Canadian Mineralogitt Vol. 20, pp. 65-75 (1982)

A MINERATOGICALSTUDV AND CRYSTAL€TRUCTUREDETERMINATION OF NONMETAMICT EKANITE,ThGa,SiaO,o

J.T. SZYMA(SKI eno D.R. owENs Mineral Sciences Laboratories, CANMET, Department of Energy, Mines and Resources, 555 Booth Street, Ottawa, Ontario KlA 1GI

A.C. ROBERTS AND H.c. ANSELL GeologicalSurvey of Canada,60I Booth Street,Ottawa, Ontario KlA 0F,8

GEORGE Y. CHAO Department ol Geology, Carleton llniversity, Ottawa, Ontario KlS 586

ABsrRilcr gemme m6tamicte trouvee pour la premibre fois A Sri Lanka (Ceylan), a 6t6 d6couverte au mont Nonmetamict ekanite, ideally ThCa2SirOro, of Tombstone (Yukon). On d6crit les propri6t6s phy- chemical composition very similar to the meiamict siques et optiques et fon en donne le diagramme ekanite originally found in Sri Lanka de poudre aux rayons X. L'6kanite cristalline (Ceylon), est has been discovered in the Tombstone t6tragonale,et fait partie du proupe spatial 1422, Mountains, Yukon Territory. The physical'the and aveca 7.483(3),c 14.893(6)A. On en a r6solula opti.ral properties are described and X-ray structure, affin6e jusqu'e R - 3.57Vo slur l3l9 powder pattern is given. Crystalline ekanite is tetra- reflexions ind6pendantesobtenues h partir de mul- gonal^ space grotp 1422, with a 7,483(3), c 14.g93 tiples..s6ries de donn6es avec rayonnement de (6) A. The structure has been solved and refined - MoKa. Sa structure est 6troitement associEeb celle to R 3,57Vo from 1319 independent reflections de la famille de composition gdn6raleThK(Na,Ca) obtained from multiple data sets with MoKa radia- SLO2g,qui cristallise dans Ie groupe spatial P4,/ tion. The structure is closely related to that of the n cc et qu'on a appel6e, i tort, '6kanite" pendant lqity nith general composition ThK(Na,Ca) de nombreusesann6es. La v6ritable €kanite'cristal- SL9.r which crystallizes in p4fmcc line possdde dimen- ,.ekanite',-for une maille centr6e dont les and which has been misnamed manv sions sont presqueidentiques i celles de la matidre years. True crystallline ekanite has a body-centred obtenue lorsqu'on chauffe i plus de 650oC f6kanite unit cell whose dimensions agree closely irith that m6tamicte. tr faut remarquer que la coordinence of the material obtained on hoating meiamict eka- autour des m6taux est la m€me dans les deux nite to above 650oC. The metal io-ordination is types de structures: le Th, en coordinence 8, se lgmqrkably similar in the two types of structures; trouve eqtour6 d'a.tomes d'oxygbne distants de Th is 8--co-ordinated in a square antiprism of 2.405(5) E et fomrant un anti-liisme i base car- oxygen atoms at 2,405(5) A, and Ca has four r&,; le Ca^a quatre premiers voisins tras proches nearest _oxygpn neighbors 12.942(5) Al in a very 12.342(5)Al en t6tpCdre difforme et quatre seconds distorted tetrahedron and four second-niarest neieh- voisins [2.688(5) A] prds de points interrr6diaires bors [2.688(5) A] near the midpoints of the faies des faces du t6traddre. Les feuillets de m6tal A of the tetrahedron. Sheets of mCtab at z = O, 14 z = 0, t/z sont #par€s par une couche ddform6e are-sepa_rated by a puckered silicate layer that ex- de silicate. La structure est caract6ris&, comme tends infinitely in r, y. The structuie ii character- celle des z6olites, par des canaux qui traversent ized by zeoliteJike channels through the silicate les coucbes de silicate, oi I'eau non structurale layers, where non-structural watei can become peut Stre pi6g6e. entrapped. Mots-cl6s: dkanite non m6tamicte ThCa2SieOro,d6- Keywordt nonmetamict ekanite, ThCarSipro, crye- termination de la structure, dispersion anormale, tal-structure determination, anomalous dispersion, analyse chimique, diagramme de poudre aux chemical analysis, powder XRD pattern. orooer- rayons X, propri6t6s, mont Tombstone (Yukon). ties, Tombstone Mountains, yukon feniiorv. INm,oDucrroN SoMMAIRE The mineral ekanite (Th,U) (Ca,Fe,pb)dia L'6kanite non m6tqmicte, en th6orie TbC-arSirOo, Oro was first reported (Anderson et al. 196l) de composition chimique trds semblable b fakanite as a green transparent to translucent metamict 65 66 TIIE CANADIAN IVIINERAI.OGIS'T gemstone. It was described in great detail by et al.' 1965, Mokeyeva & Golovastikov 1966). Giibelin (1961, 1962) n terms of its physical The present material, found in a boulder in properties, radioactivity, chemical composition, the