Canadian Mineralogist Vol. 13, pp. 22-26 (1975)

X-RAY CRYSTALLOGRAPHYOF WELOGANITE

T. T. CHEN AND G. Y. CHAO Department of Geology, Caileton ()nLversity, Ottawa, Ontario KIS 5B6

ArsrRAc"r cvystal .r-ray studies. It was hoped that once tle cell geometry of weloganite Single-crystalr-ray studies of weloganite showed was established the cell parameters of the unknown yttrium mineral tlat the mineral is triclinic, PL ot pi, with a - might be derived from tle powder q:9-8q(.1),,: 8.988(1), : diffrastion c 6.730(t)A,a : data by its l0-2.84(r), p : tt6.42e) and - isomorphous relationship to weio- ? 59.99(1)..'sub- ganite. Weloganite has a pronounced rhombohedral cell which resemblesin geometry that of rhombo- The problems of weloganite may be outlined hedral carbonates.It also displays a pseudo-mono- ao follows: clinic cell that, correspondsto the cell reported by (1) The space group P3r, s is one of the few in other authors for the monoclinic polytypo of welo- which no minerals and only a few chemical com- ganite. Twinning by [103]r:oois very common, with pounds are found, as Sabina et al. (7968) twin - cor- obliquity @ = 0 and twin index r 3. The rectly pointed out. twin cell is trigonal and is identical to the cell orig- (2) If the published chemical analyses and space inally assignedto weloganite. The ideal chemical group symmetry (Sabina et al. formula for weloganite is proposed as Narsrlr L968; Gait & - Grice 1971) (COs)B.3Hrowith Z - 1 for the triclinic cell. are correct they would require the positional disorder of Sr, Na andZr atomswhich IutnopuctroN crystal-chemically are drastically different. (3) The .r-ray precession photographs of the so- called trigonal polytype showed peculiar extra- Weloganite, a hydratcd carbonate of Na. Sr space-group systematic extinctions with and Zt from St. Michel, Montreal Island, reflec- eue- tions of the type h-k = 3n absentwhen hIl was origrnally described by and !ec, Sabina er a/. kll arc not equal to 3n. (1968). The symmslry of weloganite was con- (4) The relattve intensities of many weak reflec- sidered to b-etrigonal P3r,, with a = 8.96 and o.n the precession photographs appeared c : 18.06A. The cell contained lons to two units of be variable from crystal to crystal. SrZrzGHeOil. The authors noted the discre- (5) The deviation of the asute bisectrix by more pancy that, between crossed nicols, nearly all than 5o from the normal to the basal cliavage, (basal) fragments gave an off-centred consistently observed on grains lying on the biaxial interference figure with 2V about 15", cleavage plane, suggests that the symmetrv of The mineral was later restudied bv Gait & weloganite cannot be higher than monoclinic. Grice (1971) who reported a monociinic poly- type with C2/c, CZ or Cd symmetry and a = Oprrcar OssnnverroNs 8,95, b = 15.54,c = 37.094 and I = LO3'4S,. The powder r-ray diffraction pait"rns of the Prior to.r-ray single-crystalstudies weloganite two polytypes were indistinguishable. The rela- was examined optically using both immirsion tionships betw,een the two polytypes were given mounts of crushed grains and thin sections of 4J au= er, bu=2arSln60o, and Cu=2c, co- large crystals. Both colourless and yellow vari- sec(l8oo-B). Based on a new analysisthe chem- eties were examined. The biaxial characteristics ical formula of weloganite was revised to (Srr.se of weloganite r,eported by Sabina et at. (L96g) Nar.oZto.asCao.r)(COs)s" 2HzO with Z = 24 ftor were confirmed. tle monoclinic cell and Z = 6 for the trisonal In the immersion mounts two types of grains cell. were noted. Type I grains, although free of lndn Our inlerests in weloganite began when an lamellae, either did not show shim extinotions unknown mineral from Mont St. Hilaire was or showed no extinctions at all between crossed brought to our attention and shown to be an vt- nicols.^For the type I grains 2V ranged,from 10 trium analogue "of of weloganite. The crystals to 15o. Type II grains were charicterized bv tl,is yttrium mineraL were composed of parallel sharp and homogeneous extinction b€twee; intergrowths of a large numGr of individuats crossed nicols and considerably larger 2V (about that were too small to be separated for single- 20"). Type I was by far the morJco,mrnon. 22 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/13/1/22/3434866/22.pdf by guest on 28 September 2021 X.RAY CRYSTALLOGRAP}IY OF WELOGANITE 23

One thin section of a large crystal cut parallel ing material. Both the rim and the core, how- to tle basal cleavage showed a dbtinct core in ever, gave identical powder diffraction patterns. the form of a nearly perfect equilateral triangle surrgunded by material showing relatively coarse X-Rev Cnvsrer,rocneluv twin lamellae and patchy extinctions. Along the welldefined boundaries there was, at places, a Many crystal fragments were selected from thin yensel of very fine-grained pyrite. The core the immersion mounts, after careful examina- material gave extinctions that were sharper but tion under the polarizing microscope, for single- not homogeneous over the whole area. 2V was crystal r-ray diffraction studies. Precession pho- very small, about 2 to 5o, or nearly uniaxial in tographs of type II grains consistently showed contrast to the 2V of 10 to 15' for the surround- triclinic diffraction symmetry corresponding to

