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Ameican Mineralogisl, Volume 76, pages 257-265, 1991
Crystal chemistry and crystallography of some minerals in the tetradymite group
Ptrrn Berr-rssDepartment of Geology and Geophysics, The University of Calgary, Alberta T2N lN4, Canada
AssrRAcr
Selen-tellurium is a mixture of the minerals selenium and tellurium. The atoms instibarsen are ordered (completely or partially) so that the correct chemical formula is AsSb.Matildite AgBiSr, bohdanowiczite AgBiSer, and volynskite AgBiTer, show atomic orderingin space group P3ml with unit-cell dimensions similar to those of tsumoite, BiTe. The Sand Te atoms are apparently disordered in ingodite to yield the chemical formula Bi(S,Te)and in sulphotsumoite to yield the chemical formula Bi(Te,S). The unnamed mineral ofAksenov et al. (1968) seems to be tellurian tsumoite, (BiorrTeoor)Te. The synthetic com-position of Godovnikov et al. (1966) is equivalent to bismuthian tsumoite, Bi(Teo"Bio,').Platynite is possibly plumbian sulfurian nevskite, (Bio 65oPbo 34s)(Seo.urrSo ror).
Kawazulite, BirSeTer, is isostructural with tetradymite. Csiklovaite is a mixture of te-tradymite and bismuthinite. The chemical formula of skippenite is probably Bir(Se,Te,S)r,like that oftellurian paraguanajuatite. The Se and S are disordered in laitakarite to yield
the chemical formula Bio(Se,S).. As S and Te ordering was not observed in josEite orjosgite-B, there is possibly a complete solid solution series from tellurian ikunolite, Bio(S,Te)r,to sulfurian pilsenite, Bio(Te,S)r. The unnamed mineral of Yingchen (1986) is apparentlytellurian ikunolite. Rucklidgeite, BirTeo, has an antipilsenite, BioTer, structure type. Theunnamed mineral of Haraflczyk (1978) is rucklidgeite.
INrnonucrroN
The narrow definition of the tetradymite group wasgiven by Strunz (1970) to include only Bir(S * Se + Te),minerals. These hexagonal or trigonal minerals have anapproximately cubic closest-packed layer crystal struc-ture (Pauling, 1975) in which the five planes within thetetradymite layer are Te-Bi-S-Bi-Te. The axis [0001] ofthe hexagonal cell corresponds to [1 1 l] of the approxi-mately cubic closest-packed crystal structure, where theplanes are stacked perpendicular to [11]. The relation-ship between different ratios of Bi:(S + Se + Te) andTe:S with respect to the d-value with the strongest cor-responding intensity in X-ray diffraction patterns wasdiscussed by Spiridonov (1981). The definition ofthe te-tradymite mineral group was widened later by Bayliss etal. (1986) to include as essential those chemical elementsfrom group Va (As, Sb, and Bi) and group VIa (S, Se, andTe) to produce eight subgroups each with a different num-ber ofplanes in the basic layer repeat unit layer. Table Ilists minerals within the tetradymite group, which maybe subdivided on the basis of the number of approxi-mately cubic closest-packed planes in the layered repeatunit, where each plane contains one chemical element ora random distribution of two or more chemical elements.Differences in atomic size and electronegativity allow or-dered crystal structures at low temperature, but there aredisordered crystal structures with extensive solid solutionat high temperature.
The purpose of this paper is to compare and contrastthe various mineral species within the tetradymite group
0003-004x/9 l /0 l 02425 7$02.00
in order to investigate some of the problems associatedwith minerals in this group. The principal objectives wereas follows:
l. To obtain X-ray powder-diffraction data for mineralswhere no published data exist, or discredit the mineral(e.g., selen-tellurian, csiklovaite).
2. To use published X-ray powder-diffraction data to de-termine if the crystal structures are ordered, partiallyordered, or disordered in order to ascertain the correctchemical formula (e.g., stibarsen, volynskite, ingodite,sulphotsumoite, kawazulite, skippenite, laitakarite,jos6ite, jos6ite-B).
3. To reindex the published X-ray powder-diffraction dataand obtain refined unit-cell dimensions (e.g., matil-dite, bohdanowiczite).
