-
frich et al., 1986).
Scapoliteforms in a wide rangeof metamorphic envi-
ronments (e.g.,Deer et al., 1963).The CO, content of The scapolitesample,88 molo/omeionite, was supplied by D.
scapolite generally increaseswith increasing metamor- P. Moecher at the University of Michigan. It was found in an

phic grade. A number of scapolite-bearingassemblages ejectedblock u/ithin a tufffrom Monte Somma, Italy. The crys-

found in high-graderocks buffer CO, activities, and, as tals are translucent,white grains lessthan I mm in diameter.

with sodalite, the lack of scapolitein certain lithologies The composition, determinedby Moecher using the University

places an upper limit on the activity of CO, in those of Michigan cAMEcAelectron microprobe, is (Na"rol(otrCator-
terranes(Moecherand Essene,1985).With accuratephase Feoo,)(SiuorAl,,3)O2on,(Cl6o6(CO3e)or(SO4)o0r).
High-pressure X-ray diffraction. Rectangular single crystals of
equilibria for scapoliteand relatedminerals, petrologists sodaliteand scapolite,approximately 60 x 60 x 40 s,mon edge,
can placelimits on fluid compositionsand evaluatepro- were selectedfor study in a gasketeddiamond-anvil cell for X-ray
cessessuch as CO, flooding in the lower crust (e.g.,New-
ton et al., 1980). diffraction. Both crystalswere included in the samehigh-pres-
sure mount-a procedure that facilitates comparison of bulk
The thermochemical and thermophysical properties moduli of different materials becausepressureis identical for

enthalpy, low- and high-temperatureheat capacity, and both samples.A 4:l methanol:ethanolmixture was used as the

volume at elevatedtemperatureshave beenmeasuredfor hydrostatic pressuremedium and 10-pm chips ofruby were in-

0003{04xi88109I0-1I 20$02.00 tt20

HAZEN AND SHARP: COMPRESSIBILITY OF SODALITE AND SCAPOLITE tt2L

TABLE1. Sodaliteunit-celpl arametervsersuspressure Tneu 2. Scapoliteunit-celpl arametervsersuspressure

P (kbar) a (A) v(4,) vtvo P
(kba0
0.001 8.878(2)' 6ss.7(4) 1.0000 a (A) c(A) v(A1 VlVo
8.8s09(6) 693.4(1) 0.9910
11 686.8()1 0.9816 0.001 12.191(21 7.57s(s) 112s.8(5) 1.0013 0.6214
22 8.8228(6) 672.5(3) 0.9611 <1'
26 8.761(1) 668.2()1 0.9550 12.186(1) 7.572(1)1124.3(3) 1.0000 0.6213
8.742s(6) 5
12.172(117.563(1)1120.5(2) 0.9966 0.6213
9.5
12.144(2) 7.5s0(1) 1113.3(3) 0.9902 0.6217
11
Note; Numbersin parenthesesrepresentesd's. 12.141(1) 7.546(1)1112.3(3) 0.9893 0.6215
11
17 12.14't(2) 7.545(2111't2.1(4)0.9891 0.6215
22
25 12.113(1) 7.s30(1)1104.8(3) 0.9827 0.6217
26
cluded for pressurecalibration. Details ofdiamond-cell design, 30 '1t 22..0078671( 21 )) 7 . 5 1 6 ( 1 )1 0 9 8 . 1 ( 3 ) 0.9767 0.6218
crystalmounting, pressuremeasurementsa, nd cell operation are 41 7.s10(3) 109s.2(6) 0.9741 0.6219
given in Hazen and Finger (1982).
12.072(1) 7.506(1)1093.9(3) 0.9730 0.6218
The unit-cell parametersofcubic sodaliteand tetragonalscap-
olite were determined from reflections measuredon an auto- 12.060(4) 7.498(6)1090.600)0.9700 0.6217
mated four-circle diffractometer with Zr-filtered Mo radiation.
Each reflection was centeredin eight equivalent positions after 12.013(21 7.463(2) 1077.'t(4) 0.9580 0.6212
the method of King and Finger (1979). Sodalitewas measured
at five pressuresbetween 1 bar and 26 kbar; eight reflections ' Thepresenceof a tinycompresseadirbubbleinthesamplechamber
with 32' < 20 < 35" were used in the cell determination. The indicatedthat the pressurein the diamondcellwassignificantley ssthan
sodalite crystal deterioratedirreversibly when pressurewas in-
1 kbar,thoughmorethan1 bar.
creasedto above 30 kbar. This effectmight have resulted from
crushingbetweenthe diamond anvils, but the gasketwas signif- There were no detectablediscontinuities in the pressure-
icantly thicker than the crystal at the time of the change.We volume data, and scapoliteremains tetragonal through-
suspectthat the sodaliteexperienceda reconstructivetransition out the rangefrom I bar to 4l kbar. The c/a axial ratio
to an unknown high-pressureform. Scapolitewas measuredat remainsconstantat approximately 0.6215, reflectingthe
twelve pressuresbetween I bar and 4l kbar; as many as 16 isotropic compressibility of the phase.
reflectionswith 28" < 20 < 33' were used for cell-parameter
determination. Compressionmechanisms.Hazen and Finger (1982,
1985) demonstratedthat the principal compression
Unit-cell dimensions for both phaseswere refined initially mechanismin most framework silicatesis T-GT bond
without constraint (i.e., as triclinic). No significant deviations bending (i.e., framework distortion) coupled with
from ideal cubic and tetragonaldimensionality for sodalite and compressionof alkali-oxygen and alkaline earth-oxygen
scapolite,respectively,were observedat any pressure.Final re- bonds. Cation-oxygen bond compressionwas shown to
ported cell constants(Tables I and 2) were refined with appro- be primarily a Coulombic effect: bond compression is
priate symmetry constraints.Pressure-volumedata were fit by inversely proportional to cation charge.Thus, while in-
least-squaresproceduresto a first-order Birch-Murnaghan equa- dividual framework-forming tetrahedra are relatively rig-
tion of state based on an assumedvalue of 4 for the pressure id, the largemonovalent and divalent cation sitestypical
derivative of the bulk modulus, K' (see,e.g.,Hazen and Finger, of feldspars,feldspathoids,zeolites,and other framework
I 982). aluminosilicatesdisplay significantcompression.

