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ON the CHEMICAL COMPOSITION of GERMANITE Svetlana N

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ON the CHEMICAL COMPOSITION of GERMANITE Svetlana N 34 New ddtd on minerdls. M.: 2003. Volume 38 UDK 549.37 ON THE CHEMICAL COMPOSITION OF GERMANITE Svetlana N. Nenasheva Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow, [email protected] Germanite is a very rare mineral that commonly occurs as small segregations in association with bornite, renierite, fahlores, sphalerite, galena, and other sulfides and sulfosalts. Very fine structures of replacement of germanite for renierite are often observed. Such small segregations are difficult to study. Optical properties of germanite are slightly variable in different areas and in samples from different deposits. The chemical compo- sition (concentrations of the principal elements) of germanite varies over a wide range. In addition, the miner- al was revealed to contain a wide set of admixtures. Therefore, different researchers propose different formu- las for germanite. Chemical and electron microprobe analyses of germanite, accessible in literature, were com- piled by the author, and peculiarities of the chemical composition of germanite were studied. It has been revealed that 28 analyses from 37 ones are adequately recalculated to the formula with 66 atoms in the unit cell; 6 analyses, to the formula with 64 atoms; and 3 analyses, with 68 atoms. The Me/S ratio in the analyses varies from 32:32 to 34:32 and to 36:32; that is, this ratio in the real analyses is inconstant. This fact suggests that we deal either with solid solutions or with three different, but similar in the chemical composition and properties, minerals. The second assumption is more probable. It is concluded that there exist three mineral species close to germanite in the chemical composition. 8 tables, 3 figures and 22 references Germanite has been known from the 1920s. sometimes, of As and V together. In this case, It was discovered by G. Schneiderhu;hn (1920) concentrations of these elements were compa- at the Tsumeb ore deposit, Namibia, and was rable with that of Ge and sometimes exceeded described and named so by O. Pufahl (1920). it. We excluded these analyses from the con- Later, the mineral was found at the Iiancairoun, sideration, because they were probably as- France (Levy, 1966) and Radka, Bulgaria signed to germanocolusite or colusite. (Kovalenker, et ai., 1986) ore deposits. Finds of At all the ore deposits, germanite is associat- complex Ge sulfides were reported from some ed with bornite, renierite, fahlores, and galena, Russian ore deposits, including the Pai-Khoi, being commonly intimately intergrown with Urup, Gai, III International, and Kurumsak these minerals. Very fine structures of replace- ones, and from the Chelopech ore deposit, ment of germanite for renierite are often Bulgaria. However, these minerals contained observed. Such small segregations are difficult large concentrations of either As or V, or, to study. Optical properties of germanite are slightly variable in different areas and in sam- ples from different deposits. Its color under reflected light is pink with violet tint, being very unequal. According to the optical fea- s tures, L. Loginova (1960) distinguished three -:;;0 individual varieties of germanite. IS The chemical composition (concentrations > +... of the principal elements) of germanite varies over a wide range (in wt.%): Cu 40.9-51.0, ""+ = Fe 0.0-10.7, Ge 3.0-11.0, Zn 0.0-5.5, and N 7 ++ S 30.0 - 34.5. In addition, the mineral was M= revealed to contain a wide set of admixtures, U including As, V, Ga, Sn, Sb, W, Mo, Pb, and Ag. 6 Therefore, different researchers propose differ- ent formulas for germanite (Tables 1 and 2). These formulas are characterized by different cation/ anion ratios equal to 1 : 1 (Sclar et 5 ai.,1957; Levy, 1966); 1 : 0.95 1.052 0 1 2 (Springer, 1969); 34 : 32 = 1.062 (Tettenhorst W+Mo atoms and Corbato, 1984; Spiridonov, 1987; Go- Fig. 1. Dependence between Cu2++ Fe + Zn and W + Mo in dovikov, 1997); and 36 : 32 = 1.125 (Spi- analyses of germanite ridonov et ai., 1992). On the Chemical Composition of Germanite 35 Table 1. Germanite formulae proposed by different researchers Formula Reference Me/S Cu3(Fe,Ge)S, De Jong, 1930 1 Cu:;(Fe, Ge,Zn,Ga) (S,As), Sclar et al., 1957 1 Cu6FeGeSg---7 Cu+]Cu2+:1Fe3+Gc4+SH Levy, 1966 1 (Cu,Fe,Zn,W,Mo,V,Ge,As GaISo'", Springer, 1969 1.052 CU'26Fe4Gc4S32---7 Cu + 1GCU2+loFe3+ 4GeH 4532 Tettenhorst et al., 1984 1.062 CU+2D(CU2-i- ,Fe2-J ,Zn)6FeJT2Ge6SJ2 Spiridonov, 1987 1.062 Cu -dCu" ,Fc'- ,Zn)hFeH 2(Ce,As)hS32CU'dCu" 2Fe" 