4. To identifi unnamed minerals, incorrectly namedminerals, and synthetic phases within the tetradymitegroup.
5. To review the literature and ascertain the status ofplatynite.
MrrrroosIn the approximately cubic closest-packed layered me-
tallic minerals, the number of possible crystal structuretypes is limited. All of the crystal structure types haveatoms with symmetry-fixed values of coordinates x andy, whereas the coordinate z may or may not have a fixedvalue. Some atoms may have ordered, partially ordered,or disordered distributions. From these possible ordered,partially ordered, or disordered crystal structure types with
257
258 BAYLISS: TETRADYMITE GROUP
TABLE 1. Subgroups within the tetradymite group TABLE 2. Unit-cell dimensions obtained by least-squares refine-ment
No. ofatom Space Subgroup
planes group name Chemical formula
1
2
nel Te
R3m As
frm1 tsumoite
frm tetradymite
F3n joseite
seleniumtelluriumantimonyarsenicbismuthstibarsentsumoitesulphotsumoitenevskiteplatyniteingoditebohdanowiczitemalilditevolynskitetellurantimonytellurobismuthiteparaguanaiuatiteskippenitetetradymitekawazuliteikunolitejos6itelaitakaritepilseniteioseite-Bpoubaiterucklidgeiteprotojoseitealeksitehedleyite
Powderdiffrac_tion file
or spect-Chemical formula men no. a (A) c (A)
SeTesbAsBiAsSbBiTeBifie,S)BiSe(Bi,Pb[Se,S) (?)B(S,Te)AgBiSe,AgBiS,AgBiTe,Sb2Te3BiJe3Bi2Se3Bi,(Se,Te).Bi,sTe,BirSeTe.Bils3Bi.(S,Te)oBi.(Se,Te)3BilTe3Bi/Te,S)g(Bi,Pb)3(Se,Te)rBiJeiBis(fe,S)4 (?)BirPbSrTe, (?)Bi?Tes (?)
stibarsenmatilditebohdanowiczitevolynskiteingoditeingoditesulphotsumoitetsumoitetsumoitelaitakariteikunoliteiosEitejoseitejoseitsBrucklidgeiterucklidgeiteprotoioseitealeksite
AsSbAgBiS,AgBiSe,AgBiTe,Bi(So*Teo*Seoo,)Bi(Teo,.So*)Bi(Teo6?50s)(Bio 56Teo n2)TeBi(feo ?sBio25)Bi.(Seo",So j.Bi3 ss2oTel sBi3 es2seo rTer oBi415, eTe, oBi4 3s' 'so Je' o(Bi,Pb)sTe4BeJe4BissTe1sS16Bi2PbSrTe2
31-80 4.025s(11) 10.837(9)24-1031 4.0662(21) 18.958(17)29-1441 4.2049(14) 19.650(11)18-11734.468(7) 20.7s(5)37 4.2477(24) 23.075(22177 4.246(3) 23.26(3)38-442 4.304s(18) 23.47q14122-117 4.36413) 24.32(4)19-176 4.4sq3) 23.90(3)1+220 4.2239(14) 39.94(3)
4.258(3) 39.s4(4)22-364 4.2402(12) 39.77411s112-735 4.241s(10) 39.76906)9-43s 4.3i'26(25) 40.92(5)
29-234 4.4180(13) 41.513(15)4.389(16) 42.0(3s)4.3386(11) 57.804{18)
29-765 4.2423(25) 79.73(5)
9 frm114(?) P3m12O(?) ?
protojosaitealeksitehedleyite
values for z estimated from similar crystal structures,X-ray powder-diffraction patterns were calculated withthe program of Smith (Pennsylvania State University).The number of crystal-structure models may vary fromtwo for a two-plane structure type such as stibarsen, toten for a seven-plane structur€ type such as jos€ite.
The calculated intensities for each possible crystal-structure model were compared to the observed intensi-ties of the published X-ray powder-diffraction data. Theobserved X-ray powder-diffraction data were reindexed,and improved unit-cell dimensions were obtained by least-squares refinement of unit-cell parameters. If the calcu-lated intensities are similar to the visually estimated ob-served intensities in the literature, then a qualitativeconclusion may be made on the order-disorder to yieldprobable chemical formula, structure type, and unit-celldimensions. The calculated X-ray powder-diftaction dataof the crystal-structure models that give the best possiblefit to the observed X-ray powder-diffraction data are giv-en in the tables in order to show the goodness-of-fi1.
X-ray powder-diffraction patterns of the very smallspecimens available were obtained with a 114.6 mmGandolfi camera (Fe radiation; Mn filter). The intensitieswere measured with an automated densitometer.
Tn suscnoup
Selen-tellurium, (Se,Te) with Se:Te : 2:3, was origi-nally described from the El Plomo silver mine, OjojomaDistrict, Tegucigalpa, Honduras by Dana and Wells(1890). Only one phase consisting of both Se and Te has
been found naturally, even though at least five Se-Tephases have been synthesized. The crystal structure ofboth minerals (i.e., selenium, tellurium) in space groupP3,21 has Se or Te in 3(a) at (x,O,Yt) with x about 0.25.A complete solid-solution series occurs at moderate tem-peratures (200-450 "C) from selenium (a: 4.37, c: 4-95A, PDF 6-362) ro tellurium (a: 4.46, c : 5.93 A, pof4-554) without superstructure reflections (Lanyon andHockings, 1966). Appreciable segregation occurs if a meltis not cooled instantaneously. A phase with both Se andTe in fixed proportions would require a crystal structuretype different from that of selenium and tellurium. As yetsuch a phase has not been synthesized, and it is unlikelyto occur because of the difference in atomic radii of Seand Te.
