AmericanMineralogist, Volume63, pages448460, 1978 Is eachanalcime different? Ftonnnzo Mrzzt C.N.R. Centrodi Studioper la Cristallografa strutturale cf o Istituto di Mineralogiadell'Uniuersilh, Pauia, Italy AND ERMANNo GALLI Istituto di Mineralogiae Petrologiadell'Uniuersitit Modena,Italy bstract Precedingstudies assigned a cubic symmetry(Ia3d) to analcime,even if possiblelower symmetriesare suggested from contradictoryevidence: e.g., the occurrence of opticalanoma- liesand the presenceof forbiddenreflections in X-ray photographs. In an attempt to check the true symmetryof analcime,the crystal structuresof seven opticallyanisotropic and apparentlyuntwinned crystals from variouslocalities were refined (finalR from 0.02to 0.07for the observedreflections). They were tetragonal (I4r/acd, with cell edges:o ) c in four samplesand a ( c in one sample)and orthorhombic(/bca). These diflerentsymmetries follow from the differentordering of Al in eachof threetetrahedra and from the relateddifferent occupancy of the nearestNa site.Neither tetrahedra occupied only by Al (maximumAl fraction0.5), nor completeoccupancies of Na sites(maximum 0.84) were observed. Simplerelationships were obtained between Al fractionsin tetrahedraand (1) occupancies of the nearestNa sites,(2) latticeparameters, and (3) intensitiesof tripletsof X-ray reflec- tions. A hypothesisis given regarorngthe possiblederivation of all the crystal structuresof analcimefrom the superpositionof three different orientationsof one basic tetragonal structure,with an Al fraction of 0.5 in two equivalenttetrahedra and no Al in the third. Introduction cimesworthy of a detailedX-ray single-crystalinves- The crystalstructure of analcime,NaAlSirO.. HzO, tigation. was determinedby Taylor (1930)and refinedmany times (Calleri and Ferraris, 1964;Knowles et al,, Experimental 1965;Ferraris et al.,1972),butalways with reference Thick sectionswithout coverglasswere prepared to the cubic spacegroup la3d, althoughit has been from nearlythirty analcimesamples, and we selected known for a long time that many analcimesexhibit with the polarizingmicroscope those samples which somedeviation from cubicsymmetry (Coombs, 1955; were definitely anisotropic at least in some parts. Harada and Sudo, 1976;and quoted references). These non-cubic crystals were always poly- Before our work little was known about the real syntheticallytwinned, and hencewe were forced to symmetryof non-cubicanalcime. Mazzi et al. (1976) selectportions of twins with singleindividuals large showedthe symmetryof non-cubicleucite to betetra- enough for single-crystaldiffractometry and with gonal l4r/a and consideredit a possiblesymmetry sharpcomposition planes. alsofor non-cubicanalcime. Harada and Sudo (1976) As shown by Mazzi et al. (1976) for the related suggesteda monoclinic symmetry as probable for mineral leucite, both merohedricand pseudo-me- analcime,because the extrapolationof B valuesin the rohedrictwins arepossible, and only the latter canbe wairakite-analcimeseries gives A = 90"12' for pure recognizedand avoidedduring the data collection. analcime.We thereforeconsidered non-cubic anal- The following sevensamples were selected for the 0003-004x / 78 / 0506-o148$02. 00 448 MAZZI AND GALLI: ANALCIME completecollection of intensitydata, as typical repre- The symmetryof the intensitiesand systematicex- sentativesof about thirty specimenssubmitted to a tinctionsled to spacegroup I4r/acd for samplesANA preliminary X-ray single-crystalexamination (the l, ANA 2, ANA 3, ANA 5 andANA 6, andto space numberof the specimenin the "Museo di Mineral- group lbca for ANA 4 and ANA 7. These space ogiadell'Universiti di Modena" is in parentheses): groups are compatiblewith the presenceof diffrac- tions (hlh:2h t I : 4n -l 2) observedin the powder ANA 1: Veselinear t]sti nad Labem(also known in patternsand alsodetected in somesingle-crystal pho- German as "Wesselnbei Aussig"), Bo- tographs. These reflectionsviolate la3d symmetry hemia,Czechoslovakia (16-2-21, 47). Crust and wereconsidered for a long time to beevidence of of vitreous transparentcrystals, up to 5 deviationfrom cubic symmetry.Actually the major- mm, intergrownand twinned,filling the ity of thesereflections were "unobserved"in all ex- veinsand coveringa freesurface of an albi- aminedcrystals; when present they were in agreement tizedbasaltic rock (Rammelsberg,1858). with the correspondingspace group (e.g.020. 0100, ANA 2: Val di Fassa,Trento, ltaly (16-2-21,2l). 