RBsur-rs AND DrscussroN Sodaliteand scapoliteconform to this model. The ratio
of averagelarge-cationchargesis 1.00:1.88-almost iden-
Sodalite compressibility.Cubic sodalite experienceda tical to the ratio of bulk moduli, 0.50:0.92.Furthermore,
4.50/ovolume decreasebetween I bar and 26 kbar, cor- the 0.5-Mbar compressibilityof sodaliteis similar to that
respondingto a bulk modulusof 0.52 + 0.08 Mbar. Lin- of other Na-bearing framework silicatessuch as nephe-
ear compressibilityis 6.4 + 0.3 x l0 4 kbar-I. No dis- line (0.50Mbar; Yoder and Weir, l95l), analcime(0.45
continuities in the pressure-volumerelation were Mbar; Hazen and Finger, 1979), and albite (0.70 Mbar;
Angelet al., 1988),whereasthe 0.92bulk moduli of calcic
observed,and the material retainedcubic dimensionality scapoliteis closeto that of anorthite (0.94 Mbar; Angel
with no peak broadening or deterioration of difraction et al., 1988).
intensity to 26 kbar. It appears,therefore,that below 26
kbar, Mont St. Hilaire sodalite does not undergo a re- CoNcr,usroNs
versible "polyhedral tilt" phasetransition that is typical
of severalother high-symmetry, Na-bearing framework The high-pressurebehaviors of sodalite and scapolite
silicates,including alkali feldspar (Hazen, 1976), carne- are similar to those of many other framework silicates.
gieite (Cohenand Klement,1976), analcime (Hazen and Pressure-volumedata from this study suggest hat indi-
Finger, 1979),andzeolite4A(Hazen and Finger, 1984). vidual Si and Al tetrahedraundergolittle compressionor
other deformation, while T-O-T angles change in re-
Scapolitecompressibility.Ca-rich Monte Somma scap- sponseto compressionof Na-O or Ca-O bonds. It has
olite undergoesa 4.2o/ovolume decreasebetween I bar been shown from previous studiesthat Na-O bonds are
and 41 kbar. Scapolitebulk modulusis 0.90 + 0.12 Mbar, almost twice ascompressibleasCa-O bonds-principally
and the pressrueresponseis essentially isotropic with a Coulombic effect.It is important to remember, there-
compressibilitiesof 3.5 t 0.I x 10-aparalleland 3.7 a fore, that the equation-of-stateparametersreported here
0.1 x l0-4 kbar-' perpendicularto the tetragonalc axis. apply only to the alkali and alkaline-earth cation com-
positions of thesespecific samples.Changesin the ratio

tt22 HAZEN AND SHARP: COMPRESSIBILITY OF SODALITE AND SCAPOLITE

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Mexuscnrrr RrcErvEDFennuenv 4, 1988
M.lrsuscRrFrAcEEPTEMDav 20. 1988