2Zn,)c,FeH2(CeAs2IhS12 Spiridonov et al." 1992 1.125 Cu i-sCu2+sFeJt-2Ge4+2S16 Godovikov, 1997 1.062 Table 2. Theoretical composition of germanite (in wt. %), based on formulae proposed by different researchers Authors Cu Cu-+ Cu21- Fe Zn Ge As S Levy, 1966 49.76 24.88 24.88 7.29 9.47 33.48 Tettenhorst, Corbato, 1984 51.76 31.85 19.91 7.00 9.1 32.14 Spiridonov, 1987 43.50 39.55 3.95 6.95 4.07 13.55 31.93 Spiridonov, 1992 45.58 41.78 3.80 6.68 3.91 8.68 4.48 30.67 Godovikov, 1997 51.76 31.85 19.91 7.00 9.1 32.14 The crystal structure of germanite is deriva- The p-elements, Ge, As, and Ga, are the tive from the crystal structure of sphalerite and neighbors in Mendeleev's Periodic Table and is close to those of stannite and colusite. Based have similar electronic structures, therefore, on this fact, R. Tettenhorst and C. Corbato they can be isomorphous. Other admixtures, (1984) proposed a formula of germanite, similar V, Fe, Cu, Mo, and Ware d-elements; so, it can to that of colusite, namely CU26Fe4Ge4S32' This be assumed that V, Mo, and W can substitute formula is electrical1y neutral only on condi- Fe and Cu, occupying the sites of divalent tion that it contains 10 atoms of divalent Cu cations or trivalent Fe. The sum (W + Mo) and 4 atoms of trivalent Fe. The presence of 10 depends inversely on the sum (Cu2+ + Fe + atoms of divalent Cu is also specified by a crys- Zn), which evidences in favor of the assump- tal10chemical formula of germanite, proposed tion that Wand Mo occupy the sites of diva- by A. Godovikov (1997). CU+8Cu2+sFe3+2Ge4+'2S,6 lent cations or trivalent Fe (Fig. 1). The sum ---7Cu + 16CU2+loFe3+ 4Ge4+ 4S32'A crystal1ochem- (Cu + As) inversely proportional to the sum ical formula of germanite, proposed by E. (Zn + Ge). which is evidence for the isomor- Spiridonov (1987). Cu+2o(Cu2+,Fe2+, Zn)6FeH2 phism Zn2+ + Ge4+ ---7Cu+ + AsH (Fig. 2). Ge4+ 6S32' is not electrical1y neutral. Later, in the work on germanocolusite, E. Spiridonov with co-authors (1992) proposed another crystal10chamical formula for germanite, Cu +22(CU2+ ,Fe2+ ,Zn)6FeH 2(Ge,As)6S32' In this 28 case, the formula is electrical1y neutral. but the sum of atoms in the unit cel1 is 68 rather than 66 as in colusite whose formula was adopted by E. Ch, §' Spiridonov as the basis for examination of the Cd germanite formula; therefore, the Me/S ratio is equal to 36/32 rather than 34/32. ~+ These contradictions prompted us to make U24 an additional analysis of the literature data on germanite. 37 chemical and electron microprobe analyses 22 of germanite were found and recalculated into for- mulae with regard to necessity of their electrical neutrality (Table 3 and 4). A formula was consid- 20 ered to be electrical neutral if it had the valence 0 2 4 6 8 balance (:!:L1,the absolute value of the deviation Zn+Ge atoms from zero) no higher than 3%. To calculate the valence balance, it was necessary to understand Fig. 2. Dependence between Cu + As and Zn + Ge in analy' the positions of admixtures in the crystal structure. ses of germanite 36 New data on minerals. M.: 2003. Volume 38 Table 3. Electron microprobe and chemical (0) analyses of germanite in wt. % (upper row) and in f.u. (lower row). Analyses 1, 6, 22, 23, 24, and 36 are recalculafed based on 64 atoms in the unit cell; analyses 8, 21, and 35, based on 68 atoms; the remainder, based on 66 atoms NQos. Cu Fe Zn Ge Ga As V W Mo S L 45.1 7.4 1.3 9.7 0.00 2.6 33.4 99.5 21.92 4.09 0.61 4.13 1.07 32.17 63.99 2' 45.4 7.22 2.61 6.20 5.03 31.34 99.246 23.38 4.23 1.31 2.79 2.20 31.98 66.00 3" 42.12 7.80 3.93 10.2 1.85 1.37 31.27 99.49 21.57 4.55 1.96 4.57 0.86 0.60 31.74 66.00 4' 45.39 4.56 2.58 8.70 4.13 30.65 99.55 23.98 2.74 1.32 4.02 1.85 32.09 66.00 5' 39.44 10.7 3.56 7.04 4.86 31.44 99.98 20.38 6.29 1.79 3.18 2.13 32.19 66.00 6 44.20 6.70 1.50 9.70 3.30 34.60 100.0 21.25 3.66 0.70 4.08 1.35 32.96 64.00 7 46.5 8.5 9.4 4.2 31.6 100.26 23.50 4.89 4.16 1.80 31.65 66.00 8" 43.6 6.4 3.10 9.0 4.70 30.03 97.7 23.67 3.95 1.64 4.28 2.16 32.30 68.0 9 45.5 7.20 1.2 9.8 0.1 3.5 31.8 99.1 23.19 4.18 0.59 4.37 0.05 1.51 32.11 66.00 10 46.7 6.5 0.8 9.0 4.2 0.6 31.7 99.5 23.83 3.77 0.40 4.02 1.82 0.10 32.06 66.00 11 45.5 6.8 1.2 9.6 3.3 0.5 31.6 98.5 23.36 3.97 0.60 4.31 1.44 0.17 32.15 66.00 12 46.5 5.5 0.9 9.0 4.0 1.8 0.5 31.8 100.0 23.81 3.20 0.45 4.03 1.74 0.32 0.17 32.27 65.99 13 45.4 5.8 1.3 9.9 3.3 3.4 31.9 101.0 23.20 3.37 0.65 4.43 1.43 0.06 32.31 65.99 14 47.1 3.6 1.4 10.1 3.2 0.2 3.0 31.8 100.4 24.06 2.09 0.69 4.52 1.39 0.03 1.02 32.20 66.00 15 47.5 3.5 1.4 9.6 3.1 0.3 2.8 32.1 100.3 24.22 2.03 0.69 4.28 1.34 0.05 0.94 32.43 65.98 16.
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