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CRYSTAL CHEMISTRY of MILARITE-TYPE MINERALS Wennpx C

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CRYSTAL CHEMISTRY of MILARITE-TYPE MINERALS Wennpx C American Mineralogist Vol. 57, pp. 463 472 (1972) CRYSTAL CHEMISTRY OF MILARITE-TYPE MINERALS WennpxC. Fonens,Wnnwpn H. Blun. ANDArJAZ A. KneN. Departmentof Geological,Sciences Uniuersity of llLinoisat Chicogo Chicag o, Illinois 60680 AesrBecr Milarite-type minerals have the general formula a2l6lBzlel c trzlDIl81 ?(2)3t41 ?( 1)12Irl030, where the , -position can be occupied by Mg, Ca, Fe, Ti, Zr; lhe B-position by K, Na, Mg, IIzO (or HaO); the C-position by K, Na, Ca, Bal the D-position by HrO; the ?(2)-position by Al, Be, Mg, Fe, Li; and the ?(1)-position by Si and Al; the coordination number is given in bracketed superscripts. The B-position which pre- viously had not been recognized as being occupied in milarite-type minerals is g- coordinated with three short and six longer bonds to the surrounding oxygen atoms. This position must be occupied in all milarite-ty-pe compounds in which the number of non-tetrahedral cations and/or H2o, relative to the 15 tetrahedral cations, is greater than three. If this number is greater than five (as in armenite) the 18-co- ordinated sites (in 0, 0, 0) in the cenier of the Si*Oaodouble hexagonal ring has to be ,,Ca-cordierite', occupied. In yagiite, merrihueite, roedderite, and part of the Mg (or Fe2+) must be in tetrahedral coordination in the ?(2)-sites. The different groups of milarite-type minerals can be formally derived from a proto-milarite, 2MgO. 2A1rOs.11SiO,or MgzAleSirrAlOso,by simple cationic substitutions which alter the Al/Si ratio and which progressively fill the B- and C-positions, INrnolucrroN Milarite was found first in the Val Giuf, Graubiinden,Switzer- Iand and was describedby Kenngott (1870).Since then severalother minerals isostructural with milarite have been characLerized(see Table 2). Milarite group minerals crystallize in space group p6/m 2/c 2;/cwith approximatecell pararnetersof a = 10 A and c = 14 L. The crystal-chemicalformula of the milarite-type mineralsas deduced by Ito et al. (1952) and adopted by many subsequentworkers is ArtetgttztTt12) st4)T (l) 12t'lOao'HzO',where the coordinationnumbers of the cations are indicated as superscriptsin square brackets.The most prominentfeature of the structureis a hexagonaldouble ring of silicate tetrahedra,of compositionSirzOso. These rings are joined by tetrahedrally coordinatedcations in sites T(2) into a three-dimen- sional tetrahedral framework and further connectedby octahedrally coordinatedcations in the .A-sites.The doublerings are stackedin the c-directionthus forming the l2-coordinatedc-site betweenthem. The water moleculesfound in somemilarite-type minerals have been as- 463 464 TORBES, BAUR, AND KHAN sumed to be located inside the double hexagonal rings in large voids, with 18-coordination. The synthetic compound KzMg;SirzOgo (Roedder, 1951) contains twice as much potassium as can be accommodated in the C-sites. A structure determination of K2MgSirzOao (Khan et 'a1.,l97I) has shown that this excess potassium is located in 9-coordinated sites (with three short K-O bonds, and six longer ones) which previously had not, been known to be occupied in milarite-type structures. The general formula for milarite group minerals is therefore: /!1161g"te| gtt" Dt'"] T725""1?11;r,InI0,o. Thus, the six different sites (see Table 1) may contain up to 21 cat- ions and/or water molecules per 30 oxygen atoms in the formula unit. Since the radii of the cations which can be accommodated in these different sites range from 0.4 A (for Sin-).up to 1.7 A (for K. and Ba2*), and since the Mg and Fe atoms can enter into both tetrahedral and octahedral coordinations (Khan et al., 7971) the chemical varia- bility of milarite-type minerals is large. Cnvsrer, Cunursrnv In Table 2 we show. all known species of the milarite group with the cations and water molecules distributed among the six positions describedabove. Included in this list'are two "calcium-rich cordier- ites", mentioned by Miyashiro (1956) as being probably of the milarite-type. The assignment of the atomic species to the different Table l. Cation and/or water positions in milarite-type structures (C,N. = coordinationnumber). The list of elementsoccupying the different positions inc'ludesthose listed in Table 2, as ueil as trace el enents i dentifi ed by Eernt ( 1968) i n amountslarger than 0 0lwt% in milarites from .,vv - vezna. Site Symbol Equipoi nt C,N, Synmetry Possible0ccupants T(I ) 24(n) 4 I - C. si,A.