Enquiries for selen-tellurium at eight large mineralog-ical museums revealed only one specimen, NMNH Rl86(Smithsonian Museum). This specimen, from the onlyknown locality, Honduras, was initially examined with aKevex solid-state detector on an SEM. Distinct grainswere identified as quartz, barite, selenium, tellurium, andselen-tellurium. The mineral assemblage of the specimenis identical to that described by Dana and Wells (1390),so that this specimen probably comes from the type lo-cality. The sp€cimen itself has not been established con-clusively to be a type specimen; however, none of thetype specimen appears Io exist.
The grains were too fine for mounting in a polishedblock for electron-probe analysis. An X-ray powder-dif-fraction photograph taken of the fine grains containingboth Se and Te was identified as a mixture of seleniumand tellurium. The unit-cell dimensions of selenium andtellurium are similar to those of pure Se and Te, so thatnegligible substitution of Te occurs in selenium and neg-ligible substitution of Se occurs in tellurium.
Therefore, selen-tellurium may be discredited and is
BAYLISS: TETRADYMITE GROUP 2s9
TABLE 3. X-ray powderdiffraction data for stibarsen TABI-E 4. X-ray powder-diffraction data for volynskite
Ie Volynskite Tellurobismuthite
,e Order Disorder l*/d ,e
0031010121041 1 00150061 1 3021202o241071 1 612201800921430012503it20810.10
22003631202.10
134315o4221.1011.1210.13
045
301 0
10060701 01 01 0
540201 020405
1 01 01 0
5
5
5
1 020
5
205
1 01 01 01 0555
shown to be a mixture of selenium and tellurium. A pro-posal to discredit selen-tellurium as a mixture has beenapproved by the International Mineralogical AssociationCommission on New Minerals and Mineral Names (IMAcNMMr{).
As suncnoup
Trzebiatowski and Bryjak (1938) show a continuoussolid solution between As and Sb at 400 "C; however, aminimum melting point in a continuous solid solution ischaracteristic of a transformation in the solid state. Asthe unit-cell dimensions of stibarsen (PDF 3l-80) aremidway between those of As (PDF 5-632) and Sb (PDF35-732), the chemical composition should be approxi-mately midway between As and Sb. The specimen forPDF 3l-80 contained arsenic intergrown with stibarsen,which sr ggests that stibarsen may exsolve from parado-crasite, AsSb, (Ironard et al., 197l). Stibarsen rather thanallemontite is the correct mineral species name (Hey,1982).
Stibarsen X-ray powder-diffraction data from PDF 3l-80 were refined in space group R3m to produce the unit-cell dimensions given in Table 2. X-ray powder-diftac-tion patterns were calculated both for an ordered AsSbcrystal structure model with As at 0,0,0 and Sb at0,0,Vz,and for a disordered crystal-structure model (As,Sb). Thecalculated intensities are compared to the visually esti-mated observed intensities of PDF 3l-80 in Table 3. As
6.284.952.783.603.363.213.092.U2.3i]2.282.212.151.981.821.731.611.55
1.451.421.411.371.301.27't.29
1 1 0018019o2400.12
1 1 9208
21121321411.12
30012810.16
3.222
2.376
2192
1.8't2
1.61 1
1.490
l2 observed reflections have 1 : 0 with a disordered crys-tal-structure model, but these reflections have 1 > 0 witha completely ordered crystal structure, stibarsen is con-cluded to be ordered and have the chemical formula AsSb.The visually estimated observed intensity data are notsufficiently accurate to exclude partial disorder of As andsb.
Tsuuorrn suBGRouP
Matildite, AgBiSr, as described by Geller and Wernick(1959), shows ordering in space group P3ml comparedto tsumoite, BiTe, which has been described by Yamanaet al. (1979). The refined unit-cell dimensions based onthe matildite X-ray diffraction data (PDF 24-1031) fromHarris and Thorpe (1969) are given in Table2. The valueof Frn (Smith and Snyder, 1979) increases from2(0.060,242) to 5(0.070,84), where Fiv: overall value ofF,( lA, | ,NN) with i/ as the number of observed reflec-tions. The increase in .F, reflects a significant change inunit-cell dimensions during refinement.
Bohdanowiczite, AgBiSer, as described by Geller andWernick (1959) shows ordering in space group P.3mlcompared to tsumoite, BiTe. The refined unit-cell di-mensions based on the X-ray diffraction data fsr boh-danowiczite (PDF 29-1441) from Pringle and Thorpe(1980) are given in Table 2. The value of .Fro increasesfrom 4(0.045 ,162) to 10(0.051,59).
