American Mineralogist, Volume60, pages 1030-1040,1975

Mica Polytypism:Dissimilarities in the Crystal Structures'of Coexisting lM and 2M, Biotite

Hrnossl Terron Mineralogical Institute, Faculty of Science,Uniuersity of Tokyo, Hongo, Tokyo Il3, Japan eNn MnLcolnr Ross U.S. GeologicalSuruey, National Center-959, Reston,Virginia 22092

Abstract

Crystal-structureanalysis of coexistinglM and 2M, btotite polytypesfrom a rhyodacite lava flow, Ruiz Peak,New Mexico, shows that the two forms possessessentially identical unit-cell parameters(in the lM setting),bond length averages,and site occupanciesand probably,therefore, very similar chemical compositions. The unit layersof the two polytypes, however,are differentstructually in that the x and y atomicpositions of the 03 and 04 ox- ygensdiffer in the two forms by an averageof 14standard deviations. The averagedifference in the relativevalues of all other atomicpositions is onestandard deviation. The displacement of 03 and 04 in the 2Mrbiotitecausesa typeof deformationnot beforereported in micasand manifestsitself by a relativeshifting of the upperand lower triadsof octahedraloxygens as a unit along the aD directions(2Mt setting).This causesa reductionin symmetryof the2M, unit-layerfrom the ideal C2/m as found in the lM polytype,to Cl. It is postulatedthat the structuraldifferences between the unit layersof thelM and,2M, biotite polytypesare dueto the differentatomic and geometricconstraints imposed upon the unit layerby the adjacentlayers, the latter beingin differentrelative orientations in different polytypes.An origin for biotitepolytypism is proposedon the basisof theseunit-layer con- straintscoupled with themica platelet growth and spiralgrowth observations of A. Baronnet.

Introduction layers.'Another questionis, are the unit layersstruc- In the last decade, many reports have been turally as well as chemicallyidentical if the publishedon the crystalstructure of variousmica crystalshave differentperiodic stackingsequences? polytypes.For example,the modelstructures of lM The question of possible structural differences trioctahedralmicas were summarizedby Donnay, betweendifferent mica polytypesfound in the same Donnay,and Takeda(1964) and by Takeda(1971); rock hand specimenarose during our study of ox- the unit-layerof 2M , muscovite,refined by Burnham ybiotitesfrom a rhyodacitelava flow at Ruiz Peak, and Radoslovich(1964), was compared with that of New Mexico (Rosset al., 1966).Single crystal X-ray the 37 form by Giivenand Burnham (1967),lM and study of 42 crystalsfrom this specimenshowed that 2M, brittle of different composition were approximatelyone-third are lM forms, one-third studiedby Tak6uchi(1964), lM and2Mr lepidolites 2Mr, and one-thirdmore complexpolytypes such as werestudied by Takeda,Haga, and Sadanaga(1971), the4Mr,8M", and87c,, forms.The detailedstudy of and a 2O brittle mica was examinedby Giuseppetti the structuresof thesecrystals, however, was post- and Tadini (1972).The previousstudies of mica poned,for we did not believethat at that time crystal polytypes were accomplishedon samples from structure refinements could be done accurately diverse localities having significantly different enough to show the very small differencesexpected chemicalcompositions. It is thus difficult to decide betweenunit layersof differentpolytypes. The pres- how much the structuraldifferences found between ent study of the crystal structuresof coexistinglM various polytypes are due to chemicaldifferences, I A mica unit-layer is defined as that layer,approximately l0A differencesin temperatureand pressureof formation, thick, contained in the unit-cell of the lM polytype. Two unit and differencesin the stackingsequence of the unit- layers occur in the unit cell of the 2M, polytype. 1030 DISSIMILARITIES IN COEXISTING IM AND 2M, BIOTITE l03l and 2Mt polytypes from a single hand specimen of unoxidizedflows do not show this color and their the Ruiz rhyodacitewas made during 1972using the ferric contentis much lowqr. best X-ray diffraction techniquesthen available. With Composition this study we hope that we may develop a better un- Chemical derstanding of the nature and cause of polytypism The biotite chemical formulas based on wet and polymorphism'z in complex . chemicaland electron microprobe analyses (separates 3149-8Fand 3149-8W,respectively) are given in Mineralogy Tablel, columns(1) and(3). Electron probe analyses Specimen Description madeon the clearhomogeneouS areas of five crystals by the wet The biotite crystals examined in this study occur in revealed barium-an element missed The 0.46wt per- a rhyodacite ash flow from the Ruiz Peak area, Valles chemicalanalysis-but no calsium. found in the wet Mountains, New Mexico. The particular rock sample cent CaO and 0.34wt percentPzOu The rangeof studied (No. 3149-8)came from the top of the flow analysisis thus due to includedapatite' analyses and is reddish becauseof formation of after cation content as obtainedby microprobe with the eruption of the lava. The rock contains abundant overlapsthat givenby wet chemicalanalyses content' phenocrysts of feldspar, biotite, and magnetite and exceptionof higherMg and Al and lower Fe in less abundant phenocrysts of augite and ortho- We attributethis differenceto chemicaldifferences time of .Apatite is ubiquitous and occurs as inclu- the biotite phenocrystsdepending upon the melt. The sions in the biotite. The groundmass is composed crystallization from a fractionating these chemical mainly of feldspar, glass, and biotite; the latter is different mineral separatesreflect crystallo- much altered to hematite and other products of late differences.The biotite crystals studied andthus we stageoxidation (probably sanidine,metastable mag- graphicallycame from separate3149-8F, is closerto netite,and glass;D. R. Wones,personal communica- assumethat the Mg, Al, and Fe content The electron tion). The large biotite phenocrystsare often rimmed that given by the wet chemicalanalysis. any significant with hematite, and this phaseis also seenin the cleav- microprobe study did not detect singlebiotite crystal' age cracks. The biotite crystals separated for chem- chemicalvariation within a contains8'4, ical and crystallographic analysis, however, were The oxy-biotite sampleunder study "hydroxy", aqd nearly free of hematite but still contained apatite in- 10.6,and 81.0 percent"fluoro", (Table1, columnl)' clusionsand a small amount of rutile. "oxy" component,respectively percentis related The oxidation, which is inferred to have occurred Of the total "oxy" component52.8 during and after eruption of the lava, affected the to a couple substitutioninvolving oxidation-reduc- biotite crystalsin two ways: (l) exposedsurfaces of tion, that is, break- the biotites reacted to form hematite and other Fe2+* OH- =Fe3+ + A- + l/2Hr. down products, and (2) the crystals reequilibrated with respectto the hydrogen fugacity by the reaction, Partor mostof thissubstitution probably occurred as somewhat idealized, a solid-statereaction during eruptionof the lava' Of most KM g,-"Fel+(Si3Al the (4'7.2Vo)remaining "oxy" component, )OuH, of - KMgr-,Fe,'1"Fe'r*lsirAl)orrHr-, + | / 2yHr1. (42.2Eo)must be attributed to the incorporation 0.342atoms per formulaunit of (Table l) This reaction causedthe biotite crystals to turn dark into the crystal structure during crystal growth, reddish brown. Biotite crystals from underlying, chargebalance being maintainedby increaseof the oxyanioncontent. This coupledsubstitution may be

