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 -3I, phengile-2M2, la-4 Cr-chlorite, and 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 -2Or, llb-2prochlorite, and Ia-2 donbassite. Ordering ofoctahedral cations within the ideal spacegroups is more common and has been found for muscovite-37, -2M", -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 , , , and la-2 donbassite Four new structural determinations are reported for -2M,, amesile-2Hr,-2H", and a two-layercookeite. Mean bond lengthsindicate ordering of tetrahedralcations in all four mineralsand ordering of octahedral cations in 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- , 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. and unit cell of disordered one-layermonoclinic (lM) ,some of the one- structure are retained layer chlorites,and several 1 : I layer 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 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 - 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 within the ideal space (sl:. (oH) groups for muscovite-3Z, fluoropolylithionite-lM, rt'10. or)01o 0.8F1.2 clintonite-lM (: xanthophyllite), and two different specimensof lepidolite-2Mr. Monoclinic micas have two independenttypes of octahedra1'M(l), which lies

Leptdollte-zu2 (Sartorl, Franzini & Merllno, 1973) (Ko. (Llr. 88N'0.06Rbo. osc"o. or) e/nr. l)- (til (oH) . 36A10.64 )orO O.,* zFr . Sl

synthetlc f luoro-polyllthlonl te-11, (Takeda & Burnhara, J.969) K(L12A1)S1401OF2

CHLORITES kHnmererite Ia-4 (Brown & Batley, 1953) t+ (tl:.rsoto.ss)oreHz. @es.osF'6.r+c'o.75A10. 07) g

- ----J CI 2.082,2.074x2 L------2.O11,2.O68x2 Frc. l. Ordering pattern in vermiculite from Llano County, Tex- as Tetrahedral Al concentrates primarily in I(l) rather than in rlpldollre l.b--odd (shlrozu & Balley, 1965) I(2). ExchangeableMg'+ cations concentrate only in interlayer oc- (t"3]ro4r. (s12. (oH) tahedral sites vertically adjacent to T(l) (under dotted triangles) lzhS]osAlr.zs ) grsr. oe)010 8 rather than I(2) 178 S. W. BAILEY

ordering of the two cationsover the three octahedral positions induced by heating at 600"C causesd(060) to increaseto such an extent that the would not be recognizableby this criterion as dioctahedral. In micasthe vacantoctahedral site always has been found to be located on the mirror plane of each2'.I layer, i.e. at M(l). For this reasonthere is no reduc- tion in symmetry relative to the ideal trioctahedral analogsassumed for the derivation of polytype space groups. This will not be true for some of the kaolin @.oH and chlorite structuresto be discussedlater. Because @' AL of the linkage of tetrahedraland octahedralsheets by €="u sharing of apical ,the location of a large oc- tahedron on the symmetry plane of the 2: I layer means that the staggerbetween the two tetrahedral sheetswithin the layer is necessarilygreater than the ll' ideal value of a/3. Observedvalues of intralayershift Frc 2 Octahedralordering pattern in clintonite-lM. Octahedral range from -0.365a, in phengite-1M (Sobolevaand Al is concentratedin M(l) on the mirror plane and Mg in the two Zvyagin, 1968) to t0.386a2,e in phengite-2M, M(2) sites. (Giiven, l97la) for dioctahedralspecies with a vacan- cy at M(l) and from -0.351a, for fluoropolylithionite- on a centerof symmetryon the mirror plane for space lM to +0.364b2,sfor lepidolite-2M, for ordered grovp C2/m or on a center of symmetry for C2/c; trioctahedral specieswith the larger ion at M(l). and af pair of equivalent M(2) octahedra related to Clintonite-lM with a smaller ordered octahedral each other by thesesymmetry elements.The two OH site at M(l) has a correspondinglyreduced intralayer groups of each octahedronlie on the symmetryplane shift of -0.321ar. -lM does not show the of each individual layer, but occupy different oc- usual dioctahedral distortion because the vacant tahedral cornersin M(l) and M(2). The OH ot M(l) and occupied octahedral sites are nearly identical are in the trans orientation, whereasthose of M(2) are in size (Zvyagin, 1967). in the cis orientation and form a shared edge between Deviations of intralayer shift from the ideal value adjacent symmetry-related M(2) octahedra (Fig. 2). of a/3 may lead to deviationsof one to two degrees The patterns of octahedral ordering found are from the idealp value.Two factorsmay tend to mask primarily with M(1) larger than M(2).The one excep- the effect of intralayer shift on B, however, and tion to this rule found to date is clintonite-lM, in thereby diminish the usefulnessof the latter as a which M(1) is smaller than M(2). The relativeratio of predictor of octahedral ordering. Firstly, the direc- large to small octahedra, however, is not always in tions of intralayer shift may not be parallel to the accord with the ratio of large to small octahedral resultant XZ plane, for example as in the 2Mt and cationspresent. For example,in fluoropolylithionite- 2Mrstructures. Secondly,adjacent 2: I layersmay be lM Ihere are two large octahedral Li ions and one offset, as measured between tetrahedral cations in smaller Al ion. Yet the ordering pattern createsone adjacent2: I layers,so that the coordination around larger octahedralsite at M(l) on the mirror planeand the interlayer cation is asymmetric. This layer offset, two smaller symmetry-related M(2) sites. The com- first noted by Radoslovich(1960), has been found to positions inferred from refinement of scattering vary between values of *0.035a, in potassian powers : in thesesites are M(l) Lio8eAl0 ,, and M(2) (Burnham and Radoslovich, 1964) and = Lio5uAlo.nuX2. Similar octahedralordering patterns -0.025a, in phengite-lM (Soboleva and Zvyagin, were observed in two specimens of lepidolite-2Mr. 1968) and may cancel or enhance the effect of in- But in clintonite-lM the 2: I ratio of large to small tralayer shift deviation. octahedralcations present is mirrored in the structure by the same ratio of large to small octahedra. Chlorites Dioctahedrallayer silicatescan be assumedto have Ordering of tetrahedral cations within the ideal ordering of the vacant octahedralsite relative to the spacegroup has been confirmed for a one-layerCr- occupied sites.Lindqvist (1962) has shown that dis- chlorite that has Ia layer-interlayerstructural units in CATION ORDERING AND PSEUDOSYMMETRY IN LAYER SILICATES the terminology of Baileyand Brown (1962;T a6le2). Chlorites have two octahedralsheets, one within the 2'.1 layer and the other comprisingthe hydroxide in- -ol3 terlayersheet. All chloritesmust have ordering of oc- shift tahedral cations between the two sheetsin order to insurea positivecharge on the interlayer,and this has been observedin all structural analyses.In addition there may be ordering of cations within each oc- tahedral sheet.The Ia structural unit is characterized + b/3 by positioning certain interlayeroctahedral sites ver- tically between tetrahedral sites in the tetrahedral sheetsof the layersabove and below. A unique type of local charge balance is found in the la-4 Cr- - chlorite, in which all of the Cr3+ is concentratedin o/3 ? the interlayer in the one octahedron that is between shift the ordered Al-rich tetrahedra of layers above and below. Thus, the sourceof positive chargeon the in- terlayer is immediately adjacent to the sources of negativecharges on the 2:1 layers. - An orthohexagonal @ 90") ripidolite (or b/3 brunsvigite.in Foster's 1962classification) of the lb structural type and having semi-random stacking of structural units has been refined by use of only the sharp k : 3n reflections.This type of analysis,in which the streaked k I 3n reflections are ignored, only givesthe detailsof an averagelayer of symmetry Frc. 3. Layer sequencein s type vermiculite from Llano County, C2/m and does not permit determination of any Si, Texas Dots illustrate vertical alignment of exchangeable Mg'+ Al ordering that may be presentor of any ordering cations betweenAl-rich I(l) tetrahedraof 2: I layers above and within octahedral sheets. Electron density peak below to give local charge balance. heights indicated that 60 percent of the is con- centrated in the octahedral sheet of the 2: I layer. changeable cations are attracted to the loci of Smaller mean M-OH distances in the interlayer negativelayer charges.There are only 0.41 atoms of suggestthat most of the octahedralAl is in the in- exchangeable cations per formula unit in the terlayer. This is confirmed by the relative thicknesses specimen analyzed,and it is likely that there is further of the octahedralsheets in the interlayerand the 2: I superlatticeordering on a local domain scaleof both layer (l .98 A uersus2.18 A, respectively). tetrahedraland interlayercations, as discussedlater.