(,,14g r(2, 6(f) 4 222-02 Ar,,8e,Mg,Fe,Li,Ti ,B A 4(c) 6 32 - 03 Mg , Ca , Fe , T i , Zr ' Mn , Sr , Co , Nb B 4(d) 9(=3+6) 6 - 5: K,Na,H20,(H30?),Ms,Fe c 2(a) 12 622 - D6 K,Na,Ca,Ba,Y,Yb,Pb 'lB(=12+6) D 2(b) 6ln C[ (H20?) CHEM'ISTNY OF MILARITE 465 -SNg 3 aC 3 e 9 EO s:: -: <.i: iif f f r' !999? :;;;i ; 3 d E i i 3 3 A A i 3'd'd a'6 t'i''d a a ; -"t.9 ,Y -- o6 obb -> -:33=s3E:s3:B€SK33g a5 xg9z'fl9:a33a3:iiiiE o E NS ou6 NO na -ddd Nozz-ahz,i22o9!a'99 -,jv"-2 o 6 o a @ b 6 6 a ..:\ -6 -.ia -6 tiJli v-v-v'*'-'f *,i,i d "-Jf 2 zzzz EO oo40 a3 g:: R:30-o-o-o-o- dFd=EEts! 6t6E qzzzzzze! zv= dt r=E> FL>N> c F N6 b 6 L&o=Eehh oA6= a o Nd N6 lNUdddduoo&ub <EZZq=ou:kh>E=qo= tHd I dI J d lfl ot .-o loldlhCC .l ol o Fl u < < d o u u u tl <l dl ! 61 P i < r tsl k 6 '4lol!lo=l dodoolooF nltsl-lr'l<Lo<sEElt cldl<l!:olpseolu ul€lulc Nlc r c! c o Fil6i c a oldl.-lx@lxoldoox rf Hl =l q zl > r x g u o > o 4 ul J u x 466 FORBES,BAUN, AND KHAN sites is basedon the publishedchemical analyses and on the sizesof the ions in question.The water moleculesare assumedby us to be locatedin the 9-coordinatedB-sites. our reasonsfor this assignment are: l. The HzO molecule is of approximately the same size as the potassiumion which in K2Mg5Si1zogoprefers the B-sitesover the l8-coordinated D-sites (which would be much too large for eitherK or HzO) 2.Uponheatingtol0OOoC,andafterlosingH2O,theccellconstant of milarite increasesby approximately0.06A, while the c' cell constantdecreases by about 0.MA (eernjr, private communica- tion, 1971; Iovcheva et at., 1966).If the water moleculeswere Iocatedin the large D-sites as suggestedby Iovchevaet al., Lheit loss should not affect the cell constantsin any measurablede- gree.If they are, however,in the B-positionsthe changesin cell parameterscould be interpretedas the result of an adjustment of the oxygenatoms aroundthe B-site taking place after the re- moval of the H2o (comparethe discussionof the cell parameters of K2Mg5SilzOgoand osumilitein Khan et aI.,l97l) . This assignmentis at variancewith the findingsof Ito et al. (1952)' However,their structuredetermination is of low accuracy (R = 0.30 for 105observed structure factors) and cannotbe taken as solid proof. Bakakin and soloveva (1966) report in an abstract that they found in milarite ,,watermolecules on threefoldaxes near the face of the ca flattening octahedron".Their statementmay possibly refer to what we call the B-sites,but sincethey do not give any further details,its meaningis uncertain.The presenceof water in milarite-type minerals hasbeen the subjectof much discussion.Water is well authenticatedin milarite and armenite only. Miyashiro (1956) assumedwater to be presentin osumilite.However, o,lsen and Bunch (1970),who carefully analyzed osumilite from its three known occurrences,including the type locality, concludedthat water is not a constituentof osumilite. Litewise there is no clear indication for the presenceof water in mer- rihueite, roedderite,yagiite, or in the synthetic forms (KzMBs'SirzOso and syntheticosumilite; Schreyer and Seiferi,1967) ' The milarite group minerals may be divided into four subgroups on the basisof their AI contenk.The roedderitesubgroup is virtually CHEMISTRYOF MILANITE 467 exceptionis sodgianite,where this site appearsto be occupiedby 3- and 4-valent cations (sodgianitehas beenrecognized by Strunz (1970) to belongto the milarite-type minerals).The B-sites are usually pop- ulated by alkalis and water and occupancyof this position is very variable betweenthe groups.It is most fully occupiedin armenite. The roedderitegroup containsup to l.b atoms in this position.In the osumilitesthe B-site appearsto be empty. Brown and Gibbs (1969), in their crystal structure determination, do not report to have found any atomsto be locatedthere. The compositionof the Ca-cordierites requiresthat divalent cations,either Mg or Fe, are in this position, thus making the coordinationof these catiorisrather unusual. IIow- ever, one should keep in mind that an eight-coordinationfor Mg is commonlyfound in garnets. Becausewater moleculesand alkali ions seemto replaceeach other in the B-sites of milarite we actually may not be clealingwith water moleculesbut insteadwith hydronium ions (HrO-). If this is the case we would expectthe oxygen atom of the hydronium ion not to be exactly in the B-position (l/3 2/3 0), but slightly displacedfrom it along the threefold axis (1/3 2/3 z), so that HgO could form three hydrogenbonds in a trigonally pyramidal configurationto the three neighboringO ( 1) oxygenatoms (Khan et al., lSTl) . The C-position is occupied by alkali ions in all minerals but armenite,where it is filled by barium. The ?(2)-sites are marked by considerablevariation. They may be occupiedby Mg, Fe2*,Fea*, Be, Al, or Li. Magnesiumin tetrahedral coordinationis very rare in sili- cates.In the systemKrO-MgO-SiO however,there are two phasesin which Mg is 4-coordinated: Mg-merrihueite, K2MgS,i12Ose(Khan et aI.,1971), where the four-coordinationhas been substantiatedby the strueture determination (with a bond length Mg-O of 1.gbbA), and in K2O.MgO.3SiO2,which accordingto Roedder(19b1) has an X-ray powder pattern strongly resemblingthe pattern of kalsilite lKAlSiOr).
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