Canadian MineralogistYol. 2.4, pp. 385-397 (1986)

COMPOSITIONAL TRENDS IN TETRAHEDRITE

NEIL E. JOHNSON, JAMES R. CRAIG AND J. DONALD RIMSTIDTDepartment of Geological Scimca, Wrginia Polyechnic Institute and Stote University, Blacksburg, Wrginia 24061, U,S.A,

ABSTRACT

Available mmpositional data for l27t samples of naturaltetrahedrite and 295 of synthetic tetrahedrite weres1amingd. They show: compositional ranges of four to tenCu, zero to six Ag, and a total of two (Fe, Zn, Hg) atoms,complete substitution (up to four atems) amongAs, Sb andTe, and a total of 13 atoms of S per formula unit. Dataon contents of Pb, Bi and Cd in natural samples are insuffi-cient to define their compositional ranges, and virtually nodata exist on other substitutions, involving Co, Ni, Mn andAu. A generalized formula (Cu,Ag)6Cua@e,Zn,Cu,Hg,Cd)2(Sb,As,Bi,Te)a(S,Se)13 is proposed on the basis ofthese compositions. The cell rlimensisn is a linear functionof chemical elements in the formula errcept inAg-rich tetra-hedrite, where it increases with Ag content, up to four Agatoms per formula unit, and then decreases. Ag and Ashave a low tolerance for each other in the structure, andplots of Fe versus Ag, Zn versus Ag, and Hg versus Clsuggest that incorporation of Ag'is controlled by severalfactors. For the purpose of explaining compositional vari-ations in tetrahedrite, the simple Brillouin zone model of6eading (Jotrnson & Jeanloz 1983) is superior to any ionicmodel.

Keywords: syothetic and natural tetrahedrite, Brillouinzone, Ag-F e-Zn-Hg-Cd-As-Te-Bi substitution.

SoNaMarns

On a examinE les donn6es d.isponibles sur la composi-tion de 1271 6chantillons de t6tra€drite naturelle et 295 sp€-cimens de t6traddrite slnth€tique. Ces donndes indiquent:des domaines de composition pouvant aller de quatrei dixatomes de Cu et de z6ro i six Ag, un total de deux atomes@e, Zn, Hg), substitutibn compl&te (usqu'i quatre ato-mes) entre As, Sb, et Te, et 13 atomes de S du total parunitd formulaire. Les donn6es sur la teneur des 6chantil-lons naturels en Pb, Bi et Cd sont insuffisantes pour d6fi-nir leurs domaines de composition: quant aux donndes surles autres substitutions, impliquant Co, Ni, Mn et Au, ellessont pratiquement inexistantes. On propose ici la formulegdn6ralis6e (Cu,Ag)6Cua(Fe,Zn, Cu,Hg,Cd)2(Sb,As, Bi,Te)a(S,Se)13 qui est fondde sur ls donndes susmentionndes. Ladimension ds 14 maille est fonction lin6aire des €ldmentschimiques qui entrent dans la formule, saufpour €chanfil-lons riches en Ag, of elle augmente avec la teneur en Ag,de z6ro i quafe Ag par unit€ formulaire, aprds quoi ellediminue. Dans la structure, Ag et As ont I'un pour I'autrepeu de tol€rance; les courbes de Fe, Zn et Hg, en fonctionde Ag, Ag et Cu, respectivement, portent e penser queI'introduction d'Ag ddpend de plusieurs fdcteurs. Pourexpliquer les variations de composition dans la tdtra6drite,le moddle simple de la liaison chimique par zone de Bril-

louin (Johnson & Jeanloz 1983) est pr6f€rable d tout moddleionique,

(Traduit par la R6daction)

Mots-cl&: t€1la€drite. synth€tique et naturelle, zone de BriI-louin, substitutions Ag-Fe-Zn-Hg-Cd-As-Te-Bi.