'?We define polytypism as the existenceof two or more chemical- written ly identical, or nearly identical, phases which have structurally K(Mg,Fe)3+(Si3Al)O1o(OH)2+ Ti'+ similar unit motifs (unit layers in the case of micas) but which repeat in three-dimensional crystal space in different ways. We = K(Mg,Fe);+Ti(SisAl)O1, + (Mg,Fe)'?+ + 2H+ define polymorphism as the existenceof two or more chemically identical, or nearly identical, phases which do not possessstruc- turally similar unit motifs. Phases such as quartz-coesite and Hydrogenation of Biotite calcite-aragonite can be unambiguously defined as polymorphs Hydrogenation of the oxybiotites is easily ac- such as the biotites, zinc sulfides, car- whereas substances by passinghydrogon gas at temperatures bides, and cadmium iodides are generally thought of as showing complished polytypism. of 500' to 700'C over the singlecrystals. The reac- t032 H. TAKEDA AND M. ROSS

Tent-s l. chemical FormulaeBased on wet chemical Analysesof SeparateNo. 3149-8F and on ElectronMicroprobe Analyses of Individualcrystals from SeparateNo. 3149-8w

** (1) (2) (3) Hydrogenated At oms oxyblotlte Oxyblotite (7000C) oxyblotite (Range)

si 2.794-2.900 4.ooo 4.000 41 (rv) i.?i;l iilll , *, t.L47 t.222-L.288 A1 (vr) 0. 163 0. 186 0. 101 Ti I 0.342 0.344I o.32r 0 . 305-0. 338 I"h^. 0. 010 0.010 0.018 3.049 [ 3. 112 0. 009-0.020 0.855 o.ras( o.14r 3.066 o.742-0.762 !e 0.013 o.7uI 0.011 Mg 7.666 r.676 I 7.874 1 . 850-1.906

Ba o.022 0.022 0.020 0. 013-0.022 Na 0.t62 0.957 0.162 0. 961 0.157 0. 933 0.152-0.168 K 0.773 o.777 0.756 0. 692-0.802

OH 0.2L2 0.711 F 0.168 '0. 169 138 12. 000 r2.000 0. 0.101-0.182 c1 0. 004 0 . 002-0.004 0 rL.620 11.120

* Atonic ratios based on wet chemical-analysis, chenlstry laboratory, U.S. Geologlcal survey' sumation based on: ofF+oH = 12. Bao fron electron nrcioprobe analysls, Fractlon 3149-8F. CaO and prO. are assuned !o be in apatlte only.

** Atonic ratios based on electron nicroprobe analysls (average of flve analyses on five crystals), analyst - L.B. hltgglns (U.S. ceologlcal Survey), Sumtion based on 8.000 catlons ucluding hydrogen. Fractlon 3149-8I.I. lron 1s assumed to have the same ferrous - ferric ratlos es 14 (1). tion is reversible;by passinghot argon gas over the StructureDetermination hydrogenated crystals, hydrogen is removed and the The crystals used for structure determination crystals revert to their previous structural and measure0.23 X 0.27x 0.05mm; thecalculated den- chemical state. These oxidation-reduction reactions sitiesare 3.05g/cm3; and the linearabsorption coeffi- are of course dependent on the availability of suffi- cientsare 26.6 cm-t. cient amounts of an oxidizable metal cation such as iron within the crystal structure. The ferrous and fer- ric iron content of the Ruiz Peak biotite (separate TesLr 2. Crystal Data for Oxybiotitesand Hydrogenated 3149-8F)before and after hydrogenation at 700.C is Oxybiotitesfrom RhyodaciteNo. 3149-8,Ruiz Peak, given in Table l, Columns (l) and (2), respectively. New Mexico, and for Phlogopiteand Annite (Wones,1963) Of particular interest from a crystal-structural

