COMPOSITIONAI.VARIATIONS IN COOKEITE
PETR dERN{. Departunentof Earth Sciences,Unfuersity of Mani,toba,W'inn'i,peg, Manitobar
AssrRAcr Assuming an ideal anionic composition, the lithium-aluminum chlorite cookeite shows a fairly constant tetrahedral population AlzSio; subordinate B and Be may occasionally substitute for Al. Some Na, Ca, Mg, and K, FeP+can play the role of octahedral cations; the large cations may be accommodated between the 2:L sandwiches and "brucite" sheets. The main octahedral substitution is 3(Li, alk.)1+/Al8+, probably extending up to Li-low and Li-free donbassite and sudoite, and that involving the bivalent cations is 8R2+/2418+.Some fluorine substitutes frequently for OH. There seemsto be no need for the name tthydrocookeite".
INrnonucrrom Cookeite, a di/trioctahedral Li,Al-chlorite with idealized formula LirAle[AlrSieOzo(OH)ro],is a fairly frequent constituentof many lithium- bearing pegmatites and has been encounteredalso in other parageneses. Unfortunately, in most instancesits mode of occurrencedoes not permit thorough investigation. The separation of pure cookeite for chemical analysis is particularly difficult becauseits flakes are often intimately intergrown with other minerals. Seventeen years ago, Ginzburg (1953) was the first author to draw attention to the variations in the chemical composition of cookeite, particularly in its octahedral occupancy. However, he could discussonly two new analysesof his own and five analysestaken from literature, some of them of doubtful quality. Since that time much more information has accumulated. The crystallochemical evaluation of these and earlier data is presentedhere.
Averr.Enr,BDera ano TnBrn TnBaruBNr Chemical analysesof 16 cookeites,and one of the boron-rich manando- nite. have been collected from the available literature. Fourteen of them are used in this study, and three are rejected.Penfield's (1893) analysis indicates that the compositionof the Hebron, Maine cookeiteis more complicated than suggestedby Brush's data (1866); of two analyses
rPermanent address: Geological Institute, Czechoslovak Academy of Sciences, Prague, Czechoslovakia. COMPOSITIONAL VARIATIONS IN COOKEITE 637
published by Quensel(1,937, 1956) one refers to a cookeite contaminated with quartz, and the correction of the other for a high quartz admixture does not seem to be reliable. Some of the other analysesused here are quite old, and are questionable as to the accuracy of the lithium and aluminum determinations. However, they compare well with modern d,ataand their potential faults could not be excessive. This prescreeningof available chemicalanalyses alryays tends to be sub- jective, and the recalculationsdescribed below are inevitably basedon a seriesof assumptions.Hence all analysespublished to date are presented in Table 1, to give the reader a chance to draw his own conclusions. In this,study, the chemical analyseswere recalculated to atoms per LJayer unit cell, on the basisof 56 cationic valencesrequired by the theo- retical Ozo(OH)rscontent (Foster Lg62). For obvious crystallochemical reasons,beryllium and boron were coupled with silicon (cf strunz 1957)' and, enough aluminum was added to bring the total number of cations in the tetrahedral group to 8. The remainder of the aluminum and all other cations were allocated to the octahedralsites, more or lessrigorously as discussedbelow. Trivalent iron was placed in this group deliberately; its possible tetrahedral coordination would not influence the conclusions derived here. The alkali and alkaline earth elements are assumed to belong to cookgite itself, and no attempts were made to deduct them in some fashion since there is no sound basis for doing so. The ionic compositionsof 13 cookeites,arranged according to decreas- ing lithium content, are listed in Table 2; manandonite is also shown for comparison. Some important relations among the octahedral cations are illustrated in Figs. L and 2.