Tombstone Mountains, Yukon Territory, inslusions and gem qualities. The original re- has a totally different structure from any of the port of Anderson et al. (1961) stated that "At P4/mcc minerals to which the narnes "ekanits", temlpratur€s bef,ween650 and 10O0oC,the min- "ekanite-family" or "kanaekanite" have been eral re-crystallizes to a phase of whicb applied. Furthermore, it can be identified di- X-ray powder and single-crystal diffraction pat- rectly by its Debye-Scherrer powder pattern witb tems can be indexed on the basis of a body- the thermally recrystallized metamict ekanite of centred tetragonal cell having dimensions a Anderson el o/. (1961; priv. comm. from 7.46, c 14.96 A. The density after heating Miss E.J. Fejer to J.T. SAmar{3ki, conaerning at 1$0"C for 24 hr. rises to 3.313 and the the powder pattern from Dr. Davis's notebook). mineral becomes opaqlr putty-coloured". Thus, it alone truly deserves to carry the name Since then, the crystal structures of a family crystalline ekanite as opposed ts ,nctatnic eka' of minerals of composition similar to ekanite, nite. Nomenclature changes for the other species but with about half of the salcium ions re- are indicated or have been accepled by the placed by a sodium, and with an additional Commission on New Minerals and Mineral potassium ion, have been reported (Mokeyeva Names (Perrault & Szyma6ski 1982). As a & Golovastikov 1966, Richard & Perrault 1972). result of a re@nt decision by this Q6mmi55is11, The latter report clearly shows the incomplete the St-Hilaire "ekanite" material has been re- site-occupancies at all the metal sites, and in- named "steacyite" (Perrault & Szymariski 1982r, dicates that the (Na,Ca) and K ions occupy and will be referred to by this name henceforth. different sites in the strusturc. The cpace group of the mineral anallzed in both r€Itorts was OccunngNcs found to b P4lrncc, with c 7.58, c 14.17 A (Richar-d & Perrault 1972) for the material Ekanite was found in a glacial syenitic boul- found at St-Hilaire, Qu6bec, and a 7.58' c der within the Tombstone Mountains, Yukon l4.SZ A (Mokeyeva & Golovastikov 1966) for Territory, at latitude @o24'N and longitude the material found in Oentral Asia Iraqite 138o36'W. The boulder was located in one of (Livingstone er 4r. 1976), which is clorly the cirques situated on the north side of the but which Tombstone Mountains and was locally derived related to the latter two minerals, "from shows extensive ra^re-earthsubstitution for thor- the north-facing wall. Associated minet&ls, ium and calcium, has also ben deccribed. It identified by X-ray powder diffraction and thin too has space group P$/mcc and similar cell section study, are fluorite, garnet, quartz, per- dimensions. thitic microcline, clinopyroxene, apatite, sodic Despite the original report that metamict plagioclase, hematite, thorogummite, zircon and ekanite recrystallized thermally to a body- titanite. 'bkanite- centred phase, the name "ekanite" or family" has been retained in all cases of struc- Pnvsrcer, eNp Oprrcar, PnoPeRTIES tures found to have the primitive tetragonal lattice, space group P4/mcc. An additional Crystalline ekanite is straw-yellow to dark red name, "kanaekaoite", was coined by Povaren- in color; the darker and redder the color' the nykh & Dusmatov (1970) to describe the K- more inclusions the material contains. The ssplaining and Na-for-Ca-substituted material; is white, the is vitreous, the tenacity this name has been used in the literature (Living- is brittle, and the Mohs hardness is approxi- stone et al.1976, Hey & Embrey 1974), thougb mately 4Yz. \\e mineral does not fluoresce in I.M.A. approval never was obtained. either long- or short-wave ultraviolet light. Recently, Embrey & Fuller ( 1980) have raised The largest crystals are about 2-3 mm in questions about the validity of ekanite itself length, with the average around 1 mm. Ekanite ("Formula and status in doubt"), probably as a occunl as discrete grains or clusters of grains result of the existence of the K-containing, |'{4, that, in some calps, approximate subhedral Ca-substituled phases to r{hich the name ekanite pyramids. The vast majority of grains exa- was applied. Perrault & Richard (1973) were mined, however, are anhedral in shape. The particularly apprehensive about the use of the forms observed are {1o1}, {110}, {(F1} and name "ekanite" to describe the St-Hilaire mate- {l0O}. An ideelized crystal drawing is presented rial, but finally followed the Russian usage of in Figure 1. The ftacture is irregular; all grains that name for the Central Asian material of show some degree of fracturing even on a composition very similar to theirs (Ginzburg micro scale. The {l0l} cleavageis distincl and NONMETAMICT EKANITE 67 {001} indistinct. Crystals are striated parallel to r. The measured density by Berman balance on 9.42 mg of hand-picked sample is 3.08(5) g cm-3. The calculated density for (Tho.r*Uo*r) :o.ea( Car.errFeo.oreMno.ore ) >2.