424 werepresent both in the tetragonaland ortho- Crustof milky whiteand pale pink crystals, rhombiccrystals, whereas 002,0010, 42werc absent up to I cm across,densely intergrown with in tetragonal crystals, but present in the ortho- calciteon a weatheredalkali-basaltic rock rhombic ones)with no evidenceof broadeningor (Braccio,l95l). splitting. ANA 3. ANA 4. ANA 5 and ANA 7: Alta Val Following the X-ray data collection,electron mi- Duron, Val di Fassa,Trento, Italy (all from croprobe analyseswere made on the samecrystals $pecimen16-2-21, 100). Vitreous colorless with an Anl-Sprvreinstrument operated in the wave- transparent crystals, featuring irregular lengthdispersive mode at l5 kV, 0.1pA beamcurrent prismsup to 7 mm long, in radial fibrous (10 nA samplecurrent) and usinga defocusedbeam aggregatesfilling the cavitiesof a Middle (spotsize -50 pm), yieldingthe atomicproportions Triassicweathered basaltic andesite (Mon- reportedin Table 2. On-linedata processingbased eseand Sacerdoti.1970:Yezzalini and Al- upon ZAF corrections(Ziebold and Ogilvie, 1964) berti, 1975). usedAlbee and Ray (1970)correction factors. Syn- ANA 6: Isola dei Ciclopi, Catania,Sicily, Italy (16- theticplagioclase glass, natural albite,and microcline 2-21, 46). Vitreous transparenteuhedral wereused as standards for Si,Al, Ca, Na, and K. Fe, trapezohedron,2 cm across,in a basaltic Mg, Sr, and Ba were also analyzedfor but found to rock (Di Franco, 1926;Knowles et al., be absent.The valuesgiven in Table 2 are the aver- 1965;Ferraris et al., 1972). aged analysesof three points for each specimen, The crystalsfor X-ray measurementswere ground which were homogeneousthroughout. Water loss into spheres,whose dimensions are given in Table I was not determinedbecause of the paucity of the togetherwith otherexperimental conditions. availablematerial. Unit-cell dimensions and diffraction intensities were measuredat room temperaturewith a Philips Refinements 1100 automatic single-crystaldiffractometer and The atomic parametersgiven by Ferraris et al. graphite-monochromatizedradiation. The latticepa- (1972), properly adapted to the 14,/scd and lbca rametersgiven in Table 2 were obtained by least- spacegroups, were used as starting coordinates ofthe squaresrefinement ofthe adjustedangular settings of refinementscarried out with a modified versionof 18reflections. the least-squaresprogram Onrls (Businget al., 1962). Diffraction intensitieswere measured in the @-scan The scattering-factorcurves were calculated from the mode,and backgroundmeasurements were made at values given in International Tablesfor X-Ray Crys- both sidesof each reflection.Three standardreflec- tallography(1974, p.99-l0l) for neutralatoms. A tions, monitored at three-hourintervals, did not secondaryextinction correction(Zachariasen, 1963) show significantchanges in their intensitiesduring was includedin the refinementsof the form: the data collection. Intensitieswere correctedfor F""1"(corr): F.a./(l * Lorentz-polarizationeffects and for absorption,then 0gloo") convertedto lFon"l and o(Fop")according to Davies where Fcar" is the calculated structure factor, and Gatehouse(1973). The reflectionswith lF"o"l' F""r"(corr)the value correctedfor the secondaryex- lessthan 3o(ffioJ were considered as "not observed". tinction, 1o6"the observedintensity and g the ex- MAZZI AND GALLI: ANALCIME Table 1. Summaryof experimentaland refinementdata ANA 1 ANA 2 ANA 3 ANA 4 ANA 5 ANA 7 Sphere diameter (m) O.O84 0.1 14 0.174 o.236 o.r92 o. r98 0.236 Radiation MoKC CuIG MoKd Mol((X MoKC CuKCI MoKd , -1. AR (cm ) O.O3O 0.408 0.063 o.085 o. 069 o.7o9 o. 085 Maximum 2@(o; 60 13O 60 6o 60 13Ci 60 Scan width (o) 1.0 2.5 1.O 1.4 2,5 1.O Scan speed (o/sec) o.ozs o.o5 o.o333 o.05 o.o25 o.0625 o.05 Time for each background (sec) 5 IV J 7 ) R< Total measurements 11OO 670 I 106 449r 1111 1263 2630 Total unique reflection" 938 551 94r L9r2 942 554 1874 observed reflections t12> :g(12)) +AS 434 662 r392 677 453 t352 Maximum percent variation in 2.9 2.r 3.6 3.5 3.3 t.7 2.5 standard reflection intensiti-es Anisotropic refinenent cycles 28 ,9 ,9 28 rQ 28 28 Extinction parameter (exrOd) 67(4g) 6350 3) 1060(3 I ) r874(18) r51o(23) L6a3t(233) 506(9) Final R (A11 unique reflections) O.151 0.064 0.06r o.o4r o.o52 o.o42 0.048 Fina1 R (Observed reffections) 0.067 o. o43 o.037 o.o24 o.o3o 0.033 o.o29 E.s.d.rs on the last significant digit in parentheses. tinction parameterto be refined(Table I ). Becauseof ratios,calculated according to Jones(1968) from the the use of a graphite monochromator the expression updatedaverage Z-O distancein each tetrahedron, for B given by Zachariasen (1963) was modified as gavea sum of the products(Al fraction) X (multi- follows: plicity) much lower than the unit-cellcontent of l6 Al; hence a new line for the deduction of the Al o_- | dA* | Q*ccsa2| v * ar' fraction from the T-O distanceswas drawn through "i"n- OT7;""8 the points 1.609(Al fraction : 0 in qrrartz after wherel* is the absorptionfactor, p the linear ab- Zachariasenand Plettinger, 1965) and 1.648,{(Al sorptioncoemcient (for analcime:p(Mo) : 7.19cm-1