DrscussroN T etrah ed,r al' sub s t'itw t'i qn s The formulae calculated by Ginzburg (1953) show considerablevari- ation in the Si/Al ratio. However, this is due to an evidently erroneous formula of the Lipovka cookeite (with very low total cationic chargeand low Si content), and to the Varutriisk cookeite contaminated by quartz (rf Quensel1956). The present results show that the silicon content of the idealized for- mula, Li2Als[Al2Si6Ozo(OH)ro],is closely approached in all natural cooke- ites. The mean value is 6.08, witir variation range 5.86-6.33. Conse- quently, the AFv-content is also rather constant, but can be considerably decreasedby boron andfor beryllium substitution. The cookeites from Muiane and Radkovice show considerablecontent of these elements. 'sum The of tetrahedral (Al * Be * B) is close to 2 in both cases, 638 THE CANADIAN MINERALoGIST
Tesr,s 1. Cmurcer, ANar_vsasor.CoorBrtp
sio2 34.00 34.81 33.31 34.08 34.15 33.40 Tio: 35.49 U.92 tr. 0.019 .:.- AlrOa 45.06 45.90 M.L6 45.30 46.35 +I .+l 44.36 46.29 FerOg 015 0.72 r.72 0.L92 o.02 0.00 0.r2 0.07 BzOa 0.38 FeO 1.69 o.7r 0.24 MnO tr' 0.18 tr. tr. tr. 0.13 CaO 0.04 tr. 0.00 0.57 0.16 0.45 0.26 0.30 Mgo o.37 L.02 t? 0.20 L.32 0.24 BeO 1.06 LirO 4.02 e^5s 3.40 3.09 3.18 3.12 2.74 2.67 NazO 0.19l 0.70 0.07 o.52 0.10] 0.09 t.Lz 0.L4 KzO o.L4J 0.67 tr. rr. ) o.L4 HrO+I 14.96 14.87 13.19 L4.22 14.06 14.98 13.84 I4.L2 HzO- l 0.46 0.38 0.42 0.23 0.55 0.28 F 0.46 0.13 0.00 0.35 total 95.30 100.& 99.85 98.99S 100.30 100.65 100.28 99.05 -O :2F 0.19 0.06 0.15 95.09 100.48 100.15 1. Cookeite Hebron, Maine, U.S.A. (penfield 18g4); in pegmatite cavities. 3. 9f!9',: Buckfield,l4a.ine, u.S.4.-(L;;'d;-16rbi;--;e;;iA;iiil*luartz and . Tinorapatite,terderitein;usf";it";'c"iir,l" ;6effi;"#ffi.in'ii"r'ri7riquia". 4. uookerte northwestern u.s.s.R. (zvyagin & Nefedov lg54); replaced, aird is associatedwith,_coarse-flakd leriidoiiG in pegmatite; no admiliure" in the analyzed material. 5. cookeite Dobr5 Voda, czechoslovakia (eernf et at,, lgzo); cavity linins in pesma- trte' assocrated_withquartz, apatite, tourmaline, and cassiteritejcontamindtion of ^ ?! lyr{ sample lower tha!-L/6; t!.re,total includes 0.00b GazOs,0.052 NiO. 6. uookerte,Jvluiane, Mozambique (Sahama-et al. 1968); cavity lining in pegmatite _ yrq n9grlg_rD.le.quartz;no admixtures in the analyzed material. 7. Cookeite Kalbinski Ranger_U_.Q.Q.R.(Ginzburg 10$); from pegmatite. 8. CookeiteLipovka, Urals, U.S.S.R. (Ginzburgigb3) ;'from p.jgl;rti6.-' 9. Cookeite Manono, Katinga (Heniran a it.-1s();-i;;fi-'p."J;;ite; contains 0,00570P206, indicating that the boron and beryllium can enter the aluminum sites more easily than those of silicon. The samerelations have beenrecognized also in other beryllium-bearingsilicates (bityite-Ginzburg 19b7, Beus 1960, bavenite-Switzer & Reichen 1960, Beus 1g60, Berry 1g68; cordierite-eern!' & Povondra 1g66) and in borosilicates (e.g. reed- mergnerite-Milton et al'. rg6o, synthetic boron-gehlenite-Bauer 1962). The charge balance in the Muiane cookeite can be maintained by the [AlOa]-6/[BeOa(OH;1-rsubstitution, advocatedby Beus (1900) particu- larly for sheet silicates. However, this substitution can be proved only if accurate water determinations are possible, and this is usually not the casewith chlorite-like minerals.The high content of octahedral aluminum (Table 2, No. 6; Figs. 1 and 2) suggeststhat it may compensatefor the COMPOSITIONALVARIATIONS IN COOKEITE 639