^SirOso is 3.3 6 g cm- 3. The discrepancy of. lOVo between the calcuhted and measured densities may be ascribed to the cumulative effects of impurities and inclusions: hematite-and -thorogummiteinclusions, entrapped - air within the numerous microfractures, and non-structural $a1s1 leithin the, zeolitic channels (see Discussion). Qnlically, the mineral is uniaxial negative, with indices of refraction (determined in Na light) e 1.568(3) and co 1.580(3). The large uncertainty in the indices of refraction stems from the interference of fine grained, red to reddish brown alteration. All crystals studied on the flat stage showed poor interference figures. lgne crystals are slightly biaxial negative, with 2V lO-15'; some also show zoning. In thin section, under transmitted light and when free of inclusions, ekanite is colorless to straw_ yellow. Holever, in most specimens the reddish alteration is preponderant and makes ekanite readily identifiable in thin section. Heating of the mineral in a vacuum Ftc. l. Idealized crystal drawing of euhedral eka- for three nite. hours at temperatures in excess of 900"C did not destroy the ekanite structure. The onlv visi- ble change in the X-ray powder-diffriction Experimental conditions: accelerating voltage was the ql-lern strengtheningin intensity of those 20 specimen current -0.030 pA, counti"g diffrastion lines belonging -kY, to the admixed thoro- periods 10 seconds, diameter of electron beam gummite phase. N p^ (moving beam to minimizs beam dam- age to sample). Standards: wollastonite for Ca Cnrrvrrcer, CotvtposltloN and Si, synthetic ThO: for Th, synthetic UOz for U, rhodonite for Mn, and kaersutite for AI Initial microprobe studies showed that eka- aqd, Fe. Analysis yielded: SiO, 45.1, CaO 9.0, nite reacted strongly under the electron beam ThO, 36.0, ALOa 0.5, UO, 1.9, FeO 0.3 and (2 p,m diameter). To minimize this effest. ana_ Mn 0.2 wt. Vo;total = 929 wt. lo.2. lJnbeated lyses were performed using a defocusedelectron specimen;cations determined by electron micro- beam of 1G-20 p,m, with a closed aperture probe (at CANMET, analyst D.R. Owens, aver- except during the lO-second counting Deriod. age for three apparently similar areas). Experi- Even under these conditions, degradati6n'of the mental conditions: accelerating voltage 20 kV, surface was clearly visible. specimen current -A.O25 pA, counting periods There is a significant 'electron variation in color be- l0 seconds, diameter of beam -15 tween specimens, from deep red to ambsr. trr,m. Standards: synthetic ThO, for Th. syn- Microscopic comparison of these colors showed thetic UOr for U, rhodochrosite for Mn, and that the deep red grains are saturated with in- kaersutite for Al, Ca and Fe. Analysis yietdeA: clusions. The amber grains appear to be pure 8.1, ThOz37.t, Al,O, t.3, UO, and-monophase. Results of !i9, 11.?,CaO microprobe analyses 0.5, FeO 0.3 and MnO 0.3 vtt %; total - of the amber grains are more coniistent than of 89.3 wt. %, 3. Heated specimen; cations deter- tle r9d grains, but still yield did not a sensibie mined by electron microprobe (at CANMET, chemical composition. D.'R. Owens, average of four analyses). The microprobe results may lnalys! - be stated as Experimental conditions: acceleratingvoltage 20 follows: 1. Unheated specimen; cations deter_ kV,. specimen current -0.030 g,A, couirting mined brr elestron microprobe (at G.S.C., ana_ periods 10 seconds, electron b6am diameter lyst G.J. Pringle, average of four analyses). -10 g,m. Standards: synthetic ThO, for Th. 68 THB CANADIAN IVIINERALOGIST synthetic UOz for U, rhodochrosite for Mn' ments with precession photographs showed the kaersutite for Ca and Fe, and orthoclase for crystal structure to be body-centred tetragonal, Si. Analysis yielded: SiOg 47.6, CaO 9.6, ThOz