Vermiculite Ordering Within Subgroup Symmetry The well-known trioctahedral vermiculite from Micas Llano County, Texas, with two planes of water Perhaps the most impressiveexample of the in- moleculesin the interlayer (14.36A phase)has been fluence of cation ordering in decreasingthe ideal shown to have a two-layer structureof the s type, in space group symmetry to that of a subgroup is in which there are L type intralayer shifts of a/3 parallel anandite, the rare Ba-Fe brittle mica. Anandite has to the resultant XZ plane, incomplete Ia type in- the 2Or structure in the terminology of Smith and terlayersof Mg2+ and HrO molecules,and shiftsof t Yoder (1956),in which the intralayershifts are ra'/3 b/3 between adjacent 2: I layers (Fig. 3). The struc- with alternation of the I and II sets of octahedral ture is similar to that of the Cr-chlorite mentioned positions within adjacent layers. The ideal space previously in that the small number of Mg2+ ex- group for this structureis Ccmm, but the symmetryof changeablecations present are concentratedin the ananditeis lowered by ordering of tetrahedralSi and one interlayer position where they are vertically Feto Pnmn (Table 3). A large number otweak reflec- between partly ordered Al-rich tetrahedra in the tions are observedthat violate the C-cell centering. layers above and below (Fig. l). Thus, the ex- The octahedralcations are predominantly Fe2+,but 180 S- W, BAILEY

T,lnlr 3. Cation Ordering within Subgroups tetrahedral ordering was found, smaller M-O,OH bond lengths indicate that the octahedralAl is con- centrated in the interlayer and that there is preferen- tial ordering of cations over the three octahedralin- anandlte-2or *."I:::t 6 rad1n1, re72) (B'0. (re!+orueo.4sho. terlayer sites (Table 3). In this case, therefore, the eiho. o4Ko.osnao. oo) oo)- reduction in symmetry is due to octahedral cation

rFor cilotjtes tle first ljre refers to etahedra in ttu 2:7 faVet, two tetrahedra that would be equivalentin the ideal the secord Line to the incerlager. space group are statistically different and indicate that all of the tetrahedral Al is concentratedin one scattering factor refinements and temperature factors tetrahedron. Local charge balance between the in- suggestthe small amount of Mg present is located terlayer cations and adjacent loci of negative charge mainly in one octahedral site. Anandite is unusual on the 2 : 1 layers of the sort found in the la Cr- also in that one of the OH anion groups has been chlorite and vermiculite structuresis not realizedin replacedby (So,uClo,r), which lies close enough (3.2 this Ia donbassite,however, becauseit is the partly A; to the interlayer Ba to be consideredas a l3th vacant interlayer site that is positioned vertically coordinating anion. between ordered Al-rich tetrahedra in the lavers Chlorites above and below. One-layer chlorites of the common IID structural I:I LayerSilicqtes type may be either monoclinic or triclinic. A Kaolinite, dickite, and nacrite all have symmetries monoclinic IIb-2 prochlorite(or ferrian sheridanitein that differ from those of the ideal spacegroups as a Foster's classification) was found to belong to space consequenceof the particular locationsof the vacant group C2, rather than the ideal C2/m, as a conse- octahedral sites in each mineral. Kaolinite and quenceof tetrahedraland octahedralcation ordering. dickite both are based on the lM stacking sequence Based on differences in mean Z-O bond lengths, of layers,and would be identical and of spacegroup nearly all of the tetrahedral Al present is concen- Cm if they were trioctahedral. In kaolinite the vacant trated in the ?'(l) tetrahedron. Z(l) and (2) would octahedralsite is in the sameplace in each layer, and be equivalent in the ideal space group. From the can be located either at site B or site C of Figure 4. heights of the electron density peaks and from the Kaolinite with site B vacant is a mirror image of mean M-O, OH bond lengthsit could be determined kaolinite with site C vacant. Location of the vacant also that most of the iron is concentratedin the oc- siteat either B or C imposestriclinic symmetryon the tahedral sheet within the 2: I layer and that the oc- structuredue to lossof the mirror plane that normal- tahedral Al is in the interlayer sheet. There is ly relatesB to C in a trioctahedralstructure. Ionic in- preferentialordering of Mg, Fe, and Al over the six teractions also distort the structure to triclinic cation sites of the 2: I octahedral sheet and the in- geometry as a separateanorthic effect. In dickite the terlayer. A triclinic llb-4 corundophilite (or vacant octahedral site alternates between B and C in sheridanitein Foster's classification)was refined in successivelayers to create a two-layer structure. The spacegroup Cl insteadof the ideal CI. Although no alternation of vacant sites createsmonoclinic slide CATION ORDERING AND PSEUDOSYMMETRY IN LAYER SILICATES t8l