INrnooucrroN

In the most recent tlorough studies of the chemicalcompo$ition and physical properties ofnatural tetra-hedrite, Charlat & L6vy (1974, lms,1976) propo-sed, on the basis of a suite of 54 samples from selec-ted localities worldwide, the general formula(CuAg) to(Fe, Zn,Cu,Hg,Cd)2(Sb,As)aS 1 3. Subse-quent studies have confumed that Se substitutes forS (Johan & Kvadek 1971, Brodin et al. 1979), and,Bi + Te sub$titute for As + Sb (Oen & Kieft 1976,Mozgova et al. 1979). Other studies report the pre-sence of Pb, Co, Ni, Au and Mn (Schroll & AzerIbrahim 1958, Bishopel al. 1977, Pattrick 198, Basuet al. 1981, 1984a).

The tetrahedrite structure, as descdbed byWuensch (l9g), can be written as:. rYMl6rrrM26

lrr\XrvY?l4vrz, where Ml represents Cu or Ag, M2represents Cu, Fe, Zt,Hg or Cd, Xstands for Sb,As, Bi or Te, and Y and Z could be either S or Se.Although the existence of the six-co-ordinated Zanion in the structure (the l3th S atom) has beengenerally accepted, there has been some resistance@elov&Pobedimsftayn 1969, Kaplunnik et al. 1980,Babushkin e/ a/. 1984) to this in favor of a l2-S-atommodel. Furthermore, the distribution of the twometal (M) sites in the crystal structure does not matchthat of meta! atoms in thegeneral chernical formulaabove. Several authors (Godonikov & Il'yasheva1973, Johnson & Jeanloz 1983, Miller & Craig 1983,Pattrick & HaII 1983) have collected compositionaldata from published sourcs in order to review tetra-hedrite chemistry or to illustrate a point germane totheir research. Because the chemical composition ofa mineral reflects the underlying cry$tal chemistry,a large base of analytical data may provide infor-mation on crystallochemical relationships. Thisreport is a comprehensive compilation and analysisthat clarifies sorue reported relationships, unearthsotler ones that exist in tetrahedrite-group minerals,and thereby serves as a guide for future research. Tominimize confusion, the use of varietal names of

385

386 TI# CANADIAN MINtsRALOGIST

80

o 5 0o

! 'rod

30

e0

t 0

0{ 5 6 7 I I l 0 l l l ? 1 3 ! { 1 5 1 6

Artm

Ftc. l. Number of anions (S, Se) per forrnul,a unit.

0 . 0 0 . 5 1 . 0 , 1 . 5 2 . 0 e . 5 1 . 0 3 . 3 1 ' 0

Sb Ato0r!

Ftc.2a. Number of As atoms yersan number of Sb atomsfor samples of natural tetrahedrite. Dashed line is idealsolid-solution.

t€trah€drite-s€ries m€mb€rs (tsnnantite, freihrgite,schwatzite, elc) has been avoidd; su(h m€mbers aredescribed as As-rich tetrahedrite, Ag-rich tetrahe-drite, Itrg-rich tetrah€drite, elc.

TLre data base for this paper consisb of reportedcompositions of l27l samples of natural tetrahedritefrom various oredeposia atrd 295 of syntletic tetra-hedrite. The data were not critically evaluated exceptto remove clearly inaccurate analytisal data {e.8.,totals qualling 9390, 30 weight 9o Fe). The follow-ing informa.tion is consider€d for each composition:Cu, Ag, Fe,Zt, Hg, Cd, As, Sb, Bi, S, Fb, Te, Mnand Se contents in qeieht 90, and the length of theunit'csll edree a (fi A;, where given. Data on otherreported substitutions are sufficiently rare and theirreported inportance is sufficiently small that theycan be ignored. The weight percentage was convertdto number of atoms based on 29 atoms per formulaunit. Two additional variables were considered dur-ing the processing: BZ, the number of valence elec-trons per unit cell, and CHR, the net ionic chargeper formula unit (after Johnson& Jeanloz 1983). Theresults were then ploued as functions of each otheror as functions of their frequency. A complete list-ing of all the data may be obtained from the authors,and will be included in the Ph.D. dissertation of thefirst author.

E 2 I

0 . 0 0 . 5 1 . 0 l . s 2 . 0 2 . s 1 . 0 t . 5 c . 0

Sb Atdftt

ftc. 2b. Number of As atoms versas number of Sb atomsfor samples of synthetic tetrahedrite. Dashed line is asin Figure 2a.