witn a diffraction symmetry aPproximately 37.8, Al2Oa 0.8, UOa 1.1, FeO O.4 and MnO A/mmm. Deviations from this symmeEy in the 0.2 wt. Vo; total = 97,5 wt. Vo. layerc hkl,l = 1 to 5, were noted and compared - -1 -5. Bulk shemical analysis of hand-picked grains, with th" layers / to A clear indication chosen under a binocular microscope, gave 7.8 that Friedels Law was not obeyed was ob- vtt. Vo I{zO and 3.4 wt. Vo F. tained, and the equivalent general hkl rcflec' The observed powder pattern of ekanite shows tions of symmetry A/mmm were divided into four lines that could not be indexed on the two subseis of symmetry 422. \\ts fixed the ekanite cell, but that correspond to the strongest space group uniquely as 1422 lrom the d.iffrac- reflections of thorite or thorogummite. These tion wmmetry alone, wrth Friedels l-aw break- impurities undoubtedly affect the microprobe- ing down as a result of the strong anomalous derived composition but, provided they are in dispersion of (8.87e for MoKc ra- a finely divided form, they should not affect diation). the single-crystal diffraction pattern. Besause of No straw-yellow or amber-colored fragments the uncertainty in the chemical composition were found suitable for single-crystal data derived from microprobe analyses, srystal- collection, so a red fragment had to be used. structure determination was undertaken; iX The selected crystal was cut into a rough cube showed that the theoretical composition is ThCar and ground to a sphere A.246 mm in diameter SieOao.This structure requires SiOg 56.1O CaO using the Bond method (1951). E:ramination 13.O9 and ThOr 3O.81 wt. Vo. It is not pos- with a precession camera showed the sphere sible to say whether the minor impurities found to be a single crystal, but with a large mosaic in the microprobe analyses are present in the spread. ekanite or as part of the thorogummite. Heat' The crystal was mounted on a 4-circle dif- ing experiments on ekanite gtains showed some fractometer in a general orientation. For the 9% weight loss on heating to 900'C. By analogy determination of cell dimensions, low-angle data with steacyite (Richard & Perrault 1972), the had to be used, with a mean Kc wavelength water content may be partly zeolitic as well as (MoKa - OJIO73 A), as the large mosaic spread contained in the thorogummite. (some 3o) precluded resolving the ar, c[' coID- In summary, the microprobe analysesand the ponents within the limits of observable data. bulk chemical analyses do not reflect the com- \\e 20, ,(, ol values of 62 reflections in the position of ekanite for several reasons: 1. the range 2l'<20<3Oo were refined by least' presence of zeolitic water in ekanite, which squares (Busing 1970) to obtain the cell dimen- causes the surface of the specimens to bubble sions given in Table 1. and froth during microprobe analysis, 2. the Although normally in space group 1422, the fine grained intergrowth of ekanite with thoro' unique data-segment is l/L6 of the reciprocal gummite, and 3, the numerous fine inclusions in sphere; (hkl, h>k), the presenceof the anom- ekanite, in addition to the larger inclusions of alous scatterer made lun * /rl,l" Consequently, ya hematite. the unique segment was considered as of the The single crystal used for data collection reciprocal sphere, and three compl3p. octants in the structure determination was examined ex- of data were collected, (hkl, EH, n6l), usiag tensively with the microprobe after the data graphite monochromated MoKc radiation to a - collection was complete. It showed numerous limit of 20 80". A e-20 scan was used at a inclusions and gave overall results comparable 20 scan rate set to 2o per minute, with the to Analysis No. 2 (above). These values were not used in the crystal-structure determination, except to give the ratios for the composition TABLE1. CRYSTALDATA at the metal sites, i.e., Th:U and Ca:Fe:Mn. source: A boulder ln the TombstoneMountalns, Yukon Terfltory' The composition of the crystalline ekanite ex- --"'-iirri-r*uo.oqzcomrcsltlon: amined was derived crystallographically from )ro_ gq(c.t .gtzF"o.osgho. ozg)rz.oosl aOzo these ratios and the refined occupancies at the systemdtlc absences: hkg' hrk+t € 2t + 1 metal sites. space group: 1422 (i97). Cell dlnensions:a' 7.483(3)' a' 14.893(6)A Denslty: measured: 3.08 x 0.05 g cm-J (mlcrcfractured speclnen Pnnrtutxmv CRYsrALLocRlprrrc E:

TABLE3. ATOMICPOSITIONAL AND THERI.IALPARAMETERS

Atom Posltlon x y z Urr Uzz Ugg Urz Urg Uzr Th200 n 103(1) 103(1) 143(2) o 00 Ca4\0 n 158(6) 2e3(8) 267(8) o 00 -7(5) -15(5) si 16 ,333s(2) .2540(2) .147e(1 ) 160(5) 147(5) 220(6) -5(4) -13(21) 0(1) 16 .2544(7) .4521(5) .1251(3) 262(1.e)126(13) 523(28) 25(13) 28(16) 0(2) I .2s24(8) k+x , 606(37) 606(37) 207(26)r27(44) 6e(24) 6e(24) -25(16)-70(17) -81(16) 0(3) 16 .2553(6) .1075(6) .0818(3) 230(1S)27e(20) 2el(1e) The anlsotropic temperaturefactors are expressedin the form: T = exp[-2n2(U11a*2h2..+.2U12a*b*hk..+.)],and the valuesquoted are x LOa.

octant of these coefficients in symmetry .14. frared measurements on ekanite reveal both Because in this structure the anomalous scatterer free water and bonded hydroxyl (Farrell 1978). (Th) is at the origin, the synthesis gave a flowever, an attempt was made to locate paftial- direct determination of the position of the sili- ly occupied bonded-water sites, based on the cate molecule in the cell and its absolute con- very minor peaks in the difference map. All figuration relative to the choice of axes. No possible sites refined to zero site-occupancy in other peaks appeared in the resultant map. the least-squares refinement. The scattering curves used were the neutral The final refined structural parameters are atomic species, Th, Ca, Si and O, taken from given in Table 3. The observed (10 x F,) and International Tables (1974). This same source calculated (10 x F.) struc.ture factors are given provided values for the anomalous dispersion in Table 4, which is available at a nominal coefficients for the same atoms. The structure charge from the Depository of Unpublished was refined to R = 6.17o isotropically and R - Data, CISTI, National Research Council of 3.7Vo anrsotropically. A difference map at this Canada, Ottawa, Ontario KlA 0S2. stage revealed a significant negative peak at the thorium position, indicating incomplete site oc- DsscRlprroN oF THE Srnucrunr cupancy. In view of the unreliable microprobe anal- The structure is illustrated in Figure 2. T\e yses, no accurate mearure of the composition metals are stacked in layers al z = O, Vz and was available. The site-occupancy parameters are separated by a puckered silicate sheet. The of the metal sites were allowed to vary. That coordination of the metals is illustrated in of Ca increased by about l%o, a feature which Figure 3. Thorium has the expected square anlr- - can easily be explained by the minor substitu- prism of oxysens fIh-O(3) 2.405(5) Al. tion of Fe and Mn for Ca at that site. The The same oxygens that bond to thorium also site occupancy of Th dropped to 0.94, the re- bond to calcium in a very distorted tetrahedron sidual falling to R = 3.57%. At this stage, a 1Ca-O(3) - 2.342(5') Al. fte centres of the virtually featureless difference map was ob- tetrahedral faces are qccupied by a set of sec- tained, with only a few very minor positive ond-nearestneighbors, thesebeing O(1) at 2.688 afeas. (5) A, and this second tetrahedron is even Most of the crystallographic calculations more distorted. were done using the X.RAY system of pro- The silicate network can be described as fol- grams (Stewart et aI. 1976), The P,(r) syn- lows: the four-fold axis at vz'vz"z relates four thesis was computed using a highly modified corner-sharing silicate tetrahedra in which the y8, version of Zalkin's Fourier program FORDAP. silicon atoms are stacked in the plane z = The rapid decomposition of the crystals under These tetrahedra are linked via the bridging the electron microprobe beam (focused or defo- oxygens O(l), and each Si is connected to two cused) and the weight loss of some gVo on such O(1) atoms. Another oxygen, O(3), is heating specimensto 900oC were both indicative involved in bonding to the neighboring metal of water present in the structure, though the atoms (see above). The first square of silicate final agreement factor and the lack of any tetrahedra is linked to four other squares via significant peaks in the difference map clearly O(2), which is at z = /a. These latter squares indicate that this water is non-structural. In- are formed around the four-fold axes through NONMETAMICT EKANITE 7l

o(3) o(0

o(3) o0)

Th

Cq o o sl o0, o(o

of2] 4. 1.,,,r,,.,1r I r I r lA

Frc. 2. The structure of ekanite projected on the y-z plane. Fractional co- ordinatss (x 100) in the .r direction for each atom are given within or beside the circle representingthe atom,