smectite interstratifications(Altschuler, Dwornik, o o and Kramer, 1963;Wiewi6ra, 1973) and serpentine- rl chlorite interstratifications(Brindley and Gillery, 1954).The latter specimenactually is a random in- terstratification,but it is reasonableto expectthat regular alternationsmay be found in the future. Much more common are2: I layerswith a regular alternationof differentsorts of interlayermaterials, someof which are listedin Table4. Although the ex- actspace groups ofthese clays usually are not known, it is likely that their symmetriesare lower than those of theindividual I : I or 2 : I layers.It is importantto note herethat oneinterpretation of the regularalter- o nation of differentinterlayers in theseminerals is that @ @ the layercharges differ eitheron alternatetetrahedral sheetsor on alternate2: I laYers' I Cation OrderingWithin Local Domains -l Micas 9l Despitethe apparentlack of long-rangetetrahedral o orZ'7'16 i E l@ cationorder in muscovite-2Mr,Gatineau (1964) has reportedthe presenceof an unusualtype of short- OH ot 2.4.24 range order from study of diffusescattering in the Bragg o Al of 2.3.3O regions between the normal reflectionson 0 photographs. oHl single crystal X-ray The distribution o I at Z. z.ZO and intensity thesediffuse streaks have been inter- o) of a pretedas due to concentrationin localsmall domains Si ol Z. O,58 of Al in zigzagchains within the tetrahedralsheet. In O ot Zr O.OO a givendomain the directionof chainalignment may Frc. 4. Ideal structure of l: I layer in lM polytype. Mirror be along any one of the three pseudohexagonalX (010) planes parallel to extend through octahedral site I and inner axesof the crystal.Chains of pureAl aresaid to alter- OH of each 6-fold ring. nate along the I directionin partly orderedfashion pure planesparallel to (010) and balancesthe stressdis- with chainsof Si.This type of short-rangeorder tribution so that the cell shaperemains monoclinic is unusualin that it grosslyviolates the principleof postulated also.Thus, dickite can be consideredas a regular avoidance of Al-Al neighbors by alternationof right- and left-handedkaolinite layers, Loewenstein(1954) and found to be obeyedin a large in one sense,or as a superstructureof the ideal lM number of cases of long-range ordering in polytypedue to a particular orderingpattern of oc- aluminosilicates.The interpretationmust be viewed tahedralcations and vacancies.Nacrite, on the other with some caution becauseof the later finding by hand, is basedon an entirely differentstacking se- Kodama,Alcover, Gatineau, and Mering(1971) that quenceof layers-that of the ideal trioctahedral6R in the local domains of other layer silicatesthe structure.The vacantoctahedral sites vary regularly tetrahedral networks are distorted in linear waves in successivelayers so that the resultantsymmetry is characterizedby alternatingrows of tetrahedraof reducedfrom rhombohedralto monoclinic,and an slightly differing dimensionseven in the absenceof inclinedZ axiscan be selectedalong which there is any tetrahedralsubstitution (e.g., as in pyrophyllite, true two-layerperiodicity. talc,and ). Further study appears required to sort out the contributionsto the diffusescattering of Regularly I nterstratifed Clay Minerals Si,Al orderingas contrastedto that of the A few clay minerals are known that show a network distortion.Similar comments apply to the reasonablyregular alternationof I : I and 2: I structuresof phlogopiteand , as Gatineau and layers,which can be consideredas a differentkind of Mering (1966) have observeddiffuse scatteringin ordering.Examples of claysof this type are kaolin- theseminerals very similar to that in muscovite-2M,. 182 S. W. BAILEY

TlsLr 4. Examplesof RegularInterstratifications of 2: I Layerswith Different InterlayerMaterials

Componeat urinerals Ratio of comDonents Refereuce

pa.ragonice-snectite = rectorLte 1:1 Brown & Welr (1963)

dloctahedral nlca-smectlte = taraaovite 3:1 Lazarenko & Korolev (1970); Reynolds & Eower (1970)

i1lite-snectlte 1:L Bonorino (1959); Reynolds & Hower (1970)

biotlte-vernlcullte = hydroblotlte varlebLe Gruner (1934); Bassett (1959)

talc-saponlte = allettlte 1rL Venlale & van der Marel (1969)

trloctahedral chlorLte-rrolrelling chloriterr = correnslte L:1 Lippnann (1954)

trloctahedral chlorlte-saponite 1:1 Venlale & van der l{are1 (1969)

trloctahedral chlorite-dloctahedral aoectlte 1:1 BlaEter, Robereon & Thonpeon (1973)

tr ioctahedral chlorLte-veroicull.te 1:1 Drlts & Kossovskaya (1963)

dloctahedral chlorlte-smecClte - tosudlte L:1 Frank-Kanenetekll, Lowlnenko & Drlts (1963)

dloctahedral chlorlte-rrswel1lng chlorite" 1:1 Heckroodt & Roerlng (1965)

vernicuL lte dehydrate-hydrate 1:1 ltalker (1956)

