AmericanMineralogist, Volume 76, pages 1836-1856,1991

The crystal chemistry of the milarite-group minerals

FnaNx C. HawrnoRNE, MrrsuvosHr Krnraur* Pntn ennni, Nnrr. B.q.r,r, Department of Geological Sciences,University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada Gnoncn R. RossprlN Division of Geological and Planetary Sciences,California Institute of Technology,Pasadena, California 9 I 125, U.S.A. Jorr. D. Gnrcn Mineral SciencesDivision, Canadian Museum of Nature, Ottawa, Ontario KIP 6P4, Canada

Thecrystar srructures of six narurat ffi:JlA' ,B.clrLTr,2o30l(H,o),,p6/mcc, a - 10.40,c - 13.80A, Z:2) coveringthe rangeof known compositionshave beenrefined to R indices of -30lo using MoKa X-ray intensity data. Previously proposed split-atom models for the A and B sites are confirmed. Polarized spectroscopyin the infrared region shows HrO to be the only significant H-bearing group. Band polarization characteristics show the H-H vector to be Ic and the HrO plane to be llc; this correspondsto the type II HrO found in the related structures of beryl and cordierite. There is a quantitative correlation between band intensity and HrO content, and the observedmolar absorptivi- ties are similar to those measuredin cordierite. Milarite has a beryllo-alumino-silicate framework structure. The framework is a four- connected three-dimensional net, one of a series of seven simple nets with prominent double rings of tetrahedra previously derived by Hawthorne and Smith (1986). There are l5 minerals with this net as the basic tetrahedral arrangement;in general,Si is strongly to completely ordered into the ring tetrahedra, and the other tetrahedral cations (Li, Be, B, Al, Mg, Fe2*,Fe3*, Mn2*, and Zn) are strongly to completely ordered into the ring-linking tetrahedra. The important substitution in normal milarite is NaB + Ber2 : DB + Alr2 with the amount of Be varying between approximately 1.5 and 2.5 apfu; in (Y,REE)- bearing milarite, the important substitution is (Y,REE)A + Ber2 : CaA * AID with the amount of Be varying up to 3.0 atoms pfu. The mean bond lengths of the polyhedra involved in these substitutions vary accordingly, and linear models of this behavior are developed. In addition, there is chemical evidence of a small amount of Al = Si substi- tution at the T I site. Similar linear models are developedfor the whole group of milarite- type minerals. Milarite is found in a variety of environments, from vugs in plutonic rocks through diverse pegmatites to hydrothermal ore deposits and alpine veins; a review of specific paragenesesis given. Milarite is a low-temperature hydrothermal mineral crystallizing in the range of 200-250 "C at low pressures.Associated minerals suggestthat milarite crys- tallizes from alkaline fluids with Be being transported as fluor-complexesand carbonate complexes. Other members of the milarite group occur in peralkaline rocks, meteorites, peraluminous volcanic rocks, and high-grade metamorphic rocks, as well as other geo- chemically more unusual rocks, and form under a wide variety of P-T conditions.

INrnooucrroN ation in paragenetic sequenceand geographic locality. Milarite is a framework beryllo-alumino-silicate min- Milarite is possibly much more common than is generally eral with the ideal formula (K,Na)Car[(Be,Al)Si,,Oro]' realized, as it can be mistaken for quartz or apatite in xHrO where x - 3/qoriginally described from Val Giuf, hand specimen. It has been investigated extensively be- Switzerland (Kuschel, 1877). It is generally found in hy- causeof its anomalous optical properties,being morpho- drothermal veins and granitic pegmatites, and it shows logically hexagonalbut often having biaxial optics. It is significant variations in chemical composition with vari- the prototype mineral for a large structural $oup that includes osumilite. In the last ten years, it has been rec- ognized that osumilite is much more common than hith- * Presentaddress: Institute ofGeosciences,University ofTsu- erto realized. The crystal-chemicalrelations of the milar- kuba, Ibaraki 305, Japan. ite group have been intensively investigatedover the last

0003-004x/9l/l l I 2-l 836s02.00 r 836 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP 1837 ten years (Cern!' et al., 1980; Armbruster et al., 19891 Sarnple35. From East Siberia, USSR (researchcollec- Kimata and Hawthorne, 1989),and a generalreview and tion of P. Cernj'); this consistsof a pale pink aggregateof slnthesis are warranted. anhedral columnar crystals up to 3 x 20 mm in size; it The milarite structure is extremely flexible from a is associatedwith a pale green variety of different com- chemical viewpoint, and a considerablenumber of min- position (Novikova, 1972). erals (Hawthorne and Smith, 1986)and syntheticcom- Sample 30. From R6ssing, Swakopmund, Namibia pounds(Pushcharovskii et al., 1972;Sandomirskii et al., (National Museum of Natural History Washington no. 1977) adopt this basic atomic arrangement.In particular, C6562); this consistsof pale yellow columnar crystalsup it seemsto be a structure type that is favored by the light to 3 x 9 mm in size, with well-developedprismatic and lithophile elements.It is notable that many of the milar- basal pinacoidal faces and minor pyramidal faces.Sam- ite-group minerals have been discovered fairly recently ple 7935 from the California Institute of Technology col- and in a broad spectrumof geologicalenvironments. This lection was also used in the infrared work. is probably due to two factors: first, these minerals are Sample 36. From JaguaraEri,Brazil (researchcollection easily misidentified both in hand specimenand by pow- of P. Cernj', obtained from R. V. Gaines); this consists der diffraction; second,the light lithophile elementsthat of a translucent white columnar crystal 6 x 12 mm in are so important in many minerals ofthis group are easily size; the crystal shows prismatic facesand is terminated overlooked in an electron microprobe examination. Mi- by a basal pinacoid. Underlying the latter is a -0.75-mm larite itself usually contains l-2 wto/oHrO. Although the thick cap of transparent, pale greenish yellow material position of HrO in the structure has been located by dif- that proved to be of significantly different (Y,REE)-poor fraction methods, there has hitherto been no spectro- composition. scopic examination of the role of HrO in the milarite Sample37. From StrangeLake, Quebec(CANMET case structure.In particular, Forbeset al. (1972) suggestedthat sample SL258-71); this consistsof a pale yellow, clear, milarite may contain hydronium (HrO) groupsrather than hexagonalprism terminated by a minor hexagonalpyr- H,O groups. Obviously this possibility is of great rele- amid and basal pinacoid. The crystal was from an 8-mm vance to the polyvalent chemical substitutions operative vug in a coarse-grainedquartz-feldspar (predominantly in the structure. In addition, the complexity of the role microcline) segregationwithin the medium-grained gran- of HrO in the related structures of beryl and cordierite lte. indicates that infrared work on milarite is desirable. In Other samples used in the infrared work are NMNH view of the increasein importance of this group in the no. 135779from the Foote mine, NMNH no. 119156 last few years, we thought it worthwhile to review the from V€2n6, and NMNH no.820407 from Val Giuf. previous work done, examine the stereochemicalvaria- tions across the range of milarite compositions (sensu Chemical analysis stricto), and synthesizeboth the available crystal-chem- Subsequentto the X-ray intensity data measurements, ical and parageneticinformation. the crystalswere detachedfrom their glassfibers, mount- pyccolyte plexiglas ground ExpnnrvrnNr,lr, ed in in a l-in. disk, and and polished. Electron microprobe analyseswere performed Sarnpledescription on a Cameca instrument in the wavelength-dispersive The physical properties and chemical compositions of mode with an operating voltage of 15 kV and a beam four of the samplesexamined here were given by Cernj' current of 25 nA. A fairly large beam diameter (10 ltm) et al. (1980), whereasthe other two were only recently was used to minimize potential sample damage during reported as a new Y-rich Al-poor variety (Cernj' et al., analysis. Data for standards were obtained for 25 s or l99l). Here we shallretain the samplenumbers given by 0.250loprecision per element, whichever was the more Cern!,et al. (1980);new samplesare numbered 36 and efficient procedure. Standards used were beryl (AlKa, Jt. SiKa), diopside (CaKa), NaScSirOu(NaKa), and ortho- Sarnple 25. From Guanajuato, Mexico [National Mu- clase(KKa); no other element was observedon ED spec- seum of Natural History NMNH) no. l2l230}- this con- tra collected on the same grains. Twenty analyseswere sists of pale yellow columnar crystals up to 5 x 18 mm obtained on each crystal; the entire crystal was covered in size. These occur as slightly divergent aggregatesof in each casein order to give good averageanalyses. Data hexagonalprisms terminated by basalpinacoids, and they reduction was done with a conventional ZAF routine; are associatedwith adularia and minor qtartz. Sample averageanalyses are given in Table l. NMNH no. 120408 from the same locality was used for the infrared study. X-ray data measurements Sample 33. From Tittling, Bavaria (researchcollection Crystals for intensity data measurementswere selected of P. Cern!', obtained from C. Tennyson, Technische on the basis of optical clarity, homogeneity, freedom from Universitiit Berlin); this consists of white to colorless inclusions, and equant shape.The crystalswere mounted crystals up to 2 x 5 mm in size, with dominant prism, on a Nicolet R3M automated four-circle diffractometer, pyramid, and minor basal pinacoid; it is associatedwith and 25 intense reflections were centered using graphite- quartz,albite, and chlorite. monochromated MoKo X-radiation. Least-squaresre- I 838 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP

TleLe 1. Chemicalanalyses for milaritesamples 37

CHJ Probe sio, 71.35 72.78 70.65 72.96 71.23 71.81 72.85 70.20 67.69 Alr03 5.68 5.84 4.34 4.34 7.O2 3.40 3.31 1.50 0.26 BeO 4.58 (4.58) 6.71 (6.71) 4.52 5.07 (s.07) BeO"*' [s.18] [4.94] [s.3s] t5.371 14.281 t5.801 t5.s2l t6.301 [6.55] MnO 0.25 0.19 0.00 0.01 0.00 0.15 0.02 0.00 0.06 CaO 11.40 11.17 10.80 11.13 9.32 11.60 11.28 7.70 6.00 Na"O 0.17 0.08 0.52 0.04 1.90 0.49 0.10 0.15 0.06 KrO 4.95 4.81 4.97 5.08 4.03 4.52 4.68 4.80 4.64 Rb,O (0.03) 0.03 (0.04) 0.04 (0.02) 0.02 BaO (0.08) 0.08 (0.02) 0.02 (0.04) 0-04 Yro. (0.01) 0.01 (0.2s) 0.29 (0.00) 0.00 5.10 6.48 H"O 1.20 (1.20) 1.29 (1.29) 1.65 1.68 (1.68) Sum 99.70 100.77 99.63 100.57 99.67 98.78 99.05 96.911 94.33+ Si 11.92 11.98 11.74 11.82 11.86 12.12 12.21 AI 1.'t2 1.13 0.85 0.83 1.38 0.68 0.65 Be 1.84 1.81 2.68 2.61 1.81 2.06 2.O4 Mn 0.04 0.03 0.00 0.00 0.00 0.02 0.00 Ca 2.O4 1.97 1.92 1.93 1.66 2.10 2.03 Na 0.06 0.03 0.17 0.01 0.61 0.16 0.03 K 1.06 1.01 1.05 1.05 0.86 0.97 1.00 0.00 0.00 0.03 0.03 0.00 0.00 0.00 Sum 18.07 17.97 18.44 18.29 18.18 18.11 17.98 Si 11.83 11.93 11.95 12.03 11.90 12.00 12.14 12.09 12.13 AI 1.11 1.13 0.87 0.84 1.38 o.67 0.65 0.30 0.05 Be" 2.06 1.95 2.19 2.13 1.72 2.33 2.21 2-61 2.82 Mn 0.04 0.03 0.00 0.00 0.00 0.o2 0.00 0.00 0.01 Ca 2.O3 1.96 1.96 1.97 1.67 2.08 2.01 1.42 1.15 Na 0.06 0.03 0.17 0.01 0.62 0.16 0.03 0.05 0.02 K 1.05 1.01 1.07 1.O7 0.86 0.96 1.00 1.05 1.06 0.00 0.00 0.03 0.03 0.00 0.00 0.00 0.47 0.62 Sum 18.17 18.03 18.23 18.08 18.15 18.23 18.05 18.05t 18.01+ Note:25-: Guanaiuato;33: Tittling;35: east Siberiapink; 30: R6ssing;36: Jaguarag0green; 37: StrangeLake. * FromCernf et al. (1980). .'BeO calculatedsuch that Si + Al + Be: 15 apfu. t IncludesYb,O3 0.60, Er,O"0.30, Dy,O.0.20 wt%; Yb 0.03; Er 0.02, Dy 0.01 apfu. + fncludes Ce,O3 0.71 , Nd,O30.44, Er,O30.57, Dy"O"0.72 wtTo;Ce 0.05, Nd 0.03, Er 0.03, Dy 0.04 apfu.