COMPOSITIONAL TRENDS IN TETRAHBDRITE 387

Rssur-TS

S and Se contents

Recalculating weight go values to the mean atomic9o S indicates that the hypothesis of 12 S atoms performula unit can be rejected at the 9,9010 confidencelevel. By far the largest number ofreported tetrahe-drite compositions (8590) have 13 S atoms per for-mula unit (Fie. 1), effectively ending the debate asto whether tetrahedrite contains 12 or 13 sulfuratoms. The S content does seem to decrease slightlyas the Cu content decreases, but the maximum den-sity in population occurs near 10 Cu and 13 S atoms.The papcity of data sets (6) with detectable Se doesnot permit any generalization regarding the mechan-ism of Se substitution.

Sb, As, Bi and Te contents

Analytical data for both natural and syntheticsamples support the long-held premise ef gempletesolid-solution between the Sb and As end-members@igs. 2a,b). The relatively even distribution of com-positions indicates that the geochemical conditionsnecessary for the formation of all members of thesolid solution are widespread. Bi-rich tetrahedrite isrelatively rare, and the limited data indicate noapparent correlation of Bi with Sb or As contentsin spite of the suggestion of Bohmer (1964) that Bisubstitutes for Sb but not for As. The maximum Bicontent found in our data base is approximately twoatoms per formula unit at a Cu content of approxi-mately 10 atoms per formula unit. The relatively fewdata on Te suggest a maximum of 3 to 3.5 atomssubstituting on a l-to-l basis for Sb (Fie. 2c); thisobservation is consistent with Kalbskopfs (1974)findings.

Cu content

The ideal tetrahedrite and tennantite compositionsare usually given as Cu12SbaS13 and Cu12AsaS1r,respectively. However, the majority of compositionsexamined in this study contain l0 atoms of Cu performula unit (Fig. 3a). A significant percentage havefewer than 10, down to a minimrrm of 4, but fewerthar l2Vo contain more than 10. This is consistentwith the experimental findings of Tatsuka &Morimoto (1977), who found that synthetic tetra-hedrite with less than 0.5 atoms of Fe per formulaunit breaks down, after annealing, to a mixture offamatinite, digenite and chalcocite. This also agreeswith the generalized formula proposed by Charlat&LEvy (1974), which indicates thar Cu beyond l0atom$ per formula unit substitutes for the two Feor Zn atoms normally present.

In contrast, most synthetic compositions of tetra-

IAE|.! I. PUBLISIIED S()URCES OT OATA USEO III ftIS PAPER, II{CLIJDIIGNUfiBER OF SAI'IPT€S AIID IXEIR ORTGI{ (TAI'RAL OR SYNHFIIC)

tbulgqln8 et ql. (1975)Ar!n. (192)Araya et rl. (l9Z)Abaey (1975)Arer lbrrhlo (1958)Brsu or ql. (t981)&ru ot .1. (1984r)Edsu ot !1. (1984b)BlEh (1981)Bl3hop ot r l . ( l9Z)BoldyEva & BorcdlyeY (1974)Brodln * 61. (1979)qurkhsd-86@nn (1984)Cdh & Hrl (1979)Corn9 & tlsrls (1978)chrrl!! & t6!ry (1974)Chon & Petruk (19&l)Cher et al. (1980)C!tulozl & Eeran (1973)Cor (197?)Cz@nske & H.ll (1975)tloelbr (t925)ltrlnln-Barkovskly €t al. (1970)Eld@ or !1. 0984)Godovolkov & Ilrydsheva (1973)Hlt & Tuppsr (1979)Hatl (1972)flall & Cz@rrl€ (1972)Hlneo (1904)finng & il€yer (1982)lul A L€e (1980)tndolev ot al. (19t1)Ix6r & SbBley (19&))llor e suiloy (19tlt)Jrstnikl (1983)Johsn & Kvacol (1971)Johnsoa (1982)lGrop-luller ('1974)(ovalenker ot rl. (1981)rvrtol ( 197)Krdol ot !1. (1975)Llnd & Hlkovlcky (1982)