Intensitymeasurements were made with a Picker- from the total amountof Fe in the chemicalformula, FAcs-lcomputer-controlled four-circle, single-crystal lessthat in Ml. The occupancyof Mg in Ml andM2 diffractometer,using MoKa radiationwith a graphite is fixedby the value(l-Ti-Alvr-Mn-Fe) for eachof monochromatorand a scintillationdetector. Inten- thesesites. sitiesof 1202reflections for the lM form and 3539 In the last stagesof refinement,reflections for - reflectionsfor the 2M, form were measuredin the whichlFol < l5 and| | Fol | F"l l/o(Fo)>5 werere- 4"-60o 20 rangeusing cir-2d scans. Each reflection was jected. After six cyclesof refinement,during which scannedthrough 1.2o, using dispersion factors ofthe the scale factor, the atomic coordinates,and the systemfor a, and a2 separationat a scanningrate of anisotropictemperature factors Bij wete varied,no onedegree per minute.The backgroundwas obtained further change of parameters took place. The by countingfor 20 secondsat both sidesof the peak. observedand calculatedstructure factors fot the lM Integratedintensities were corrected for Lorentzand and 2M, polytypesare listed in Tables 3a and 3b, polarizationeffects and werereduced to structurefac- respectively.3The final conventional unweighted tors lF,l . The absorptioncorrection was madeby residualsfor structuralparameters given in Tables4 Burnham's(1966) subroutine with a linearabsorp- and 5 are R:0.044 for 649observable reflections of tion coefficientof 26.6cm-l for MoKa radiation.The the lM form andR:0.056 for 875reflections for the estimatedstandard deviation of an observationwas 2M, form. computed on the basis of the counting statistics The variousbond lengths(Tables 6,7, 8), selected (Burnhamet al, l97l). bondangles (Table 9), andassociated standard devia- Of the intensitiesmeasured, 649 nonequivalent tions werecomputed from the atomicpositions given reflectionsof the lM biotite,with the indiceshkl and in Table5. The variance-covariancematrices (Finger, hkl, and875 reflections o[ the 2Mrbiorite,withthe in- 1969)of the refinementsand the standarddeviations diceshftl and Ekl, were utilized in the final refine- of the unit-cellparameters were usedfor thesecom- ments.The spacegroups C2/m for lM andC2/c for putations.The observedlM coordinatesare com- 2Mrwere assumedand, on refinement,turned out to paredin Table5a with the observed2Mt coordinates be correct. in the lM setting.The latter coordinatesare those of The full matrix least-squaresprogram, RntNn the unit layer of the 2Mt polytype in the crystal- (Finger,1969) which includesthe provisionfor site lographicorientation of the lM form and arederived occupancyrefinement, was used for the lM and2M, by a matrixrotation of 120"about c* of coordinates biotite crystalstructure determinations. The weights of atoms in a 2M, unit layer (Table5b) and then usedin refinementare l/o(Fo)2, where o(Fo) is the translationof the origin to that of the lM polytype. standarddeviation of lFol . The atomic-scatteringfac- tors usedwith RrINr were taken from Cromer and Comparison of Structures Mann (1968),the oxygenatoms being considered Chemical Similarity half-ionized(O'* ), andall cationscompletely ionized. Anomalous dispersioncorrections were made by The similarity of the unit-celldimensions and bond usingthe coefficientsof Cromer (1965). lengths of the lM and 2Mt crystals indicate that thd Least-squaresrefinement of the lM structurewas crystals are very close in chemical composition. In initiated by using atomic coordinates of a lM Table 2 the measured unit-cell dimensions of the lM phlogopite(Joswig, 1972). The trial structuremodel form are compared with the calculated unit-cell of the 2M, form was derivedfrom the atomic coor- dimensions of the 2M' polytype in the lM setting' dinatesof the abovelM phlogopiteusing a program The agreement between these two sets of unit-cell (Twuc) written for the Hluc 5020Ecomputer dimensionsis within one standard deviation for all (Takedaet al, l97l). The hydrogenatoms were not four parameters. Likewise, the mean tetrahedral, the includedin theserefinements. The following cations octahedral, and the inner and outer sets of werefixed during refinement: K i Na in the interlayer potassium-oxygenbond lengthsare essentiallyiden- sites, Si + AlIv distributedequally over the tical for both structure types. The comparative values tetrahedralsites. and Ti + AlvI * Mn distributed for average bond lengths (Tables 6, 7, 8) for the lM equallyover the octahedralsites. The relativeamount 3 To obtain a copy of Table 3a and 3b, order document AM-75- of Fe * Mg in the two octahedralsites was allowed to 008 from the BusinessOffice, Mineralogical Society of America, vary during refinement,During the site refinement, 1909 K Street, N.W., Washington, D.C. 20006.'Pleaseremit the occupancyfactor of Fe in theM2 sitewas derived S1.00 for the microfiche. r034 H. TAKEDA AND M. ROSS

Trslr 4 Site Occupancy Factors for Coexisting TlnI-s 5b. Atomic Coordinates of the Hydrogenated Biotites after Hydrogcnation at 700'C+ Ruiz Peak 2M, Biotite in the 2M, Setting

ML M2x* Atom* z

l-iV Polytype M1 0.75 o.25 0.0 (2) -0 . 00004(8 Mg 0.537 o.546 M2 0.2406(3) 0.0802 ) (3) Fe 0.290 0.281 K 0.0 0.0840 0,25 (21 (9) Tl+t"ln+Al 0.173 o.r73 TI 0.462r(3) o .249e 0.13798 rt 0.9635(3) o .4169(2) o.L37e9(9) 2M, Polytype dl1 0.7410(8) 0.3r4o(s) 0.1662( 3) d)1 0.2430(8) 0.3532(5) v.Lotz\5) 0. 568 0.531 u2z 0.4337(8) 0.0840(s) o . ]-666(2) Fe 0.259 0.296 031 o.43L4(7') 0.2406(5) u.uf,c)(2, Tl+l,In+A1 0.173 0.173 o32 0.9375(7) 0.4090(6) o.0547(2) * 04 0.9348(7) 0.0739(5) 0.0508(2) During Leas t- s quate s re finenent, te tz.ahedtal si te e uez'e at Si=0.71, 41=0.29 qnd the intexLayen site fi.aed * Standard deviations are given in parentheses and are at Na=0.22, K=0.78. TotaL cati.ons in 141+172aye no?- expressed in wits of last digit stated. nalized to 3.000 atons per fonmtla unit uith Mg=1.630, Fe=a.852, and Ti+l4n+AL=o.519. Anotmt of Ti+I,ln+ALin each site i.s fired at 0.173, The oaLues fon t"l1+142 differ slightly fron tVnt g"ioen in Tohle 1, coLwnn (2) after nomnalization to 3.000 (vIz. Mg=1.617, Fe=0.864, and 2M1forms, respectively,are: tetrahedral(1.659, ytd Ti+Itn+AL=0.521). 1.659A), octahedral-MI (2.086,2.086), octahedral-M2 *xTln occupancy of Mg in t42 ia derLued by 0.826-0.5r (occupartcy of Mg in Yl7). Eryon in octahedral occupanci,es (2.068, 2.06'7),inner K-O (2.972, 2.972), and outer uith regatd to Mg and Fe nag be as Lange as 10% con- K-O (3.318, 3.321). The site occupancy factors sidering the possible errors in the chemical analgses. (Table 4) are also in good agreement for the two polytypes even though the errors are large. The tetrahedral rotation angle,oL, which reflectsthe degree of deformation of the ring from hex- Tenlr 5a. Atomic Coordinates of the Hydrogenated agonal to ditrigonal symmetry, is 7.61' in Lhe lM lM and 2M, Biotites from Ruiz Peak, New Mexico* form and 7.63" in the 2M, form (Table 9), these values being identical within the accuracy of the Atom** (Form) x )L refinements.This similarity of rotation anglesis ex- pected l.{1 (1M) 0 0 if the composition of the octahedral and M1 (2M1) 0.0002 0. 0002

tr2 (1M) 0 0.3392 (1) 0.s Test-e 5c. Equivalent Isotropic Temperature Factors B M2 (2q) 0.0002 0.3398 0.5 and Anisotropic Temperature Factors+ X K ( IM) 0 0.5 0 Bii 104of Ruiz Peak Biotites K (2M1) 0 u.) 0