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Fro. 1. Precessionphotographs (0-1eve1, !r. : 30o, MoKa radiation) of a single crystal of weloganite showing(a) the a*-b* net and (b) the D*-cx netwith a pseudo-mirrorplane normal to 08. Frc. 2. [103]- precessionphotographs (O-level, p = 30o, MoKa radiation) of weloganite(a) singlecrys- tal and (b) crystal twinned by [103]rzo"resulFng in trigonal symmetry. Pseudo-rhombohedralsymmetry is evident in both (a) and (b) trom the strong reflestions.

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spac€igoups Pt or pI (Figs. 1 and 2). The pa- with materials used in this study are identical tameters of ,the triclinic cell presented in Table to that given by Sabina et al. (t968). The in- I are results of least-squares refinement using dexing of the powder pattern based on the powder diffraction data. The cell, although not present triclinic cell is presented in Table 2, a reduced one, \tras chosen because the relatiou- shiq between the present cell and the trigonal cell of Sabina et al. (L968) may be describe-din sim- hkr, dcarc(R) aots(R) r hkr, dcalc(A) dobs(ff) r ple (ot = -b*u terms r" arro, bao : and c,r '100 yo = -3ctr"r") and it brings out clearly pseu- 2.012 2.012 the 001 420 2.012 do.symmetry of the mineral (a-b i_6O; and r0r I 003 2.010 0I1 c8=90o). Due to the high pseudo-symmetry 335 'l1.966 of 't20orT 305 .966 the lattice there are two equally acceptable 03e 1.966 choices for the reduced ziT 8 032 1.966 cell, which mav 6e de- lT0 301 I.966 j.9t the p5esenr_ce-llby the 127 331 1.966 ::u"d transforma_ tTr rions101/ I 10/001 241 1.909 'I anOiOiZOtO,/001. The para- 'l0l210 427 1.908 .907 meters of both reduced cells are compared 'I 'I 2n I.908 in II 54T r.691 Table 1 with those of the chosen cell.^ 121 FlZ r,691 rTl 5 ]40 1.69r ztz 150 t.691 002 4Tr 1.691 'tIZ211 45r 1.691 10 214 1.677 122 ]T3 1.677 1.676 337 515 r.5e0 8.988(1 't 8.987 8.988 c30 7 141 ,590 6.730( 'l5l l O. /JU O. /JU 30r I .590 r02.84(1 102,870 102.840 JJU 4m 1.590 116.42 '108.74 3oz 108.74 033 '1.588t.589 Y s9.e9_fl 110.63^" l l 0.67^2 mr 03t 421.624" 42't.624' 031 302 1.588 300 a* 304 t.s88 o.l 3ess(2)fi-l o.r:sBF-1 o. r sggl-l at5 b* 0.12848(2) 540 1.538 0.i 285 0. I 285 172 4Tt I.537 c. 0.r 6592(4) 0.1696 0. 1695 122 'tET qt 90.01(llo 67 a"o I .537 67.740 2'T 152 t.537 64.01 64.0I ?47 510 I.537 6Z 427 33T I.498 'I ce,, 220 36T 1.498 .497 iii':'?ffiil3io[?':ll ii"hio:'i3t3i;)oli. ""o,ced 240 632 1.498 3I0 363 I.341 'I rIr 33t 1.34r 212 Plus The weloganite lattice has a nearly ideal manymore llnes rhombohedral sub-cell with a,r._ = Vzi*r,/sit_ - 60" Vzatu/pin6}o - 5.161A and c"h;_ = c*rg = 17.9014. This sub-cell is eviclent from the fact that reflections that do not conform to which includes indices only of those reflections t}te requirement of rhombohedral symrnetry are tlat make substantial contributions to the pow- either absent or very weak (Fig. 2a). Tlne g"onr"- der diffraction lines as judged frona preceision try of the rhombohedral sub-cell resembl6sthar and Weissenbergx.-ray photographs. of the cells of the rhombohedral carbonates such as (a - - 4.9898 and c 17.060A: An- TwtNNnIc drews 1950) and bariun --"uit,l*r". (Cao.sBao.r)_ CO', (a =. 5.02 and c = lT.ggAl Weloganite may represent an ideal textbook Donnay 1963, p.'801). This--Gg*t" tiut"lile illustration of twinning of crystals in the triclin- structure of weloganite is probably derivable ic system. The twinning condition for triclinic rrom the structure of the rhombohedral carbo_ crystals requires that the following ratios must nates. approach rational numbers (International Tabks The weloganite lattice also possessesa pseudo- lor X-Ray Crystallography, fr, 1959, p 106): plane-perpendicular to 64.r" @igs.'lb and T1rro{ a2 a b2 : c2 za), gtvrng fise to a pseudo_monoclinicsymme_ : bccosot: cacxp : abcosy, pseudo-monoclinic TI._-Ihg cell has a : a.td"= This condition is beautifully 8.988, , = met for weloganite 2btausin60o = 15.567,c* =" r/z as the required ratios are: c*torc of c = 36.865A and B = 1b3.8o. cor- responding 6.013 tA exactly to the monoclinic ,"- : 6.01.3: X rc.Lt4 : 1.000 : po$d by Gair & Grice (1.971). ""U 2.003 : 3.O07. The powder patterns of weloganite obtained In fact, all type I grains selected from the in-