finement of the setting anglesproduced the (hexagonally and Cary l7I (near IR) spectrophotometers.Spectroscop- constrained)cell dimensions given in Table 2. Reflections ic measurementswere obtained on sample regions from were measuredto a maximum 20 atgle of 60" according 100 pm to 2 x 4 mm that were as free as possible from to the experimental procedure of Hawthorne (1985); inclusionsand cracks.In the 3600-cm-' region,samples numbers of reflectionsare given in Table 2, togetherwith were ground as thin as 15 prm to obtain on-sslle mea- other information pertinent to X-ray data measurement surements.Low-temperature spectrawere measuredwith and structure refinement. Ten strong reflectionsuniform- the sample attached to a liquid N-cooled cold finger in a ly distributed with regard to 20 were measured at l0o vacuum dewar. intervals of ry'from 0 to 350". These data were used to refinement calculate an ellipsoidal shape for the crystal, which was Structure then used for absorption corrections on the whole inten- Scattering curves for neutral atoms together with sity data set; azimuthal R indices were of the order of anomalous dispersion corrections were taken from the 1.0-1.50/o(Table 2).Data werecorrected for Lorentz,po- International Tablesfor X-ray Crystallography(1974)- R laization, and backg.roundeffects, then averagedand re- indices are of the form given in Table 2 and ate expressed duced to structure factors. A reflection was classed as as percentages. The SHELXTL system of programs observed if its intensity exceededthat of 4 sd based on (Sheldrick,l98l) was usedfor the computationalproce- counting statistics; final numbers of observedreflections dures. are given in Table 2. Refinementswere initiated using the atomic positions of milarite from King's Mountain (Cern! et al., 1980) Spectroscopicdata together with site occupanciessuggested by typical chem- Samplesfor infrared spectroscopywere prepared as thin, ical relations of the speciesconcerned. Full-matrix re- doubly polished slabsparallel to [001], following the pro- finement of all variables for an isotropic displacement ceduresof Goldman et al. (1977). model converged to R indices of approximately 4.5o/o Spectrawere measuredwith a combination of Perkin (Table 2). Upon conversion to an anisotropic displace- Elmer model 180 0R), Nicolet 60SX 0R and near IR), ment model, the A cation(s) showed exaggeratedanisot- HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP 1839

Trsr-e 2. Miscellaneousdata regarding milarite refinements

25 35 a (A) 10.41 0(1) 10.404(2) 10.415(3) 10.396(1) 1O.342(2) 10.340(1) 13.845(3) 13.825(5) 13.763(5) 13.781(4) 13.777(61 13.758121 v(A1 1299.3(5) 1296.0(7) 1293.0(7) 1289.8(5) 1275.8(9) 1273.9(4) Spacegroup P6lmcc P6lmcc P6lmcc P6lmcc P6lmcc P6lmcc z 2 2 2 2 2 2 Crystal 0.20 x 0.24 O.20x 0.24 0.18 x 0.20 0.14x 0.16 0.22x 0.30 0.15x 0.15 Size (mm) x 0.36 x 0.30 x 0.30 x 0.18 x 0.32 x 0.25 Radiation MoKo MoKq MoKa MoKa Mottu MoKa Mono. graphite graphite graphite graphite graphite graphite R..1" 1.4 1.3 1.3 0.9 1.4 1.0 Total lFl 1527 152'l 1515 1516 1503 1501 Noof lFol 629 581 607 555 561 561 R*% 4.6 4.3 +.c 4.1 4.5 c.o Fl o/^ 2.9 3.1 3.2 3.4 3.3 4.1 Note:25: Guanajuato;33: Tittling;35 : east siberiapink; go: R6ssing;36: Jaguarag0green; 37: strange Lake.

ropy. This was taken as evidencefor significant displace- Hawthorne and Smith (1986) developed46 four-con- ment of the cation off the special position; the electron nected three-dimensionalnets basedon insertion of two- density was modeled as a split site with an isotropic dis- connected vertices into the three-connectedtwo-dimen- placement factor. This gave the same R indices as were sional nets 63,3.122,4.8,, and 4.6.12.Hawthorne and obtained for the ordered anisotropic displacementmodel, Smith (1988) show that one of the topological conse- but we prefer the split-site model on physical grounds; quencesof developing four-connectedthree-dimensional both models gave consistentresults across the series.Full- nets by out-of-plane linkages between stacks of two-di- matrix least-squaresrefinement of all variables (including mensional nets is that the initial three-connectedvertices site occupancies)converged to the R indices given in Ta- must lie on infinite paths within a single prototype two- ble 2. Final atomic parametersare given in Table 3, and dimensional net. Such infinite paths are of two types: structure factor tables (Table 4) are deposited;r selected open paths (which do not return to their starting vertex) interatomic distancesand anglesare given in Table 5. and circuits (closed paths). Thus the family derived by Hawthorne and Smith (1986) can be divided into two GrNnn-c.L srRUcruRAL RT,LATIoNS types according to the graphical characteristics of the In the Bragg classification of the silicates, milarite is three-connectedvertices in the prototype two-dimension- considered as a double-ring structure. However, if the al nets: nets with open paths and nets with (infinite) cir- chemical identity of the tetrahedrally coordinated cation cuits of three-connectedvertices. The milarite framework is not considered,as suggestedby Zoltai (1960) and Lie- has double six-membered rings of such vertices and is bau (1985), milarite has a tetrahedral framework struc- just one of a family of possiblestructures based on sigma- ture. Smith (1977)instituted a systematicexamination of transformednets (Shoemakeret al., 1973)containing such the derivation and characterization of four-connected double n-membered rings. three-dimensional nets as topological models for frame- The simplest case is net 278, the basis of the beryl work silicate structures.Hawthorne and Smith fl 986) de- structure (Fig. I, Table 6). Two-connected vertices are veloped a new family of nets basedon the insertion of inserted into the edgesof the plane net 63 such that al- two-connected vertices into alternate edges of two-di- ternate six-membered rings of three-connectedvertices mensional three-connectednets combined with the usual are preserved (Fig. la). If topologically equivalent nets out-of-plane operations; the milarite framework is based are stackedorthogonal to the plane ofthe prototype two- on one ofthe resulting nets. This topological approach is dimensional net such that the (initially) two-connected of interest here for two reasons:first, the milarite frame- vertices link to vertically adjacent nets, a four-connected work is seen as just one of a series of possible atomic three-dimensional net results. If the six-membered rings arrangementsof related topology; second,the relative or- of (initially) three-connectedvertices contrarotate in ad- dering of tetrahedral cations over the vertices of the nets jacent layers, the edgesconnecting to the (initially) two- seemsto be related to the local connectivity of the frame- connected vertices take on a tetrahedral arrangement (Fig. work. Thus a further examination of the relationships lb). The polyhedral representation of this net is essen- within this family of nets is of interest. tially the tetrahedral framework of beryl and indialite. The key topological featuresof the framework are ac- tually encapsulatedin Figure la, and for simplicity we I A copy ofthe structure factor tables (Table 4) may be ordered will use this type of representation here. All nets with as Document AM-91-483 from the BusinessOfrce, Mineralog- ical Society of America, 1130 SeventeenthStreet, Suite 330, rings of (initially) three-connectedvertices are shown in Washington,DC 20036, U.S.A. Pleaseremit $5.00 in advance Figure 2, in both casesordered according to the number for the microfiche. of tetrahedra in the r-membered rings; note that there is l 840 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP

Trale 3, Finalatomic parameters.for milaritesamples 25 36

x 1/a '/3 V3 V3 V3 Vs v ry3 .f3 ry3 zf3 rya q3 z o.2437(41 0.241O(2) o.2372(2) 0.2390(2) 0.23572(7) 0.2314(6) u4 189(4) 154(4) 1se(s) 141(71 46(3) 84(12) B x th '/3 v3 Y3 V3 V3 24. zf3 v ry3 43 rya z 0.0404(19) 0.0349(15) 0.025004) 0.0298(18) 0.0499(13) 0.050(1) u* 68s(e7) 585(77) 645(79) 674(106) 602(62) 267(50) Ur, 685(120) 427(70) 361(32) s48(88) 578(76) 238(55) U"" 685 427 361 548 578 238 695(160) 900(18s) 1214(227) 927(264) 649(107) 327(104) U,, 343(60) 213(35) 180(16) 274(441 28e(38) 119(27l, c x 0 0 U 0 0 0 0 0 0 0 0 U z V4 V4 V4 V4 V4 184(8) 193(7) 1 81(7) u4 205(4) 190(6) 18s(6) 't72(sl ur, 206(5) 183(7) 186(7) 182(e) 172(8) u", 206 183 186 172 182 172 us 204(8) 204(10) 196(1 0) 207(15) 216(1 3) 197(1 3) u., 103(3) e1(4) e3(4) 86{4) sl(4) 86(4) T1 x 0.08239(6) 0.08168(7) 0.07988(7) 0.0811(1 ) 0.08132(8) 0.08085(9) 0.33642(71 0.3362s(8) 0.33s42(8) 0.3363(1) 0.33844(9) 0.3380(1) z o.11217(4) 0.112s7(4) 0.11 324(4) 0.1127(1) 0.11 249(5) 0.11 262(6) u* e4(2) 88(2) s2(21 81(4) 87(3) 72(31 uf 100(3) 87(3) 95(3) 75(4) 77(3) 6s(4) II 117(3) 103(3) 112(3) 95(4) 107(3) s5(4) 67(2) 7312) 6s(3) 74(41 78(3) 51(4) Ut. 53(2) 47(3) 52(2) 43(4) 47(31 41(4) -5(2) -3(2) -6(2) -5(4) -4(3) -2(3) Ur" -11(4) - -11(4) u23 -8(2) -7(21 -1 1(3) 13(3) T2 x 0 0 0 0 0 0 th '/2 '/2 Y2 V2 '/2 v 1/a z V4 t/4 V4 V4 '/4 u4 94(7) 90(10) 81(1 2) 73(17) 97(21\ 45(2ol uu 110(10) 98(15) 80(17) 94(25) 104(30) 58(31) 101(7) 93(11) 79(14) 48(18) 105(30) 48(21) 74(9\ 81(13) 83(16) 93(24) 83(2s) 32(291 U," s5(5) 4s(7) 40(s) 47(13) 52(1s) 29(15) o1 x 0.0954(3) 0.0946(4) 0.0929(4) 0 0940(6) 0.0946(4) 0.0941(s) v 0.3835(3) 0.3830(3) 0.3822(3) 0.3837(5) 0.3873(4) 0.3872(5) 2 0 0 0 0 0 0 u* 194(1 0) 190(1 1) 211(12) 200(18) 205(14) 187(1 5) ur 290(13) 298(16) 332(16) 330(26) 319(1 s) 315(221 222(121 208(13) 234(141 225(221 206(16) 203(18) uss 71(8) 68(e) 73(10) 67(14) 80(11) 42(14) II 128(11) 129(12) 146(1 3) 15s(21 ) 12305) 129(1 7) 02 x 0.1957(2) 0.1951 (2) 0.1939(2) 0.1949(3) 0.1959(3) 0.1959(3) v 0.2761(2) o.2759(21 0.2758(21 o.2759(4) 0.2779(3) o.2776(31 z 0.1342(1 ) 0.1340(1 ) 0.1347(1 ) 0.1342(2) 0.1333(2) 0.1327(2) ,t 176(71 167(71 18s(8) 173(12) 180(s) 162(1 0) ur 178(8) 169(9) 190(e) 159(1 4) 168(11) 156(1 3) 234(13) Uzz 244(9) 230(10) 256(11) 235(16) 244(12) us 160(7) 162(8) 172(8) 188(13) 197(1 0) 175(121 IT 146(7) 145(8) 1ss(g) 146(13) 154(10) 157(1 1) -17(7) -22(71 -33(8) -341121 -36(s) -37(10) ute -52(10) -44(10) U"" -2s(7) -24(81 -45(8) -46(13) o3 x 0.11 60(2) 0.11 54(2) 0.1142(21 0.1147(3) 0.11 s8(2) 0.11 55(2) v o.4730(21 0.472s(21 o.4727(21 0.4733(3) o.4751(21 0.4747(21 z 0.18040) 0.1804(1) 0.1807(1) 0.1809(2) 0.1821(1) 0.1822(21 128(6) 120(6) 129(6) 108(1 0) 11 8(7) s7(8) uu 145(7) 131(8) 157(8) 120(12l, 118(7) 108(10) uzz 138(7) 12s(8) 132(8) 110(12) 129(1 0) 112(10) U"" 104(6) 112(7) 112(71 s8(10) 11 3(8) 88(10) ur" 74(6') 7o(7) 82(7) 6000) 6s(8) 6s(9) -3(6) -6(7) -2(71 -6(10) 5(8) -2(s) Ut" -28(8) -27(91 u4 -28(6) -24(7) -30(7) -23(10) ' llr: U,, x 10a. HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP l84l