Aton -11 Bzz Btz T (IM) 0.0745(2) 0.1671(1) 0.2242 (r) T1 (2M1) 0.A742 0.1673 0.2240 lM biotite T2 (2Mi) o.o74l 0.1570 0.2240 Ml LI7 (4J 85 (s ) 79 (2) 50(2) o 23(2) 0 (r) (1) (r) o 01 (lM) 0.016s(7) 0 0.L67 4 (4) M2 L.L1 14) 64 (3) 33 4r 0 12 2.2914) 180(7) s3(2) 7e12) 0 28(3) 0 011 (21.{1) 0. 0168 0. 0005 0 .L676 K r 0.75(2) 55(3) 12.1Ie) 30.e(8) 0(1) 13(1) -o. s (e) 01 1.46(6) r72(r3) 24(4) 4r(4) 0 13(6) 0 -6(2) 02 (lM) 0.3240(s) 0.2310(3) 0 . 1660(3) 02 1.s9(s) r15(8) 44(3) so(3) -77(4) 11(4) 021 (2q) o.324L 0.2300 0.1656 03 0,99(4) 14\1) 18(2) 33(2) 3(4) r5(3) o(2) o22 (2Mi) o.32s3 0. 2303 0. 1668 04 r.03 (5) 79 (11) 2814) 3l (3) 0 8 (s) 0

03 (1M) 0.1310(4) 0 .1684(3) 0.3906(2) -"1 ------031 (2M1) 0.1179 0.1733 0.3910 o.85(4) ss(7 25(2) 6.9(5) 0 3(1) o n2, Ml ) (2Mi) 0.1173 0. 1639 0. 3906 r1t2 I.05(3) 1r0(s) 26(2) 6.3(3) 20(4) 2 (1) 2 (1) K 2.2I 14) r82(8) 65(3) 14.6\7) o 3(2) o 04 (rM) 0.1312(6) u.) 0. 398s(3) 11 0.70(3) 52(s) 2312) 4.7(4\ 3 (3) r (r) 1(1) -6(3) (1) -2 (1) o4 (2M1) 0.1180 0.4957 0. 3984 12 0.7r(2) 53(5) 22(2) 5.0(4) 2 011 1. s4(8) 136(r7) 54(6) 8 (r) -le (8) 0 (4) r(21 o2f 1. 53(8) r08(16) 60(6) I (r) 25(8) 0 (4) -2(2) 't Both forre are Llsted in the LM settlng to facilitate o22 1.48(7) 1el (16) 38(5) 7G) 3(9) 4 (3) -0 (3) comparlson. The atonic coordinates for the 2M. polytype 03r 0.57(6) 4't t72) 7 14) 6 (1) I (7) 3 (3) 4 (3) in the 2M1 setEing are given in Table 5b. o32 0.69(6) 7O(r2) 14 14) s (1) rr (8) 2(3t 5(3) 04 0.80(6) 88(13) 11(4) 6 (1) e (8) 0(3) -0(3) ** Standard deviations for the lM foms are glven in * is in the expression parentheses; those of the 2I4. form are listed in Table 8ii !*p-(Brr!2 + B2zk2+ B.rL2 * Brrzvt * Brrrl! + Brz2lu ' DISSIM ILARITIES IN COEXISTING IM AND 2M, BIOTITE r035

TesLs 6. Bond Lengthsfor TetrahedralSites of T,nnt-s 8. Bond Lengths for Interlayer Sites of Ruiz PeakBiotites Ruiz Peak Biotites

IM biotite 2Ml biotite lM biotite 2Ml biotite

(E) (fl) Bond Length (i) Lenqth Lenqth Lenqth (.&) Lenqth (i)

(21 r.55e(4) T-0r L.65'7 fl-011 r.649(4) T2-011 (inner) r-02 1.655(3) r1-02.1 1-656(4) T2-O2L r.656(4) (4)x2 K-011 2 .975 (5)x2 r-o2' r. 65?( 3) T1-O22 1.644 (5) T2-O22 1.662 (s) K-01 2,97L 2.971 (5)x?. r-03 1.657(3) 11-03r r.6'75 14) r2-A32 1.6'74 14) K-02 2 .913 (3]-x4 K-021 (4)x2 Mean 1.659 Mean 1.656 Mean 1.663 2.97I (Mean 1.659) Mean 2.972 Mean 2.912

01-02 2.686(3) 011-021 2.6A2(6) 011-021 2.695(6) (ouler) ol-o2' 2.69r (3) ot7-o22 2.619 16) Orr-022 2 .100 (61 K-01 3.331(4)x2 K-0ll 3. 329(5) x2 (I) 2.69916) o2-o2 2.688 o2r-o22 2.679(6) O2l-O22 K-02 3.312(3)x4 K-021 3.313(5)x2 Mean 2.644 Mean 2.680 Mean 2.694 K-o22 3.322 14)x2 Mean 3.318 Mean 3.32r 03-01 2.73L(4) 031-011 2.15r(6) 032-011 2.1O2161 03-02 2.72414) 031-021 2.76116) 032-O2r 2.1L616) (lateral ol-oz' 2.72a1:) 031-022 2.6'13(6) 032-022 2.719\6',) ) (41 ' 4.].66 (9) Mean 2.729 Mean 2 .72a Mean 2 0 1-02 4 .144 x4 011-01t "732 OII-O22 4.I48(6)x2 o2-o2', 4 .143(5) x2 O2!-O22 4.L45(6)x2 o2L-O2r' 4.r29(9') Tnslt 7. Bond Lengths for Octahedral Sites of Mean 4.L44 Mean 4.147 Ruiz Peak Biotites (basa1 ) 01-02 4.26L (4)x4 ot1-021 4.260 (6)x2 (61 lM biotite 2MI biotite 011-02 2 4 .257 x2 02-02' 4.265 (6\x2 o2).-022 4 .26]. (61x2 .259 Length (i) EOnO Length (A) Mean 4.262 Mean 4

