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Scapolite Crystal Chemistry: Aluminum-Silicon Distributions

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Scapolite Crystal Chemistry: Aluminum-Silicon Distributions American Mineralogist, Volume 61, pages 864-877, 1976 Scapolitecrystal chemistry:aluminum-silicon distributions, carbonate group disorder, and thermal expansion LoutssLrvtnu a,No J. J. Peprrr Department of Earth and Space Sciences State Uniuersily of New York, Stony Brook, New York 11794 Abstract Crystal structure parameters have been determined for a compositionally intermediate scapolite (Na, nrCa,.rrK' ro) (Si8.0sAl3e5)Or4Cl' u"(COr)0.(SOr)o on in space group P4r/nbefore and after a heating cycle. In addition, unit-cell parameters have been determined with room- temperaturedata collected before and after heating, and with data collectedat 400",600o, 700', 800',900o and 1000"C. Both a and volume increasewith temperaturewhile c remains constant.Thermal expansionof a resultsfrom rotation of four-memberedrings of tetrahedra in the (001) plane. Tetrahedral bond distances suggestthat this sample has a highly ordered Si-Al distribution with Tl and 73 occupied by Sio+ and T2 occupied by Al3+. The irreversible change in tetrahedral bond distancesafter the sample was heated to 1000'C suggestsa slight amount of disordering has taken place. This change also affectedthe unit-cell parameters.The carbonate group was refined as a rigid body with realistic bond distancesand anglesusing the least-squarescomputer program RrtNE4. The refinement confirms the model of Papike and Stephenson(1966) and suggeststhat the group tilts only slightly out of the (001) plane. Normalized primitive reflections which violate 14/m symmetry decreasein intensity with temperatureup to 1000'C but do not dissappear. Introduction and that of a Ca-richscapolite by Papikeand Ste- phenson(1966). Lin and Burley(1973a,1973c,1975) Scapolites,a group of rock-formingsilicates that reportedthree additional structural refinements. The exhibitmany structuralcomplexities, can, as a first spacegroup of scapoliteis a functionof composition. approximation,be consideredas solid solutions The theoreticalend-members, marialite and meion- between marialite, NaoAlrSi"O2oCl,and meionite, ite, display diffraction symmetry consistentwith CaoAl.SiuO2nCO..These formulae are written in the spacegroup 14/m, bfi intermediatecompositions form mostconsistent with the scapolitestructure, but havereflections that violatethis symmetryand reduce can also be written as marialite,3NaAlSirOr.NaCl the spacegroup to a primitive type. Papikeand Ste- and meionite,3CaAlrSirOr.CaCO, to showan anal- phenson(1966) reported diffuse reflections violating ogy with plagioclasefeldspar chemistry. Three thebody-centered symmetry but did not treatthem in coupledsubstitutions are evidentin this solid solu- the structuralrefinement. Lin and Burley (1973a, tion series:Na'+ c Ca2+'Si4+ = Al3+;Cl'- c CO3-. 1973b,1973c,1975)state that the true spacegroup is All threesubstitutions are activein the more sodic P42/n,and their threerefinements are reported in this part of the series[Ca/(Ca + Na) < 0.75]while in the spacegroup, eventhough they did not observereflec- lesssodic part lCa/(Ca * Na) > 0.751,the sub- tions violating 14/m symmetryin their meionite-rich stitutionis NaSi z=lCaAl, the sameas in the plagio- scapolite.Ulbrich (1973)also statesthat the space clasefeldspars (Evans et a|.,1969).AtCa/(Ca * Na) group for intermediatecompositions is P4"/n. Pha- > 0.75 the anion site is filled with CO3- (papike, key and Ghose(1972) and Buseckand Iijima (1974), 1964;Evans et al.,1969). however,conclude from electrondiffraction studies The first reasonablycomplete structure models of that thespace group for intermediatecompositions is scapolitewere reported by Pauling(1930) and Schie- P4 or P4/m. bold and Seumel(1932). The structureof a Na-rich Infrared spectra (Papike, 1964; Schwarcz and scapolitewas first refined by Papikeand Zoltai (1965) Speelman,1965) suggest that the carbonategroup is, 864 SCAPOLITE CRYSTAL CHEM ISTRY 865 in fact, a trigonal group similar to those in other controlled Picker four-circle diffractometer with substances.The roleof a trigonalcarbonate group in MoK" radiation(0.7107A) monochromatized by a a tetragonalmineral was first addressedby Papike graphitecrystal. A total of 1778independent reflec- and Stephenson(1966), who suggestedthat the car- tionswere measured in the before-heatingexperiment bonate group in each unit cell is displacedoff the and 1657in the after-heatingexperiment. Intensities four-fold axis(14/m) and disorderedover four differ- werecollected using an o-N scan(2olmin) with five- ent positionsin the (001)plane. Because scapolites secondbackground counts on eachside of the peak, are volatile-containingsilicates, there has recently and then convertedto structurefactors by applying beenrenewed