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 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-', occupied. In , 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 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 E=qo=

tHd I dI J d lfl ot .-o loldlhCC .l ol o Fl u < < d o u u u tl 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 .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 determination, do not report to have found any atomsto be locatedthere. The compositionof the Ca- 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). Consequently,KrO.MgO.BSiOz may be formulatedas K(Mgo.sSio.r)SiO+,with Mg and Si substitutingfor AI. Schreyerand Schairer(1962) have shownthat alkali-free milarite- type phasescan be synthesizedwithin the system MgO-AlsO3-SiO2. As theseauthors recognized, the milarite structuretheoretically could form from the composition2MgO.2AlzOe'11SiOz, i.e'., only lhe A, T(2), and,?(1) positionswould be filled. If we consider2:2:ll to representa "proto"-milarite composition,the main groupsof milarite minerals may be derived by simple cationic substitutions.This is shownschematically in Table 3. As can be seen,numero:l.ls variations are possible,simply by altering the AfSi ratio and consequentlythe 468 FORBES, BAUR, AND KHAN

Table 3, Fornal derivation of nllarite-type ninerals fton the proto_nilarite-type'

D[18] r(2) T(1) oso Type neptacenent lj6l "tel-2 c[12]

(starting conposition) M1- Mi si11Ar oSO proto- nilarile

* (si+2M1*+3M2+)for 4M3* M:' NII M]' v!' sirz o:o "i;Tl:::::::,.1,'"'| "*.,"u1' uI* u]. Siu o:o sodgianite milarite (si+M' +2M" ) for 3M- ''2M2 tr{20) u1* ul*us* si' oso

(Ax*M1') for si "zM2 vl* v!- siroAxz oso

(zAx+I{2+) for 2si ul (ilzo) v2* lurol* vl* sigAt3 o:o alkali content. It should be mentioned,however, that this starting compositiondoes not producea milarite-type phaseunder the experi- mentalconditions investigated by Schreyerand Schairer(1962) ' olsen and Bunch (1970) studiedthe compositionsof na;tural osu- milites and found that the K/(K * Na * Ca) ratio valies with the occupancyof the c-position. Actually their diagram can be extended to include the contentsof the B-position as well, thus allowing the additionof yagiite (Fig. 1).In Figure2, we haveplotted total Al * Fe3*against >(K + Na * Ca).It appearsthat as occupancyof the B-

MountArci

Sokkobiro

oCenlrolAsb Vol Giuf MILARITES from YAGIITE Ault

lo 09 0a o.7 0.6 05 0.4 03 02 0.1 oo K/(K.No.Co) - Frc. 1. Ratio K/(K + Na f Ca) versus total K f Na * Ca located in the B- and C-positions in ozumilite and yagiite (crosses) and milarites (circles)' CHEMISTRY OF MILARITE 469

Mounf + Arci Soklobiro YAGIITE

o.2

o.o 3:l 3.8 3.9 4.O 4.t 4.2 4.3 4.4 4.5 4.6 47 > (Al.Fe31

Frc. 2. Total Al * n'e* versus ratio K,/(K + Na f Ca) in osumilite and yagiite.

and C-sites incteases,the Al + Feu* content decreasesand the content increases (see Fig. 1). Thus, natural osumilites and yagiite are members of a solid solution series, related by the substitution K + Al * 2Na * Mg. Idealized end members of this series are KMgrAls(Si1sAl2)Obs and NasMg2(AIzMe) (SiloAIz)O80.The sodic end member may be related to roedderite through the substitution 4Al : 2Si * 2Mg. However, intermediate members of the series are lacking. A similar relationship between the K,lK * Na * Ca ratio and total cation occupancy of the B + C sites is found for milariies as well (Fig. 1). The Ca representsthe excessover 2 in the.4 site. However, there are no corresponding changes in the abundances of other cations as was found for osumilites. Possibly this means that variation in K/K * Na * Ca ratio is correlated with the postulated hydronium content of the B-site. Unfortunately, there is no direct evidencebearing on this possibility. OccunnnNcn Milarite-type minerals are typically found in low pressure environ- ments. Milarite itself as well as armenite occurs as a fissure-filling or pegmatitic mineral. Osumilite is found only in volcanic rocks, where it oscurs within vesicles as well as in the groundmass. Milarite-type phases found in synthetic systems usually occur at low pressures. Roedderite, NazMgsSirzO36,w&s found as a decomposition product of richterite only at, pressures below 5O bars P(H2O) (Forbes, 197f). -bearing roedderite, again the decomposition product of a sodic amphibole, occurs only at pressures below 500 bars P(HzO) (Ernst, 470 FORBES.BAUR. AND KHAN