M1 octahedron /ch^rfact interlayer lengths) 01-01 3.380(7)x4 011-021 3.364(6)x4 o2-o2 3.348(5)x2 O22-O22 3.355 (8)x2 Ml-03 2 . IO2 (2) x4 Ml-032 2.O34(4't x2 MI-031 2.LO4(4)x2 MI-04 2.o53(3)x2 Ml-04 2.L2o (4)x2 Mean Mean 2.046 tetrahedralsheets of each polytype is similar. The (Shared edges) tiltineof thetetrahedra, resulting in thecorrugation UJ-UJ 2.82e (51x2 o3r-032 characteristicof the2Mt o3-04 2 .769 (4',x4 o32-O4 i.'rlii?i:t oflthi tetrahedralsheets so 031-04 2 'a6'7('7'tx2 dioctahedral micas' is not pronounced in either of the Mean 2.789 Mean 2.749 biotites. (Unshared edges) A comparisonof the atomic coordinatesof the lM o3-03 3.109(5)x2 o3l-o32 layerof rhe2Mt struc- 03-04 3.098(4)x4 031- 04 i:i19lilX3structure with those of a unit o32-O4 3.osl(s)x2 ture in the lM settingreveals a remarkableagreement 3.103 Mean 3.102 Mean betweenall atomic coordinates,except for the x and y M2 Octahedron values of the octahedral oxygen atoms 03 and 04. coor- M2-03 2.118 (3)x2 M2-031 2.Os4(s) For example, the displacement of the .r atomic rrl2-o32 2.r24 (4) M2-03 ' 2.OA3(2') x2 M2-031, 2.]-46(4) vt2-032' 2,L4't(5) Tlsre 9. SelectedAngles for Computing Tetrahedral t42-O4 2.004(3)x2 ti2-04 1.934(5) Rotation Angles a* M2-04' 1.99e(4 ) Mean 2 .068 Mean It't biotite 2MI biotite

achrra^ oidac) 03-03 2 .799 (5)x2 031-031' 2.792(7) Anqle, aleq. Angle, deg. o32-O32' 2,895 (8 ) (17 \o4.69 (20\ 03-03' 2.828(5)xl o3r-032 2.73O(6) o2',-o\'-o2" LO4.A2 ) x2 021',-011'-022 104.53(10)x4 011-o2r-02 2 r04.71 (r8) 04-03 2.769 (4)x2 04-032' 2.'770(5) 0l-02-02' 104.76(19) 04-031' 2.86't(7) orr-o22-o2r' Mean LO4.69 Mean LOA.7 2 04-04 2.69! (7| xL o4-o4 2.s92(al Mean 2.774 Mean 2.7'16 o2-or"-02 " 135.18(19)x2 o2!-oIL-022' 135.30 (22) 02-02'-01, 135.08(I7)x4 otL'-o2r'-o221 I35.20 (22\ (Unshared edges) oLr'-o22-o2r L35 .27 (20',1 (s) 03-03 3. 062(2) x2 031-032' 3.064 Mean 135.t r Mean 135.26 o31'-O32 04-03 3.o76 (4) x2 o4-032', 3.073(5) o 7.6r q 04'-o31' 3.075 (5) 04-03' 3. 062(3) x2 04- 03r 3.O58(5) 04 '-032 3.06s(s) * -mean Mean Mean 3.066 s = 1120" anglesl x 0.5. t036 H. TAKEDA AND M. ROSS

dinateof oxygenatom O32 of the2M, structurefrom thex coordinateof O3 of the lM structureis 0.0137, whichis l9 timesthe standard deviation of theO32 x coordinate(0.0007). The averagedifference in the x and y parametersof the 03 and 04 oxygensin the two crystalstructures is l4 standarddeviations. The averagedifference in the relativEx, l, and z positions of all other atoms,including z of 03 and 04, is one standarddeviation. Deformation of 2M, Octahedra Previouscrystal structure studies of 2M, micas haveshown that displacementof the 03 and 04 ox- ygen atoms from the ideal octahedralarrangement causestwo typesof octahedraldeformation: (l) flat- .\).o tening.ofthe octahedraalong the pseudo3 axes(com- Fra. l. The structureof 2M, biotitefrom Ruiz Peak,New Mex- mon in disorderedtrioctahedral micas, Donnay et al, ico, projectedon (@l). The tetrahedralring can be seenbelow the 1964), and (2) lengtheningof the edgesshared octahedrallayer. Oxygen atoms are at the apicesofthe polyhedra. between Ml and M2 octahedra containing Cationsare shown as cireles. The K-K stackingvectors are shown monovalentions or vacancies.The latter type of as dashedlines. The potassiumatom designatedK' is in the unit deformation is found in partially ordered layerabove the oneshown. The2M, axialsetting is shownas solid lithium- lines,the lM settingas dashedlines. bearingtrioctahedral micas (Takeda and Burnham, 1969)and in dioctahedralmicas(Radoslovich, 1960). The octahedraof the 2Mrbiotite of this studypos- sess a new mode of deformation. A convenient methodof depictingthis deformationis to projectthe sharedoctahedral edges 031-O3l and O32-O4onto the (001) plane. Theseprojected edges are inclined approximately6o from the a axisof the2M, unit cell (Fig. l). The equivalentoctahedral edge projections of the lM biotite are almostexactly parallel to the a axis when viewed in the 2M, setting. ln 2M, muscovite,these same projected edges are also almost exactlyparallel to c. The inclination of the projectededges of the oc- tahedraof the 2M, form relativeto thoseof the lM polytypeis presentedin Figure4. Note especiallythe inclinationof the O4-O4 and 03l-O32 projections \" with respectto the equivalentprojections of the lM Frc. 2. The structureof 2M, muscovite(Rothbauer, l97l). Notationas given Figure positions octahedra(solid and dashedlines, respectively, Fig. in l. The hydrogenatom deter- minedby neutrondiffraction are shown as small solid circles. Note 4). This distortionof the 2M, octahedrais causedby that thepotassium positions (large open circles) are shifted slightly a "sliding" of theupper triads of oxygenatoms (O31, from the centersof the tetrahedralrings (c). K' and H' denote O32, and 04) as a rigid group parallelto the 6 axis atomsin the unit layer abovethe one shown. (2M, setting)and a "sliding" of the lower triads of symmetricallyequivalent oxygen atoms in the op- above causesfour of the twelve Ml-O and M2-O posite direction (Fig. 4); the z coordinatesof these bond distancesto becomeshorter, four to become atomsdo not changesignificantly as a result of this longer,and four to remainalmost the samerelative to deformation.a the equivalentbond distancesin the lM structure. The deformationof the 2M, octahedradescribed Also, four of the six edgesshared between octahedra 4 ln the 2M, setting, this octahedral deformation is reflectedonly changein the 2M, form with respectto thoseof the by a change in the y parameters of 03 and 04; in the lM setting, it lM structure type (Fig. 4, Table 7): two become is reflected by a change in both x and y (Table 5a, Fig. 4). shorter (O4-O4, O3l-O32), two become longer DISSIMILARITIES IN COEXISTING IM AND 2ML BIOTITE t037