TABLE5. Selected interatomic distances (A) and angles (") in milarite samples

25 JJ A-O3 x3 2.322(1) 2.310(2) 2.299(2) 2.297(2) 2.253(2) 2.327(21 A-03a x3 2.393(1) 2.41't(2) 2.44't(2) 2.420(3) 2.411(2) 2.327(2) (A-O) 2.357 2.361 2.370 2.359 2.332 2.327 O3-O3a x3 2.617(2) 2.612(31 2.5s5(3) 2.58s(s) 2.537(4) 2.s36{4) o3-o3b x3 3.164(2) 3.170(4) 3.180(3) 3.172(4) 3.137(4) 3.13s(4) O3-O3c Xb 3.724(2) 3.729(3) 3.747(4\ 3.730(s) 3.686(4) 3.692(4) (o-o)A 3.307 3.310 3.312 3.304 3.261 3.264 O3-A-O3a x3 67.4(1) 67.1(1) 66.3(1) 66.4(1) 65.8{1) 6s.5(1) o3-A-O3b x3 84.3(1) 84.3(1) 84.2(11 84.4(1) 84.5(1) 84.10) O3-A-O3c x3 106.6(1 ) 107.6(1) 109.2(1) 108.5(1 ) 109.8(1) 111.3(1) O3a-A-O3 x3 102.2(1) 101.3(1) 100.3(1) 100.8(1) se.7(1) 98.3(1) (o-A-o) 90.1 90.1 90.0 90.0 90.0 89.8 B-O1 x3 2.799(5) 2.790(41 2.784(3) 2.774(5) 2.790(5) 2.793(6) B-O3 x3 2.894(17) 2.946(15) 3.045(13) 2.996(17) 2.801(11) 2.798(141 (B-o) 2.847 2.868 2.915 2.870 2.796 2.796 c-o2 x12 3.021(21 3.017(2) 3.008(2) 3.012(3) 3.021(2) 3.022(2) T1-O1 1.614(1) 1.616(1) 1.618(1) 1.614(2) 1.614(1) 1.615(2) T1-O2 1.618(3) '1.621(2) 1.617(3) 1.617(3) 1.619(5) 1.619(4) 1.620(4) T1-O2d 1.618(2) 1.618(2) 1.616(4) 1.618(4) 1.592(4) T1-O3 1.s93(2) 1.s88(2) 1.588(2) 1.593(3) 1.594(2) 1.609(3) (T1-O) 1.612 1.610 1.610 1.611 1.611 1.609 01-o2 2.638(3) 2.633(4) 2..633(4) 2.636{6) 2.633(s) 2.630(s) 01-o2d 2.654(2) 2.648(3) 2.651(3) 2.648(41 2.648(3) 2.637(41 01-o3 2.$q2) 2.634(2) 2.629(2) 2.632(31 2.639(2) 2.636(3) o2-o2d 2.560(1) 2.556(1) 2.555(1) 2.s54(1) 2.558(1) 2.55s(1) o2-o3 2.646(3) 2.645(4) 2.645(4) 2.651(6) 2.643(4) 2.646(5) o2d-o3 2.649(2) 2.652(3) 2.660(3) 2.654(4) 2.658(3) 2.6s5(3) (o-o)T1 2.631 2.628 2.630 2.629 2.630 2.627 01-T1-O2 109.4(1 ) 109.0(2) 109.0(2) 10s.2(3) 109.1(2) 108.8(2) o1-T1-O2d 110.3(1) 109.9(1 ) 11 0.0(1) 110.1(1) 110.q1) 109.7(2) o1-T1-O3 110.6(1) 110.6(1) 11 0.2(1) 110.3(2) 110.7(2) 110.5(2) o2-r1-o2d 104.4(1) 104.3(1) 104.3(1) 104.2(1) 104.40) 104.6(1) o2-T1-O3 110.90) 11 1 .2(1) 11 1 .2(1) 11 1 .2(1) 110.7(1) 110.9(1) o2d-T1-O3 11 1 .0(1) 11 1.6(1) 112.1(1) 11 1.5(1) 111.7(1) 112.1(1) (o-T1-O) 109.4 109.4 109.5 109.4 109.4 109.4 T2-O3 x4 1.675(2) 1.670(21 1.6s6(2) 1.654(3) 1.638(3) 1.636(3) 03-O3a x2 2.617(31 2.612(3) 2.595(3) 2.585(s) 2.537(4) 2.536(7) O3-O3€ x2 2.739(4) 2.728(51 2.708(s) 2.706(7) 2.689(6) 2.689(6) o3-o3f x2 2.844(3) 2.835(3) 2.808(3) 2.809(s) 2.7s4(4) 2.786{41 (o-o)12 2.733 2.725 2.704 2.700 2.673 2.670 O3-T2-O3a x2 102.8(1) 102.9(1 ) 103.1(1) 102.7(1) 101.5(1) 101.6(1) O3-T2-O3e x2 109.7(1 ) 109.6(1 ) 109.6(1) 109.8(2) 110.3(2) 110.5(2) o3-T2-O3f x2 116.2(1) 11 6.2(1) 115.9(1) 116.2(1) 117.00) 116.7(1) (O-r2-o) 109.6 109.6 109.5 109.6 109.6 109.6 A.A 0.'174(31 0.248(s) 0.3s3(4) 0.304(6) 0.393(2) 0.51(2) A-B 2.82(3) 2.85(2) 2.92(2) 2.88(3) 2.56(2) 2.75(21 B-B 1.1 2(5) 0.s6(4) 0.69(4) 0.82(s) 1.38(4) 1.39(4) T1-T2 2.9s6(1) 2.947(11 2.933(1) 2.937(1) 2.914(1) 2.912(1l ------x,1 Note:a: x,1 y I x,1/z z;b: y x,y,r/" z;c:1 y,1 y + x, zid: x f, x, zie: - y, z;t: -x, y - x,y2- z. only one type of ring in any one member of this family of nets (in more complex nets, this condition is relaxed). s The symmetry, topological and geometricalproperties of r these nets are given in Table 6. \_. l'-\ The first point of interest is the occurrenceor nonoc- ! >c- currence of these possible structure types. Two of these nets represent known structures: net 279 is the milarite structure type, and net 291 is the framework of steacyite, (a) (b) (K,tr)Th[(Na,Ca)rSirO,o] (Richard and Perrault, 1972 Perrault and Szymanski,1982). There are as yet no known Fig. 1. Net 278: (a) projection down the c axis; successive minerals basedon the other nets. Also indicated in Table lattice-translationally equivalent plane nets are stacked along the 6 is net 278, the basis for the beryllo-silicate framework c axis and are linked by the two-connectedvertices inserted into the prototype plane nets; in this net, the T2 polyhedron is square- of beryl. Figure 3 shows the tetrahedral frameworks of planar; (b) projection down the c axis; alternate plane nets along beryl and milarite, together with their correspondingnets the c axis are contrarotated 30'in opposite directions such that and the sigma-transformationthat generatesnet 279 from the T2 polyhedron is tetrahedral. Note that the difference be- net 278. The range of chemical compositions found for tween a and b is geometrical but not topological, and so the the milarite-type framework is very wide (see Table 7), essentialtopological information is contained in a. r842 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP

TABLE6, Simple four-connectedthree-dimensional nets containing double rings of tetrahedral vertices

Net Circuit symbol Spacegroup a (A) c (A) Ringsize 278 (465),(4'6'8'), 18 ftlmcc 10 o 40.4 b 279 (4'64)4(6491' 30 ftlmcc 10 14 40.4 b 287 (34'63).(6.10'), 30 ftlmcc 13 14 68.3 3 289 (4'6.),(66), 40 P+lmcc 11 14 42.4 I 291 (4363)4(60101' 20 P4lmcc I 14 44.8 4 297 (4'64)4(66)1 60 ftlmcc 16 14 51.7 12 14 51.7 b 299 (4'61.(66), 60 ftlmcc 16 -l 301 (4363)4(649')' 60 ftlmcc 16 14 11 4 Note; Net : net number assigned by Hawthorne and Smith (1986); Z: number of four-connectedvertices in unit cell; D: volume per tetrahedral vertex.

yet the minerals all prefer this framework over the re- hedra) must have some feature that prevents them from maining frameworks. What factors are instrumental in collapsing in on themselves.Net 279 (mllaite) has six- the apparent stability of the milarite framework as com- membered silicate rings and nine-membered silicate-ber- pared with the other frameworks of Figure 2? One could, yllate rings, each of which is arranged coaxially along in principle, calculate the total energies of the frame- [001] to form a hollow tube. As indicated in Figure 3, works, find some basis to normalize these for compari- these tubes contain the A, B, C, and D sites; these are son, and see if the milarite framework has the lowest occupiedby alkali and alkaline earth cations and by HrO energy.However, this gives no insight into the structural groups, which thus support the fairly open framework. reason(s)why one framework is to be preferred over an- Net 291 (steacyite)has four-membered silicate rings and other. It is of more use to see if the topological charac- l2-membered silicate-sodiaterings that are stackedcoax- teristics of the nets give any clues as to this relative sta- ially along [001] to form hollow tubes.The four-mem- bility. Inspection of Figure 2 shows one very obvious bered rings seem small enough not to need internal but- feature that varies significantly from structure to struc- tressing,and the l2-membered rings contain K and Th, ture: some of the structures are much more open than both quite large cations. The other lower volume net is others. As a measure of this, the volume per vertex for 289, which has eight-membered (silicate-type)rings and eachnet is given in Table 6. Three of the nets(279,289, both six- and l2-membered mixed rings; the similarity and29l) have significantly higher vertex densities(lower of this to the steacyite structure suggeststhat this is a volume per vertex), and two of thesethree nets (279 and potential structure type of similar (but probably more 291) arerepresented by natural minerals (Table 7). Struc- complex)composition. tures that are very open (i.e., have large rings of tetra- The other nets are very different; their types of rings

Fig. 2. Simple four-connectedthree-dimensional nets with double n-membered rings derived by systematicinsertion of two- connected vertices into three-connected two-dimensional nets followed by signa-transformations; nets are from Hawthorne and Smith(1986). HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP I 843

Tnele 8. Site occupanciesin the milarite-groupminerals

Equi- Site point C.N. Occupancy T1 24m 4 Si, Al 12 6f 4 Li, Be, B, Mg, Al, Si, Mn,*,Zn A4c 6 Al, FeP*,Sn4+, Mg,Zr,Fe2', Ca, Na, Y, REE B4d 9 Na, H,O,!, Ca?, K? C2a 12 K, Na, Ba, U, Ca? D2b 18 a,?

