I A merican M ineralogist, Volume60. pages175-187, 1975 Cation Ordering and Pseudosymmetryin Layer Silicates' S. W. BerI-nv Departmentof Geologyand Geophysics,Uniuersity of Wisconsin-Madison Madison, Wisconsin5 3706 Abstract The particular sequenceof sheetsand layers present in the structure of a layer silicate createsan ideal symmetry that is usually basedon the assumptionsof trioctahedralcompositions, no significantdistor- tion, and no cation ordering.Ordering oftetrahedral cations,asjudged by mean l-O bond lengths,has been found within the constraints of the ideal spacegroup for specimensof muscovite-3I, phengile-2M2, la-4 Cr-chlorite, and vermiculite of the 2-layer s type. Many ideal spacegroups do not allow ordering of tetrahedralcations because all tetrahedramust be equivalentby symmetry.This includesthe common lM micasand chlorites.Ordering oftetrahedral cations within subgroupsymmetries has not beensought very often, but has been reported for anandite-2Or, llb-2prochlorite, and Ia-2 donbassite. Ordering ofoctahedral cations within the ideal spacegroups is more common and has been found for muscovite-37, lepidolite-2M", clintonite-lM, fluoropolylithionite-lM,la-4 Cr-chlorite, lb-odd ripidolite, and vermiculite. Ordering in subgroup symmetries has been reported l-oranandite-2or, IIb-2 prochlorite, and llb-4 corundophilite. Ordering in local out-of-step domains has been documented by study of diffuse non-Bragg scattering for the octahedral catlons in polylithionite according to a two-dimensional pattern and for the interlayer cations in vermiculite over a three-cellsuperlattice. All dioctahedral layer silicates have ordered vacant octahedral sites, and the locations of the vacancies change the symmetry from that of the ideal spacegroup in kaolinite, dickite, nacrite, and la-2 donbassite Four new structural determinations are reported for margarite-2M,, amesile-2Hr,cronstedtite-2H", and a two-layercookeite. Mean bond lengthsindicate ordering of tetrahedralcations in all four mineralsand ordering of octahedral cations in amesite and cookeite. Lower subgroup symmetries were found for margarite-2M, and amesite-2Ilr. The conclusion is reached that if a structure determination results in a disorderedcation distribution on the assumptionof an ideal spacegroup, then evidencefor a domain structure should be sought or the structure should be relined in subgroup symmetry Introduction tural energythat may contain cluesto the conditions of crystallization. When one atom or ion substitutesfor another in a Atomic ordering may take place on a strict unit- crystal structure, it may do so in several different cell-by-unit-cell basis in a true single crystal. The structural ways. It may do so in disorderedfashion, resultantspace group may be the sameas that of the i.e., atom,4 substitutes for atom .B randomly at disorderedcrystal or loweredto that ofa subgioup. lf differentbut structurallyequivalent locations in adja- the subgroupbelongs to a differentcrystal system, the cent unit cells. This is our standard view of sub- crystal often simulates the higher symmetry of the stitutional solid solution, in which the averageunit parent disorderedstructure, sometimes by twinning, cell is consideredto contain "half-breed" atoms that with the result that detection of ordering is made are statistically part A and paft B. For favorable more difficult unless a superlattice is evident. An ratios of I to .B atoms, and if A and B are of different alternative or concurrent form of ordering is within sizes or bonding tendencies,the substitution may local out-of-step domains, usually with different take the form of a partly or completely regular, directions of ordering patterns in adjacentdomains. ordered distribution of A and B atoms over the The ordering patterns within domains may be availableatomic positions.Ordered structuresare of repetitiveon a unit cell scaleor may createa superlat- special interest to mineralogists and petrologists tice in one or more directions.Ordering within local becausethey represent stable sl.ructuresof low struc- domains can be studied by means of diffuse non- Bragg spotsor streaksthat occur betweenthe normal ' Presidential address, Mineralogical Society of America. Delivered at the 55th Annual Meeting of the Society, l9 November Bragg reflections on monochromatized X-ray patterns t974. or, if the domains repeat on a regular basis, by t16 S. W. BAILEY means of non-Bragg satellite reflections. These rect in many cases.But it should be realizedthat this ordering possibilitiesare summarized below. assumptionautomatically precludes the finding of ca- A. Ordering within an homogeneoussingle crystal tion ordering in certain spacegroups. For example, l. Space group and unit cell of disordered one-layermonoclinic (lM) micas,some of the one- structure are retained layer chlorites,and several 1 : I layer minerals have 2. Symmetry is lowered to that of a subgroup only one independenttetrahedron in the asymmetric of disorderedstructure unit (Table 1), so that all tetrahedralcations must be a) Superlatticepresent equivalent to one another on a statisticalbasis, i.e. b) No superlattice disordered,for the ideal symmetry.Octahedral order- B. Ordering within local domains ing, on the other hand, is possiblein the ideal space l. Unit cell is sameas that of disorderedstruc- groups of most layer silicates,only some of the I : I ture layer polytypesbeing exceptions. Table 2 summarizes 2. Unit cell is superlattice relative to drs- those layer silicatesin which ordering has been con- ordered structure firmed by use of the ideal spacegroups. 3. Non-Bragg satellitereflections are present Micas This paper will review known ordering patternsof Ordering of tetrahedral cations, as judged by tetrahedral,octahedral and interlayer cations within differences in mean Z-O bond lengths for non- the hydrous layer silicate minerals, with special equivalenttetrahedra, has beenfound within the con- reference to the results from several previously un- straints of the ideal spacegroups only in muscovite- published structural determinations.Examples of all 3T and phengite2Mr. The pattern of ordering,which the ordering possibilitieslisted in the precedingtable is the same for all layer silicatesdiscussed here, is exist and have been recognizedin theseminerals. A regular alternation of Z(l) and Z(2) tetrahedra major point of this paper will be, therefore,that if a around each 6-fold ring of the sheet (Fig. 1). structuredetermination results in a disorderedcation Although muscovite-2Mt has two independent distribution on the assumption of an ideal space tetrahedra, all well-refined structures for this mica group, then evidencefor a domain structure should show relatively small differencesin mean Z-O bond be sought or the structure should be refined in sub- lengths for the two tetrahedra. In contrast to the group symmetry. This has not been done in many casesin the literature. Tesr-B I Layer Silicate Polytypes for which ldeal SpaceGroups Do Not Permit Cation Ordering Within the Ideal Symmetry Orderins in the Sites Listed Ideal Space Groups Tetrahedral 0ctahedral Interlayer The particular sequenceof sheetsand layers pre- sentin a layer silicatestructure is said to havean ideal M1cas symmetry that is usually basedon the assumptionsof LM IM 20r 2M. trioctahedralcompositions, no significantdistortion, I and no cation ordering. Ideal spacegroups of this 6H no restrlctlons -"2,M sort have been worked out for the simple mica 20r polytypesthat are theoreticallypossible by Smith and 3T Yoder (1956), for chlorites by Bailey and Brown od (1962) and Lister and Bailey (1967), for I : I layer l-Layer Chlorlteg silicates by Zvyagin (1962) and by Bailey (1963, Ia-2 tD-r 1969), and for talc-pyrophyllite by Zvyagin, IIa-I no reatrlctlons no reatrlctlons Mishchenko, and Soboleva (1968). Dioctahedral IIb-2 compositions have been considered for I : I layer 1:1 Layer Slllcateg silicatesby Newnham (1961) and Zvyagin (1962)and LT IT for chlorites by Drits and Karavan (1969). LM 2T Probably the majority of the structural deter- 20t 2,L minations of layer silicateshave been made on the -"1 2r2 not. appropriate assumptionthat the ideal spacegroup is valid for the J( natural crystal. Undoubtedly this assumptionrs cor- 6R CATION ORDERING AND PSEUDOSYMMETRYIN LAYER SILICATES t1l Tnus 2. Cation Ordering within Ideal SpaceGroups Tesrr 2, Continued MICAS VERMICIJLITE mu6covlte-3? (GUven & Burnhan, 1957) 2-layer s E)pe (Shirozu & Balley, 1966) -r+1Lr+ (K0.9N"0. (o1r. (*er (ttz (oH) ooR6.oz) srF"6.o+Meo. ogF'-o. o4rto. or)- . ss"'6. orAlo. ts ) . e6o1t. t4 )or0 2- (sis (0"), (Hzo) .rrAlo. s9)oro .,rro. o, wo. +eKo.or ,r.lz r,973, 2.084,2.O75,2.088 2.073, E , A Phenglte-2,lt 2 * For chforites and vetmicuTite the first Tine (Zhoukhllstov, Zvyagtn, Soboleva, & Fedotov, 1973) tefer s to oatahedra in the 2:1 1aget, the second '(K0.68N"0. (s1:. (oH) Iine to the interlager. og)A1r.gr sAlo.s)oro. o6 r..94 order found in phengite-2Mr,two determinationsof the structure of lepidolile-2M, show no tetrahedral cllnronlte-lrtl (Tak6uchl ,! Sadanaga, 1966) ordering. As mentioned previously, tetrahedral t"r. ("g2. Aro.t) (ttr. (0H) r 18 osfl2. gs)oro 2 ordering is impossible in lM micas of spacegroup C2/m. This includesmost of the trioctahedralmicas. Octahedral cation ordering, based either on differencesin mean M-O,OH bond lengths or on Leptdoltte-2u, (Takeda, Ilaga & Sadaaaga, 1971) refinementof octahedral scatteringpowers, is more (Ko. (Llr 7L 8zN"o.rzRbo. o6t"o. oz ) . osAlr. +R6. rs )- common and has been found