Tenlp I (Continued')
10 11 12 13 L4 15 16 t7 18 u.7 34.68 34.72 32.00 25.20 35.20 35.25 38.22 U.52 0.03 0.02 48.4 48.r7 43.56 45.87 47.80 44.91 42.58 43.20 48.82 'j 0.29 o.20 tr. o.25 0.08 1.66 9.25 0.90 0.70 0.07 0.10 0.06 0.03 o.L2 0.58 1.63 0.51 0.36 0.05 : 0.83 0.78 0.59 0.M 2.45 2.45 2.28 2.ro 3.97 2-.82 0.80 4.33 2.86 0.01 0.04 0.07 0.65 0.48 0.00 0.00 0.06 0.20 2.57 1.48 0.42 13.44 L4.2t) 13.84 L2.46 13.80 14.1 , L7.25 14.10'II13.4L t30 'J ' 0.38 3.59 0.16 o.o2 0.34 0.34 0.33
99.93 100.00 100.00 100.40 100.22 99.63 100.00 99.84 100.00 0.01 0.15 0.15 0.14 100.39 99.48 99.85 99.70 10. Cookeite North Little Rock, Arkansas, U.S.A. (Miser & Milton 1964); in pegtna' tt. 3i:*n'ffi1rlr:.:'xtn*'*ni;::'fffi"#: (Brammaretar'. re*e); in gord- b"a.i-r,e ot-"- i"ii* ;- il-u'1,- tt.*, S.bZn' aolomite deducted from the original u.'-' ?]i:f; Radkovice, czechoslovakia (eernf et aI. r9z0); -replacesIepidotite in of the ;"gr;;tii"l-it" i" ttt" €mple deducted on the basis k;-o, N;ro, "a*iit"i"r'; itu tot"t-contain!"".tv^d 0.0Lt7o'Gazoa, 0.052 Geoz, 0'016 Crzor, 0.042 Nio. ""d 18. C;d6-Wait-a-bit Creek, B.C., Canada (Hoffman 1895); in cavities of quartz veins. l . i\iar[ndonite, Sahatany Valley, Madagascar (Lacroix L922); replacesrubellite in pesmatite. 15. b;%k-;i;; Hebron, Maine, U.S.A. (Brush 1866); cavitv lining in pegmatite' i6: a;[;i;; V"iutra"t,Swtja"n iOu,]n.el 1937);'riplace3spodumenein pegmatite; 52.4V" of.quartz deducted from the analyzed sample. fZ.-" b-."i!it" V"irt.a.[, S.'eden (Qu"n""l fsgZ, fgSOi; pseudomorpho-usajter rubel- lit"!;;;L-i""t"a *itfti;.t""tiionte11 gf-gqirtz-^-;iontains 0.rrToPrOu,O'037oCl' 18. Id;l compositionof cookeiteLizAl8[Alesi60ro(OH) to]' grven rn The original ahalysis in Norrish (1952) was not availabl:; atomic Potti:It$ fabte Zi No. 2, w'ererecalculated'from those quoted by Radoslovich(1962)'
heterovalent tetrahedral substitution-a mechanism somewhat similar to the mutual AlYr/Alrv equilibration in magnesiumchlorites' The fairly constant (Al, B, Be)Iv/Si ratio is somewhat surprising in a chlorite mineral, but it seemsto be well supported by all the thirteen analyses available. It may indicate that a particular type of si, Al- ordeiing is preferred in the Li, Al-chlorite structure. Actually, the stack- ing sequenc"of layers in the Ia chlorite polytype, typical of cookeite, is well suited to local charge balance between ordered Si, Al-sheets and ordered,,brucite" sheets (Lister & Bailey 1967, p. 1630; an ordered 640 THE CANADIAN MINERA]-OGIST
6tmo: doo
8*TE8E8N=85:8R co (o (o ro co tQ ro co eo co (o € ro 5bs8s$3pdEE$J3 H i H Cil H i H 6l i i d i c\ i O o FI C\HH6 ioro@ z a 11 ooci6i o ? !1 t ci 2 o F n X g sffi$R8gNR$88$=R @ @ 6t n 9.292 z (De o f cg Ol6O6a iHd+s =n E;iE; {6 3 ^X !O t'- --: F o.HO.r E33R83p8S388FR lqHC oa .{> E 'i!s.c b- r'. r'- r.- t- € f'- l.- t- co 0o t- f.- @ 8.5 I k .t- c, I 9l E 8.o ,\t t-^{ -"'r- -:7 ! 