K*-vern1cul1te-ca2*-vernlcullte 1:1 Sawhney (L967, 1972>

Gatineauand Mering(1966) also have studied the cupiedby water molecules.The questionthen arises diffuseX-ray scatteringfor a lepidolite of unstated as to the distribution of exchangeablecations and structuraltype. The compositionis not given,but by water moleculesover the availablesites in adjacent implicationis that of polylithionite.Diffuse spots are unit cells. The structure determinationby Shirozu observedat positionsthat are satellitic(I a*/3) to and Bailey(1966) of Llano vermiculitefound that the positionsof certain of the h + k = odd Bragg 0.41 atoms of Mg2+ occupy statisticallyone of the reflectionsthat are forbidden by C centering.The three availableoctahedral interlayer sites, and under spots are interpreted to indicate short-rangeoc- the relativehumidity conditionsof the laboratoryat tahedralordering of lAl + 2Liin local domains.The the time of investigationthat 0.62 HrO statistically orderingtakes the form of rows of pure Al and pure occupyeach of the six interlayer"anion" sitesfor a Li aligned along one or the other of the three total of 3.72H2O per formula unit. No long-range pseudohexagonalY axes [01], [31], or [31],such that orderingof the interlayercations and H2Oover adja- each domain is characterizedby only one ordering cent unit cells was noted by Shirozu and Bailey in direction.The orderingis two-dimensional in that the their study. Al rows are spacedregularly within a layer at inter- Mathiesonand Walker (1954) suggested a dynamic valsof 3a/2 (everythird row), but irregularlybetween interlayersystem should exist that would allow cation layers.The extent of eachdomain is small, perhaps and water migrationbetween the availablepositions. due to antiphaserelations. They further suggesteda tendencyto maintain a regular,hexagonal distribution of the occupiedcat- Vermiculite ion sitesover threeunit cellsas a resultof mutual The 14.36A phaseof vermiculitethat is stableun- repulsion,with the water clusteringaround the oc- der atmospherichumidity conditionshas an in- cupied cation sites as octahedralhydration shells. completeinterlayer sheet, relative to a chlorite in- Bradleyand Serratosa(1960) have suggested another terlayer,in that onlyabout l/6to l/lO of thepossible modificationof the interlamellarregion in which a C- cationsites are occupied by exchangeablecations and centeredsuperstructure would extendlaterally over onfy about 2/3 of the possible"anion" sitesare oc- three unit cells for the lower observedlevel of ex- CATION ORDERING AND PSEUDOSYMMETRY IN LAYER SILICATES 183 changeablecations and a regulararray over one cell for the highestlevel of exchange.The orderly arrays were not believedto extend throughout the entire crystal.Alcover, Gatineau, and Mering (1973)have confirmedexistence of a three-cellsuperstructure in local domainsby study of diffusereflections present on singlecrystal X-ray patternsof Kenyavermiculite. The abnormal scatteringis concentratedalong lines parallel Io Z* and disappearsupon dehydrationat 500oC,so that it must be attributedto the arrange- ment of the interlayermaterials. The interpretation by Alcover et al confirmsthe generaloutline of the local ordering patternspredicted by Mathiesonand Walker and by Bradley and Serratosain which ex- changeableMg2+ and associatedwater moleculesare distributed over the lattice points of a large C- centeredcell having dimensions 3a X b, wherea andb are the dimensionsof the normal vermiculiteunit Frc. 5. Distribution of exchangeableMg2+ over sitesof a C- cell. The orderingis two-dimensionalin that thereis centeredsuperlattice (3a x b) in Kenya vermiculite.lnterlayer sitesoccupied by Mg'* in the threedomain types are indicated by no coupling between successiveinterlayers. The threedifferent symbols. From Alcoveret al (1973). possiblepositions for the exchangeablecations in the local supercellare divided into threesets, as shownin l:l of Si to Al or Fe8+, (2) have been reported Figure5. Only one setcan be occupiedin a givendo- previously as disorderedwithin their ideal space main, but differentsets can be occupiedin different groups,and (3) can be obtainedin good crystals.A domains.Full occupancyof one set of possible fourth structure,that of a two-layer cookeite,also positionsrequires 2 Mg"* per supercellinterlayer, or was selectedbecause the ideal spacegroup imposes 0.33Mg'z+ per formulaunit. The precisedistribution very few constraintson ordering. of the interlayerwater moleculesover the threecells, Although refinementof two of thesestructures has which would be requiredto distinguishbetween the not beencompleted, the residualvalues ranging from Mathieson-Walkerand Bradley-Serratosamodels, R : 7.5 to 10.9percent with isotropicB values was not determinedin this study. It is inferred,but suggestthe structuraldetails are essentiallycorrect. not demonstrated,by the authors that local Si,Al The results indicate that ordering of tetrahedral ordering accompaniesthe interlayerMg2+ ordering cationsexists in all four minerals.Evidence for do- so that the local chargebalance is retained.It should main structurehas not been investigated,but non- be noted in this connectionthat the amount of long- Braggsatellites were observedin cronstedtite.More range S|Al order found in Llano vermiculite by detailedresults will be presentedin laterpublications, Shirozu and Bailey lauerage composition of but are summarizedbelow and alsoin Table 5. tetrahedronf(l) : Sio.6uAL.su]is almost ideally suited for an additionalshort-range order in which one set Margarite of (l) positionsin eachsupercell domain (adjacent Tak6uchi(1965) found the brittle mica margarite- to the positions occupied by the exchangeable 2M, from Chester, Massachusetts,[(Cao..'Nao.") cations)is occupiedonly by Al and the other two sets Alr(SirAlr)O'o(OH)rlto be disorderedwith respect of Z(l) only by Si. to tetrahedralcations within its ideal spacegroup C2/c, despitethe favorableI : I ratio of tetrahedral Test Structures cation species.Since then, Gatineauand Mering In order to testthe conceptthat orderingmay be a (1966)from the absenceof diffuseX-ray scattering more common phenomenonin layer silicatesthan and Farmerand Velde (1973) from thesharp infrared previously suspectedbecause of the difficulty in spectraand the absenceof Al-O-Al vibrationshave detectiondue to reducedsymmetry or domainstruc- suggestedthat margariteshould be orderec.. ture, three mineralswere selectedas test examples. Margarite-2M' from Chester County, Penn- Margarite, amesite,and cronstedtitewere selected sylvania,was selected to test the possibilityof order- becauseall (l) havefavorable tetrahedral ratios near ing in a lower symmetry.As a first step,refinement 184 S. W. BAILEY