1960).Seifert and ,Schreyer(1969) have synthesizedMg-merriheuite at pressuresup to 32.5Kbar; however,from considerationof the phase equilibria of this phase,they concludethat this compoundis restricted to low pressureenvironments. While field relations and experimentaldata indicate that milarite- minerals form under low-pressureconditions, it is surprisingto note that the molar volume changefor the breakdownof osumilite under volcanicconditions (Schreyer and Seifert,1967) : *t*,*llPi"*r,o3oY],?,t9' + Mq?*k:l:o"* 378.8 chs,/molo 108.98 232,a1 25.74"'_?.t9",,.0",,." is *14.5 cm3/mole,which suggeststhat osumilite in this instance would be stable at higher pressures.If orthoclaseand d-quarLz are used,instead of sanidineand cristobalite,AV is reducedto 8.4 cm'/ mole but it is still positive. It is possiblethat changesin the phase compositionsand/or nature of the phasesmight alter the sign of the volume change.The molar volume data for osumilite were obtained from Schreyerand Seifert (1967),for the other phasesthe compilation of Robie et al. (1966)was used. As Miyashiro (1956) has recognized,milarite-type minerals,par- ticularly osumilite,may be quite commonin nature but are mistaken for cordierite. Several instancesof optically anomalouscordierites were discussedby him. Two additional reports of optically positive "cordierite" (Rutherford, 1933; Conant, 1935) support the view that osumilitemay be a fairly commonmineral. It might be profitable to study suchoccurrences further.

AcrwowmlcnunNr We thank Dr. P. Cernf for communicating to us in advance of publication the results of his heating experiments on milarites and Dr. Olsen for a review of the manuscript.

Rnnonnlvcns Berarrrv, V. V., exn L. P. Sor,owve (1966) The crystallochemical analysis of compounds with beryl and mila"rite structures. Acta Crystallogr. 21, L4l. Bnown, G. E., .lNl G. V. Grses (1969) Refinement of the crystal structure of osumilite. Amer. Minerar. 54, 101-116. BuNcrr, T. E., eNl L. Fucus (1969) Yaeiite, a new sodium- analogue of osumilite.Amer. Mi,neral.s4,14-18. ennNt, P. (1968) Berylliumminerale in Pegmatiten vonYezn6 und ihre Umwand- lungen. Ber. Deutsch. Ges. GeoI. llldss. 8.13, 56b-578. Cnrsrvexove, M. 8., G. A. Sor,olrrxe, rNn %. P. Rezuar.rove (1964) Milarite from central Kazakhstan. Dold. Akad.. .ly'azft SSS8, 159, 1305-1308.[transl. Dokl. Acad,. Scd. USSft, Earth Sci. Sect. 170, 102-1051. CHEM'ISTRY OF MILARITE 471