9.7 7,5 E.t f r, a o , lr E + td a 3.0 2,5 2.O t.5 LI Li Frc. 1. Lithium content of cookeite plotted against the total Rn, total Rn-Li, total RYr-Rrl and (Al * Fsr+)lT contents, in atoms per 1-layer unit cell. Full lines follow the theoretical 3lil+/(Al, Fe)8+substitution; the plot of ttre composition LizAle[AlzSio- Ozo(OH)ro]is marked by crossbar on these lines. The dashed lines separate specimens with very high contents of Na, K. l-layer Cr-chlorite displaying a regular Ia sequenceshows the sameAlzSis tetrahedral content, Brown & Bailey 1963). However, the mere existence of a very closelyrelated manandonite vr'arnsagainst overemphasizingthis fact: manandoniteis usually idealizedas Li2Als[AlzBzSiOzo(OH)16]and its actual tetrahedralpopulation is Alr.orBr.osSia.zo. Octak e d,r al, sub s titut'i, ou s In the idealizedformula of cookeiteLisAls[AlzSioOzo(OH)ro], Brammal et al,. (L937), Zvyagin & Nefedov (1954), and Strunz (1966) allocated Ala to a dioctahedral layer within the 2:1 sandwich, and Alaliz to a triocta- hedral interlayer "brucite" sheet. However, Norrish (in Eggleton & Bailey L967) assignedAla.rolio.72 and Alalis to these respective layers, and the octahedral occupancy derived here from chemical analyses shows broad variation between 10.70 and 9.80. This indicates that the distribution of cations between the two octahedral layers is not so simple as indicated earlier; for thesereasons the octahedral population is treated here only as a whole group. Generally, the octahedral occupancy is close to 10, as required in a di/trioctahedral "chlorite. However, the highest octahedral contents of 6+2 THE CANADIAN MINERALOGIST + d ; E E n + E a. + o o l! o TL t + a 2.5 2.O t.5 3.5 3.O 2,5 2.O Rlf ( R'*+ 2Rt*) Frc. 2. Total alkali content (left side) and total alkali * doubled alkaline earth con- tent (right side) of cookeite plotted against the total Rn and (Al * Fer+;vr contents, in atoms per lJayer unit cell. Full lines follow the theoretical 3(Li, Na, K)t+/(Al, Fe)a+ substitution in all four plots; the composition LirAle[AlzSieOro(OH)ru]is marked by crossbar on these lines. The dashed line separates specimens with high contents of Ca, Mg, Fe9+. 10.70 and 10.62 deviate considerably. Besides this, the single manando- nite analyzed to date shows the octahedral occupancy of 11.23. This sug- gests that broader variations might occur in cookeite, particularly in connection with possible changes in the Si/Al ratio. The fairly constant tetrahedral charge (Alr5iu;+ao recognized in the previous paragraph suggests that-ideally-all substitutions within the octahedral sheets should maintain the total charge of *26, The mutual relations among the octahedral cations comply with this requirement. The total octahedral occupancy decreases rapidly, and the AlYr-content increases slowly, with the decrease in lithium (Table 2), indicating the substitution 3Li1VAl3+. This mechanism preserves the charge balance and seems to be not restricted by space limitations, at least within the observed range of the octahedral occupancy 10.70-9.80. The four plots illustrated in Fig. 1 show that most analyzed specimens follow closely the relations expected from the 3Li1+/Al3+ substitution. There are, however, important exceptions, namely, the cookeites with high Na, K, Ca, and Mg contents. Some doubts may arise about the COMPOSITIONAL VARIATIONS IN COOKtrITE 643 purity of these specimens,but at least some of them are reported as free of u"V mineral admixture (No. 4, 5, 12 in Tables L, 2). Magnesium(and Fe2+) can undoubtedly enter the Li,Al-chlorite structure quite easily. However, this is not the casewith the large cations, and a question arises about their accommodation. Belov (1950) suggested that small amounts