Tlst-s 5. Resultsof Test Structures

Mineral Reflectlons ueed Method Residual (Z) Mean bond lengths r--o (i) M--O,OH (;,)

Bargarlte-2U1 1,071 Counter 7.5 (lso. B) L.747, I.624 1.910,1.900,El 1.640, L.730

aneslte-2H2 722 Counter 10.9 (lso. B) L.67L, L.7L7 2.064,2.073,L,977 (ln progress) L.736, r.649 2.062,1.988, 2.060

cronstedtlte-2!, 300 Flln, visual- 10.2 (lso. B) 1.61_7,1.801 2.097x 3

cookelte 900 Film, visual 10.3 (lso. B) L.622, L.620 L.932,1.906, E (in progress) 7.652, L.673 L.892, L.928, 2.L25

of the structurein the ideal spacegroup C2/c con- the same 2: I layer in the ideal spacegroup C2/c firmed Tak6uchi's finding of tetrahedral disorder proveto be compositionallydifferent when refined in (Guggenheimand Bailey,in preparation).Because of subgroupCc. Alateral (pseudo)two-fold axisnormal systematicabsences, the only possiblesubgroup is Cc to the direction of intralayer shift and passing and Farmer and Velde (1973)have suggestedthis through the two octahedralAl atomscan be seento may be the truespace group. In thislower symmetry, relatecompositionally similar tetrahedrain the two the two tetrahedralsheets within the 2 : I layer no sheetsinstead. Giiven (1971b)has pointedout for longer are equivalent,and two different ordering muscovite-2Mrlhatordering may be inhibitedin the modelsmay be postulatedthat are consistentwith idealspace group C2/c becauseordering would cause disorderin the parentspace group but full orderin two apical oxygens(of Al-rich tetrahedra)along the the subgroup.Because of the high pseudosymmetry,sameoctahedral shared edge to be electrostatically initial attemptsto refinethe orderedstructure start- unbalanced.This unstablesituation is avoidedin ing from thecoordinates ofthe disorderedphase and varyingthe coordinatesof one sheetat a time proved unsuccessful.It was necessaryto model the coor- dinatesfor the two possibleordered structures by us- ing the distanceleast squaresprogram Oprors of Dollase. For one model, refinementof coordinates predictedby Orrus from assumedbond lengthsand bond strengthsconverged smoothly to a satisfactory structure with consistentbond lengthsand angles. Refinementof the secondmodel was unsatisfactory in that bonds around the postulatedAlrv became smaller and around the postulatedSiIv became larger,thus proving an incorrectmodel. Application of Hamilton's(1965) residual ratio testindicates that the successfulordered model is a significantimprove- \ ment over the disorderedmodel at betterthan the I percentsignificance level. /,'rl The resultant mean ?'-O bond lengthsof 1.624, 1.747,1.640, and 1.730A in the successfulCc model suggestnearly completeordering for the indicated ,l electron probe composition of (Cao..,N3o.rrKoor) i (AI,.rrMgo.orFeo.o,)(Si2.r,Al1.8r)Oro(OH)2.The order- Ftc. 6. Tetrahedral ordering pattern within 2..1 layer of margarite-2M, = ing patternis illustratedin Figure6. Tetrahedrain in subgroupCc. Al-rich tetrahedra ruled lines and dots; octahedralAl = solid circles;inversion centers of i.deal the lower tetrahedralsheet that would be relatedby spacegroup : small open circles;pseudo 2-fold axis = dashed inversioncenters to tetrahedrain the upper sheetof arrows. CATION ORDERING AND PSEUDOSYMMETRY IN LAYER SILICATES r85 muscovite-32,where the two tetrahedralsheets are Cronstedtite relatedby a true two-fold axisinstead of by inversion The iron-rich I : I layer silicate cronstedtite centersand whereordering of tetrahedralcations has - [(R'+r-,Fe3+")(Si2-"Fe3+,)Ob(OH)4with x 0.5 to beenconfirmed within the idealspace group P3'12 1.0]shows a greatvariety oflayer stacking sequences. (Table2). The Cc orderedpattern in margarite-2M, Partial refinementof severalof theseby Steadman also avoidstwo unbalancedoxygens on the sameoc- and Nuttall (1963, 1964) showed no evidenceof tahedral sharededge, but the two-fold axis relating orderingof tetrahedralor octahedralcations. For compositionally similar tetrahedra in the two comparison with the amesite-2H, structure, a tetrahedralsheets does not hold for the structureas a