Volynskite, AgBiTer, was described by Bezsrnertnayaand Soboleva (1965). The XRD data for volynskite (PDFI 8- I I 73) were indexed using space group P3ze l; the unit-cell dimensions in Table 2 correspond to a value of Froof2(0. I 50,78). Reflections that could not be indexed werelargely those of tellurobismuthite, BirTer, as shown inTable 4 (PDF 15-863), in addition to weak reflections for
D
1 01 0 25 25 2
80100 1001 0 130 11 050 4030 361 0 130 211 0 620 0.220 13
10 0.35 1
20 181 0 1 61 0 71 0 1 31 0 6
101012013
104105107
00
1oo t39320700
1 71 10
1 51 3
o0
1 0400333o636036700
6't5t003528
c
6I4
1 382
1 084163332223323324422
25
25
I
6
I
260 BAYLISS: TETRADYMITE GROUP
TmLE 5. Atomic coordinates of volynskite, space group PBml
Atom Equipoint
TABLE 7. X-ray powder4iffraction data for ingodite, sulphotsu-moite. and tsumoite
IngoditeSulpho- (Bio*- Bi(Teo7ttsumoite Teo4)Te Bio*)
/-" ,* l*o ,* /*.,* l*
1(b) 02(d) V31(a) 02(d) l/s2(c) 02(d) 1/g2(d) t/s
AgAgBiBiTeTeTe
0ry30+50%.+5
/e
V2+500.163V40.4170.917
which d : 6.28 ard2.28 A. An X-ray powder-diffractionpattern was calculated for an ordered crystal-structure ofvolynskite, as in the cases of matildite and bohdanowicz-ite; atomic coordinates are given in Table 5. The calcu-lated intensities in Table 4 are similar to the visuallyestimated observed intensities of PDF l8-1173, indicat-ing an ordered crystal structure.
Ingodite, BirSTe, was describedby Zav'yalov and Be-gizov (1981) as having space group P3*1. The range of12 chemical analyses given by Zav'yalov et al. (1984)varies from Bir 'oPbo o2S, ,rSeo orTeo ,u to Bi, ,.Pbo ,oSo nr-Te, or. The refined unit-cell dimensions of their specimen37 (B i rooPboorS , , rSeoo rT€o ru ) and spec imen 77(Bi,.nrSo noTe, ,o) are given in Table 2. An ordered crystal-structure model of BirSTe with atomic coordinates sim-ilar to those of matildite, AgBiTer, produced a calculatedX-ray powder-diffraction pattern significantly differentfrom the observed X-ray powder-diffraction pattern ofingodite. A crystal-structure model of Bi(S,Te), similar tothat of tsumoite, with z atomic coordinate adjusted forthe smaller S atom has atomic coordinates given in Table6. The intensities of the calculated X-ray powder-diffrac-tion pattern given in Table 7 are similar to the visuallyestimated observed X-ray powder-diffraction intensitiesof ingodite. Therefore, the chemical formula of ingodite(specimen 37) is inferred to be Bi(S,Te), whereas speci-men 77 is inferred to be tsumoite, Bi(Te,S). The visuallyestimated observed intensity data are not sufficiently ac-curate to exclude partial ordering ofS, Se, and Te.
Sulphotsumoite, BirTerS, was originally described byZav'yalov and Begizov (1982) as having space groupP3ml. The refined unit-cell dimensions of sulphotsu-moite (PDF 38-442) are given in Table 2. Disordered Teand S in a crystal-structure model similar to that of tsu-moite, with formula Bi(TeourSorr) have atomic coordi-nates similar to Te * S of ingodite as given in Table 6.The intensities of the calculated X-ray powder-diffractionpattern given in Table 7 are similar to the visually esti-
TABLE 6. Atomic coordinates of ingodite, space group P3m1
Equipoint
Notej The numbers 37 and 77 are specimen numbers from Zav'yalov €tal. (1984).
mated observed X-ray powder-diffraction intensities ofsulphotsumoite. Therefore, the chemical formula of sul-photsumoite is inferred to be Bi(Te,S). The visually es-timated observed intensity data are not sufrciently ac-curate to exclude partial ordering ofS and Te.
The unnamed mineral of Aksenov et al. (1968) has Bi:Te : 2:5. The X-ray powder-diftaction data (PDF 22-I 17) were indexed assuming space group P-3m1, as intsumoite. and the refined unit-cell dimensions are listedin Table 2. Crystal-structure models were constructed withboth Bi and Te ordered and disordered in the tsumoitestructure type. Many of the calculated X-ray powder-dif-fraction intensities are similar to the visually estimatedobserved intensities in Table 7; however, some signifi-cantly higher observed intensities are probably due toincorrect atomic coordinates. Although the exact natureof the Bi-Te order-disorder cannot be established, themineral seems to be a tellurian tsumoite, (Bio rrTeoor)Te,even though the chemical composition is far removedfrom BiTe.