or H-bonded complexesof HrO groups(e.g., Bissert and Liebau, 1987). The cation occupanciesofthe two types oftetrahedral vertex for the minerals of the milarite group are shown in Table 8. It is immediately apparent that Si orders at the three-connectedvertices ofthe prototype nets,where- as the other tetrahedrally coordinated cations order (al- most exclusively) at the two-coordinated vertices of the 1 prototype net. The unusual characteristic of this partic- I ular structure type is the wide variety of tetrahedrally coordinated cations incorporated into the structure. F-a_-i Fig. 3. Nets278 and,279and their relationshipto the struc- turesof beryl and milarite, togetheru/ith the characterof the HrO rN MTLARTTE sigma-transformation. Typical spectra for milarite in the midinfrared region are shown in Figures 4 and 5; the spectraare similar for all milarite samples,but with different absolute intensi- are summarized in Table 6. It is notable that most have ties of the absorption bands and, with minor shifts in large rings (up to l8-membered mixed rings) that stack band position. The bending mode aI 1624 cm-r and the along [001] to form very large open tubes. Open tubes of combinationmode at 5180cm-' can ariseonly from HrO this sort are not known in natural minerals. Such large groups,and the stretchingmodes around 3550 cm-, can tubescan occur(e.g., cacoxenite, Moore and Shen,1983), also be assignedto HrO. Thus the H in the milarite struc- but they are filled with (ordered or disordered)cations or ture occursas HrO and not OH. The band at 5180 cm ' complex oxyanions that hold the tubes open. This sug- results from a combination of the fundamental bending gests that structures based on these nets could occur if (zr) and asymmetric stretching (zr) modes, and it is po- the open tubes were filled, possibly with large oxyanions laized in the H-H direction of the HrO group (assuming

TABLE7. Mineralsbased on the nets of Table 6

Formula a (A) c (A) Space group Net no.- Reference Armenite BaCar[Al6SirOso]2H,O 10.69 13.90 ftlmcc 279 (1) Brannockite KSnr[Li3Si,rOso] 10.002(2) 14.263(3) ftlmcc 279 (2) Chayesite K(Mg,FeB.),[(M g, Fe,*),Fep*Si1,O3o] 10.1s3(4) 14.388(6) ftlmcc 279 (3) Darapiosite KNa,Zr[Li(Mn,Zn),Si,,Os] 10.32 14.39 ftlmcc 279 (4) Eifelite KNa3MgIMg3Si,rO30] 10.1 37(5) 14.223(61 ftlme 279 (s) Merrihueite (K,NaXFe,Mg),[(Fe,Mg)3Si1,O@] 10.1 6(6) 14.32(6) ftlmcc 279 (6) Synthetic KrMg?[Mg,Si,,Os] 1O.222(2) 14.152(21 relmcc 279 (7) Milarite KCar[AlBe,Si, roso]H,O 10.396{1) 14.781(4) ftlm@ 279 (8) Osumilite (K,NaXFe,M g),[(Al, Fe)3(Si,At)1,Oso] 10.1 50(2) 14.289(2) ftlm@ 279 (s) Osumilite-(Mg) (K,NaXM g, Fe),(Al, Fe)3(Si,At),,O3ol 10.078(2) 14.319(2) ftlmcc 279 (e) Poudretteite KNa,I93Si,2030] 10.253(1) 13.503(4) ftlmcc 279 (10) Roedderite (Na,KXMg,Fe),[(Mg, Fe)sSi,,O30] 10.142(s) 14.319(3) ft2c 279 (11) Sogdianite (K,Na),(zr, Fe,Ti)[Li,(Li,At)Si,,O30] 10.083(5) 14.24(1) ftlmrc 279 (1) Sugilite (K,NaXFe+,Al)JLi3Si,rO3ol 10.009(2) 14.006(3) ftlmcc 279 (2) Yagiite (Na,K)Mg,(Al,Mg)3(Si,AD,,Osl 10.09(1) 14.20(3) ftlmcc 279 (121 Steacyite (K,!)Th[(Na,Ca),Si60,o] 7.s8(1) 14.77(21 P4lm@ 291 (13,14) ,v9fli-!-efergnces:(1) Bakakinet al. (1975);(2) Armbruster and Oberhansti (1988b); (3) vetde et al. (1989);(4) Semenov et at.(1975); (S) Abraham (ts$);10)Dodd et (1965I(7) 9Jgl. al. Khanet al.(1971)i (8) this study; (9) Armbruster'ani ooernansti (i9BB;); (io)' Griceet at.119bz1; 1ii jArmoruster (1989);(12) Bunch and Fuchs (1969); (13) perrautt - Richardand.Periautt ttgtblitiCl andSzymanski (i982).' The net numberis fromHawthorne and Smith (19g6). 1844 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP

symmetric stretch (vr) arc polarized along the (ideal) two- fold axis ofthe H,O group; identification ofthis direction, in combination with the already known direction of the H-H vector, fixes the orientation of the HrO group. The o o2 o-- (Fig. indicat- c bending mode is polarized parallel to c 5b), o 0 ing that the twofold axis of the HrO group is parallel to L o a c; this fixes the HrO plane as parallel to the c axis. This correspondsto the type II (H'O) found in beryl (Wood and Nassau, 1967) and cordierite (Goldman et al., 1977). The one anomalous aspect of this assignment is the 00 band at 3520 cm-' assignedto 2,. Unlike the analogous 6000 5500 5000 4500 4000 bands in the spectra of beryl and cordierite, which were !,,lavenumbens(cm-1) highly polarized, this band in the milarite spectra shows polarization dependencewhen the contribution of Fig. 4. Infraredspectra ofmilarite from VEZndin the near- little ' by numerical curve infraredregion for a crystalthickness of 150pm. the 3580-cm band is removed either fitting or by visual estimation. The incomplete polaiza- tion of the 1620-cm-'band suggeststhat the diad axis of Cr" symmetry). The polarization of this band in a direc- the HrO group is not perfectly aligned with the c axis in tion perpendicular to the c axis shows that the H-H vec- milarite. In other minerals such as joaquinite (Rossman, tor of the HrO group is aligned perpendicular to c. Like- 1975), in which an HrO group was proposedon the basis wise,the asymmetricstretch (2,) at 3581cm ' is polarized ofthe infrared spectra,the polarizations ofthe individual perpendicular to the c axis (Fig. 5a), confirming the H-H HrO bands were distinct. Likewise in cordierite (Gold- orientation as perpendicular to c. Next we must identify man et al., 1977),the type II HrO that most resembles the orientation of the plane containing the three compo- the HrO in milarite has distinct polarizations in its infra- ' nent atoms of the HrO group. This may be done from red spectra.It is difrcult to assignthe band at 3580 cm the fundamental modes, as the bending mode (zr) and the convincingly to another HrO species.If we try to do this,

(b) Milanj'te (a) Mi taF ite Foote Mine Foote Mine 1.5 K 296 K 296 o o o c €E @ o{n L o a 3 r.o o 0.5

00 0L 1300 4000 3800 3600 3400 3200 1800 1700 1600 1500 1400 (cn-1) l,{avenumbens(cn-l) v{avenumbens

(d) ite (c) t'4ilanite Mi lan Mine I Foote Mine 20 Foote 15 78K ll 78K o o U tl o c c @ tl o !to L'" I L I 3 ro o ! I 05

0 OL 1300 4000 3800 3600 3400 3200 3000 1800 7700 1600 1500 1400 1) l.lavenumtlens (cm- wavenumbens(cm-1) Fig. 5. Typical infrared spectraof milarite: (a) in the H.O principal stretchingregion, solid line llc, dotted line I c, sample 24 pm 296 K; (c) as a at p- rhi.k, 296 K 0) in the H.O symmetric bending region, solid line llc, dotted line l-c, sample 24 thick, 78 K; (d) as b at 78 K. HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP I 845

Tlele 9. Absorption band positions and values for H2O in mi- laritesamples Milarile Band position € values Polari- 52oo cm-1 E zation Val Giuf Rossing Foote E 0) V1 3516 llc, rc 93 55 (J c 1624 tlc 240 211 (d V3 3581 Ic 240 172 ../' v2+ v3 1922 5203 -Lc 4.3 a (v. * v")? 1948 5133 -Lc sh- sh- sh- 050 a Overtone 1477 6770 Ic 0.05 Overtone 1438 6954 llc,-Lc 0.11 Overtone 1403 7128 llc 0.08 'Weak overlappingshoulder on the 1922-nm band. 0 00]z 00 05 10 15 20 25 Watercontent (wt Va\ ite. From line in Figure 6, the e value Fig. 6. The correlationbetween the intensityof the 5200 the trend shown cm-' band(polarized Ic) and the HrO contentof milarites. from Beer's law is 4.2, a value significantly higher than that for (liquid) H,O. Goldman et al. (1977)have previ- ously noted that the intensity of this combination mode the lack of an asymmetric stretch in the Ellc spectrum is inversely related to the strength of the H bonding for would force the H-H direction to be llc; the observedEllc which the HrO group is the donor anion. The efect of spectrum has no significant absorptions at -5200 and this is to enhance the sensitivity of the spectroscopic -1620 cm ', and hence is not compatible with such an method to weakly bonded HrO compared with HrO in orientation. fluid inclusions. Two samples (Foote and Riissing) were made thin The intensities of the midinfrared fundamental absorp- enough so that the spectra of the fundamental bands (2,, tions of the HrO group were determined for the Rdssing v2,and z.) could be recordedon scale.With thesesamples, and Foote samples.Using the HrO content obtained from it was possible to test whether the polarization behavior the trend line in Figure 6, molar absorptivity (e) values ofthe 3520-cm 'band could be due to an unrecognized were calculatedand are given in Table 9; they are similar contribution from an OH component that, by coinci- in magnitude to those found for HrO in cordierite (Gold- dence,was superimposedon the 3520-cm-' HrO band. man et al., 1977). Although minor OH componentsare common in silicate minerals, their absolute intensity varies geatly among Crrntlr c,l'r RELATIoNS FoR MIr-ARrrE samples from different localities (e.g., olivine, Miller et Palache(1931) gave the ideal formula of milarite as al.,1987; pyroxene,Skogby et al., 1990).Thus it is to be (K,Na)CarBerAlSi,rOro.xHrO,x : 0.75. Cernf et al. expectedthat the ratios of the intensities of an HrO band (1980) showed that Be is a variable component through to those of a hypothetical OH band would vary from the substitution BNa * r2Be + B[ * r2Al. However, Cer- locality to locality. This is not the casewith milarite. The nj'et al. (1991)show REE cationsto be a significantsub- intensity ratios of the z, band and the two polarizations stituent into the milarite structure through the substitu- A(Y,REE) r2Be ACa of the /3 band were identical within error of measure- tion + + * "Al. Thus the largecation ment, indicating that it is unlikely that an OH component substitutions can be quite complicated, and the tetrahe- is the causeof the unusual polarization behavior of the dral framework can have significant variation in Be/(Be z, band. The spectraof these two sampleswere recorded + Al) ratio. Nevertheless,the total tetrahedral cations at 78 K. The peak height ofthe v, andv, bands(Figs. 5c must be 15 apfu in this structure type if vacanciesare to and 5d) nearly doubled; this was not just due to narrow- be avoided at the tetrahedrallv coordinated cation sites. ing ofthe bands becausethe integratedarea also increased by about 25o/oin both polarizations. The peak height and Framework-cationsums integrated area of the z, band increasedby about 330/0. It is apparent from the analysesofTable I that, based Thus these polarization and intensity anomalies remain on a formula unit of 30 anions, the sum of the fourfold- unexplained. coordinated speciesis not constant but varies in the range Using the HrO contentsgiven by Cern!'et al. (1980), 14.85-15.26apfu. There are two possibilitieshere: (l) there is a correlation between the intensity ofthe 5200- data for one or more components in the chemical anal- cm ' band (E perpendicular to c) and the (HrO) content ysesare in error; (2) a significantcomponent has not been (Fig. 6). In milarite from Rcissing,Namibia, inclusions of considered in the analytical procedures.As the analyses other hydrous minerals are common, and henceit is not spanboth sidesof ideality, the omission of any important surprising that data for it deviate from the trend line to component cannot be the problem becausethis would the side of higher analytical (H,O) content. Similarly, shift all the sums either up or down and would not result many inclusions were observedin the Guanajuato milar- in a wide spreadaround the ideal value, as is the casefor 1846 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP data for milarite. This leaves us with the possibility of O--O? error in one or more components, an error that is com- 12.O --j- l mon to many analyses.As some of the milarite samples I I ^'-' were analyzed both previously and as part ofthis work, z-' rt.g we can get some indication of errors that could causethis I = deviation from the ideal sum of 15 apfu. The milarite - sample furthest from ideality is sample 33, and both pre- -o 11.8 vious and current analysesindicate a fourfold-coordinat- a ed cation sum of 15.26apfu. As the analyseswere done by different methods in different labs, this indicates that the sourceof the error is unlikely to lie in the components o.4 0.6 0.8 points analyzedin both studies. This essentially to BeO Be/(Be+AD -+ as the culprit. Values of Be (and BeO) contents were cal- culated such that the sum of the framework cations is Fig. 7. Variationin Si (apfu)as a functionof Bel(Be+ Al) 15.00apfu for a 3O-anionrenormalization procedure, ex- in milarites;the chemicaldata are taken from Grn! et al. (1980). cluding HrO; these values are given as BeO. in Table l. The open circle is the analysisof Tittling milarite (no. 33 of given We note that this problem is not restricted to this study Cern! et al., 1980);the analysisof this milaritesample in doeslie on the observedtrend. but is alsofound in the data ofCern!'et al. (1980),which Table I showsthat this sample were measured in several different labs and also taken from the literature. The variation in framework cation sumsis 14.7l-15.25 apfu, reasonablysimilar to that ob- served in the present work. analysesof all these milarites have a grand mean Si con- An obvious correlation is apparentin the data ofCernj, tent of I 1.93apfu. The chemicalanalyses of Cernf et al. et al. (1980).Figure 7 showsthat the Si content(in atoms (1980) suggesta more complicated situation. Figure 7 per formula unit) is apparently related to the Be/(Be + shows the variation in Si apfu as a function of Bel(Be + Al) ratio, as a very well developed correlation is ob- Al); there is a well-developed positive correlation be- served. A similar correlation is apparent in the data of tween these parameters,indicating that the Si content of Table I for both Be and Be. values. We consider the milarites is not fixed at 12 atoms pfu but is a function of possibility ofBe + Si substitution in the hexagonalrings the Be/(Be + Al) ratio. It may be significant that the Be/ as unlikely; when Be substitutes for Si in a structure, it (Be + Al) ratio of normal milarites does not greatly ex- invariably totally replaces Si at one structural position ceed0.8, the point at which the observedtrend intersects (often causinga changein spacegroup) with no significant the maximum possible value of 12 Si apfu. Only the Si Be-Si solid solution at any one site. Alternatively, Al is value for the Tittling milarite sample (no. 33 of Cernj' et known to substitute for Si in other minerals of this struc- al., 1980) does not plot on the trend, and the reanalysis ture type, and the data do suggestsmall amounts of Al of this material given in Table I shows that it does in substitution for Si with variable Be content. fact plot on the observed trend. One particular milarite sampledoes not plot on the trend of Figure 7; no. 13 of Cnvsrar, cHEMrsrRY oF MILARTTE Cernj'et al. (1980),originally reportedby Iovchevaet al. It is desirableto know the steric responseof the struc- (1966), has only 10.76 Si apfu and an extremely large ture to variations in mineral chemistry, as this is very amount of Al (2.60 apfu), suggestingextensive substitu- useful for indicating chemical complications in a mineral tion of Al for Si at the Tl site. However, as this particular (for example, the unusual chemical nature of the Y-rich analysis is the only one of its type, further examination milarite from Jaguaragi,Brazll, was first recognizedfrom of this material is desirable. its unusual stereochemistry)and also for understanding How does the trend of Figure 7 agreewith our conclu- the structural controls on chemical substitutions in the sions relating to the (Tl-O) distancesin milarites? The mineral. Consequentlywe will examine in detail the ste- maximum Al substitution in the analysesof Figure 7 is reochemistry of the cation sites in milarite. 0. 16 Al apfu; this amount of Al would produce an in- creasein mean bond length of -0.0017 A, a very tiny The T1 tetrahedron amount that is within the spread of +0.002 A observed This is the tetrahedron of the [Si,rO3']double six-mem- for all the refined milarite structures.Thus, although the bered ring (Fig. 3). It sharesthree corners with adjacent chemical analysesdo suggesta very small amount of Al Tl tetrahedra and one corner with a T2 tetrahedron that + Si substitution at the Tl site in milarites, it is not links the [Si,roro] clusters into a framework. For the mi- enough to affect significantly the size of the Tl tetrahe- larites refined here and in the refinementsof Cernf et al. dron. (1980),the grand (Tl-O) distanceis l.6l I A with a total spread of +0.002 A. tnls indicates that, in all of the The T2 tetrahedron milarites examined here, there is no significant variation This is the tetrahedron that links the [Si,rO3.]clusters in the site occupancy of the Tl position. The chemical into a framework (Fig. 3); it sharesfour corners with ad- HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP t847