O,A,z, l.O,z, 0,1,2 and 1,1,2. Within the latter layers of silicon atoms, occurs at c/2 away four squares, the silicon atoms are stacked in from the first sheet; in this sheet, the silicon the plane 7 = 3/a. In terms of topology, the atoms are at z = s/a and Ze. Further details of structure has a puckered, singly connected in- bond lengths and angles in the structure are finite sheet of silicate tetrahedra, though within given in Table 5. this single sheet, the puckering results in tc'o Throughout the structure, there are infinite distinct layers of silicons at z = th and 3/s. A intersecting channels in the .r and y directions, symmetry-related second sheet, with its two about 4 A in diamet€r, at x,O,O.2; rOO.8; 1) THE CANADIAN MINERAL@IST

Frc. 3. A view down the e axis showing tbe co,ordination of the metals along the r axis.. The thermal eltipsoids are drawn at 5OVo probability (Iohnson 1965). Legend for the equivalent positions is given in Table 5.

TABLE5. BOIIDDISIANCES ND ANGLESIITH SIANDARDDEVIATIONS explanation for the non-strucfural water content (a) of the mineral. These channels are large enough - th 0(3) 2.405(5)l to provide easy passagefor any water molecules Non-bondedcontacts, (l), Angles(" ) in a zeoliteJike fashion. Furthermore, as the 0(3)r... 0(3)3 2.932(7) o(3)r - Th- 0(3)2 74.012) 0(3)t... 0(3)2 2.895(6) 0(3)t - Th- 0(3)8 74.s(2) "walls" are composed of oxygens, this water 0(3)1...0(3)8 2.e20(6) 0(3)2- Th- 0(3)8 75.r(2) hydrogen-bonded network 0(3)r - Th- o(3)4 140.9(2) can easily form a o(3)r - Th- 0(3)6 142.0(2) It need not be structurally - - within the structure. o(3)r Th 0(3)s 11e.1(2) repetitive and regular, and will thus not affect (b) diffracted intensities significantly other than - the ca 0(3) 2.342(5)A the contribution to thermal diffuse scattering Non-bondedcontacts, (A). lnsl es(. ) and to the background. 0(i)r... 0(3)o 2.e20(6) 0(3)! - ca - 0(3)€ 139.8(2) a 0(3)r...0(3)e a.000(7) 0(3)r- ca - 0(3)e 1li.3(2) When heated at 900'C for three hours, 0(3)r...0(3)r0 4,398(7) 0(3)r- ca- 0(r)to tt.t,r, specimen of the mineral lost weight ftom 65 to powder pattern. (c) Slllcat€ tstrahedrcn. 59 mg with no change in the sr - o(1)- sr 147.8(4). This Correspondsto a composition of about-9-5 Forother bond lengths and anglss' see Fig. 5. water molecules per unit cell. Certainly, there Non-bondedcontacts, (A). is room in the strusture to accommodate more 0(1) ... o(2) 2.624(51 o(1) ... o(1)t2 2.64e(7) this, and it may be that the water mole- 0(1)... o(3) 2.6s8(5) 0(2) ... 0(1)12 2.6e0(7) than o(2) ... o(3) 2.62e15, o(3) ... o(1)t2 2.533(7) cules within the cell are mobile. The only significant nonstoichiometry oc- supeEcrlpts used abov€ and ln the dlagraB r€fer to the followlng eoulvalent msltlons:- curs at the thorium site, which has a site oc- wlrtJrinthis remainder, l. a , , , s 7. V .4. a cupancy of about 94Vo; 2, ! t I t'a L a r-V.-a about SVo uranium substitutes for thorium. The 3' a , -o , a 74'-v, a 4. -8, U.-a 10. 74. U,-a thorium is contained within a square antiprism 5, -a,-9, a 11. X.-Ar-a '-a 6. 'Y .4 ,'a L2. I'!t 6r a of O(3)oxygens, and the sides of the square are about 4.15 A. Radioactive decay of both thorium and ura- nium lelds isotopes of radon which, not being coordinated to water, could easily escapethrough x,/z."A3i x,!z,O,7i 0,y,0.2; 0,y,O.8; t5s'O.3i the O(3) square and into the infinite channels. soluble, radon b,y,0.7. The "walls" of these channels consist Being both a gas and also water of all tlree types of oxyggns in the structure: could easily escape from the stnrcture through O(l), O(2) andO(3). either gaseous emanation or water leaching, prior to its decay to lead. No lead was found to say D$cussloN in the analyses. However' it is impossible with any certainty whether the mineral actually The channels mentioned above provide the crystallized with some deficiency in the thorium NONMETAMICT EKANITE 73 siteo or whether the incomplete site-occupancy the .r co-ordinates given beside each atom. The at this site is the result of radioactive decay. principal difference between these slices of the two structures is that steacyite contains potas- CoupenrsoN wrrn THE Srnucrune sium at O,O,Yz,whereas in ekanite thd cor- oF STEACYITE responding site, OnO,r/+,is vacant. The square antiprism co-ordination of Th, the distorted The structure of steacyite (Richard & per- tetrahedral co-ordination of the nearest neigh- rault 1972) has been redrawn in Figure 4 to bors, and the second-nearest neihbors around r/q extend from e = to LVa, Tbe segment of the Ca are very similar. There are differences in structure of ekanite from z : 0 to z: rA is the bond lengths in the two minerals: Th-O(3) virtually identical with the segment of the : 2.405(5), 2.413(5), Ca-O(3) - 2.342(5), structure of steacyite from e - r/t to z = y2. 2.447(5), Ca-O(second-nearest neighbors) : This can be seen readily from a comparison of 2.688(5), 2.631(5) A for ekanite and steacyite,