A synthetic phase with Bi:Te : 5:3 was cooled from a
30 1220 't2
1 0 6
005 20101102
2 0 41 0 2't2 0.3
25 1
25 0.1100 100
25 138 4325 138 33
3 0 32 0 2
1 0 8
100 1003
2 0 360 37
50 351 0 41 0 2
3 0 820 1
40 181 0 1
1 0 4
1 0 130 111 0 11 0 230 1310 0 .3
103007104105
10610800.11
1 1 01 1 110.1 000.12,1 1 5
1 0 . 1 11172042061 1 910.13
1 1 . 1 020810.14
20911.12
21121321410.1611.1420.13
218300
100 100 100< 1 0 1
< 1 0 1 0< 1 0 1 0 1
50 60 39
100 100
70 38
80 37
25 18 40 19
50 34
30 40 14< 1 0 1 0 1< 1 0 1 0 2
20 30 18
5 0 1 0 1< 1 0 1 0 4
1 010 0 .1
20 20 11
10 30 151 0 1 0 21 0 1 0 120 30 15
2 0 3 0 11 0 1 0 230 20 113 0 2 0 5
50 12
38 18
25 15
25 11
50 12
70 't4
40 15 40 164 0 5
30 111 0 6
1 0 11 0 130 102 0 6
Bi 2(c) 0 0Bi 2(d) l/a +5Bi 2(d) +5 Yzs+Te 2(c ) 0 0s+Te 2(d l h ry3s+Te 2(d) % v3
0.1240.2910.4590.3620.056o.2'11
BAYLISS: TETRADYMITE GROUP 261
molten solution at700-270 "C at 5'C/h by Godovnikovet al. (1966). The X-ray powder-diffracriondata (pDF lg-176) were indexed by those authors on a unit cell ofthehedleyite type; however, the large difference between theobserved and calculated d-values indicates that the unit-cell dimensions are incorrect. The data were reindexedon a structure of the tsumoite type and the refined unit-cell dimensions are given in Table 2. A crystal-structuremodel for Bi(Teo.rrBio rr) was inferred having coordinatessimilar to those of tsumoite, BiTe. The intensities of thecalculated X-ray powder-diffraction pattern given in Ta-ble 7 are similar to the visually estimated observed X-raypowder-diffraction intensities of the synthetic phase (pDF19-176). Therefore, the composition of the synthetic phaseis equivalent to bismuthian tsumoite, Bi(Teo rrBio rr).
Platynite from Fahlun, Sweden, was originally de-scribed by Fink in l9l0 (Palache et al., 1944). Althoughthe specimen has been lost, Strunz (1963) quotes from a1933 private communication from F. E. Wickman whostates that the unit cell is hexagonal with dimensions c :8.49 and c: 20.80 A; however, the structure is not known.As this c dimension suggests a tsumoite-type structure,the original analysis may be recalculated without chal-copyrite and insolubles, and expressed as (BiouroPboror)-(SeourrSoro*). Therefore, platynite may be plumbian sul-furian nevskite.
Berry and Thompson (1962) stated that platynite issimilar to selenjos€ite. Ramdohr (1980) gave data for fourX-ray powder-diffraction reflections for platynite, but hecould not remember (Ramdohr, personal communica-tion, 1983) the locality of his specimen and was unableto supply either the original X-ray film or a specimen.The composition of platynite by Nikitin et al. (1989) issimilar to that of poubaite; however, the X-ray powderdata could not be indexed on a cell for any mineral ofthe tetradymite group. Therefore, platynite remains aquestionable species.
Tnrn-lnravrrrE suBcRoup
Tetradymite, BirSTer, has been synthesized by Glatz(1967), Evdokimenko and Tsypin (1971), and Abrikosovand Beglaryan (1973). The compositional limits of te-tradymite have been shown to be BirSTer-BirS, rTe,.rAsdetermined by Kuznetsov and Kanishcheva (1970) withinternally consistent X-ray powder-diffraction data. Pau-ling (1975) has explained why the substitution of Te byS increases the chemical stability.
Kawazulite, BirSeTer, was originally described by Kato(1970) as the Se analogue of tetradymite. The crystalstructure of synthetic BirSeTe, was solved by Bland andBasinski (1961), and confirmed by Nakajima (1963), inspace group R3m (166) with the atomic coordinates ofSe 0,0,0, Te 0,0,0.211, and Bi 0,0,0.396. As Se does nothave a significantly greater radius than Te, there is thepossibility that the Se and Te may be disordered. Thecalculated intensities of an X-ray powder-diffraction pat-tern of kawazulite with the ordered tetradymite structuretype are similar to the visually estimated observed X-ray
Tlau 8. X-ray powder-diffraction data of kawazulite and skip-penite
Skippenite
Kawazulite L""Dis- Or-
/* ordered deredI*/*"003006101009, 104015107018
00.121 0 .1001 .1 1
1 1 000.1 5
1 16 , 10 .1301.14,021
20510 .16 ,00 .18 ,20811 .1202 .100 1 . 1 71 1 . 1 501.20
12502.16, 1 1 .1821.10
16 331 1 71 0 86 5
100 1000 07 1 00 0
35 365 3
32 325 6
1 1 1 02 2
1 6 1 6c c0 09 90 0
1 0 1 03 3
1 1 1 15 57 7
10 3 5040 10 603 0 9 6 01 0 4 2 0
100 100 1001 0 22 0 3 1 01 0 150 38 701 0 5 5 050 34 801 0 6 3 01 0 9 3 010 2 20820 18 401 0 5 3 06 1
10 11 401 0 110 13 508
4 3 010 14 4081 0 5 2 0
4 4 0
powder-diftaction intensities of kawazulite (PDF 29-248)listed in Table 8. The visually estimated observed inten-sity data are not sufficiently accurate to exclude partialordering ofSe and Te.
