Cation Ordering in Lepidolite
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American Mineralogist, Volume 66, pages 1221-1232, 1981 Cation ordering in lepidolite SrBpnBN GucceNFIetNI Department of Geological Sciences University of lllinois at Chicago Chicago, Illinois 60680 Abstract Three lepidolite-lM and two lepidolite-2M2 mica structures from three localities have been refined using single crystal X-ray data to determine cation ordering schemesin both ideal and subgroupsymmetries. The lM (Rr = 0.035)and 2M2 Gr : 0.048)crystals from Radkovice, Czechoslovakiaare ordered in their respectiveideal spacegroups so that M(l) : Li6.e1(Mn,Mg)s0efor lepidolite-1Mand M(l) : Lir o for lepidolite-2M2.In contrast,the lepidolite-lM from Tanakamiyama,Japan (R1 : 0.062)is similar topologically to zinnwal- dite in subgroup symmetry C2. The octahedra related by the pseudo-mirror plane are significantly different in size (mean M-O,F,OH: 1.88A) and electron count (11.5 and 6.0). The difference in electron count between these two octahedral sites is more substantial than in zinnwaldite. The previous 2M2 refinement of Sartori et al. (1973) is confirmed but the lM structure (Sartori, 1976)is better describedas similar to that of the Tanakamiyamalepidolite, althoughdue to systematicerrors in the data an ordered model is not unequivocally established.Cation ordering similar to that found in the Tanakamiyama lepidolite is promoted by a high fluorine content, but parametersof crystallization other than fluorine content are important. Introduction several octahedral ordering schemes. The lepido- Three recent refinements of layer silicate struc- lite-3T structure(Brown, 1978)crystallizes in space tures have shown that additional cation ordering group P3l2 and has two sites that are large and may be present when symmetry constraints are lithium-rich and a small aluminum-rich third site. relaxed from an assumedhigher order spacegroup This type of ordering scheme is analogousto the to a lower one. Lower order space group refine- zinnwaldite structure. In other lepidolite structures, ments have shown that margarite-2Mr(Guggenheim when the ideal symmetry has been used in the and Bailey, 1975,1978) has an ordered arrangement refinement procedure, one large site is located in of tetrahedral cations while zinnwaldite-lM (Gug- the trans arrangement at M(1) and two smaller genheim and Bailey, 1977) and a dioctahedral lM equivalent octahedra in cls orientation. This order- mica (Sidorenko,et al., 1975)showed both tetrahe- ing pattern with M(1) larger than the two M(2) dral and octahedral ordering. In lM polytypes of octahedra appearsto be adopted even in structures ideal space group (CZlm) symmetry, octahedral where the octahedral composition might suggest ordering is possiblebetween the M(1) site on the alternate ordering models. Fluor-polylithionite-lM mirror plane and the two equivalent sites related by (Takeda and Burnham, 1969), lepidolite-lM (Sar- the mirror plane (both designated as M(2)). In toi, 1976),lepidolite-2M2 (Takeda, et al., l97l; zinnwaldite, the spacegroup has been shown to be Sartori, et al., 1973) and lepidolite-2M1(Sartori, C2 and the two M(2) sites are not symmetrically- 1977;Swanson and Bailey, 1981)are all lepidolite related by a mirror plane as in space group C2lm. micas that appear to have such an ordering scheme They are, therefore,designated as M(2) and M(3). where the M(1) site contains the larger lithium ion The M(2) site was shown to be occupied by alumi- and the two smaller symmetry-related M(2) sites num whereas lithium, iron and vacancies are dis- have an averagecomposition near Lis.5Als.5. tributed randomly over the M(1) and M(3) sites. The novel ordering pattern for lepidolite-3T is The lepidolite micas have been shown to have significant in that such an ordering schemeindicates 0003-004x/8l/l I I 2-t 221$02.00 1221 1222 GUGGENHEIM : ORDERING IN LEPIDOLITE there is no a priori reason why a similar scheme tetrahedra in the supergroup refinement (A ": should not be found in the more common lepido- 0.0374) and in the subgrouprefinement (A : 0.054) lites. Experience has shown that some ordering appear unreasonably large for chemically similar schemesmay not be observedbecause of refine- atoms in an identical chemical environment. These ment in the incorrect space group. Alternatively, results suggestserious systematicerrors in the data crystallization history and environment may affect and therefore ordering cannot be unequivocally cation ordering. The purpose of this study is to established in C2 symmetry. Two ordering models examine cation ordering as a function of both are possiblein Cm symmetry,but were not investi- possibilities.In addition, coexistinglepidolite poly- gated because the 1:3 ratio of Al to Si is not types (1M and 2M2) are re-examinedbecause sys- compatible with one Al ion per tetrahedral site tematic errors exist in data reported previously. (maximumpossible ordering is Als.25Sio.zs). All possible