CoNawr, L. C. (1935) Optically positive cordierite from New Hampshire. , zzer. Mi.neral.20,310-3l l Dono, R. T., W. R. Verv Scrorus, eNo U. B. MenvrN (19&5)Merrihueite, a new alkali-ferromagnesian silicate from the Mezij-Madras chondrite. Science, 149, 972-574. Dusuerov, V. D, A. F. Ynrruov, Z. T. Kerevnve, L. A. KnonosHrlovA,eNo K. P. YeNur,ov (1968) Sodeinaite, a new mineral. Dokl. Akad. Nauk SSSR, taz, ll7LLl77 lTransl. DokL Acad,. Sci. USS,REarth Sci. Sect., 782, 137-139]. Ernst, W. G. (1960) Stability relations of magnesioriebeckite.Geochem. Cos- mo chim. Acta, 19, 10-40. Fonnns, W. C. (1971) Sl'nthesis and stability relations ol richlerite. Amer. Mineral.56, 997-1004. Fucrrs, L., C. Fnorvnnr,,exn C. Kr,nrrv (1966) Roedderite, a new mineral from the Indarch meteorite.Amer. Minerar. 51,949-955. Hiicr, T. (1956) Verbreitung des BeryIliums uud der Beryllium-mineralien in den Schweizer Alpen. Schweiz. Mineral. Petrogr. Mitt. 36, 457-510. Iovcrrnve, E. I., I. f. KupnrvewovA, AND G. A. Srnonewro (1966) Milarite from central Asia. DokI. Akad.lfazk SSSE,170, 1394-1397 lTransl. DokI. Acad,.Soi. USSR Earth Sci. Sect.170.160-1631. ho, T., N. Monrnor:o, elro R. Salexace (1952) The crystal structure of milarite. A cta Cry stallo gr. 5, 2CE-213. KnNNcmr, A. (1S70) Mittheilungen an Professor G. Leonhard. Netrcs. Jah'rb. Mineral. Geol. 80-81. KueN, A. A., W. H. Beun, exn W, C. Fonnos (1972) Synthetic magnesiummerri- hueite, dipotassium pentamagnesiumdodecasilicate: A tetrahedral magnesio- silicate framework crystal structure. Acta Crgstallogr. B28, 267-272' Mrvesnrno, A. (1956) Osumilite, a new and its crystal structure. Ame,r.M ineral. 41, 104-116. NnulrewN, IL (1941) Armenite, a water-bearing barium-calcium-alumosilicate. Norsk GeoI. Tid.sskr.21.19-24. Or,soN,E. (1967) A new occurrenceof roedderite and its bearing on osumilite- type minerals. Amer. M'ineral. 52, lslg-1523. eNn T. E. BuNcrr (1970) Compositions of natural osumilites' ' zzer' Mineral.55, 875-879. Parecun, C. (1931) On the presenceof beryllium in milarite. Amer. Mi'neraL 16, 469470. Ronru, R. A., P. M. Bnrnrn, M. S. Tour,rrrN, ANDJ. L. Enw*us (1966) X-ray crystallographic data, densities, and molar volumes of minerals. Geol. Soc. Amer. Mem.97,27-73 Rononnn,E. (1S51)The system K,O-MgO-SiOn.Amer. J.9ci.249,81-130; 2,94-24& Rutuonronn, R. L. (1933) Optically positive cordierite from the Northwest Ter- ritories, Canada. Amer. M ineral. 78, 216. Scnnnrnn, W., ewo J. F. Scnernon (1962) Metasta.ble osumilite-and petalite-type phases in the system MgO-ALOe-SiO". Amer. Mineral. 47, 90-104. _, arvo F. snrrnnr (1967) Metastability of an osumilite end member in the system K,O-MgO-ALO"-SiO"-ILO and its possible bearing on the rarity of natural osumilities. Contrib. Mi"neral. Petrology, 14, 343-358. Snrnnnr,F., ern W. Scnnnyon (1969) Stability relations of K,MgSi-Oo, an end member of the merrihueite-roedderitegroup of minerals. Comtrib. Mineral. PeIrologg, 22, 190-2A7. 472 FORBES, BAUR, AND KHAN

Sosooro, T. A., eNn R. L, Tpr,psnnve (1962) Chemical composition of milarite. Dokl. Akad,. l/auft. 8888, 146, 437439 lTransl. DokI. Acad. Sci. USS/?. Earth ScienceSecti,on, 146, ll2-ll4l. Srnuwz, H. (1970) Mineral,o,gisch,eTabellen, 1th ed. Akademische Verlagsges., Leipzig.

Manuscript receiued,,Au4rct 23, 1971; accepted tor publication, October 28, 1971.

Note ad,d,edin proof. Professor Schreyer called to our B,ttentiona paper (Borchert and Petzenhauser,1966) in which the synthesisof four additionalmembers of the milarite groupis reported.These could be formulatedin accordwith the generalformula proposedby us, as Iul tnl tt'] Inl 16l t'l t'l Mg, Na Rb Mg, si12o3o,Mg2 Bat Al, Al'sisoao,

MnrtolgurtztAlrtolAlrsino.o, and Mgrl6lBart2lFerlulAlrSinoro. The Iast compound could also be Fert6lg4rl'r (MgsFe) ratAlsSisOso. Of particular interest is the Na-Rb compoundbecause the authors report that they were not able to synthesizethe correspondingpure Rb compound.This may mean that Rb is too large to enter the nine coordinatedB sites,which howevercan be occupiedby potassiumions.

Boncrnnr, W., eNn I. PnrzpNneusnn (1966) Osumilithbildung in verschiedenen silikatischenSystemen. Ber. Deutsch. Kenam. Ges. 43, 572-576.