of calcium, frequently found in Mg,Fe-chlorites, may occupy the octahedral sites between the 2:L units and "brucite" sheets,adjacent to the vacant positionsin the latter. The same would apply also to sodium and potassium. However, the Io lJayer polytype typical of cookeite shows a superposition of tetrahedral and octahedral cation right around this proposed"intersheet" octahedralsite (Brown & Bailey 1963,Lister & Bailey 1967), and the closeproximity of tetrahedral cation would seemto inhibit the occupation of this position even if the regular octahedral site in the "brucite" sheet is vacated. Besides this position proposed by Belov (1950), there is another "intersheet" site in the Ia structure, suitable for accommodationof large cations. The cavities between the six-sided openings among the tetra- hedral basesand, "brucite" cations facing them have an irregular 9-fold (O, OH) coordination(cf Brown & Bailey 1963,Fig. 3; Lister & Bailey 1967,Fig. 6b). They can be undoubtedly easily enteredby Ca, Na, K substituting for the adjacent regular "brucite"-sheet cation, the site of which becomesvacated. Besidesthis possibility, it seemsprobable that structural imperfections in natural crystals would enable small amounts of the large cations to enter even those sites that are highly unfavourable in ideal siructures, i,e.,regular octahedral positions and the "intersheet" sites proposed by Belov. Thus the small Na, Ca, K-contents found in cookeite have a good chanceto be incorporated in its structure, and the mode of their substitution for Li and Al should be found. In Fig. 1, the alkali-rich specimensshow very high totalRu andRE-Li contents, and very low AIE figures, as comparedwith the other cookeites concentrated along the theoretical 3Li1+/Al3+ line. This suggests that large cations play the same role in the charge relations as lithium. Actu- ally, the left side of Fig. 2 indicates that all specimenscan be satisfactorily correlated with the 3R1+/Al3+substitution. In the left part of Fig. 2, the samples falling appreciably above the theoretical line in the total Rw plot, and much below that in the (Al + Fe'+)* plot contain considerable(Ca,Mg,Fd+). Since the sub- stitution 3R1+/Al3+seems to be the principal one' a 3R2+/2A18+relation is more probable for bivalent cations than any other involving alkalies (4.g. RL++ Ilz+/Als+).The right side of the Fig. 2 shows the octahedral rela- tions with R2+-contentsdoubled, to simulate minerals with all bivalent 644 THE cANADIAN MINERALoGIST cations substituted by alkalies. The almost perfect agreement of the plotted points with the theoretical BR'+/A1B+line suggeststhat the above assumption is correct, and that the bivalent cations substitute for AIE in the ratio 3P.+f2Ala+. Actually, major irregularities in the right side of Fig. 2 are causedby the variations in the si/Alrv ratio. specimens with si < 6 prot on the upper side of the theoretical line and those with si ) 6 on the lower side, compensatingthe slightly varying tetrahedral charge. The substitutional series of cookeite seemsto be extended to much lower (Rt+'r+/AuYr ratios in some of the donbassites(Lazarenko 1g6g, Drits & Lazarenko 1967, Drits & Alexandrova 196g), and sudoites ,,end-member', k.C,. Miiller 1963). Their idealized composition is Als.6?