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.