Llco d rl. (192)ilsske & Sllnrer (1971)lhfu6n ( 1984)ntllor A cn'lg (1983)ilorulc & Kublcr (1972)lilougovo ot al. (1979)llozgov! ot !1. (1980)iipoeto3 (1983)ilalk (1975)N$h (1975)lrnHn Gszg)illshlsll ot !1. (1971)llorgorodovr ot !1. (1978)&tr st al. (1973)oon A Kloft (1976)Plttrlck (1978)Pstrrtcl (1984)9r$rtct & Hall 0984)Petnk et.al . (1971)Rlabo & Srck (1984)Rans@ (l$9)Rl ley (1974)Srnd6tl & teoft (1981)shl@d. & lllMqrl (1972)Shl@kl (1974)Sprlngor (1969)SUnlry & Iror (1982)Sugakl ot al. (1975)Suglkl ot rl. (1984)Tlt80h & [ort@tq (1973)Torlutr ! liort@io (19n )norpo ot al. (1976)ft@ (1982)Tl@feyorskly (1967)Teplr ot sl. (l9Z)T3splr or 81. (1979)Vslll'yov & l-lvrentry€v (1973)Yrlghatr & lror (19d))Iu & PehEo (1977)Yul (1971)Y6hkln (1978)Zolrzskl & Nugtorei (1984)

3 5 S1 4 S4 t t

32 l14 tl

184 tl2 8 N1 l {4 t l8 N

2 5 [5 l {

41 t{I N5 N

1 6 N2t il2 5 S9 t l

25 tlt ! l

24 tl2 t It d Nu t {e 9 r4 l l

3 8 S3 N

1 6 S

2 N8 N7 t l3 t l2 f r2 NI N7 t {

t l t l3 N

14 ll

I t lI t l

1 7 i l3 t {7 N5 N5 t {

1 2 !l t lI N2 N3 N6 N5 l {4 t

161 N14 tl4 N1 t l9 t {T N

t38 N3 N

73 l{88 tl1 2 N1 0 s3 N

26 il8 N

t0 il9 t l6 t {

2 7 i l2 N1 t {

137 S6 N2 ! lt t l2 l l

F

! e .

F

t .

0 . 0 0 . s 1 . 0 1 . 5 2 . a 2 . 5 3 . 0 3 . 5 { . 0

Sb Afom

Ftc.2c. Number of Te atoms versas number of Sb atomslor samples of natural tetrahedrite. Dashed line is asin Figure 2a.

388 THE CANADIAN MINERALOGIST

o

Io

F

o

0 t 2 3 u 5 6 7 0 I l 0AO Alomg

Ftc. 3a. Number of Cu atoms yersras number of Ag atomsfor samples of natural tetrahedrite. Uppermost dashedline is for six Cu atoms in Ml site, middle line is forfive Cu atoms in Ml site, and lowermost line is for fourCu atoms in Ml site.

A9 Atoms

Ftc.3b. Number of Cu atoms versr number of Ag alomsfor samples of synthetic tetrahedrite.Dashed lines areas in Figure 3a.

hedrite fall into three groups with 4' 5 or 6 Cu atomsper Ml site (Fig. 3b), correspoading to a total cucontent of 10, l l or 12 atoms. This results from themany syntheses of tetrahedrite compositions in therange of l0 to 12 atoms per formula unit. In con-trast, tetrahedrite with as few as three total Cu atomswas synthesized by Pattrick & Hall (1983).

Ag content

The work of Wuensch (196/), which showed thatthere are six three-co-ordinated sites in the structure,and that of Kalbskopf (1972), suggest a maximumof six Ag atoms per formrila unit. Charlat & L6vy(1974), however, proposed a maximum of l0 Agatoms per formula unit. The data gathered in thisstudy @g. 4a) indicate that although there are rarecompositions that approach l0 atoms, the vastmajority indeed have fewer than six Ag atoms.

Several authors (Charlat & L6vy 1974, Wu &Petersen 1977, Miller & Craig 1983) have noted thatthere is a distinct correlation between the Sb and Agcontents. Figure 4a, a plot ofAs versus Ag contents,shows that, in fact, Ag and As have little tolerancefor each other in the tetrahedrite structure; thepresence of only one atom of As decreases (neglect-ing scatter) the maximum number of Ag atoms con-tained from six to one.