cronstedtite-2H2crystal from Pribram, Bohemia, whole. kindly provided by C. Frondel, was selectedfor In the caseof muscovite-2Mywhere the Si:Al study.This particularpolytype had not beenrefined ratio is 3: 1, only one orderedmodel is possiblein previouslyby Steadmanand Nuttall. subgroupCc that would be disorderedin the ideal Becauseof its jet-black color, optical searchfor spacegroup. This is the sameordered model found to (001)twin sectorssimilar to thosein amesiteproved be presentin margarite-2Mr,except that the max- difficult, but no clear evidencefor sectorswas ob- imum order possibleis alternationof Si and Sio.6Alo.6tainedeither by reflectionmicroscopy or by transmis- tetrahedra.Refinement of the neutron diffraction sion microscopywith ultra-thin sections.Refinement data of Rothbauer(1971) according to this model by Henry and Bailey (in preparation)in the ideal provedunsuccessful, and it canbe concludedthat the spacegroup P6, indicatedordering of tetrahedralSi orderedmodel of margarite-2M,does not extendto and Fe3+according to mean Z-O bond lengthsof muscovite-2M,. 1.62and 1.80 A. Orderingis confirmedalso by obser- Amesite vation of 06/ reflectionswith / odd (asindexed on an orthohexagonalcell). Radoslovich and Norrish Prismaticsingle crystals of the aluminianserpentine (1962)have shown that thesereflections should have (SiAl)O6(OH)n] amesite[(Mg,Al) usually crystallize zero intensitiesin a truly hexagonalstructure in polytype as lhe 2Hz and exhibit apparenthexagonal which the tetrahedralcations have y coordinatesthat diffraction symmetry.Partial refinementwithin the are exact multiples of b/12 and are disordered. group ideal space P6, by Steinfink and Brunton Cronstedtite-2I/z appears truly hexagonal, and (1956) indicatedboth tetrahedraland octahedraldis- calculationsshow that the observedintensities of the order for amesite from the . 06/ reflectionswith / odd mustbe dueto differencesin Microscopicobservation, however, invariably shows X-ray scatteringpowers ofordered tetrahedral Si and that amesitecrystals are composed of six-foldbiaxial Fe3+plus the differencesin position of their coor- (001). twin sectorson Individual twin sectorscut dinating oxygens as a consequenceof ordering. from the larger aggregatehave 2V optic anglesnear Orderingof octahedralFe2+ and Fe3+is not possible geometry,and triclinic 20o,slightly triclinic unit cell in the idealspace group. progress diffraction symmetry.Refinement in by S. Non-Braggsatellites were observed on X-ray films H. Hall of the structure of a sector cut from a of nine cronstedtite crystals of several different previouslyunreported amesite from Antarctica,kind- polytypes,including the 2H, crystalused for refine- ly provided by T. S. Laudon, indicates both ment. The satellitesare associatedwith only a few tetrahedraland octahedralcation ordering in sub- Bragg reflectionson ftkOWeissenberg photographs, (actually group Pl refined in Cl for convenience).and do not conform to hexagonalsymmetry. The pattern The of tetrahedral ordering is such that satellitesvary in positionand sharpnessfrom crystal tetrahedra of adjacent layers that should be to crystal,and someare connected by diffusestreaks. equivalentto eachother in the idealP6s symmetry ac- They cannot be accountedfor by physicalimperfec- judged tually havedifferent Si,Al contents,as by the tions or twinning in the crystal. Although not in- differencesin mean ?n-Obond lengths(Table 5). Of vestigatedfurther, they presumablyrepresent some the six independentoctahedra in the two-layerstruc- non-hexagonalmodulation of the structurethat is ture, two are observedto be smallerthan the other not reflectedin the averagerefinement of the struc- four and are interpretedas Al-rich and Mg-rich, ture reportedhere. respectively.Although this refinementis not com- plete,no structuralfeatures exist that would explain Cookeite the observedtriclinic symmetryand geometryother Cookeiteis a di.trioctahedralchlorite in which the than ordering. 2:l layer is dioctahedraland the interlayersheet is 186 s. l4/.BAILEY