t I 1.67 I I I ^ 0.4 I1.66 o, .g : .E o.s o o. A 1.65 o sv .o E o.z I

0.6 0.8 1.0 -+ 1.65 1.67 Be/Ge+AD (r(2)-o)d) - Fig. 8. Variationin (T2-O) asa functionof Be/(Be+ Al) in milarites;filled symbolsindicate site occupanciesderived from unconstrainedsite-occupancy refrnement, and hollow symbols +I denoteoccupancies derived from the chemicalanalyses. I o.u € jacent Tl tetrahedra and two edgeswith adjacent A oc- o, tahedra. Previous refinements have shown that this site E o.+ E is occupied by variable amounts of Be and Al, and the o scattering observed at this site in the present study is c consonant with this assignment. € o.t The T2 site occupanciescan be determinedin two ways: 1' (l) Unconstrained site-occupancyrefinement can be used to determine the occupancy in terms of Be and Al. (2) 0.5 0.7 0.9 Based on data from the chemical analyses,all Be can be assignedto T2 and the remainder of the site filled with B.7(gg+AD+ Al; when Be + Al : 3, the contents must be normalized Fig.9. The variationin A-cationsplitting in milarites:(a) as to 3.0 apfu. a functionof the (T2-O) distance;(b) asa functionof Be/(Be+ Figure 8 shows the (T2-O) distance as a function of Al). the Be/(Be + Al) ratio for the structures refined in this study, using the results of the unconstrained site refine- ments to determine the Be/(Be + Al) ratio (solid circles); with three flanking T2 tetrahedra (Fig. 3), edgesthat are the ideal relationship for a hard-sphere model is also extremely contracted relative to the other edgesof both shown. Thus the size of the T2 tetrahedron is linearly the A octahedron and the T2 tetrahedron. In normal mi- related to the Be-Al contents.If (T2-O) is plotted against larite, this site is ideally completely occupiedby Ca; both the chemically analyzed,values of the Be-Al content (Fig. chemical analysesand the unconstrained site-occupancy 8, hollow circles; previous literature data are also shown refinementsconfirm this. In all the refinementsdescribed in this manner), there is far more scatter, although the here,the A cation showedextremely anisotropic displace- values do lie about the relationship defined by the refined ment parameterswith the long axis of the ellipsoid ori- site compositions and the hard-sphere model in Figure ented along the c axis; similar behavior was noted by 8. There are two possibilities here; either the milarite Cernj' et al. (1980). In the presentwork, this was modeled samplesare usually quite heterogeneousand the crystals by a split atom (cf. Kimata and Hawthorne, 1989; Arm- used for the structure work where in general not repre- bruster et al., I 989). The amount of splitting varies across sentative, or the Be analysesleave something to be de- the series(Fig. 9), using either (T2-O) or Be/(Be + Al) sired. Whichever is the case,it is apparent that the T2 as a measure of the Be content. In milarites 36 and 37, tetrahedral size respondsto variations in Be-Al occupan- the A site is partly occupiedby Y and REE. Nevertheless, cy- the constituent cation(s) still shows the splitting; and the amount of splitting still increasesas a function of the Be/ The A octahedron (Be + Al) ratio. The A octahedron lies between the [Si,rOro] clusters, In normal milarites, with a split-site model and vari- sharing corners with the Tl tetrahedra and further able anisotropic displacementparameters, unconstrained strengtheningthe framework linkage. It also sharesedges site-occupancyrefinement converged to complete occu- I 848 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP

TABLE10. Comparisonof refinedand chemically assigned B-site occupanciesfor the milarite samples refined here

Species 30 2.36 Na' 0.46(4) 0.64(4) 1 10(4) 0.89(6) Na 0.06 0.18 0.46 0.13 HrO 0.67 0.72 0.92 0.94 Ca 0.04 0.11 6 z.es K 0.05 0.06 (Na + H,O)"- 0.46 0.61 1.01 0.69 (Sum)t 0.61 0.71 1.01 0.88 ' Indicatesrefined occupancy. -. Effective scattering power of chemically analyzed (Na + HrO) (ex- 0.5 o.7 0.9 pressed as Na at sin d/I : 0.30). t Effectivescattering power of all scatteringspecies assigned to B from Be/(Be+Al) + the chemical analysis Fig. 10. The variationin (A-O) as a functionof Be/(Be+ Al) in milarites.

trosymmetric hexagonalsubgroups of P6/mmc. Thus there pancy of the A site by Ca. For the yttrium-REE milarite must be additional factors (e.g., spontaneousstrain) af- samples,the A-site occupancyconverged to values con- fecting the breaking of hexagonalsymmetry. sonant with the microprobe analysesof the crystalsused polyhedron for the X-ray intensity data collection. For normal mi- The B larites,the grand (A-O) distanceis 2.366 A wittr a total The B site lies on the threefold axis,between the [Si,ro3o] spreadof +0.010 A, but there is a pronouncedcorrela- clusters,and directly above and below the A octahedron tion (Fig. l0) with the Bel(Be + Al) ratio of the milarite, (Fig. 3), surrounded by nine O atoms. The ideal B site a significant inductive effect.The (A-O) distancesfor the occursat z : 0, and it has three O neighborsat -2.78 L yttrium-REE milarites are significantly less than the (A- and six O neighborsat -3.30 A. In the refinementsde- O) distancesin normal milarites, the effectof substituting scribed here (and in the refinements by Bakakin et al., Y3+(r: 0.90 A) for Ca (r: 1.00A) (Shannon,1976). 1975, and Cernj'et al., 1980),the B cationsshowed very Armbruster et al. (1989) have studied the reasonfor anisotropic displacementbehavior that was modeled by the A- and B-site splitting in milarite by refining the a split-atom position; note that for the B cation, the split- structures of both natural and dehydrated milarite. In ting is much greater than that observedfor the A atoms. dehydrated milarite, both the A- and B-site splittings do Bakakinet al. (1975)and Cernj,etal. (1980)have shown not occur. This can be rationalized as follows (Armbrus- that HrO is an important constituent at the B site. How- ter et al., 1989).If H,O occupiesthe split B site and the ever, small amounts of alkali and alkaline earth cations B cations occupy the central B site, dehydration will re- are also assumedto occupy this site. Indeed, Cernf et al. move the HrO from the split site but will not affect the (1980)show that the incorporationofcations in the ide- B cations at the central B site. This is what is observed ally empty B polyhedron is the only charge-compensation in dehydratedmilarite, leadingto the conclusionthat HrO mechanism for the incorporation of Be at T2 in excessof occupiesthe split B site. On dehydration, the splitting of the ideal amount of 2 apfu. Note that this does ignore the A site also doesnot occur. This suggeststhat the A-site substitutions of the type (Y,REE)A + Ber2 + CaA * Alr'?; splitting observed in milarite is the result of a ACa-BH,O however, theseare not of significancein normal milarites. interactionalong the 6-axis, and Armbruster et al. (1989) Armbruster et al. (1989) have shown that the split B site propose that the A site (: Ca) is coordinated by seven is occupied by HrO and that the B cations occupy the anions, with a Ca-HrO bond of 2.84 A augmentingthe central B site. In the structure of the dehydrated milarite six shorter (2.35 A) Ca-O bonds, and that the lowering sample,they also showedthat the B site doesincorporate of symmetry that gives rise to the well-known optical K. Table l0 comparesthe refined scatteringpower at the anomalies in milarite is due to the polar nature of the B site with the excesscations and HrO assignedto the B A-site coordination and the resulting disorder along ac. site from the chemical analysis; by and large, there is If this is the case,it is possible that there is a high-tem- reasonablygood overall agreementbetween the two sets perature ferroelasticphase transition in milarite. At high of data. temperatures,the A cation is eightfold coordinated; at a specifictemperature, there is the onset ofdisorder at the The C polyhedron A and B sites that breaks the symmetry and gives rise to The C site occurs on the sixfold axis at z : t/q,sand- the polar nature of the A-cation coordination. This tran- wiched betweentwo [Si,rOro]clusters (Fig. 3); the central sition is suggestedby the fact that the directional disorder atom is bonded to 12 O atoms, all of which are at a of the Ca-HrO bond is not sufficient to break the hexag- distanceof -3.0 A. ttris site is occupiedcompletely by onal symmetry, as such ordering is possible in noncen- K, as indicated by both chemical analysesand uncon- HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP 1849