In a study of Ag-rich tetrahedrite from the Mt.Isa deposit, Riley (1974) noted that the cell expandswith increasing Ag content, up to a maximum of fourAg atoms per formula unit; samples with progres-sively higher Ag contents show a decreasing lengthof the cell edge. Fieure 4b shows that this is a generaltrend for all Ag-bearing tetrahedrite, but thisdecrease contrasts with the relatively linear cell-dimension variations resulting from other composi-tional substitutions.

The plot of the unit-cell dimension of composi-tions of synthetic tetrahedrite (Fig. 4c) is basicallylinear up to seven atoms of Ag, and does not dis-play the break at four Ag atoms nor the subsequentdecrease from four to six atoms that samples ofnatural tetrahedrite show.

Fe, Zn, Hg, Cd and Mn contents

Figures 5a, 5b and 5c are plots of Fe, Zn and Hgconcentration as a function of Sb. The only appar-ent relationship is that of higher Hg content withhigher Sb content, which may be an artifact of thelimited number of data on Hg-tetrahedrite. Theredoes appear to be a fairly well-defined limit of twoatoms per formula unit for each element. This is con-sistent with the limits proposed by Charlat & L6vy(1974). The Cd data that do exist are insufficient tomake the same observation definitive, but those datathat are available, and the chemical similarity of Cd

COMPOSITIONAL TRENDS IN TETRAHEDRITE 389

o

Eo

o

.A//_,/ // a /

2t ,{A o /

",/ /'/ ./-( r / o

. / -

oo' /aE

7/o

n/

t 0

l 0

o I z 3 oooo lor " t t I I I

Fro. 4a. Number of As atoms yersrs number of Ag atomsfor samples of natural tetrahedrite.

1 0 ,

1 0 .

0 I e 3 q 5 6 ? 6 I t 0Ag Atffi

Frc. 4b. Plot of cell dimension (in A) versas number of Agatoms for samples of natural tetrahedrite.

o I 2 3 * * i r - r u ? I e l o

Frc. 4c. Plot of cell dimension 0n A) versas number of Agatoms for synthetic tetrahedrite; the lower line is forFe- and Zn-barng tetrahedrite, the upper line is forCd-bearing tetrahedrite.

to Znand Hg, suggest an analogous situation. Asshown in Figure 5d, Hg content is greatest when theCu content is 10 atoms, and decreases sharply if Cuis higher or lower. The decrease on the low-Cu sideappears to indicate an antipathetic relation betweenHg and Ag.

Figures 5e and 5f display relationships between Feand Ag, and between Zn and Ag, respectively. BothFe and Zn display complete (up to two atoms) sub-stitution where there are fewer than two atoms ofAg; if Ag increases beyond this value (to a maximumof six), Zn decreases, whereas Fe remains betweenone and one-halfand two atoms. This effect has beennoted by Pattrick & Hall (1983) for selected naturalsamples. This positive correlation of Fe withAg maylead to a secoudary positive correlation of Fe withSb, consistent with that found by Raabe & Sack(1984) for natural samples.

Reports of Mn-rich tetrahedrite have recently beenpublished (Basu el al. 1984a, Burkhart-Baumann1984), but the available data are too meager to domore than suggest that Mn substitution is analogousto that of Fe, Zn, Hg and Cd. Experimental dataon Fe, Zn, Hg, Cd and Mn content are scarce andappear consistent with the data on natural material.

1 0 .

t 0 .

0

!

!U

o

1 0 .

t 0 .

Eo

390 TIIE CANADIAN MINERALOGIST

0 . 0 0 . 5 . 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . 5 q . 0Sb Almt

Frc.5a. Number of Fe atoms versas number of Sb atomsfor samples of natural tetrahedrite.

f ^ ^" a , r r

0 . 0 ! . 5 1 . 0 1 . 5 2 . O 2 . 5 3 . 0 3 ' 5 { ' o

Sb Atm

Ftc.5c. Number of Hg atoms versus number of Sb atomsfor samples of natural tetrahedrite.

I

t"t

1b:

dl

0 . 0 0 . 9 1 . 0 1 . 5 2 . 0 2 . 5 3 . 0 3 . S 1 . 0

3b Atffi

FIc.5b. Number of Zn atoms versrs number of Sb atomsfor samples of natural tetrahedrite.

o r 2 1 q 5 6 ? € 9 l 0 l l 1 2

Cq Afoms

Frc.5d. Number of Hg atoms versus number of Cu atomsfor samples of natural tetrahedrite.