[Al2Si6Oro(OH)ro],displaying the sametetrahedral Si/Al ratio as in cookeite. Low percentagesof FezOa,MgO, CaO, NazO are usual, the Li2o content reachesup to 0.78 wt.7o, and the recalculatedanalyses plot very well along the right-side extensionof the theoretical lines in rig. z. The r-ray powder patterns of some donbassitesagree well with those of cookeite(Dr. s. w. Bailey, personalcommunication 196g). If the struc- tural identity is proved-and there seemsto be little doubt about it in some cases-the existence of donbassite would be an additional proof for the cookeitesubstitutional seriesproposed here, and Li-poor cookeites approaching dioctahedral composition could be expectedto occur also in pegmatites.Maybe this is the caseof the Li-poor cookeitefrom varutrdsk (anal. No. 16 in Table 1). Hyd,roxyl, and.fluorin e The difficulties encounteredin the determination of water in micaceous minerals are well known and are the main reasonfor using the theoretical anionic charge as the basis of crystallochemical calculations (c/. Foster 1962). However, the low fluorine content of somecookeites and the term "hydrocookeite" (Ginzburg 1g5B)require at least brief comment. In contrast to the Mg,Fe-chlorites, chemical analysesof cookeite quite frequently show some fluorine. Besidesspecimen Nos. L, 3, 6, and 18 recalculated in Table 2, fluorine was reported also by Brush (Is66) and Quensel(1937, 1956) (Nos. 15, 10, L7 in Table 1). At least the Muiane cookeite (Sahama et al,. 7968) and the quartz-contaminated specimen retriortedby Quenselwere quite pure, without any admixture of mica or other fluorine-rich phases.Thus the ability of the cookeite structure to incorporate fluorine seemsto be proved. The name "hydrocookeite" was applied by Ginzburg (lgbg) to the cookeite froiir Lipovka (and other unspecified samples) which shows a COMPOSITIONAL VARIATIONS IN COOKEITE 645 slight low-temperature dehydration efiect at around 150oC, refractive indic"s and birefringence lower than other "normal" cookeites, and slightly higher d-spacingsin x-ray powder records.Strangely enough,the Lipovka,,hydrocookeite" seemsto contain a quite normal H2O percentage lGbtes I and 2, No. 8), and there is no sound reasonfor referring to the ii-poor Varutriisk cookeite (Quensel 1937) as an altered mineral, as Ginsburg does. All properties given as characteristic for "hydrocookeite" by Ginzburg can be explainedin other ways,without assumingchanges in"hydration." Thus there seemsto be no need for the term "hydrocookeite" at present. No crystallochemical basis for such a phase was found or proposed,and there is no mineral known to date which showssuch a departure from the usual range of the HgO contents as to require special classification. Coucr-usrous Much more work remains to be done on cookeite but any progress dependsmore on better laboratory techniquesrather than on accumulat- ing data in the present way. Chemical analysesyielding reliable informa- tion about anionic water, and methodsaimed at identification of adsorbed cations and minor admixed phasescould eliminate most assumptionsthat are necessarytoday. As for the present, the available chemical analysesshow that cookeite has a fairly constant tetrahedral population Al$io, with Be and B substituting occasionallyfor Al. Octahedral occupancyvaries from 9.8 to L0.7, concentrating around but deviating substantially from the strict di/trioctahedral figure of 10. The