Csiklovaite was introduced by Koch (1948) for a min-eral from Csiklova, Rumania, with the chemical formulaBirSrTe. The validity of the mineral has been questionedby some compilers; e.9., Fleischer (1987). To obtain X-raypowder-diffraction data of csiklovaite, eight specimensconsisting of several small grains and two small vials wereobtained from G. Grasselly, who supplied the type spec-imens used by the late Sandor Koch to describe csiklo-vaite.
In reflected light, three areas with distinctly differentcolors can be seen as follows: (l) creamy yellow, (2) lightblue-gray, and (3) darker blue-gray. This descriptionmatches that of Koch (1948). The eight specimens wereanalyzed, for As, Bi, Pb, S, Se, and Te with an electronmicroprobe under operating conditions of 20 kV, 15 mA,3 pm beam size, and 5 s count times. Grains from thevials were identified by X-ray powder-diffraction as amixture of tetradymite, BirS, ,Seo,Te, , (creamy yellow),galenobismutite (light blue-gray), and bismuthinite (darkblue-gray).
Bismuthinite and galenobismutite occur as lamellar andmyrmekitic intergrowths within tetradymite. These fine-grained minerals are therefore difficult to separate. Amixture of 60 wto/o tetradymite @irSTer) and 40 wto/o bis-muthinite (BirS.) would approximately reproduce thechemical analysis of csiklovaite reported by Koch (1948).
No evidence has been found in studies of svnthetic
262 BAYLISS: TETRADYMITE GROUP
phases (Glatz, 1 967 ; Kuznetsov and Kanishcheva, 197 0;Evdokimenko and Tsypin, 197l) to suggest a phase witha composition near that of csiklovaite. A phase with thecomposition of csiklovaite does not exist among the syn-thetic phases or type specimens. A proposal to discreditcsiklovaite as a mixture has been approved by the IMACNMMN.
Skippenite, BirSer(Teo nSo ,), was described by Johan etal. (1987), and was inferred to be the Se analogue ofcsik-lovaite. The crystal structure determination of BirSerTeby Nakajima (1963) shows a partially disordered crystalstructure with atomic coordinates for Se at 0,0,0,(Seor,Teor) at 0,0,0.2115, and Bi at 0,0,0.3985 in spacegroup R3m, yielding a chemical formula similar to thatof kawazulite, BirSe(SeTe). X-ray powder-diffraction pat-terns were calculated for a partially disordered crystal-structure model and an ordered crystal-structure modelwith the atomic coordinates Te 0,0,0, Se 0,0,0.212, andBi 0,0,0.396 in space group R3m. The calculated X-raypowder-diffraction intensities of both crystal-structuremodels as given in Table 8 are similar to the estimatedobserved X-ray powder-diffraction intensities of Johan etal. (1987). The chemical formula of skippenite is proba-bly either BirSe(Se,Te,S), or Bir(Se,Te,S)r, as is true fortellurian paraguanaj uatite.
Josiirrn suBGRouP
Laitakarite, BioSSer, was described by Vorma (1960);however, Nakajima (1963) stated that the Se and S couldbe either ordered or disordered within space group R3n.The refined unit-cell dimensions of laitakarite (PDF 14-220) are given in Table 2. An X-ray powder-diffractionpattern was calculated for a disordered crystal-structurehaving space group R3m wirth atomic coordinates similarto those of ikunolite, (BioSr; Kato, 1959) and pilsenite,(BioTer; Yamana el al., 1979). The SorrrSeo..., occupiessites with coordinates 0,0,0 and 0,0,0.426, whereas Bioccupies sites at 0,0,0.142 and 0,0,0.287. A second cal-culated X-ray powder-diffraction pattern was calculatedfor an ordered model with the S atom at 0,0,0, and thetwo Se atoms at 0,0,0.426. The calculated data for boththe ordered and disordered crystal-structure models, whichare given in Table 9, can be compared to the visuallyestimated observed intensities (PDF 14-220) of Vorma(1960). The calculated data of the disordered crystal-structure model are similar to the observed data (PDF14-220), especially for 003 and 006, so that the correctformula of laitakarite is Bio(So,S)r.