COMFO,SITIOh}AL TRENDS IN TETRAHEDRITE 391

Ph cantent

Reports of Pb in tetrahedrite 4rs errceedingly rare,a somewhat surprising fact, given the @tnmon oc.cur-rence of tetrahedrite with and within galena, and theapparent solid-solution befween the two minslsls,as evidenced by the submicroscopic exsolution-related inclusions oftetrahedrite in galena (Gaspar-rini & Lowell 1983, 198t. The only discernible trendin the Pb-bearing compositions reported is a sfuhttendency for tlern to b€ associated witlr Sb-.rich com-positions.

Brillouin zone (BZ) ond ianic-chorge (CHG) models

Johnson & Jeanloz (1983) proposed the use of asimple Brillouin-zone (BZ) model to account for thecompositional variations in tetrahedrite. In thismodel, each atom contributes one or more valenceelectrons to a conduction band, and these electronsare summed over a unit cell. That paper, and a sub-sequent one (Jeanloz & Johnson 1984), use thismodel to account for several observations: l) Com-positional variations correspond 1s s pnge ofbetween 204 and 208 valence electrons per unit cell.

2) The larg€st proportion of compositions is foundat the upper limit of 208 valence electrons per unitcell..3) The electrical resistivity incteases as compo-sitions approash theupper limit of 208 electrons perunit crll. 4) The optical absorption decreases as com-positions approach the upper limit of 208 electronsper unit cell.

The data base collected here is ideal for testing theIirst two premiss; hence, the quantities BZ, the num-ber of valence electrons per unit cell, and CHR, thenet ionic charge per formula unit, were calculatedfor each composition. The number of electrons andthe formal charges used for each atom were takendirectly from Johnson & Jeanloz (1983). Figure 6ashows that the largest number of natural composi-tions do in fact occur at 208 valence electrons perunit cell, andT2Vo of all the compositions fall withinthe limits (2&l-208 valence electrons) predicted. Theanalogous calculations ofnet ionic charge (Fig. 6b)have the largest number of compositions (3590) at0, which represents a balance of charge, but theremainder of the compositions have a net electriccharge, which violates the premises of a purely ionicmodel. The calculations for compositions of syn-thetic tetrahedrite show the same tendencies (Figs.

d.aa a

tr odl f " rl * a

Ag Atoms

Ftc.5e. Number of Fe atoms yersar number of Ag atomsfor samples of natural tetrahedrite.

0 l 2 x { 5 6 7 8 9 1Ag Atoms

Ftc.5f. Number of Zn atoms yersas number of Ag atomsfor samples of natural tetrahedrite.

TI{E CANADIAN MINERALOGIST

@ @ o N C o o o N s @ o o Ne o R R R R R - i l N * * N S

Volenc€ Elgclrons Psr Unlt Cell

Ftc.6a, Number of valence electrons per formula unit forsamples of natural tetrahedrite.

N.l ldlc Ch6g! Per Unll Call

Ftc.6b. Net ionic charge per formula rtnil fs1'samples ofnatural tetrahedrite.

Frc.6c. Number of valence electroos per fonnula udt forsamples of synthetic tetrahedrite.

Nol Chqrgs P6r Ut*l C.ll

FIc.6d. Net ionic charge per formula unit for samples ofsynthetic tetrahedrite.

{5

q0

it

a0

l 0

0

EE€HHHHR$: :H$$volenc€ Elaclrons Pe Unll Coll

- f o - 8 - 6 - 4 - 2 0 2 4 8 E O

COMPOSITIONAL TRENDS IN TETRAHEDRITE 393

6c, 6d). It is clear that these data substantiate thepremise that the concept of a Brillouin zone is con-siderqbly more effective than a simple ionic modelin explaining the nature of tetrahedrite.

Regression onalysis of cell dimensions

Charlat & L€W (1975) gathered cell-parameterdata to complement the compositional data fromtheir previous work (Charlat & L6vy 1974) and fita linear equation to the results. Their equation relatesthe changes that Ag, Hg, Cu2* and As contributeto a "standard" tetrahedrite of compositiqnCu16(Fe,Zn)rSbaS1, and cell dimension of 10.386 A.Their result is useful under their stated constraint ofless than 20 weight 9o Ag and only for those substi-tutions available to them for examination. The muchlarger data-base in this study allows us to furthergeneralize their results and to provide quantitativestatistical information concerning the fit of the modelto the data.