existenceof manandonite shows that even larger variability in both the Si/AFv ratio and in the octahedral o..upun"y may be encountered,although the AlzSio tetrahedral popula- tion seemsto be preferredby the prevailing lJayer Ia structural polytype. BesidesLi and Al, appreciableamounts of Na (*K), Ca, Mg, (+Fd+)' and possibly Fea+can play the role of octahedral cations. The vacant b"t*"en the 2:L sandwichesand "brucite" sheetsseems to be suit- "pu""ubl" to accommodate the large cations, as suggestedby Belov (1950). With the tetrahedral content AlzSionearly constant, the main octahedral substitution is that of 3 (Li ) Na >> K)l+/(Al ) Fe)8+, occasionally modified by the introduction of 3 (Ca,Mg ) Fe)2+ for 2 (Al ) Fe)0+' The substitution 3lil+/Ala+ requires considerable space for lithium accommodation. This is provided in the dioctahedral sheet in cookeite, and in vacant spaceof eucryptite-the only example of this substitution known to date (Mrose 1953). 646 TIIE CANADIAN MINERALoGIsT Taking into account fluorine which may substitute to some extent for the (oH) groups' the generalformula of cookeitemay be written as (Li,Na )) K)r+a,(Ca,Mg)) Fd+)B/(Al )) Fes+)s_,_2r1, X [(Al ) B,Be,)2SiuOro(OH) F)ru], -0.15 wherer: to *0.30, !:0 to *0.18, and,z:0 to +0.44. The cookeite substitutional seriesseems to be extended up to the di- octahedralAl-end member in donbassiteand sudoite, with r : -0.66, y and z : 0. Further chemical analysesmay broaden the variations even in true pegmatitic cookeite,and the detailed studies carried out at the university of wisconsin (Lister..lg66;Eggleton & Bailey Lg67;Lister & Bailey 1967) will undoubtedly clarify the structural relations among these minerals. AcrNorunocMENTS This study was carried out in the Department of Earth sciences, university of Manitoba, under a fellowship supported by this university and N.R.c. researchgrants to Drs. R. B. Fergusonand A. c. Turnock. The writer is most grateful to Dr. R. B. Ferguson and Dr. s. w. Bailey (Dept. Geology,univ. of wisconsin) for critical reading of the manuscript and for help in collecting some of the data. RBn'BnBNcns Beupn, H. (1962): Uber Diadochie zwischen Aluminium und Bor in Gehlenit, N. Jahrb. M,imer.,Mh. L27-L40. Brr,o1,_N. V. (1950): Research in the field of structural mineralogy, Mineral,. sbornih L.G.O. Ltsot, 4,2L-34 (in Russian). Bennv,.L, G. (1963): The compositionof bavenite, Arner. Mineral,.,4g, 1166_116g. Bsus., A.- A. (1960): Geochemistry.ofberyll,iwn and, genetic types oj beiyil..iumil.eposits, Acad. Sci. U.S.S.R.Moscow (in Russian). Bnemral., A-, LEecH, J. G, c. & Ba'xrsran, F, A. (1gBZ): The paragenesisof cookeite and hydromuscovite associated with gold at cigofau, cannartien"hire, Mineral,. Mag.,24, 507-520. Bnowu, P. E. & Bur.ev, S. W. (1968): Chlorite polytypism:- - II. Crystal structure of a _ one-layer Cr-chlorite, Arner. Mineral.., 49,424L. Bnusn, G. J. (1866):Amer. Jour. sci.,41,246'(quoted in Dana's system of Mineralogy, , L94l,6thedition,62b7. cann_!, P' & Povorone, P. (1966): Beryllian cordierite from vEZn6: (Na,K) * Be - - Al, N.Jahrb. M,ineral,.Mh.,BH4.' CenNf_, P., Povo*ona, P. & Sre*ir, J. (1920): Two cookeites from Czechoslovakia; * a b-o_ron-richvariety and a IIb polytype (in print). DnIrs, V. A' & AlpxeNrnova, V. a. (trioa):'stiucture of a mineral of the donbassite group-dioctahedral chlorite from Novaya zemlya, M'iner. sbornik Lztot.stnte unit. 22, t62-r67. Dnrts, v. A. & Lazenonro, E. K. (1967): Structural and mineralogical characteristics of donbassites, Miner. Sborn'ih Luou, State Unh. 21, 4Mg. 647 COMPOSITIONAL VARIATIONS IN COOKEITE chlorite' Eccr-sroN, R. A. & Benev, s. w. (1967): Structural aspects of dioctahedral Amer, Mineral,.' 