trioctahedral.Bailey and Brown (1962)and Lister was supportedin part by the Earth SciencesSection, National (1966) have shown that cookeiteusually adopts ScienceFoundation, NSF grantGA-34918, in partby grantAC2- oneof two stackingsequences based on the Ia struc- 4899 from the PetroleumResearch Fund, administeredby the part tural unit. The American ChemicalSociety, and in by a grant from the regular2-layer variety from North GraduateResearch Committee of the Universitvof Wisconsin. Little Rock, Arkansas, [-(Al4.orlioa.)(Sia ozAlo.rr)O,o References (OH)rl, kindly suppliedby C. G. Stone,was chosen for study becauseof its good crystalsand relatively ALcovnn, J. F., L. GATTNEAU,lr.ro J. Mrnmc (1973) Ex- rare occurrencein nature. changeable cation distribution in nickel- and - vermiculite. Clays Clay Initial study (Lister, 1966) Minerals, 21, l3l-136. showedthe 2-layer ALEKSANDRovA,V. A., V. A. Durs, lNo G. V. SoKoLovA (1972) structureto be of the r-type(Mathieson and Walker, Structural features of dioctahedral one-packet chlorite. 1954),for which the ideal spacegroup is Cc. This K ristallogr. 17, 525-532. [transl. Soulet Physics-Crystallogr. 17, spacegroup imposes few constraints on cationorder- 4s6-46t1. ing andallows the two tetrahedralsheets within a 2: I ALrscuulen,Z. 5., E. J. Dwonurr, eNoHsNny Kn,lurn (1963) layer to be non-equivalent. Transformationof montmorilloniteto kaoliniteduring weather- Refinement(Table 5) ing. Science,l4l, 148-152. suggestsboth tetrahedraland octahedralordering Bntlev, S. W. (1963)Polymorphism of the kaolinminerals. lm. havetaken place in this specimen.The Li is localized Mineral. 48, | 196-1209. primarily in one position in the interlayersheet - (1969)Polytypism oftrioctahedral l: I layersilicates. C/ays (M-O,OH : 2.125A;. ffre patternof tetrahedral CIay Minerals, 17, 355-37l. -, ANDB. E. Bnowx (1962)Chlorite polytypism: orderingis surprisingin that meanZ-O I. Regular bondlengths and semi-random one-layer structures.Am. Mineral. 47, suggestone tetrahedral sheet contains only Si in 8l 9-850. tetrahedral coordination and that all of the Bessrtr, Wrr-lreu A. (1959)The origin of the vermiculitedeposit tetrahedralAl is concentratedin the othertetrahedral at Libby, Montana.Am. Mineral.4,282-299. sheetof the same2:l layer.This is sucha surprising Burrrn, C. L., H. E. RonrnsoN,lNo G. R. Tsonpsou (1973) Regularlyinterstratified chlorite-dioctahedral and interestingresult that it is being smectitein dike- checkedpresent- intrudedshales, Montana. ClaysClay Minerals,2l, 207-212. ly by a secondrefinement based on densitometer BoNonrNo,FeI-rx GoNzlir-nz (1959) Hydrothermal alteration in measurementof the film datarather than the original the Front Rangemineral belt, Colorado.Bull. Geol.Soc. Am. visualmeasurement. If verified,this resultwill have 70. 53-89. implications as to the origin of regular in- Buorny, W. F., lNo J. M. Snnurose (1960)A discussionof the water content of vermiculite. terstratificationsof the type listedin Table Clays Clay Minerals, T, 260-2'10. 4 because BnrNorrv, C. W., eNo F. H. Grr-lrny (1954)A it will mixed-layer mean asymmetricdistributions of negative kaolin-chlorite structure. Clays Clay Minerals, 2, 349-353. chargeare possiblein 2:l layers.Assumption of a BnowN, B. E., eNo S. W. Blruy (1963)Chlorite polytypism: IL centrosymmetricspace group, as in many previous Crystal structureof a one-layerCr-chlorite. Am. Mineral. 48, studiesof 2:l layer silicates,necessarily precludes 42-6t. BnowN, determinationof suchasymmetry. G., lxo A. H. Wsrn (1963)The identityof rectoriteand aflevardite.Proc. [nt. Clay Conf.Stockholm, 1,27-35. Conclusions BunNHnlr,CHlnr-ss W., ANDE. W. Rnoosr-ovrcn(1964) Crystal structuresofcoexisting muscovite and paragonite. Carnegie Inst. The finding of cation order in all four test struc- Wash. Year Book, 63, 232-236. tureslends support to the viewthat orderingis more Dnrrs, V. A., eNo Yu V. KARAVAN(1969) Polytypes of the two- common in layer silicatesthan previouslybelieved. packet chlorites. A cta Crystallogr. 825, 632-639. Structurespreviously determinedas disorderedin -, ANDA. G. Kossovsrnve(1963) Sangarite, a new with an ordered structure.Dokl. Akad. their ideal space groups merit reexaminationfor mixedJayer Nazft ,S,SSR,l5l,934-937. ltransl Dokl. Acad. Sci. U.S.S.R., lower symmetry or for non-Braggscattering. It Earth Sci.Seo. l5l, 122-125(1964)1. should be kept in mind that, as for margarite-2Mr, Fnnurn, V. C., eNo B. Vrlor (1973)Effects of structuralorder successfulrefinement in subgroupsymmetry by least and disorderon the infraredspectra of brittle micas.Mineral. squaresprocedures may require modelingof all Mag.39,282-288. possibleordered structures. Fosrnn, Mlncnnsr D. (1962)Interpretation of the composition and a classificationof the chlorites. U. S. Geol.Suru. Prof. Pap. Acknowledgments 4t4A, l-31. FnnNr-KnivsNETsKrr,V. A., N. V. LocvrNENro,eNo V. A. Dnrrs I am indebted to my co-workers S. J. Guggenheim, S. H. Hall, (1963)A dioctahedralmixed-layer clay mineral,tosudite. Zap. C. J. Peng, Diane L. Henry, and Judith S. Lister for their aid in Vses.Mineral. Obschch.92, 560-565 [in Russian]. refining the four test structures; to T, S. Laudon, C. Frondel, and GlrrNeeu, L. (1964)Structure r6ele de la muscovite.R6partition C. G. Stone for supply of specimens;and to W. Dollase for use of des substitutions isomorphes.Bull. Soc. Franc. Mineral. program Oprols. Previously unpublished research described here Crystallogr.87, 321-355. CATION ORDERINGAND PSEUDOSYMMETRYIN LAYER SILICATES 187