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m€r$ion mounts and fragments removed from the coro material in the thin section were found to be twinned. The twin law .may be stated as [103]rzo" or [@1]*rzoowith twin obliqutty ctl:O and twin index n=3. It falls in the class of twin bv twin- latticd symmetry (ILS) according to the;impli- fied clpssification of twins by Donnay & Donnay (1974), The complete twin symmetry is 3. The twinn€d cell corresponds exactly to the trigonal cell reported by Sabina et al. (1,968) with the same type of peculiar extra-space-group syste- mafis extinctions wilh h-k=3n absent when h*l and K+l are not equal to 3r. The 0-level [103] preces-ionphotograph of a twinned crystal

Frc 3. D-precesrion photo€raphs (0-level, u, - 30o, MoKa radiation) of weloganite (a) single crystal, (b) twinned crystal and (c) twinned crystal displaying diffuse streaks along c*.

is shown in Figure 2b which is to be compared with that of the untwinned crystal (Fig. 2a). It is interesting to note that pronounced dif- fuse streaks along cE or the twin axis, [103], were observed on precession photographs of some twinned crystals (Fig. 3c). However, they were not observed for all twinned crystals @ig. 3b) and were never observed for untwinned crystals (Fig. 3a). Similar diffuse streaks were observed and described by Gait & Grice (1971) as "streaking" of reflections and were inter- preted as due to stacking disorder along the cs direction. Although stacking disorder could not be completely ruled out, the development of the diffuse streaks may be adequately explained, without introducing stacking disorder, by twin- ning of individuals of submicroscopic thickness repeating along [103]. The adjacent twin indivi- duals, if small enough to diffract x-rays cohereat- ly, would act as antiphase domains and produce diffuse streaks similar to the effect of stacking disorder. The submicroscopic size of the twin individuals is testified by the fact that many fragments of weloganite crystals that showed no twin lamellae under polarizing microscope with high magnification (500x) were nevertheless found by r-ray diffraction to be twinned.

C\nnarcar. Fonrrrwe Recalculation of the chemical analysis of weloganite reported by Gait & Grice (1971) on

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the basis of 6 oxygen atoms (excluding COz and nite used in their studies and for their permission HrO) per formula gave Naz.:il(o.orSrz.7eC4.r4o.* to quote their unpublished materials. We would (COs)s.rrO0.se.4.49FIIA or ideally NasSrZr(COs)r. also like to thank Mr. V. Boyko for his analyses 4-5HrO. Several new analyses of the water con- of H0. This work is supported by research tent by the Penfield method gave 6.78 and grant A5113, from the Natiohal ResearchCoun- 6.8OVo (Jambor, private communication, t974), cil of Canada (to G.Y.C.). 6.9 and 6.9Vo (analyst: V. Boyko). The consis- tency of results of these independent analyses RnrsRENcss suggests that the water content (9.66Vo) re- ported by Gait & Grice is probably in ANDREwS,K. W. (1950): An x-ray examinationof error. If pure tle new values are accepted, a sample of calcite and of solid-solution ef- the ideal formula fects in some of natural calcite. Mineral. Mae. 29, weloganite becomes NazSrZr(COs)u.3H2O. 85. This formula is confirmed by results of a pre- DoNNey,G. & DoNNey,J. D. H. (L974): Classifi- liminary analysis (Grice, private cation of triperiodic twins. Can. Mineral. 12, 422- communication, 1974). Assumns Z = 1, tle 425. density of weloganite calsulated from the ideal DoNNal l. D, H, (Ed,) (1963): Crystal Data, De- formula using the volume of the triclinic cell terminative Tables, 2nd Edit. Amer. Crystall. is 3.208 g,/cms which compares favorably with Assoc. the measured values 3.22 (Sabina et al, 1968) GArr, R. I. & GHcs, J. D. (1971): Someobserva- and 3.20 g/cma (Gait tions on welogariite. Can, Mineral, 10, 912 & Grice 1971). (abstr.). Sarnre, A. P., Jevson, I. L. & Pr,exr, A. G. (1968):Weloganite, AcrNowreocEMENTs a new zirconium carbonate from Montreal Island, Catada. Can, We wish to thank Drs. J. L. Jambor and J. Mineral,9, 468477. D. Grice for making available to us the single Manuscriptreceived August 1974,emended Septem- crystal r-ray diffraction photographs of weloga- ber 1974.

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