1.64 A .s

I 1.63 "s o 1.62 : * o F N 1.61 I

0.26 o.27 o.28 0.29 (,)r(r)tAl-* o.20 0.40 0.60 (r) (A) -----+ Fig. 11. Variationin (T1-O) with constituentcation radius Fig. 12. Variation in (T2-O) as a function of constituent at the T1 site for the milarite-groupminerals; the filled circles cationradius at theT2 sitefor themilarite-group minerals; filled arethe meanvalues of the datafor milaritesand osumilite, and circlesare data pointsthat we considermore reliable,and the the line is drawnthrough these two points;other data are marked starsindicate data for the relatedstructures of lithium cesium by hollowsymbols, except the opensquare : armenite. beryl(Hawthorne and Cernj', 1977) and tuhualite (Merlino, 1969); datafor zektzeite(Ghose and Wan, 1978)also lies on the line; opencircle : sogdianite,open square: armenite,open diamond strained site-occupancyrefinements. Unlike the A and B : zinc milarite. cations, the C cation is not significantly displaced from the high-symmetry position at the center of the C poly- hedron. The Tl tetrahedron The polyhedron D Most of the milarite-group minerals have the Tl site The D site occurs at the center of the [Si,rOro]cluster ideally occupiedby Si. However, armenite, osumilite, and (Fig. 3), surrounded by 18 O atoms arrangedat the ver- yagiite incorporate significant amounts of Al at Tl, and tices of an augmentedhexagonal prism. In this study, no the (Tl-O) distanceshould reflectthis substitution.Fig- significant density was detected within this cavity, al- ure I I shows the variation in (Tl-O) as a function of Al though if small amounts of atoms were disordered about assignedto Tl from the chemical analyses.There is a this cavity, it would be extremely difficult to detect them well-developedlinear relationshipbetween tetrahedral size with conventional diflraction techniques. and Al content. The line shown in the frgure is not a least-squaresline but was drawn through the averageval- SrnnnocrrnMlsTRy oF THE MTLARTTE ues for milarite (this study; Cern! et al., 1980)and osumi- STRUCTURX, TT'PE lite (Armbruster and Oberhzinsli, 1988a; Hesseand Sei- As is apparent from Table 7, the group of double-ring fert, 1982).The slopeof this line is 1.0, the ideal value silicates with symmetry P6/mcc and general formula for a hard-spheremodel, and the relationship is given by t6lA2lelB2tr2lctr8lDI4lT23t4lTI r2O30(HrO), is quite large. Much (Tl-O) : 1.609* 1.0 (rr,) where(r,,) is the meancon- of the earlier mineralogical work on this group refers to stituent ionic radius of the Tl cations. This may also be the minerals of the milarite group, whereassome of the expressedas a predictive relationship for the amount of more recentwork refersto minerals of the osumilite group. Al at theTl site:Al,, : 7.71,t1-O) - 1.6091.This equa- Frequencyof usageinclines us to the former designation. tion could be refined somewhatby further structural work In addition, milarite has more of the possiblecation sites on well-characterizedsamples of armenite. filled [A, B, C, Tl, and T2] than does osumilite [A, C, Tl, and T2l, which makes milarite more useful as the The T2 tetrahedron type structure. Consequentlywe designatethese minerals The T2 site shows a tremendous range in occupancy as belonging to the milarite group. The cation sites and (Table 8) both with regard to size (B3* r Fe2+)and formal the range of speciesthat occupy them are summarized in valence(Li* ' 3r*;. Figure l2 showsthe (T2-O) distance Table 8. as a function of the constituent cation radiusl a linear 1850 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP

the analysisof the Wesselsmaterial, Li was not analyzed but was assumedto fill the T2 site; the equivalent isotro- pic displacementparameter supports this assignment.The original analysis of the Iwagi Islet sugilite indicated sig- nificant Al and Fe3"at the T2 site, but the observed (T2- O) distance was not compatible with this. Reanalysisof material (T. Ifuto, personalcommunication, 1984) I the I gave a significantly higher value for Li, but it was not I I sufficient to fill T2; however, the refinement indicated Li I | 2.20 essentiallycompletely occupyingT2. Refinementsof lwa- .3 gi sugilite and an Indian sugilite (T. Kato, personal com- munication, 1989)give values closeto the relationship of o I Cernf et al. (1980)and henceclose to the valuesfor the Wesselssugilite. Thus the values for sugilite also seem reliable. A reinvestigation ofsogdianite is desirable. 2.OO To summarize the situation for brannockite and sugi- lite, the T2 site is occupied completely by Li in both structures,with (T2-O) distancesof 1.922and 1.97I A, 0.60 0.80 1.00 respectively. The principal difference between these two structuresis that the B site is unoccupied in brannockite (r)d)+ and completely occupiedby Na in sugilite. Thus the pres- Fig. 13. Variation in (A-O) as a function of constituent cat- ence of Na at B in sugilite could inductively affect the ion radius at the A site for the milarite-group minerals; legend size of the T2 tetrahedron. Such inductive effectsare much as for Figure 12. The data points seem to define two separate more pronouncedfor low-valencecations such as Li than correlations. for higher-valencecations such as those that occupy T2 in the other milarite-group minerals; presumably this is correlation is apparent, and linear regressiongives the why such effectsare less noticeable or possibly absent in relationship the rest of the milarite-group structures. Data for poudretteite deviate from a linear relationship (T2-O): 1.344+ 1.039(r,,) R:0.992. in Figure l2 by -0.02 A and ye1the structure is very Despite this apparently good result, there are significant well refined.In fact, the data below (r) : 0.35 A (pou- discrepanciesthat suggestthat the situation may be more dretteite, milarite samples)define a very well developed complicated than it seems; there are significant devia- linear trend with a slope of -1.0, slightly less than the tions from linearity for several well-refined stnrctures, and slope for the total data set. It is possible that the rela- a more detailed discussionseems warranted. tionship is slightly nonlinear over the whole range. The (T2-O) values for brannockiteare -0.05 A less than those for sugilite and sogdianitewhen the T2 site in The A octahedron all theseminerals is occupied solely by Li. Irt us consider The A site also showsa wide rangeofcation occupancy the data for these refinements.There are two precise re- (Table 8) with a concomitant range of polyhedral size. finements for brannockite (Armbruster and Oberhiinsli, Figure 13 showsthe (A-O) distanceas a function of con- 1988b) giving similar results. In further support of this, stituent cation radius; ignoring the imprecise values for Hawthorne (unpublished data) refined the brannockite armenite and zinc milarite, we can fit a linear model with structure some years ago, and these results are in good a correlation coefficient of 0.983 and a form (A-O) : agreement with those of Armbruster and Oberhiinsli l.3ll + 1.066(r") where(ro) is the meanradius of the (1988b).The observedbond lengthsin brannockitewould constituent A cations. However, the deviations from lin- seem to be reliable. White et al. (1973) show that bran- earity are considerable;for example, data for both bran- nockite (from the Foote mine, the only known locality) nockite and osumilite deviate from the line by -0.04 A, is of ideal composition, indicating that the T2 site should an amount far greater than the precision of the values. be completely occupied by Li. This is also supported by On the other hand, a good linear model occurs if the the refinements,which give reasonableequivalent isotro- relationship is broken into two segments,as shown by pic displacementparameters for T2 completely occupied the full lines in Figure 13. The reasonfor this is not clear. by Li. Next we considersugilite. The structure of sugilite from The B polyhedron the Wessels mine, South Africa, was refined by Arm- In milarite itself, the B site plays a key role in the prin- bruster and Oberhiinsli (1988b), and that of sugilite from cipal substitution in the structure NaB * Ber2 + EB + Iwagi Islet, Japan,was refinedby Kato et al. (1976).The Alr2. In some of the other milarite-goup minerals, the B (T2-O) distancesin both refinements are essentiallythe site is empty (e.g.,brannockite); in others (e.g., sugilite), same,suggesting that the value given above is reliable. In it is full. When occupied by a cation, it is (always?)oc- HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP l85l cupied by Na, which often shows a displacement from es abundant celadonite and minor bavenite in the VEZn6 the center ofthe polyhedron along the c axis. complex pegmatite(Cernj', 1960, 1968),but no associ- ated minerals are evident in this generationof milarite at polyhedron The C Radkovice (Cernj', 1967).At Mar5ikov, Czechoslovakia, This polyhedron is occupied primarily by K (and pos- milarite crystallizeson schorl within an open fracture in sibly minor Na) in all the milarite-group minerals except a minor offshoot of the main pegmatite,accompanied by armenite, in which it is occupied by Ba. The occuffence bertrandite and adularia (Stanek, 1964); this occunence ofBa at the C site is relatedto the occurrenceofsignifi- seemsto be unrelated to alteration of any primary Be-rich cant amounts of Al at the Tl site; the higher valence minerals. In contrast, alteration of abundant beryl could cation at C helps satisfy the local bond-valenceefficiency have contributed to the crystallization of milarite in the at the 02 anion causedby the substitution of the lower- Jaguaragirpegmatite, Minas Gerais, Brazil, although no valenceAl at the Tl site. direct spatial relationship is apparent. Yttrian milarite is found here in an alpinelike replacementunit, coating al- P.c,RAcnNnsrsoF MTLARTTE bite in associationwith adularia. late albite. hematite ro- Milarite is found in a variety of environments, from settes, muscovite, and quartz, and locally with musco- vugs in plutonic rocks through diverse pegmatitesto hy- vite, quartz, and acicular tourmaline; minasgeraisiteis a drothermal ore deposits and alpine veins. Before we can late phase covering the milarite crystals (Foord et al., make any generalobservations, a briefreview ofspecific 1986). The parageneticrelationships of milarite in the localitiesis in order. amazonite pegmatiteat R6ssing,Namibia, are not known (von Knorring, 1973). Principal milarite localities Occurrencesof milarite in hydrothermal vein deposits Postorogenicto anorogenicintrusions of alkaline affin- are rather uncommon. In east Siberia, milarite is an al- ity, together with their interior pegmatites, constitute a teration product of phenacite and bertrandite, both pri- distinct type of milarite-generatingenvironment. Vugs in mary phases of the main Be-CaF, mineralization. The nordmarkite at Ramnes and in the Grorud and Tysfiord milarite formed during later sulphide-carbonate alter- districts of Norway (R. Kristiansen, personal communi- ation. It has bastnaesiteinclusions, grows on albite and cation, 1989; Oftedaland Saeb6,I 965; Raade,I 966)and calcite, and is subsequentlyaltered to calcite or opal + miarolitic pegmatitesin the Strzegomgranite, Poland (Ja- chalcedony pseudomorphs (Novikova, 1972). In central neczek, 1986), the Kent granite in Central Kazakhstan Asia (Iovchevaet al., 1966),phenacite-bearing albite veins (Chistyakova et aI., 1964), the Conway granite, New with fluorite, sulfides, epidote, tourmaline, cassiterite, Hampshire(J. S. White, personalcommunication, 1985), calcite, prehnite, and bavenite have milarite as a late and the Strange Lake syenite, Labrador (Cernf et al., mineral. Its crystallization was accompanied by recrys- 1991),all have milarite on a potassiumfeldspar-quartz tallization of adjacent phenacite,fl uorite, and potassium substrate associatedwith albite, zeolites, fluorite, stilp- feldspar,the latter being transformed into adularia. In the nomelane, wulfenite, and rarely other Be minerals such epithermal Ag-Au deposit of the Valencia mine, Guana- as beryl, bazzite, helvite, bertrandite, phenacite,or bave- juato, Mexico (Jensenand Bateman,l98l), milarite crys- nite. Milarite is also found in marble xenoliths in a neph- tallized with adularia and quartz and is overgrown by eline-syenitebreccia at Mont St. Hilaire, Quebec (Grice calcite; fluorite, barite, and zeolites are also present in et al., 1987). subordinate quantities. Low-temperature veins at Hen- In peraluminous rare-element pegmatites, milarite is neberg,Thuringia (Heide, 1953),have milarite in asso- locally generatedas an alteration product of beryl. This ciation with barite, fluorite, chalcopyrite, calcite, and mi- mode of occurrenceis well documentedin the quartz oli- nor beryl, bertrandite, and bavenite. All of these four goclasiteatYELn6, Czechoslovakia (Cernj', 1963, 1968), diverse occurrenceshave a common denominator, a ge- where milarite is associatedwith bavenite and cerolite. netic connection with subalkalineleucogranites, or in the Much less information is available on one of the two case of Guanajuato, A-type rhyolites of related compo- generationsof milarite recogrrizedat Radkovice, Czecho- sition (Burt and Sheridan,1988). slovakia (Cernj', 1967),but it also seemsto be generated In contrast to the apparent low frequency of milarite by alteration of beryl. No obvious relationship with al- in mineralized hydrothermal ore' deposits, milarite is tered beryl was reported from Tittling, Bavaria (Tenny- widespread in the alpine mineral veins of Switzerland son, 1960),or from the Kola peninsula,USSR (Sosedko, and Austria. This is the type environment of milarite, 1960),although pseudomorphsafter beryl have been rec- one that is representedby a large number of localities. ognized at both localities. At Tittling, milarite is associ- They have been extensively reviewed by Parker (1954, ated with albite, muscovite, and zeolites;in the Kola peg- 1973)and Hiigi and Rowe (1970)for Switzerlandand by matite, milarite occurs on microcline and albite and is, Niedermayr (1978, 1979) for Austria. In general,milar- in turn, overgrown by chabaziteand bavenite. ite-bearing veins are formed in syenitic, granitic, and ap- Another mineral for which milarite is a breakdown litic rocks. Milarite is associatedmainly with quartz, ad- product is beryllian cordierite (Al3* + I - Be2* + Na+; ularia, and chlorite; associationswith apatite and fluorite Cernj'and Povondra, 1966).This alterationalso produc- are lesscommon, and thosewith smoky q:uartz,hematite, l 852 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP albite, calcite, actinolitic asbestos,and monazite are rare. and nature ofthe coprecipitating phasesthat compete for In addition, replacement or alteration features are not the relevant cations. observed. P,rucnNnsls oF orHER MEMBERSoF THE General synthesis MILARITE GROUP This brief review of important localities clearly indi- Some minerals of the milarite group are low-tempera- catesthat there are four principal types of milarite para- ture, low-pressurephases as is milarite itself. However, genesis.It is less conclusive in terms of specific parage- the compositional flexibility of the basic structural frame- netic relationships, as these are usually insufficiently work is reflected in the broad variety of environments described.Nevertheless, the broader mineral associations hosting other milarite-type minerals, in many casesre- are fairly well characterized,and it is possible to draw sulting in stabilization in very diverse P-Z regimes. some general conclusions about the circumstancesand Most of the minerals listed in Table 7 occur in peral- conditions of milarite crystallization: kaline rocks: darapiosite, sogdianite, sugilite, eifelite, l Milarite is a low-temperature mineral of hydrother- poudretteite, roedderite, and the closely related species mal origin. The composition of the associatedadularia tuhualite, zektzeite, and emeleusite. Host rocks range (Cernjrand Chapman, 1984, 1986)and typical crystalli- from protomagmatic assemblagesin plutonic intrusions zation conditions of other associatedminerals suggestthat (sugilite and emeleusiteat their type localities; Murakami milarite crystallizesat temperaturesbelow 300 "C. et al., 1976;Upton et al., 1978)through pegmatiticas- 2. Milarite is a low-pressure mineral, typically found semblages(sogdianite and darapiosite; Dusmatov et al., in vugs of shallow-seatedalkaline intrusions or their in- 1968; Semenovet al., 1975) and miarolitic pegmatites terior pegmatites,in mesothermaland epithermal ore de- within parent intrusions (zektzeriteand sogdianite;Dunn posits, and in cavities in alpine veins. Rare-elementpeg- et al., 1977 Boggs, 1986) to melt-coated cavities in matites seem to be an exception, as the characteristic gneissicxenoliths ejected in leucite-tephrite lavas (roed- pressure range of crystallization is 4-2 kbar (London, derite and eifelite;Hentschel et al., 1980;Abraham et al., 1986). However, subsolidusprocesses at <300 oC can 1983).Poudretteite is found in marble xenolithswithin a take place long after thermal equilibration of the peg- nepheline-syenitebreccia (Grice et al., 1987).The only matites with their host rocks (usually of lower amphibo- exception from the overall magmatic afrliation is the Af- lite or, rarely, upper greenschistfacies) during incipient rican occurrence of sugilite in an obviously metamor- erosion and regional uplift of the host terrane (London, phosed manganeseore deposit (Wesselsmine, Kalahari; 1986).Under thesecircumstances, pressures below 2 kbar Dunn et a1.,1980). However, closeassociation with ac- are to be expected. mite indicatesan alkaline environment analogousto those 3. The composition of milarite, an alumino-beryllo- of the igneousoccurrences. silicate of K, Na, and Ca, and the nature of its common In accord with their affiliation to peralkaline magmas, associatessuch as adularia, albite, calcite, fluorite, chlo- all of the above minerals are Al poor to Al free, and most rites, and zeolites strongly suggestthat milarite crystalli- have (K,Na) from 2 to 4 apfu. Sugilite is the only excep- zes from alkaline fluids. Other beryllo-silicates and tion with I (K,Na) pfu, correspondingto the ideal milar- alumino-beryllo-silicates (such as bavenite or epididy- ite formula. Heterovalent substitutions involving Li, Mg, mite-eudidymite) tend to be contemporaneouswith mi- Fe, Zn, Mn, Zr, and Ti compensatefor the Al, except in larite, whereassilicates of Be that are indicative of a low- the case of homovalent B cations in poudretteite. The pH environment (euclase,phenacite, bertrandite) precede presence of Li in darapiosite, sogdianite, sugilite (and or follow the crystallization of milarite (Cernf, 1968, zektzeite) should also be emphasized: low concentra- l 970). tions of Li in peralkaline igneous suites commonly find 4. Fluorite and calcite are among the most persistent their mineralogical expressiononly in pegmatitic stages associatesof milarite in all environments (although they (e.g.,protolithionite) to which most occurrencesof these are not common in milarite-bearing alpine veins). This four minerals are related. The pressureof crystallization supportsthe suggestionsofBeus (1960) that fluor-com- rangesfrom epizonal plutonic to extrusive volcanic, and plexes and carbonate complexes could have been in- early magmatic to late-stagepegmatitic temperaturescan volved in the transportation ofBe in the fluid phase. be assumed. 5. Y is a typical minor-to-substantial component in Another well-defined genetic group consists of three milarite (Cern!'et al., 199l). Y and HREE are also com- rare minerals from meteorites: yagiite in a small silicate plexed and mobile in F- and COr-rich fluids (Taylor and inclusion in an iron meteorite, merrihueite as inclusions Fryer, 1983). in clinopyroxenes,and roedderite (besidesits only terres- 6. In contrast to the above conclusions,there seemsto trial occurrencementioned above) in enstatite chondrites be no simple relationship between parent environment and octahedrites(Dodd et al., 1965;Fuchs et al., 19661' and the extent of the principal Be2* + Na* = Al3* + tr Bunch and Fuchs, 1969). All three minerals are Al poor substitution in milarite (Cernf et al., 1980). The Be con- to Al free with I (K,Na) pfu and are distinctly rich in Mg tent of milarite is evidently governed by such factors as and Fe. No specific data are available for the conditions the activities of its principal componentsand the number of crystallization, although the frequent associationwith HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP r 853 tridymite may be significant for estimates of minimum (Euler and Hellner, 1957; Schreyerand Seifert, 1967). temperature of formation. Experimental work indicates This led the latter authors to conclude that metamorphic very extensive stability fields in the anhydrous systems osumilite might be metastable. It was the more recent relevantto most meteorites(stable to - I 100'C and - 32 work of Hensen(1977) and Oleschand Seifert(198 l) in kbar; Seifertand Schreyer,1969).This would explainthe conjunction with petrological studies (Berg and Wheeler, occurrencesof these minerals in the broad range of me- 1976;Ellis, 1980;Grew, 1982;Arima and Gower, 1987) teorite categoriesmentioned above, from those solidified that establishedthe stability of osumilite in both types of from high-temperature melts at high pressures(such as environments. Osumilite is stableunder near-to-total an- octahedrites) to probable low-temperature condensates hydrous conditions of relatively high-temperature gran- (chondrites). ulite facies metamorphism (>750 "C, P,o,< 8-9 kbar; The remaining mineralsare decidedlyindividual in their Grew, 1982)and in hydrousmagmas at < I kbar (Olesch modes of occurrence,a feature that seemsdictated pri- and Seifert, l98l). Thus the stability fields of osumilite marily by geochemicalconsiderations. resemble those of roedderite-merrihueite (Seifert and Armenite requires Ba-rich environments. Thesecan be Schreyer, 1969). Experimental evidence (Schreyer and provided by hydrothermal Ag-bearingsulfide mineraliza- Seifert. 1967: Oleschand Seifert. l98l) and natural oc- tion (Neumann, l94l), hydrothermalveins in altereddi- currences(Olsen and Bunch, 1970;de Michele, 1974;Berg orite (Pouliot et al., 1984), hydrothermal alteration of and Wheeler, 1976:Armbruster and Oberhiinsli,1988a, granitoid and volcano-sedimentaryrocks (Semenenkoet 1988b) indicate that osumilite is not stablein hydrous al., 1987),and hydrothermalveining of chemicalor vol- low-temperature conditions. The breakdown products caniclastic sedimentshosting metamorphosedsulfide de- seem to be similar to those generatedat the expenseof posits (Balassoneet al., 1989; Zik and Obst, 1989).In cordierite in comparable retrogressiveenvironments. two of the above occurrences,the temperature of arme- nite crystallization was estimated as less than 400-300 Suvrvr.q.nv 'C. The only exception to this low-temperature environ- Milarite is the prototype mineral for a large structural ment seemsto be the gneiss-likecelsian-rich assemblages group of tetrahedralframework minerals involving a wide from Broken Hill (Mason, 1987). variety of other tetrahedrally coordinated cations in ad- Brannockite is known only from its type locality, the dition to Sia*.The following points are of importance: Foote mine spodumene pegmatites at Kings Mountain, 1. The general formula for this structure type can be North Carolina (White et al., 1973).It is found in vugs written ast6rA2terB2rr2rct'81D[t4]T23r4lTl r2O30lwhere A : Al, and open fissureswith bavenite, tetrawickmannite, py- Fe3*,Sna*, Mg,Zt, Fe2*,Ca, Na, (Y, REE);B: Na, HrO, rite, stannian titanite, albite, and qvartz, an assemblage !, Ca(?),K(?);C: K, Na, tr, Ca(?);D: tr;Tl : Si,Al; of apparent low-temperature hydrothermal origin. The andT2: Li, Be, B, Mg, Al, Si, Mn2*,Zn. presenceof considerableLi in this mineral is not surpris- 2. The general formula of milarite can be written as ing in view of the spodumene-rich wall rocks; however, (K,Na),K(Ca,Y,REE)r(Be,Al)3SirrO3o(HrO),. Al was evidently not mobilized in significant quantities 3. The important substitutions in milarite are BNa + by the low-temperature fluids, despite the peralumrnous r2Be = BE + r'?Al and A(Y,REE) + r'zBe= ACa * r2Al. envlronment. The first is more common, but the secondcan be signif- Last but certainly not least,osumilite deservesa special icant in REE-enriched environments, and a REE species note becauseof its (Mg,Fe)-rich peraluminous composi- seemspossible. tion, considerable range of composition, and the wide 4. H2O is always presentin milarite and armenite; it is variety ofits occurrences.On the one hand, it occursboth rarely (ifever) present in other speciesofthis group. as anhedral phenocrystsin the groundmassand as cavity- 5. The (T2-O) distanceis linearly dependenton the lining crystals associatedwith tridymite and quartz in Be/(Be + Al) ratio, supporting the structure refinement acid volcanicrocks (Miyashiro, 1956;Rossi, 1963;Olsen results that indicate direct Be = Al substitution at Ihe T2 and Bunch, 1970; Stankevich.1974: de Michele. 1974: srte. Yokomizo and Miyachi, 1978; Hochleitner, 1982., 6. In milarite, the A cation (Ca,Y,REE) always shows Schreyeret al., 1983;Parodi et al., 1989)and in buchites positional disorder about the central position, even when (Chinner and Dixon, 1973; Schreyeret al., 1986;Arm- the site is completely occupied by Ca. The observedav- brusterand Oberhiinsli,1988a). On the other hand, sev- erageseparation is a function of the Be/(Be + Al) ratio. eral localities have recently been identified in high-grade 7. In milarite, the B site is occupied by (HrO,Na,K) metamorphic rocks (Berg and Wheeler, 1976; Maijer et and shows positional disorder that is a function of the al., 1977; Hensen, 1977; Bogdanovaet al., 1980; Ellis, Be/(Be + Al) ratio. 1980;Ellis et al., 1980;Grew, 1982;Arima and Gower, 8. In milarite, the C site is completely occupied by K, 1987). Thus the conditions of crystallization range from and there is no positional disorder at this site. near-solidusvolcanic to granulite-faciesmetamorphic. 9. In the minerals of the milarite group, there is Al + The stability of osumilite was a contentious problem Si substitution at Tl and a wide variety of substitutions until the early 1980s,as initial attemptsat synthesisgave at T2. The sizes of the coordination polyhedra are lin- results seemingly irreconcilable with natural occurrences early related to the ionic radii ofthe constituent cations. l 854 HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP

10. There are four principal types of milarite paragen- Crystal chemistry of double-ring silicates: Structure of natural and de- esis: (a) postorogenicand anorogenic intrusions of alka- hydrated milarite at 100 K. EuropeanJournal ofMineralogy, l, 353- line affinity, together with their interior pegmatites, (b) 362. Bakakin, V.V., Balko, V.P., and Solovyeva,L.P. (1975)Crystal structures peraluminous rare-elementpegmatites, (c) hydrothermal of milarite, armenite, and sogdianite.Soviet PhysicsCrystallography, vein deposits, and (d) alpine-type veins. 19,460-462. I l. Milarite is a low-temperature(<300'C), low-pres- Balassone,G., Boni, M., Di Maio, G., and Franco, E. (19E9) Armenite sure (usually <2 kbar) mineral of hydrothermal origin. in southwest Sardinia: First recorded occurrence in Italy. Neues Jahr- buch fiir Mineralogie Monatshefte, 1989, 49-58. 12. Milarite occurs in alkaline environments, and its Berg, J.H., and Wheeler, E.P. (1976) Osumilite of deep-seatedorigin in persistent association with fluorite and calcite suggests the contact aureoleof the Nain complex, labrador. American Miner- that fluoro- and carbonate complexes were involved in alogsst,6l, 29-37 . the transportofBe. Beus,A.A. (1960) Geochemistry of beryllium and geneticrypes of beryl- Akademia Nauk SSSR,Moscow (in Russian). 13. Y and REE can be significantcomponents in mi- lium deposits. Bissert, G., and Liebau, F. (1987) Crystal structure of [N(n- larite. CoHr)41H,[Si.O,o]5.33H,O-a zeolite AJike double-ring silicate with 14. For the milarite-group minerals, the compositional protonated water clusters [Ho,O,J'*. Zeitschrift fiir Kristallographie, flexibility of the basic structural framework is reflectedin 179,357-37L M.K., the broad variety of environments hosting these miner- Bogdanova,N.G., Troneva, N.V., Zaborovskaya,N.8., Sukhanov, and Berkhin, S.I. (1980) The first find of metamorphic osumilite in the als. There are four principal parageneses:(a) Most min- USSR. Doklady Akademia Nauk SSSR,25O,690-693 (in Russian). erals occur in peralkaline rocks, ranging from protomag- Boggs,R.C. (1986) Miarolitic cawityand pegmatitemineralogy of Eocene matic assemblagesin plutonic rocks through various anorogenicgranite plutons in the northwesternU.S.A. FourteenthGen- classesof pegmatitesto xenolithic assemblages.(b) A small eral Meeting, Abstractswith Programs,58. Intemational Mineralogical Association, Stanford. group (yagiite, merrihueite, roedderite) occurs in mete- Brown, G.E., Jr., and Gibbs, G.V. (1969) Refinementof the crystal struc- orites. (c) The occurrenceofa small group ofvery unusual ture of osumilite.American Mineralogist, 54, l0l-116. composition (e.9., armenite, brannockite) is dictated by Bunch, T.E., and Fuchs, L.H. (1969) Yagiite, a new sodium-magnesium geochemicalfactors. (d) Osumilite showsa wide range of analogueof osumilite. American Mineralogist, 54, 14-18. with topaz occurrences in both acid volcanics and in high-grade Bun, D., and Sheridan, M. (1988) Mineralization associated rhyolites and related rocks in Mexico. In R.P. Taylor and D.F. Strong, metamorphic rocks. Given the appropriate bulk compo- Eds., Canadian Institute of Mining and Metallurgy Recent advances in sition, a great diversity ofconditions can give rise to the the geologyofganite-related mineral deposits,39, 303-306. crystallization of minerals with the milarite structure. New Cernj', P. ( I 960) Milarite and wellsite from V€2n6.Prace Brnen Zakladny specieshave recently been found at a considerablerate, Csav.32.1-14. - (1963) milarite-alteration products ofberyl from and increasing Epididymite and interest in alkaline rocks has shown the VEZnd, Czechoslovakia. Mineralogical Magazine, 33, 450-457 . importance of theseminerals not only as accessoryphases -(1967) Notes on the mineralogy of some West-Moravian pegma- but also as rock-forming minerals. tites. Casopispro Mineralogii a Geologii, 12, 461-463 (in Czech). - (1968) Berylliumminerale in Pegmatitenvon V€Zndund ihre Um- AcxNowr-nocMENTS wandlungen Berichte der Deutschen Gesellschaft fiir Geologische Wis- senschaften,Bl 3, 565-578. would (CANMET, We like to thank J.L. Jambor Ottawa) for the Strange - (1970) Review ofsome secondaryhypogene parageneses after early Lake sample,R.V. Gaines for the Jaguaragirsamples, and John S. White pegmatite minerals. Freiberger Forschungshefte C270, Mineralogie und (Washington). for the samplesfrom the NMNH-Smithsonian Institution Lagerstettenlehre,47-67. W.W. with of the spectroscopicwork. Gould assisted the early stages Cernf, P., and Chapman, R. (1984) Paragenesis,chemistry and structural provided Engineer- Financial assistancewas by the Natural Sciencesand stateofadularia from granitic pegmatites.Bulletin de Min6ralogie, 107, (operatinggrants P.C.; a ing ResearchCouncil of Canada to F.C.H. and 369-384. gmnt, major equipment a University ResearchFellowship, and an Inlia- -(1986) Adularia from hydrothermal vein deposits: Extremes in installation grant P.C.) and the structure $ant to F.C.H.; and a major to structural state.Canadian Mineralogist, 24, 7 17-7 28. NSF grantEAR-8618200 to G.R.R. Cemj', P., and Povondra, P. (1966) Beryllian cordierite from V€zne: (Na, K) + Be - Al. Neues Jahrbuch fiir Mineralogie Monatshefte, 1966, RnrpnnNces crrED 36-44. Abraham, K., Gebert, W., Medenbach,O., Schreyer,W., and Hentschel, Cern9, P., Hawthorne, F.C., and Jarosewich,E. (1980) Crystal chemistry G. (1983) Eifelite, KNarMgSi,,Oro, a new mineral of the osumilite of milarite. Canadian Mineralogist, 18, 4l-57 . group with octahedral sodium. Contributions to Mineralogy and Pe- Cerni, P., Hau'thorne, F.C., Jambor, J.L., and Grice, J.D. (1991) Yttrian trology, 82, 252-258. milarite. Canadian Mineralogist, in press. Arima, M., and Gower, C.F. (1987) Osumilite-bearingmetasedimentary Chinner, G.A., and Dixon, P.D. (1973) Irish osumilite. Mineralogical gneiss in the Grenville Province, eastern labrador. Geological Asso- Magazine, 39,189-192. ciation of Canada/MineralogicalAssociation of CanadaAnnual Meet- Chistyakova, M.B., Osolodkina, G.A., and Razmanova, Z.P. (1964) IN{i- ing, Saskatoon,Program and Abstracts, 12, 22. larite from Central Kazakhstan.Doklady Akademia Nauk SSSR,159, Armbruster, Th. (1989) Crystal chemistry ofdouble-ring silicates:Struc- 1305-1308(in Russian). ture ofroedderite at 100 and 300 K. EuropeanJournal ofMineralogy, de Michele, V. (1974) Qualche osservazioneparagenetica sull'osumilite 1,715-718. del Monte Arci (Cagliari).Natura, 64, 39-45. Armbruster, Th., and Oberh2insli,R. (1988a)Crystal chemistry of double- Dodd, R.T, van Schmus,W.R., and Marvin, U.B. (1965) Menihueite, a ring silicates:Structural, chemical and optical variation in osumilites. new alkaliferro-magnesian silicate from the Mezo-Madaras chondrite. American Mineralogist, 73, 585-593. Science.149. 972-974. -(1988b) Crystal chemistry of double-ring silicates: Structures of Dunn, P.J., Rouse,R.C., Cannon, B., and Nelen, J. (1977) Zektzeitei A sugilite and brannockite. American Mineralogist, 73, 594-600. new lithium sodium zirconium silicate related to tuhualite and the Armbruster, Th., Bermanec,Y., Wenger, M., and Oberhiinsli, R. (1989) osumilite group. American Mineralogist, 62, 41 6-420. HAWTHORNE ET AL.: CRYSTAL CHEMISTRY OF THE MILARITE GROUP I855

Dunn, P.J., Brummer, J., and Belsky,H. (1980) Sugilite,a secondoccur- from central Asia. Doklady Akademia Nauk SSSR, 170, 1394-1397 rence:Wessels mine, Kalahari manganesefield, Republic of South Af- (in Russian). rica. Canadian Mineralogist, 18, 37-39. Janeczek,J. (1986) Chemistry, optics, and crystal Srowth of milarite from Dusmatov, V.D., Efimova, A.F., Kataeva, 2.T., Khoroshilova, L.A., and Strzegom,Poland. Mineralogical Magazine, 50, 27 1-27 7. Yanulov, K.P. (1968) Sogdianite,a new mineral. Doklady Akademia Jensen,M.L., and Baternan, A.M. (1981) Economic mineral deposits. Nauk SSSR, 182, ll76-1177 (in Russian). Wiley, New York Ellis, D J. (l 980) Osumilite-sapphirine-quartzgranulites from Enderby Kato, T., Miura, Y., and Murakami, N. (1976) Crystal structure of sugi- Land, Antarctica: P-T conditions of metamorphism, implications for lite. Mineralogical Journal, 8, lSzt-192. garnet-cordieriteequilibria and the evolution of the deep crust. Con- Khan, A.A., Baur, W.H., and Forbes,W.C. (1971) Synthetic magnesium tributions to Mineralogy and Petrology, 74,201-210 merrihueite, a dipotassium pentarnagnesium dodecasilicate: A t€tra- Ellis, D.J., Sheraton, J.W., England, R.N., and Dallwitz, W.B. (1980) hedral magnesiosilicate framework crystal structure. Acta Crystallogra- Osumilite-sapphirine-quartz gfanulites from Enderby Land, Antarcti- phica,B28, 267-272. ca-mineral assemblagesand reactions. Contributions to Mineralogy Kimata, M., and Hawthorne, F.C. (1989) The crystal chemistry of milar- and Petrology,72, 123-143. ite: Two split-site model. Annual Report of the Institute of Geosci- Euler, R., and Hellner, E. (1957) Hydrothermale and roentgenographische ences,University of Tsukuba, 15, 92-95. Untersuchungenan gesteinbildendenMineralen-V. Uber hydrother- Kuschel, H. (1877) Mitteilung an Prof. G. konhard. Milarit. NeuesJahr- mal hergestelltenOsumilith. 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