petrologic interest in them as possible Lorentzand polarizationcorrections, but no absorp- storagesites for volatilesin the lower crust or upper tion corrections(pr : l2.2cm-'). A standardreflec- mantle.Lovering and White (1964)described sulfur- tion, (040), which was repeatedevery 20 reflections rich scapoliteas a primary constituentin granulite duringdata collection, showed less than one standard inclusionsoccurring with eclogiteand ultrabasic deviationvariation in its structurefactor. rocksin the breccia-and basalt-filledpipes ofeastern Initial positionalparameters and temperaturefac- Australia.Newton and Goldsmith (1975) report that tors for all atomsexcept the CO3- group weretaken meionite,CanAl.Si.OrrCOs, has an unusuallyhigh from Lin and Burley(1973a). Original carbonate po- thermal stability, exceeding1500"C at 20 kbar. sitions were estimatedfrom Fourier and difference Meionitethus may play a role asa primary mineralin Fouriermaps. deep-seatedmagmatic processes, and may be a major Both isotropicand anisotropicrefinements were storagesite for COzin the lower crust. executedusing scatteringfactors from Doyle and In this study we haveinvestigated: (l) the Al-Si Turner (1968).The structurewas initially refinedin distribution in a scapolitederived from a slowly the space group P4r/n using the least-squarespro- cooledmetamorphic rock; (2) the preciselocation of gram,RrtNn2, written by L. W. Finger.Final refine- the carbonategroup within the scapolitestructure; mentsused RnINn4, (Finger and Prince,1975) a pro- and (3) the thermalexpansion of this mineral. gram which permittedrefinement of the CO!- group asa rigidbody. The carbonategroup was constrained distanceof 1.34. The Experimental to be planarwith a fixedC-O refinedparameters for the group includedthe posi- Data werecollected on a singlecrystal from sample tion of the center,the rotation and tilt of the triangle ON-6 (Naz.r7Ca,.rrKo.ro) (SiE.06AL.eu)O24C10.6e(COs)o.s? of oxygens,and thermal parameters corresponding to (SO,)o.on(Shaw, 1960) (0.25 mm X 0.15mm X 0.10 the motionassociated with uniaxialtranslation of the mm). Thissample is from a scapolite-pyroxenegran- entiregroup. ulite outcrop in Monmouth Township, Ontario Errors on isotropictemperature factors were calcu- (Lin and Burley, 1973b). latedusing the programBcoNv, and errorson bond The crystalused was found to beof suitablequality distanceswere calculatedusing the programBoNux. by examiningWeissenberg photographs, which dis- Both programswere written by B. A. Wechslerand played sharp spotswith no split reflections.Next it usediagonal matrix elements only. was mountedparallel to its a axis on a quartz fiber In the anisotropicrefinement of the before-heating with specially prepared high-temperaturecement data, all 1660reflections of the P4z/n data set were (Brown et al., 1973).The crystaland fiber werethen acceptedand resultedin a final unweightedR of 0.072 placedin a silica-glasscapillary (0.5 mm in diameter) and a final weightedR of 0.052.In the anisotropic which was evacuatedand sealed. refinementof the after-heating data, again all reflec- Becauseof the scapolitespace-group controversy, tions wereaccepted resulting in a final unweightedR the first data set was collected assumingno ex- of 0.080and a final weightedR of 0.054.The weight- tinctions. All h + k : odd for / : 0 reflectionswere ing schemewas W : l/or2 whereap is the standard found to be indistinguishablefrom background,and deviation of the structurefactor basedon counting wereassumed to be of zero intensity.This suggestsstatistics(Prewitt and Sleight,1968). P4r/n as the most likely choicefor the scapolitespace Cell-parameterdeterminations at eachtemperature group. The primitive reflectionswhich violate I4/m weremade using 2d valuesof 13 to 17 independent symmetry(h + k + / : odd) werefound to be present reflections.2d values were collected at eachsuccessive and were treated in the refinement.Integrated in- temperatureas temperaturewas increased,and then tensitieswere measuredusing a PDP-15 computer- at room temperatureafter the heatingcycle. The cell L. LEVIEN AND J. J. PAPIKE Tlnt-s 2. Final positional parametersand equivalentisotropic TesI-r 2, continued temperaturefactors (A) for scapolitebefore and after heating. Theseparameters are (/r, t/e,%) greaterthan thosereported for Before After the 14/m spacegroup by Papikeand Zoltai (1965) Heating Heating Before After x 0.36(7) 0.36(7) y Atom Heating Heating 0.28(7) 0.28(7) z 0.24(7) 0.24(7) B 4.3(4) Na,Ca x 0.6136(l) 0.6127(l) 4.3(4) y 0.5361(l ) 0.5358(r) ^^ z 0.7641(2) 0.7634(2) vr x 0.17(7) 0.17(7) B 1.ee(2) 2.14(3) y 0.28(7) 0.28(7) z 0.24(7) 0.24(7 ) cl x 0.7500 0.7500 B 4.3(4) 4.3(4) y 0.7500 0.7500 z 0.7500 0.7500 B 3.87(e) 5.02(e) parameterswere refined using the program CELRT, x 0.5892(l) 0.ssel ( I C. T. Prewitt'smodified version of Poonx. y 0.6592(l) 0.659s
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