octahedral ordering models were Preliminary refinements considered for the subgroup refinement in Cc sym- Initially, Weissenbergfilm data (Sartori, 1976and metry for lepidolite-ZMz and involved varying the Sartori et a1,,1973)were used to refine the lepido- size of the M(2) site so that it was larger than its lite-lM and 2M2 structuresin their respectivecen- pseudosymmetry-relatedM(3) site and, in an alter- trosymmetric space groups and noncentric sub- nate model, smaller than the M(3) site. Each refine- groups. Starting parameters were obtained for the ment produced a large number of parameterinterac- lM structure from the refinementby Sartori and for tions with correlation coefficientsranging from 0.80 the 2Mp from Takeda et al. (1971). The scattering to 0.99, which clearly indicate a centric structure. factor tables were from Cromer and Mann (1968). In addition, bond lengths involving the octahedral Each refinement in the ideal space group agrees cations indicate that the coordination polyhedra with the results of Sartori (1976)and Sartori et al., were more highly distorted than is reasonablefrom (1973); however, the isotropic thermal parameters a crystal chemical viewpoint. The R values for the of several atoms difer and the R values are consid- isotropic refinement for Cc symmetry were Rl : erably lower, presumably becauseof the effect of 0.143and Rz : 0.188and R1 : 0.136and R2 : the different scattering factor tables. 0.200. The refinement procedure in subgroup symmetry procedures in each case followed the method given by Guggen- Experimental and refinement heim and Bailey (1975,1977)and is reviewedbelow. .Because of apparent systematic errors in the For lepidolite-lM, startingmodels in C2 symmetry lepidolite-lM data and the importance of an accu- were obtained from zinnwaldite (Guggenheimand rate comparison of coexisting lepidolite structures, Bailey, 1977) or derived from the distance least- intergrown lM and 2M2lepidolitepolytypes (R1-43 squaresprogram oprDrs written by W. A. Dollase in eerni et al.,l97O) from nearRadkovice, Czecho- of the University of California at Los Angeles.Each slovakia were examined. Microprobe analyses of model was then refined by using the least-squares flakes of each did not show significant chemical refinementprogram oRFLs(Busing et aI.,1962). differences. The resulting formula unit (Table 2) is The model with the smallerM(3) site was rejected assumed to be the same for both. In addition, a becauseit has more than double the R value of the lepidolite-lM (Genth and Penfield, 1892)obtained alternate model. In addition, octahedralsite compo- from the F. A. Genth Collection at The Pennsylva- sitions, as determined from bond lengths and scat- nia State University was examined. The electron tering power, differ from the chemical analysis and microprobe analysis (Table 2) is an averageof nine those determined in the parent spacegroup. How- individual analysesfrom an energy-dispersivesys- ever, convergenceappeared successful for the mod- tem (from an ARL microprobe) and a wavelength- el analogousto the zinnwaldite structure (Table l). dispersive system (from a MAC-5 microprobe), Calculated octahedral compositions based on bond both using the correction procedures of Bence and distancesare consistentwith those determinedfrom Albee (1968)and the alpha values of Albee and Ray bond lengths in the parent space group, but bond (1970).In all casesthe electron beam was broad- lengths from the subgroup refinement are inconsis- ened and the sample current kept as low as possi- tent with the chemical analysis(as was also noted in ble. the supergrouprefinement by Sartori,1976). Differ- The three lepidolite samples were examined ences in the T-O(2) bonds from within individual (Horsey, 1979, personal communication; Bish, GUGGENHEIM: ORDERING IN LEPIDOLITE t223 Table l. Summary of residual values for the lepidolite models Total ***Goodness- Goodness- Reflections Parameters *& **R^ of_fi t Parameters R, R^ of- fi t Ideal c2ln Syrmetry Subgroupc2: Zinnwaldite-type orderinq model Lepidolite - lM (reflection data from Sartori, 1976) i sotropi c 400 27 0.067 0.068 2.57 44 0.056 0.058 2.69 ani sotropi c 400 52 0.0s4 0.0s7 2,22 Lepidolite - lM (Radkovice) isotropic I 164 23 0.062 0.074 1.38 ****40 0.043 0.052 LI I ani sotropi c I 154 JJ U.UJ3 0.043 0.08 Lepidolite - l,v (Tanakamiyama) i sotropic 807 25 0.131 0.142 4.70 42 0.084 0.082 2.00 ani sotropi c 807 53 0.t00 0.106 2.84 92 0.062 0.064 1.74 Ideal cZlc Symetry Lepidolite - 2rv, (Radkovice) i sotropi c 2764 40 0.082 0.078 5.5/ anisotropi c 2764 93 0.048 0.050 3.42 * R,= (rllrol-lrci |)/rlro1 2 2L ** = \ {trw(lFol-lFcl)l/6ylFol 12 = #* Goodness-of-fit: [xwllfol-lrcll'/r(n-m)]t whe.ren = numberof independentdata and m numberof parameters **** based on a partial data set of 838 reflections 1979, personal communication) with a neodymium 0.35 x 0.03 mm size fragments. Ten specimens glass laser to observe second harmonic signals were examined before choosing one for study.