(O3l-O4, O32-O32),and two remainabout the same (O3l-O31, O32-O4). If we designatethe shared edgesof the M2 octahedraof the lM form asm-m-m- m-m-m (observedin a clockwisesequence from the upperleft of Fig. 4), the sequenceof sharededges of the M2 octahedraof the 2MLform becomess-l-m-s-l- m, where / indicates a lengthenededge and a -b'" s shortenededge relative to the m edgesof the lM polytype.The sequenceof theseshared edges is s-/-s- l-s-lin the2M, muscovitestructure. The unsharedoc- tahedraledges have the samelengths in the lM and 2M, biotites (Table 7). The 03 oxygensof the octahedraare also the "apical" oxygensof the tetrahedra.In both the lM and 2M, biotites. the basesof the tetrahedraare Ftc. 3. The structureof lM biotite from Ruiz Peak,NewMex- nearly parallel to (@l). In the (001) projection of the ico, projectedon(001). Notation as givenin Figure I lM biotite structure(Fig. 3), the tetrahedralcations liejust belowthe 03 apicaloxygen atoms. In thepro- jection of the 2M' biotite structure(Fig. I ), however, the apical oxygens03l and O32 aredisplaced away from Il and 72, respectively.This shifting of O3l '.ii and O32 parallel to b causesa shorteningof the f, - bzur O3l-O22 distanceand a lengtheningof the O32-O22 ot distancerelative to that found in the lM structure /i (Table6). Iru i In the 2M, muscovitestructure (Fig. 2), the mode E o cr\ 1 /l o o o O1 of deformation of the octahedralsheets is quite o N different from that found in the 2M, biotite. The vl N N I (001)projections of the octahedraledges 03l-O3l +l o31 and O32-O4 arc parallel to c in 2M, muscovite, whereasin 2M, biotite they are not. In 2MrbiotiIe, the octahedraledges O32-O3l and 03l-O4 arepar- r+ allelto D,whereas in 2M, muscovitethey are not. The pseudomirror planes5of the unit-layersare essential- F o ly retained in the 2M, muscovitestructure ; in 2M, biotite, they are not. N The structure of the tetrahedral sheet of 2M, muscoviteis also quite differentfrom that found in 2M, biotite. In muscovite,the (Ol) are shiftedtowards the vacantoctahedral sites by 0.4 A, I causinga distinct tilting of the tetrahedron.In 2M, 0zrut biotite, the tetrahedraremain essentiallyuntilted. Orientation of the O4-H Groups 2Mr (1M) Neutron diffraction study of a lM phlogopite Frc. 4. Projectiononto (001) of theMl andM2 octahedraledges (Joswig,1972) shows that the axisof the O4-H bond of the lM (dashedlines) and 2M, (solid lines)biotite structures. is normal to the unit-layer,the hydrogenatoms being The metal-oxygen and oxygen-oxygenbond lengths are given, positionedexactly above (or below)the O4 oxygens. with thoseof the lM polytypeenclosed in parentheses.Explana- tion of the letters,n, s and / is givenin the text. Distortionof the 2M, octahedrawith respectto those of the lM form is depicted o The pseudo mirror planes include the O4-O4 octahedral with arows, whichindicate the relative displacement of theO3 and shared edgesand are perpendicular to the b axis of the unit layer in 04 oxygenatoms in the tb direction. the lM setting. 1038 H, TAKEDA AND M. ROSS