Techniques of multiple linear and nonlinear regres-sion were used to analyze the relationship betweencell dimensions and composition for this, data set.Preliminary results for natural tetrahedrite indicatethat an equation of the form:

a = Po+ PtAg+ P2Fe+ fuZ^+P4Hg+B5Sb + 9aAg2

provides the best fit. The regressor variables areatoms,/formula unit divided by the number of sitesavailable (six for Ag, two for Fe, Zn and Hg, fourfor Sb), B6 is the intercept, and gr-e are the weightsfor the regressors. The resultant overall R2 is0.961833; the values for Bq6 and their associatedpartial F-tests are listed in Table 2. Predicled valuesof a have been found to be within 0.08 A of theiractual values. Further research on refining these rela-tionships is underway and will be reported at a laterdate.

CoNcLUSIoNs

The body of tetrahedrite data indicates that thereare a great many interrelated effects on tetrahedritecompositions, some of which are easily explained,whereas others are not yet understood. A general-ized compositional formula of the form: (Cu,Ag)6Cua(Fe,Zn,Cu,Hg,Cd)r(Sb,As,Bi,Te)a(S,Se)13 se€rrsto best account for the range of composition ofnaturally occurring tetrahedrite. This suggests thatthere are three distinct metal sites in the structure,instead of the previously determined two. The dataon synthetic tetrahedrite agree in general with thosefor natural tetrahedrite, but there are some markeddiscrepancies, especially concerning Ag-bearing tetra-hedrite. More research is needed using the approach

TABLE 2. VALUES FOR REGRESS0R ITIEIGHTS' Bo-s' Al{DIHEIR CoRRESP0NDING Fobs TESTS

Parameter F . + l m t a F:t:::;:-=========="==:s!!==

B0 (lntercept)

q (As)

s2 (Fe)

83 (zn)

B, (Hs)

Bs (sb)

eu (rs2)

' l 0 .2 l l

0.459 577.29

0.021 3 .10

0.017 2 .76

0.142 160.8I

0.147 680.09

-0.346 276.09

of Tatsuka & Morimoto (1977), who studied the sta-bility of synthetic tetrahedrite, particularly if we areto understand the graphic intergrowths of tetrahe-drite, arsenopyrite and gudmundite that apparentlyresult from tetrahedrite breakdown, as reported byJuve (1974), Sandecki & Amcoff (1981), Shadlun(1982), Miller & Craig (1983) and Basuet al. (1984b).Further research on less common and coupledschemes of substitution is also desirable.

AcrNowrsocEMENTs

The authors are indebted to Dr. Mary L. John-son for her permission to use her unpublished dataon synthetic tetrahedrite. We would also like to thankJ.L. Jambor for his careful review of the manuscript,and for providing additional analytical data that sup'port our conclusions, but are not included in thefigures. This research was supported by a VirginiaMining and Minerals Resources and Research Insti-tute (U.S. Bureau of Mines) allotment grant.

REFERENcES

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Anar.re, R. (1977): Nota sobre la tennantita de Molvi-zar (Granada), Estud. Geol. 33,219-222.

Aneva" R.A., Bowr.rs, J.F.W. & SINPsott, P.R. (1977):Relationships between composition and reflectancein the tennantite-tetrahedrite series of the ElTeniente ore deposit, Chrle. Neues Jahrb. Minerol,Monatsh., 461482,

Araresov, V.A. (1975): Argentian mercurian tetrahe-drite, a new variety, from the Chiprovtsi ore deposit,western Stara-Planina Mountains, Bulgaria.Mineral. Mag. 40, 233-237.

Azrn ISRAHIN4, N. (1958): Beitrag zur Kenntnis ostal-

394 TIIE CANADIAN MINERALOGIST

piner Fahlerz. II. Chemische Untersuchungen vonFahlerzen. Tschermoks Mineral. Petrog. Mitt. 6,238-245.

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Received fune 12, 1985, revised manuscript acceptedNovember 20, 1985,