Jos0ite was named after Safi Jos6, Minas Gerais, Brazilby Kenngott in 1853 (Palache etal.,1944). The chemicalformula and refined unit-cell dimensions for space groupR3m are given in Table 2. The data in PDF 9-435 (Li,1957) are for a typeJocality specimen. Calculated X-raypowder-diffraction patterns were produced for orderedcrystal-structure models for BioSTe, and BioSrTe, and dis-ordered crystal-structure models for Bio(TerS) andBio(SrTe). The atomic coordinates of Bio(S,Te) were takenas Bi at 0,0,0. 1433 and 0,0,0.2856, and SouurTeor' at0,0,0 and 0,0,0.4260 for space group R3z. The atomic
coordinates of Bio(STer) were taken as Bi at 0,0,0.1447and 0,0,0.2842, and Sorr.Teouu, in 0,0,0 and 0,0,0.4260for space gloup R3ru. Jos6ite with ordered S and Te has006 strong, whereasjosdite-B with ordered S and Te has003 strong. The calculated intensities in Table 9 are sim-ilar to the visually estimated observed intensities ofjosEi-te (PDF 12-735) given by Peacock (l9al) and joseite-B(PDF 9-435) as consistent with the disordered crystalstructure types, Bio(S,Te), and Bio(Te,S)r.
The original X-ray powder-diffraction photographs(57.3 mm diameter Debye-Scherrer) of jos€ite by Berryand Thompson (1962) were borrowed from the RoyalOntario Museum, and new X-ray powder-diffraction filmswere taken of specimens ROM Ml9602a (os€ite) andROM Ml9602b (os6ite-B). As neither 003 nor 006 wasobserved in any ofthese photographs or reported in theliterature, the S and Te are inferred to be disordered. Inaddition, a wide solid-solution range between BioTe, (pil-senite) and BioS, (ikunolite) is suggested by the chemicalformulae in Table 2 and by so-called jos6ite-C(Bio,S,,Teo,) from Godovikov et al. (1970) and josEitefrom Minster et al. (1968). Therefore, jos6ite, Bio(S,Te).,is inferred to have a chemical formula similar to that oftellurian ikunolite, whereas jos€ite-B, Bio(Te,S)', is in-ferred to have a chemical formula similar to that of sul-furian pilsenite.
The unnamed mineral of Yingchen (1986) gave achemical analysis with 8i75.42,Te 19.2, and S 6.65 wto/0.The X-ray powder-diffraction data were indexed in spacegroup R3m as for ikunolite, and the refined unit-cell di-mensions are given in Table 2. As 003 is not observed,a disordered crystal structure is suggested in which thechemical formula is (BirrTeor)(SroTe,.o); i.e., this phaseis tellurian ikunolite.
Rucklidgeite [(Bi,Pb)rTeo] was originally described byZav'yalov and Begizov (1977); however, the Pb does notappear to be essential so that the end-member chemicalformula is taken as (BirTeo). The unit-cell dimensions(PDF 29-234) were refined and are given in Table 2. AnX-ray powder-diffraction pattern was calculated using acrystal-structure model similar to that for BioSe, of Sta-sova (1968). The model is for space group R3m, $iith Biat 0,0,0 and0,0,0.429, and Te at 0,0,0.143 and 0,0,0.286.The intensities of the calculated X-ray powder-diffractionpattern given in Table 9 are similar to the visually ob-served intensities of PDF 29-234 (Zav'yalov and Begi-zov,1977) and PDF 38-458 (Roberts and Harris, 1988).
The unnamed mineral (BirTeo) of Haraflczyk (1978) isrucklidgeite. The unit-cell dimensions were refined as-suming space group R3m and are given in Table 2.
Wehrlite was reexamined by Ozawa and Shimazaki(1982) and found to be a mixture of pilsenite, BioTer, andhessite, AgrTe. The discreditation was approved by theIMA CNMMN.
DrscussroN
The data ofBrown and Lewis (1962) show a wide rangeof solid-solution of Bi and Te from 32 to 600/o Te. Thedata of Godovnikov et al. (1966) for synthetic phases
BAYLISS: TETRADYMITE GROUP
TABLE 9. X-ray powder-diffraction data of laitakarite, joseite, joseite-B, and rucklidgeite
263
Laitakarite
l^. Jos€iteB Rucklidgeite
/*" Ordered Disordered L. l*/*t ,* le
II2
3
c
20
20
100c
360601 05
1 025
316
8
1 0
1 0
1 0
1 0
42I
1 4
1 50.1
1 1c2117
5 1 0100 100
1 0 1
3030
5
5
1 033
1 053
1 01 0
1 03
1 03
1 033c
3 83 07 82 1
1 4 1 51 2
1 2 1 7100 100
1 1
1 21 71
1 57
1 151117
1 21 71
1 57
1 1D
11,|6
31 5
003006009101012104015,00 .1210710.100 1 . 1 110.1301.14
1 1 010.1601.17
1 1 900.2110.19,20211.12,205
0271 1 . 1 501.2220.1411.2102.16
21710.2812.14
30021.16
30911.3330.21
1 51
1 0100 100
0.1
5 4100 100
10 0 .10.1
413510.11I
45 1235 18
30 151 0 725 12
40 4150 351 0 45 0.2
42 0 8
1 930
41 4134 343 4
5 68 85 57 7
1 8 1 80 0
20 121 0 1 7
20 157
1 0 1 11 0 6
41u40.16I23
1 810.2
1 2'17
c
4040
c
1 033
301
1 51 51
1 53I15'|3B5
fvote.' The ws, vs, s, ms, m, mw, w, vw, and ww were taken as 100, 40, 30, 15, 10, 8, 5, 3, and 1 , respectively
indicate a chemical formula of Bi, ,rTeo rr, which extendsthe solid-solution range far beyond the ideal compositionof BiTe. In contrast, the data of Brebrick (1968) for theBi-Te system shovi a series of individual phases with 50-60 at.o/o Te after annealing for several days at 525 "C,near the melting point. The identification of additionalphases may be due to the use of more sophisticated X-raytechniques having better resolution.