The potassium atoms lie immediately above (or gestthat thesetwo polytypeshave differentfields of below) the hydrogen atoms. We assumethat the stability.Since we cannot demonstrate that thestruc- orientationof the O4-H vectorin the Ruiz lM biotite tural differencesare causedby differencesin chemical is the sameas that found in lM phlogopite. compositionor differencesin temperatureand pres- The hydrogenposition of 2M, muscovitehas also sure of crystallization,we propose that they are been accuratelydetermined by neutron diffraction causedby strain introducedby the geometricaland (Rothbauer,l97l). It wasfound that the O4-H vec- atomic restraintsthat one unit layerplaces upon the tors are inclined toward the vacant octahedralsite adjacentunit layers. (Ml) andthat thepotassium atoms do not lie directly In the biotite series,individual layers can superim- above(or below)the 04 andH atoms(Fig.2) asthey poseupon one anotherin threeways such that adja- do in the lM phlogopitestructure. Also, the centers cent layersare relatedby a relativerotation aboutc* of the (Si6O,E)ditrigonal rings are displacedfrom of 0", 120o,or -120".6If we assumethe structureof positionsdirectly above(or below) the 04 oxygens. any particular unit-layerof a sequenceis influenced The O4-H, K-O4, and O4-O4 vectors,however, all only by the orientation of the layers immediately lie within the pseudo-mirrorplane of the unit layer aboveand below,there are only four possiblebasic (Fig.2). structuresfor a unit layer.These four typesof layers The 2M, biotite structure possessesthe same are positionedin the middle of four differentthree- + 120' stackingsequence as found in 2Mrmuscovite; layer packets.The relativeorientation of the three however,the potassiumatoms do not lie within the layerswithin thesepackets is dgfinedby the vector pseudomirror planesas found in muscovitebut are stacking symbols [00], [02], l2l), and f22), whete: displacedalong a line parallelto the 6 direction(2M' 0 : 0o, 2 = l20o,j = -120o, and the first and last setting,Fig. l). This displacementof the potassium numberswithin the bracketrefer, respectively, to the atomsand the "sliding" of the upperand lowertriads rotatibn of the first relativeto the second,and the sec- of oxygenatoms of the octahedrallayer along the tb ond relative to the third layer of the packet.?We directionreduces the symmetryof the unit layerfrom designatethese four kinds of unit-layersas type [00], the ideal C2/m to CT. f nis is in markedcontrast to l12l, 1221,and [22]. Mica polytypes of the form the unit layersof muscovitewhich to good approx- lMl}l,2M, [n], 3T12221,and 6T[020202]contain imation retain the ideal C2/m symmetry.Note the only type 1001,1221, 1221, and [02] unit layers, pseudo-mirrorplane oriented along an O1-O4 vector respectively. and perpendicularto bru of the muscovitestructure The more complex Ruiz polytypescontain more depictedin Figure 2. thal one type of unit layer; for example,the 3Tc' Our X-ray data do not give us any directinforma- [022]form contains[02] and [22]type layersand the tion on the positionof the hydrogenatom inthe 2M, 4M, 122201formcontains [22] and [02] type layers. biotite structure.From the positionof the potassium The Ruiz biotite \Tc," 10002D021contains all four atoms and octahedralcations, however, we predict typesof three-layerpackets, which appearin the se- that the axis of the O4-H bond will be tilted away quence:[00], [00], lul,l22),tnl,Eol, [02],and [20]. from a perpendicularto the unit layer and in a direc- This polytype,thus, theoretically,possesses all four tion away from the potassiumatom and in a plane typesof unit layerswithin the unit cell.It would be nearlyparallel to the b axisof Lhe2M, unit cell.The informativeto undertakea very carefulcrystal struc- vectorsbetween K-H, H-O4, O4-O4, and K-K of a ture analysisof polytypessuch as theseto further unit layerall lie within the mirror planesand pseudo- delineatethe effect of first (and second)nearest- mirror planes of lM biotite and 2M, muscovite, neighborlayeis on the structureof anyparticular unit respectively.The O4-O4 and K-K vectorsin the 2M ' layer. biotite structuredo not lie within sucha plane.When Platelet Growth viewed in the (001) projection(Fig. l), the O4-O4 Primary vectoris rotatedcounterclockwise with respectto the Baronnet(1972, 1973,1974), from his studiesof K-K vector. syntheticmicas, proposesthat mica crystalgrowth 6 In most lithium-free trioctahedral micas relative layer rotations Biotite Polytypism of 60o, 180o, and -60o are not permitted becauseof.the presence of-pseudo-trigonal symmetry at the layer interfaces. Unit-Layer Stntctures 1 The vector stacking symbols (Ross et al,1966) for the lM and 2M, polytypes are lMl}l and 2M,1221, respectively. Note that the The structuraldifferences between the unit layers srackingsequences t02l : I0Zl: [20]= P0l,l22l = PTl,andl22l of the lM and2M, biotitesstudied here would sug- - taat DISSIMILARITIES IN COEXISTING 1M AND 2M, BIOTITE 1039 starts as a simple nucleation of one layer upon growth.The exactsequence of unit layerswithin the another.If thereexists in the Ruiz lavabiotite nuclei stepsdefine the polytype.Primary plateletswith unit one unit-layer thick, a secondlayer may nucleate layersin an ordered lM sequencecan developonly upon the first in one of two ways:(l) with a relative the lM[0] form through spiral growth. Primary layer rotation of 0o to initiate a lM stackingse- plateletswith unit layersin an ordered2M, sequence quence,and (2) with a relativelayer rotation of l20o develop by spiral growth polytypes of the form (or - 120')to initiatea2Mrstacking sequence. In the tMl\l, 2M,1221, 3Tc[220]. 5Tcl(22)201, Ruiz samplethese two sequencesappear to be equally (2n+l)Tcl(22)o01,depending upon the number of probable, for about one-third of the crystals ex- unit layerswithin the dislocationsteps and within the aminedare lM and one-third2Mb Oncea stacking primary platelet.ePolytypes of the series(n *l)Zc sequencehas started,we predictthat the atomicand [(22)"0]have not yet beenfound in the Ruiz specimen geometricconstraints at the interlayerinterface will but have been reported in syntheticmuscovites by distort the unit layers in different ways depending Baronnet,Amouric, and Chabot(in preparation). upon whetherthe sequenceis of the lM or 2Mrtype. Polytypesforming from a platelethaving only type Further, we suggestthat theseinterlayer constraints l22l unit layers can form three different polytypic will tend to propagatean orderedsequence of layers, seriesdepending upon whetherthe dislocationstep is producingthe basiclM[0] or 2M,122)stacking ar- 3 mod 3, 4 mod 3, or 5 mod 3 unit layersin height. rangementswithin the primary platelet. These three types of stepsform, respectively,the Occasionally,a unit layer will nucleateon a grow- series3Z[222], (3n + |)MIQ22),01, and (3n+ 2) ing plateletin a "faulted" positionwith respectto the Ml(222)"221.The latter two seriesare represented ordered lM or 2M, sequence,giving rise to unit by the Ruiz polytypes 4M,122201,8M{(222),221, layersof the type [02] or [22]. Continuedordered and I lM,[(222)r22].Another important polytypic crystalgrowth of plateletscontaining only type [02] seriesfound in the Ruizspecimen, as well asin other or l22l unit layerswould produce a 6T10202021or types of micas, has the form (n'12)TcI(0)"221.Ex- 3f1222)polytype. No 6Z mica, nor any relatedto it, ampleshave been reported with n : | , 2,6,7 , 12,and have been reported, but a number of polytypes 2l (Baronnet,1975, Table 7). Such polytypescan closelyrelated to the3?n form&have been found in the originatefrom a primaryplatelet that hasa disloca- Ruiz specimen.It would appear from our present tion step of (n+2) layersand possessesa single knowledgeof the Ruiz biotites,that interlayerstruc- "faulted" unit layer of the [22] type. tural control during the initial stagesof platelet Much more complex polytypes can arise if the growth is very strongfor orderedlM[0] and 2ML[22] primary plateletdoes not possessa simpleordered se- sequences,moderate for the 3TI222l sequence,and quenceof layers.Also, if the exposedupper and weak or nonexistentfor the 6T[020202)sequence. lower stepscontain different stacking sequences, two differentpolytypes can form in one crystal.For ex- Spiral Growth ample,consider a six-layermica platelet with a layer Baronnethas also shownthat mica plateletsonce stackingsequence [02022] and containingupper and formed by nucleationof one layer upon anotherwill lower stepswith the sequences[02] and [Z], respec- often continue their growth by the Frank spiral tively. Spiral growth will then generatethe 3Tcl022l mechanismthrough the introductionof a screwdis- and 3T@l polytypeson the upper and lower sur- location.The screwdislocation perhaps arises by an faces,respectively. Baronnet's theory thus explains edgewiseencounter of dendriticarms of a single one possibleorigin of "coalescence"of polytypes. crystalplatelet. In an eleganttheoretical treatment of micapoly- Summary typism Baronnet (1975) consideredthe relation- l. The crystalstructures of the unit layersof lM ship between:(l) the layersequence of the primary and 2M, biotite are significantlydifferent and this platelet, (2) the layer sequencewithin the spiral differencedoes not appear to be related to any steps which are introduced above and below the chemicaldifferences between polytypes, nor to any plateletby a screwdislocation, and (3) the type of differencesin conditionsof crystallization. polytype generated by subsequentspiral crystal " The upper and lower spiral stepswithin an orderedplatelet, " A 3Tl222lmuscovite has been carefully studied by Glivenand whichare of equalheight and identical structure, generate identical Burnham(1967). Detection of the 3I form is difficult,for its single polytypeson the (001) and (001) faces.The polytypepairs are, crystalX-ray patternis similarto the twinnedlM polytypeand to however,in l20otwinned relationship (twinning abbout [310] or the 37c, form. [310])if the plateletsare 2 mod 2 unirlayersthick. 1040 H. TAKEDA AND M. ROSS