z o(3) oe) II L-----*v

o(2) o(3)

o sl

r\ o o(3) o(2) n No, Cc

() Th

(t K

o(2) o(3)

ot2 34 1,,,,1,,r,1 r I ' l' ll

Frc. 4. The crystal structure of steacyite (Richard & Porrault 1972) pro- jected on &" y-z plane from I - Va to lYq. Other than the presence of K at 0,0,r/2, the slice of the unit cell from 7 : la to z : Vz can be nearly superimposed on the corresponding slice (z = 0 ta Yt) of. the ekanite structure. 74 TTiE CANADIAN MINERAL@IST respectively. The square antiprism of co-ordi- (5), 1.625(51 AI. rhe Si-o bonds of o(3) nating oxygens around Th is somewhat more (the Th and Ca co-ordinating oxygen) and of regular in ekanite than in steacyite. The trian- O(2) (the oxygen ft6f links squat€s of tetra- gle of three oxygens that make up the side face hedra) are virtu-ally identical in ekanite [1.584 of the antiprism has sides2.932(7),2.896(6), (5), 1.589(2) Al but are somew-hatdifferent 2.902(6) A, and angles 60.6(2), 59.3(2\ and in'iteacyite Lr.s7t(j), 1.601(3) Al. There are 60.1(2)' in ekanite. The corresponding values significant differences between the two strus- in stiacyite are2.891(7) , 2.966(1) ,3.043 (7) A tures in the angular distortions of the silicate and 57.5(2), 59.9(2) and 62.6(2)". tetrahedra, but this is not unexpected in view The principal difference in the structures of of the two different ways in which the squares the two minerals is found in the silicate net- of tetrahedra are linked. work. In ekanite, the first square of silicate Steacyite also contains channels through the tetrahedra is virtually identical to that found silicate network where nonstructural water can in steacyite, but in the latter structure, a centre be entrappd (3-t47a by weight: Perrault & of symmetry at (Yz,r/z,r/z) grvesrise to a second Richard 1973), However, ekanite has twice as square above the first. The tetrahedra of the many channels as steacyite, because in the latter two squares share corners to(l)l to form dis- structure, half the corresponding channels are crete SLO:o cubes (Fig. 4). As already ex- blocked by potassium ions. This makes ekanite plained, in ekanite, oxygens O(2) connect even more porous than stearyite on an atomic squares of tetrahedra within the same sheet, scale. but at two different levels in e. These squares are formed around different four-fold axes. AcKNowLEDGEMENTS There are, therefore, no discrete silicon-oxygen units in ekanite, but rather infinite sheetsin tle The authors thank the following who con- r-y plane. tributed to this paper: Drs. R.T. Bell and W.D. The geometry of the silicate group in ekanite, Goodfellow for securing tle ekanite samples, shown in Figure 5, is very similar to that found D.M. Watson for mineral separates,G.J. Pringle in steacyite. In the present structure'there are for additional electron-microprobe analyses, two longer Si-O bonds [1.&O(5), 1.632(5, J.L. Bouvier for water and fluorine analyses, Al, and these are to the oxygens that are in- P. Pint for the thermal analysis of ekanite, R.N. volved in the corner-sharing to form Si-O-Si- Delabio for X-ray po,wder-diffractometer traces G- links within a given square of tetrahedra. of ekanite, and especially Professor Guy Per- In steacyite, these bonds are also longer [1.639 rault, Ecole Polytechnique, Montr6al, for valu- able insight into problems of ekanite nomen- clature.