-, ANDJ. MERINc(1966) Relations ordre-desordre dans les Snwnrrv, B. L. (1967)Interstratification in vermiculite.Clays substitution isomorphiquesdes micas. Bull. GroupeFranc. Clay Minerals, 15' 75-84. Argiles, 18, 67-'74. - (1972)Selective sorption and fixation of cationsby clay Grusrpprrrr, G., nNo Cnnln Tlorr.rr(1972) The crystalstructure minerals:a review.Clays CIay Minerals,20' 93-100' of 2O brittle mica: anandite. Mineral. Petrogr. Milt. lt, Surnozu,Hrnuo, ANDS. W. Blllev (1965)Chlorite polytypism: 169-184 III. Crystalstructure of an orthoheiagonaliron chlorite.Am GnuNrn,JouN W. (1934)The structures of vermiculitesand their Mineral.50, 868-885. collapseby dehydration.Am. Mineral. 19, 557-575. -, AND- (1966)Crystal structureof a two-layerMg- GirvrN, Nrcrn (l97la) The crystalstructures of 2M1phengite and vermiculite.Am. Mineral. 51, 1124-1143. 2M, muscovite.Z. Kristallogr 134, 196-212. SnrrH.J. V., ANDH. S. Yoonn, Jn. (1956)Experimental and - (l97lb) Structuralfactors controlling stacking sequences in theoreticalstudies of the mica polymorphs.Mineral. Mag. 31, dioctahedralmicas. Clays Clay Minerals,19, 159-165. 209-235. -, ANDCHlnr-es W. BunNHnu(1967) The crystalstructure of SogoLrvr. S. V., lNo B. B. ZvvecIN (1968)Crystal structure of 3T muscovite.Z. Kistallogr. 125, 163-183. dioctahedralAl-mica lM. Kristallografya,13' 605-610'[transl. Heurlror, Wllrnn C. (1965) Significancetests on the Sou.Phys.-Crystallogr 13, 516-520(1969)1. crystallographicR factor.Acta Crystallogr.18, 502-510. SrreopteN.R.. lNo P. M. Nurrell (1963)Polymorphism in Hrcxnooor, R. O., enn C. RonruNc(1965) A high-aluminous cronstedtite.Acta Crystallogr.16' l-8. chlorite-swellingchlorite regular mixedlayer clay mineral.Clay -, AND--- (1964)Further polymorphism in cronstedtite. Minerals,6.83-89. Acta Crystallogr. 11, 404-406. Konnun, H., J. F. ALcovER,L. GlrrNr,ru,AND J. MERTNG(1971) SrerNrtNx,Huco (1958a)The crystalstructure of chlorite.I. A Diffusions anormales (Rayons X et electrons) dans les monoclinicpolymorph. Acta Crystallogr.ll, 19l-195. phyllosilicates2 - l. Structureset Prcprietesde Surfoce des - (1958b)The crystal structureof chlorite. II. A triclinic Mineraux Argilew Symposiwn,Louuain, l5-l'1 . polymorph.Acta Crystallogr.l1' 195-198. LlzlneNxo, E. K., ANDYu. M. Konolrv (1970)Tarasovite, a -, ANDGEoRGE BnurroN (1956)The crystalstructure of new dioctahedralordered interlayeredmineral. Zap. Vses. amesite.Acta Crystallogr.9' 487492. Mineral. Obschch.99, 214-224 [in Russian]. Trxrnn, HtnosHt,AND CHARLESW. Burxsnu (1969)Fluor- Lnoevrsr, Brucr (1962) Polymorphic phase changesduring polylithionite:a mica with nearly hexagonal(SirO.)'- heatingof dioctahedrallayer silicates.Geol. Fbren. Stockholm ring. Mineral. J. 6' 102-109. Fdrhand.84,224-229. -. N. Hecn. ,lno R. Slolucn (1971)Structural investiga- LrPPunNN,FnrennrcH (1954) Uber einen Keuperton von tion of polymorphictransition between2M,', lM-lepidolite, and Zaisersweiherbei Maulbronn. HeidelbergerBeitr. Mineral. 2Mr muscovite.Mineral. J. 6,203-215. Petogr.4, 130-134. Trrrucsr, Y. (1965) Structuresof brittle micas. Clays Clay LtsrrR, Junnu S. (1966)'TheCrystal Structure of Two Chlorites. Minerals, 13' l-25. Ph.D. thesis,Univ. Wisconsin-Madison. -, AND R. SrolNlce (1966)Structural studies of brittle -, ANDS. W. Brrr-ry (1967)Chlorite polytypism: IV. Regular micas.(I) The structureof xanthophylliterefined. Mineral. J.4' twoJayerstructures. Am. Mineral.52, l614-1631. 424-437. LorwnNsrerN,Wllrrn (1954)The distributionof aluminumin VrNrlLr, F., eNo H. W. Ver.rDER MAREL (1969) Identification of the tetrahedraof silicatesand aluminates.Am. Mineral. 39, some I: I regular interstratifiedtrioctahedral clay minerals. 92-96. Proc Int. Clay Conf. Tokyo,1,233-244. MlrurrsoN, A. McL., ANDG. F. Wllren (1954)Crystal structure WeLxrn, G. F. (1956)The mechanismof dehydrationof Mg- of magnesium-vermiculite.Am. Mineral. 39' 231-255. vermiculite.Clays Clay Minerals,4'101-ll5' Nrwmrlna, Rosnnr E. (1961)A refinementof the dickitestructure Wmwtonl, A. (1973)Mixed-layer kaolinite-smectite from Lower and some remarks on polymorphism in kaolin minerals. Silesia, Poland: Final repott. Proc. Int. Clay Conf Madrid, Mineral. Mag. 32, 683-704. 75-88. Rnoosr-ovtcu, E. W. (1960) The structure of muscovite, ZsoursLIstov, A. P., B. B. ZvvlctN, S. V. Sorolrvl, ANDA. F. KA l,(SiBAI)O 1o (OH)". A ct a Cry s t al I o g r. 13, 9 l9-9 32. Fnootov (1973)The crystalstructure of the dioctahedralmica -, ANDK. Nonnrsn(1962) The cell dimensions and symmetry 2Mzdeterminedby high voltageelectron diffraction. ClaysClay of layerJatticesilicates. I. Somestructural considerations. lrz Minerals,2l, 465470. Mineral. 47, 599-616. Zvvectr, B. B. (1962)Polymorphism of double-layerminerals of REyNoLDs,Rosnnt C., Jn., lNo JoHr Howrn (1970)The nature the kaolinitetype. Kristallografiya'7,51-65. [transl. Sou' Phys.- of interlayeringin mixed-layerillite-. C/ays Crystallogr.T' 38-511. Clay Minerals, lE, 25-36. - (196'l)Electron-Difraction Analysis of Clay Mineral Struc' RorHaluen, R. (1971)Untersuchung eines 2MrMuskovits mit ,rres. PlenumPress, New York, 364 P. Neutronenstrahlen.N euesJahrb. M ineral. M onatsh., 143-154. -, K. S. MrsHcseuro,nno S.V. Sorolrve (1968)Structure Senronr, Fn.lNco, Mlnco FrelzINI, AND STEFANoMrnltNo of pyrophylliteand talc in relationto the polytypesof mica-type (1973)Crystal structure of a 2M" lepidolite.Acto Cryslallogr. minerals. Kristallografiya, 13, 599-604. [transl. Sots.Phys.- R29.s73-578. Crystallogr. 13, 5l l-515(1969)1.