2. The structure of the unit layer of the 2ML - (1973)Sur lesorigines des dislocations vis et desspirales de polytypeis characterizedbya shifting,relative to the croissancedans les micas../. CrystalGrowth, 19' 193-198. - (1975) growth aspectof polymorphism/polytypismin polytype, The unit layerof the lM of the upperand lower syntheticmicas of petrologicalinterest. Fortschr. Mineral. (in triadsof octahedraloxygen atoms as a unit alongthe press). *b directions.This causesa reductionof symmetry - (1975)Growth spiralsand complexpolytypism in micas of the 2M, unit layer from the idealC2/m, as found I-polytypic structure generation.Acta Crystallogr. A31, in the lM form, to CT. 345-355. W. (1966) of absorptioncorrection proposed particular BunNHltvt,C. Computation 3. It is that the structureofa and the significanceof end effect.Am. Mineral.5l' 159-167. unit layer of a polytype is directly related to the -, Y. OunsHr,S. S. Hrrnrn, eNnD. Vtnco(1971) Cation dis- atomic and geometricconstraints imposed on it by tribution and atomic thermal vibrations in an iron-rich the adjacentunit layers,the constraintsvarying with orthopyroxene.Am. Mineral. 56' 850-876. the relativeorientation of the adjacentlayers. Thus, -, ANDE. W. Rloost-ovrcH(1964) Crystal structures of coex- isting muscovite and paragonite. CarnegieInst. Wash. Year the unit layerof lM polytypewith adjacentlayers in Book,63,232. 0o relativeorientation has a structuredifferent from Cnourn, D. T. (1965) Anomalousdispersion corrections com- that of the unit layer of the 2M, polytypewhich has puted from self-consistentfield relativisticDirac-Slater wave adjacentlayers in Ll20o relativeorientation. functions.Acta Crystallogr.lt, l7-23. -, 4. It is further proposedthat onceinterlayer con- ANDJ. B. MANN(1968) X-ray scatteringfactors computed from numericalHartree-Fock wave functions. Acta Crystallogr. straints form between adjacent layers these con- A24,321-324. straintswill tendto controlthe orientationof the next DoNNev,C., J. D. H. DoNNnv,eNo H. Texrpe (1964)Trioc' nucleatedlayer so as to givean orderedstacking se- tahedralone-layer micas. IL Predictionof the structurefrom quence,usually of the lM or 2M, type,more rarely composition and cell dimensions.Acta Crystallogr.17, the 37ntype. I 374-138 I . FrNcrn, L. W. (1969)Determination of cation distributionsby 5. Once a sequenceof layershas formed through least-squaresrefinement of single-crystalX-ray data. Carnegie layer-by-layernucleation within a growing mica Inst. Wash Year Book, 67,216-21'1. platelet,further crystalgrowth oftenoccurs by means Grusrrrrttr, G., eNoC. Teorxt (1972)The crystal structure of 2O of a spiralgrowth mechanism,the polytypicform of brittle mica: anandite(abstr.). Acta Crystallogr.A2E' 570-71. the final crystalbeing controlled by the sequenceof GUvrN, N., ANDC. W. BunNHlu (1967)The crystalstructure of muscovite.Z. Kistallogr. 125, 163-183. platelet 3T layerswithin the primary and within the dis- Joswlc, W. (1972)Neutronenbeugungsmessungen an einem lM- locationstep. phlogopit. NeuesJahrb. Mineral. Monatsh. ln2, L-lL. RADosLovIcH,E. W. (1960) The structure of muscovite, KAl,(SisAl)Or0 (OH)2. A cta Crystallog r. 13, 9 L9-932. Acknowledgments Ross, M., H. Trrroe, eNo D. R. WoNrs (1966)Mica polytypes: Systematicdescription and identification. Science, l5l' 19l-193. We wish to thank Dr. David R. Wones (U. S. Geological RoTHBAUER,VoN R. (1971)Untersuchung eines 2Mr-Muskovits polytypes, Survey) for his collaboration in studies of the mica Mr. mit neutronenstrahlen.Neues Jahrb. Mineral. Monatsh. 1911, (U.S.G.S.) L. B. Wiggins for making the electron microprobe 143-154. analyses,Dr. Robin Brett (U.S.G.S.), Professor R. Sadanaga,and TAKEDA,H. (1971)An improvementon the geometricalmodel (University given Professor Y. Tak€uchi of Tokyo) for the interest structureof trioctahedralmicas (abstr.), Geol. Soc. Am. Abstr. work, us during the course of this and Dr..Alain Baronnet for fur- Programs,3, 727-728. nishing us with preprints of his mica studies. -, ANDC. W. Bumtsrt"t(1969) Fluor-polylithionite: a lithium performed The computations were both at the computer center mica with nearly hexagonal(SLO'f- ing. Mineral. J. 6, of the NASA Manned Spacecraft Center and at the Computer 102-109. work was carried Center of the University of Tokyo. Part of the -, N. Hlcl, luo R. SlolNnce (1971)Structural investiga- while was out at the NASA Manned SpacecraftCenter H. Takeda tion of polymorphictransition between 2Mz-, lM' lepidolite, supported by a National Research Coqncil Senior Resident and 2M, muscovite.Mineral. J. 6,203-215. Associateship. T,rr6ucHr,Y. (1964)Structures of brittle micas.Proc. Nat.Conf. Clays Clay Minerals, 13, l-25. properties biotiteon the References WoNss,D. R. (1963)Physical of synthetic join phlogopite-annite.Am. Mineral.,48, 1300-1321. Brnormr, A. (1972)Growth mechanisms and polytypism in syn- thetic hydroxyl-bearing phlogopite. Am. Mineral., 26, Manuscript receiued,August 13, 1974;accepted t2'72-t293. for publication,June 23, 1975.