RsrnRENces AxpnnsoN, B.W., CLARrNGEur,r.,G.F., Dlvrs, R.J. & Hrrr,, D.K. (1961): Ekanite, a new metamict mineral from Ceylon. Nature I'g0, 997. BoNo, W.L. (1951): Making small spheres.Rev. Sci, Instr. 22, 34+345.

BusrNc,W.R. (1970): Least-squaresrefinement of lattice and orientation parameters for use in automatic diffractometry. /n Crystallographic Computing (F.R. Ahmed, ed.). Munksgaard, Copenhagen. Er{BREy,P.G. & Fur,r-rn, J.P., eds. (1980): I Manual of New Mineral Names. Orford Uni- versity Press,Oxford, England. FARRELL,D.M. (1978): Infrared investigationof oF) an unidentified hydroxo calcium thorium silicate from tlre Yukon Teritory. CANMET Rep. Flc. 5. The SiOa group viewed parallel to the [110] MRP/MSL 78-115CfR). Dep. Energy, Mines, direction, showing bond lengfhs and angles. Res.,Ottawa, Ont. NONMETAMICT EKANITE 75

GtNzrunc, I.V., SeurNov, EJ., LnoNove, L.L., MorEyEvA, V.I. & Gor,ovesnxov, N.I. (1966): StooRnNro, G.A. & Dusvrerov; Y.D (1965): The crystal structure of ekanite ThK[Ca,Na]rSi, Alkali-rich crystalline ekanite from cenfal Asia. Or' Dokl. Akad. Nauk ^9J.S.R. t67, ttSl-1134 In New Data on Minerals of the U.S.S.R. Aca- (in Russ.; translation h Dokl. Acad, ,Sci. demy of Sciences of the U.S.S.R., Moscow (in U.S.^!.R., Earth Sci. Sect. 757, 106108). Russ.). Oreve, Y., Serro, Y. & PeprNsKy, R. (1955): Gilnnr-rN, (1961): E.J. Ekanite. Gems Gemrnology New method in X-ray crystal structure deter- 10, 165-179. mination involving the use of anomalous disper- (1962): Ekanite. Gemmologist 8t, 142-159, sion. P&ys. Rev. 98, 1857-1858. 165-169. PERRAULT,G. & Rrcrnno, P. (1973): L'ekanite de Her-1, S.R. & Szvrr,re(srr, J.T. (1975): Powder pat- Saint-Hilaire, P.Q. Can. Mineral. Ll, 913-929. tern generation from single-crystal data, Amer. Ctyst. Assoc. Winter Meet., Abstr. F13. Now & Szvuerisrr, J.T. ( 1982): Steacyite, a incorporated in the XRAY-76 system of pro- new name, and a re-evaluation of the rromen- - grams [see Stewart et al. (1976)]. clature of "ekanite" group minerals, Can. Mineral. 20, 59-63. Hrv, MlI. & Etrtnnv, P.G. (1974): Twenty-eighth list of new mineral names, Mineral. Mag. 89, PoVARENNyKH,A.S. & Dusuerov, V.D, (1970): 903-932. lnfrared absorption spectra of new minerals from alkaline pegmatites of central Asia. Kozsr. Irrns, J.A. & HeunroN, W.C., eds. (1974): In- Svoistva Mineralov, Akad. Nauk U&r. ,S.S-R., ternational Tables for X-ray Crystallography. IV. Respub. Mezhvedom. Sb. 4, 3-9. Revised and Supplementary Tables. Int. Union Cryst., the Kynoch Press, Birmingham, England. RIcHARD, P. & Prnreulr, G. (1972): La structure cristalline de I'ekanite Thr-"(Na,Ca)r-"Ko_ngiru JoHNsoN, C.K. (1965): ORTEP: A FORTRAN O^. Acta Cryst. B,28, 199+1999. thermal ellipsoid plot program for crystal struc- ture illustrations. Rep. ORNL-3794, Rev. 2nd STEWART,J.M., MecurN, P.A. DrcrnvsoN, C.W., ORTEP-I addition 1971, Oak Ridge Naa Lab., ArvrMoll, H.L., Hrcr, H. & Fr-ecr, H. (1976): Oak Ridge, Tennessee; modified for use in tle The X-RAY system of crystallographic pro- X-RAY STEWART System by J.F. Gu€don, S. grams. Univ. Maryland Comp. Sci. Ctr. Tech. Hall, P. Richard and S. Whitlow. Rep. TR--446. LrvrNGsroNE, A., Atxn, D., HurcrrnrsoN, D. & AL-HERMF?r, H.M. (1976): Iraqite, a new rare-earth mineral of the ekanite group. Mineral, Received October 1981, revised manuscript accepted Mag. 40, 441445. December 1981.