The suggested disorder of S-Te in the natural seriesBioSr-BioTe, (ikunolite-jos€ite-C-jos€ite-jos€ite-B-pil-senite) is difficult to explain. Most of the relatively fewobserved specimens have approximately integer ratios ofS:Te, which would favor an ordered structure at low tem-peratures.
The unnamed mineral of Gamyanin et al. (1980) isBirTe. The X-ray powder-diffraction data were indexedwith a hexagonal unit cell with a : 4.476(5) and c :
5.997(12) A; however, two reflections with d-values of4.16 and 3.57 A could not be indexed either with thisunit cell or a supercell.
Imamov and Semiletov (1971) described BioSe, (c :4.21, c : 51.54 A;, SboTe, (a : 4.27, c : 53.79 A), andBioTe, (a : 4.41, c: 54.9 A) as having space group P'3m1.Protojos6ite was redefined by 7,av'yalov and Begizov(1983). Their X-ray powder-diffraction data were in-dexed using space group P3ml, and the refined unit-cell
dimensions, which are listed in Table 2, imply a structurewith a repeat unit ofnine closest-packed planes.
Aleksite was originally described by Lipovetskii et al.(1978). Their X-ray powder-diffraction data (PDF 29-765) were indexed using space group P3ml, and the re-fined unit-cell dimensions are listed in Table 2. Hedley-ite, which was described by Warren and Peacock (1945),was given the chemical formula BirTer; however, bothBirTe, and BirTe, are in better agreement with the chem-ical analyses. The hexagonal subcell has refined dimen-sions a : 4.475(4) and c :5.367(3) A.
Imamov and Semiletov (1971) consider that syntheticequivalents of the tetradymite-group minerals have anodd number of approximately cubic closest-packed planes,including the structures of BirSe, BirSer, BioSer, BioSer,BiuSer, BirSer, and Bi.Ser, which have all been synthe-sized and characterized by Stasova (1968). The well-doc-umented minerals listed in Tables I and 2 basically fitthe relation involving an odd number of planeq however,the crystal structures of protojos6ite (9 atom planes),aleksite (14 atom planes?), or hedleyite (20 atom planes?)have not been established (Table l).
The published X-ray powder-diffraction data were ob-tained using a variety of kinds of equipment with differ-ent degrees ofprecision, and with intensities recorded onfilms with different exposures and estimated visually by
264 BAYLISS: TETRADYMITE GROUP
different people. Published intensities tend to be very in-accurate, and are subject to preferred orientation in thephases under discussion because of the common {0001 }perfect cleavage. Most investigators overestimated the in-tensity of weak reflections by using a scale of 5 to 100instead of 2 to 100. The intensities at the lower 20 angJesoften reflect the degree oforder-disorder, whereas the in-tensities at the higher 20 angles often appear to be inde-pendent of such effects. In contrast, the majority of well-refined crystal structures of the tetradymite group wereobtained with a carefully measured series of 0001 reflec-tions (nine to 36 of them).
The discrepancies between the calculated intensities andthe published observed intensities may be accounted forby the poor accuracy of the visually estimated observedreflections, the errors in estimated atomic coordinates,the possibility of partial order rather than either completeorder or disorder, or the possibility that further structuralcomplexities exist because of the hierarchy of subcellspresent in each X-ray powder-diffraction pattern.
Members of the tetradymite group present complexproblems, many of which remain unresolved due to in-complete data. To confirm the structure type and the de-gree of order-disorder in the mineral species within thetetradymite group, a series of crystal-structure determi-nations must be undertaken either by single-crystal orRietveld methods.
AcrNowr,nocMENTS
Dr. E. Nickel, vice chairman of CNMMN, provided valuable advice.Critical reviews were provided by J.E. Post, D.C- Harris and J.L. Jambor.Financial assistance was provided by the National Scientific and Engi-neering Research Council of Canada.
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BAYLISS: TETRADYMITE GROUP 265
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Memrscnrp'r RrcErvED Mencr 19, 1990Mexuscnrrr AccEprED Nowrvrsen 10, 1990