52, 673-689. of the Fomnn, M. D. (1962), i"t"rp.ut ti"n of the composition and a classification chlorites, U.S,Geol'.Santey Prof. Paper 414'A, A1-A33' Sci. A.S.S.R. 90' GrNzsunc, A. I. (lgb3): On lithium chltrite-cookeite, Trans. Acad., 871-874 (in Russian). Grrzsunc,e.l.trsrz),Bityite-alithium.berylliummargarite,Trud'yMinero]', Museum Acait'. Sci,,U.S.S.R., 8, 12&131 (in Russian)' a I'dtude des HERuAN, P., VlNnrnsleu"t*, d. & Hn"e,ti, A. (1961): -C^ontribution miniraux congolais, Ann. Soc.Geol'. Bel'gi'gue,84,297-309' ^^ . (quoted in Brammal Horruerr, C. C.'ffASSii inn. ir4t. Geol'.-sltroeyCanail'a,6, 22 et al',, L937). Lncno*,'e. Qilzz)z Mi,nArahgia d.eMad'agascar, Vol, I, Paris' of central Maine, ienoes,'f. f. (fbZg), itt" irtuguo*is"of the granite pegmatites Amer, M'i,neral'.10, 35H11. Miner. Soc, Lezenpwro, B. K. (1969): On donbassites and sudoites. Mem. Al'l-un'ion 9E, No.3' 318-320. chlorites, Ph.D. Thesi's, Unht. of Lr5ar", l. S.'tiSOOlr The crystal structure,of.two W'isions'in (qttoted in Lister & Bailey, 1967). polytypism: IV. Regular two-layer LrsTER, J. S. &'Ban-ov, s. w. (1967): chlorite struitures, Amer. Mineral'., 52' 1614-1631. Mn-roN, C., Cueo, E. C. T., Axunoo, J. M. & Gmller'or, F' S' (1960):Reedmergnerite' Utah, NaitSirOr, the boron anrlogue oi-albite, from the Green River formation, Amer. Mineral'., 45, 188-199. Mrson,-d;;;, H. D. & Mir-ton, C. (196a): Qoartz, rectorite, and cookeite-lrom-the Jeffrey near North i_ittl" Ro"L, Fubski County, Arkansas, Bul,l,.Arkansas Geol'. Commiss'i'on,No. 21' 29 P.P. 353. Mnosn, M. E. (ig53): Alpha_eucryptite problem_(abstr.), A.rner.M4neral...38, (Sudoit- Mtrrir'*,- G. (rsos):'Zui'run"t"i" aiokiaedrischer Vierschicht-Phyllosilikate Reihe dei Sudoit_chlorit-Gruppe), proc. Int. clay conf , stockkol,m,I,_121-130. Un'i'v-Lond'on No*o;;, K. (1952): The crystai of cookeite, Ph.D. Thesi's, "tiucture (quoted in Radoslovich' 1962). prnrr'er-o, S. L. (1898): Aier. Jiur. Sc7,.,45,393(quoted in Dana's System of Miner- alogy,6th ed., 1914,APP' I, 19). Geol',Foren. i. Stockhol.mForh., 59' O**i,fi'p. ifbiil r O,' ittL'o""urrun"u of cookeite, 262. a review of OuBNi"r,, P. (1956): The paragenesisof the Varutriisk pegmatite' including its mineral assemblage,Ark&t Miner. Geol'.,2,Vl25' silicates Raoosr-ovtcu, E. W. (fSdZj: The cell dimensions and symmetry of layer-lattice -636' I I. Regre*sion relations, Arner, Mineral'., 47, 617 the Muiane Seneua, G.C., X"o*o*o, O.u. t I-r"trNEN, i\{. (1968): Cookeite from pegmatite' Zambezia, Mozambique, Lilhos-,-l, I2-L9' Ital'i'ana, Srsr;z-, H. (19b2): So.iJB"tfti"t" i" Phyliosilikaten, Rend,.Soc. M'i'neral'. 19,372, Sr*n*i, it.'(r966): M'i,neral'ogi,scheTabel'l,en, 4th-ed.,.Aka$e11e;Verl' l:ip?ig' identification SwrrzsR, G.'& RercssN, i. i. tiSOOl, Re-examination of pilinite and its with bavenite, Amer. M'ineral,., 45,757162. I/.'S.S.R.,95' Zwncr*, B. B. & Nens;o" E. i. ifSSa), On cookeite, Trans. Acail'. Scd. 1305-1308 (in Russian). Manuscri'ptreceiiled, January 99, 1970,erne'nded APri'l' 8, 1970