ON the CHEMICAL COMPOSITION of GERMANITE Svetlana N
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#05_nenash_en_0802:#05_nenash_en_0802.qxd 21.05.2009 20:05 Page 34 34 New data on minerals. 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. Schneiderhцhn (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 Bancairoun, sideration, because they were probably as - France (Levy, 1966) and Radka, Bulgaria signed to germanocolusite or colusite. (Kovalenker, et al., 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 PaiKhoi, 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- tures, L. Loginova (1960) distinguished three individual varieties of germanite. atoms 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 S 30.0–34.5. In addition, the mineral was revealed to contain a wide set of admixtures, including As, V, Ga, Sn, Sb, W, Mo, Pb, and Ag. Therefore, different researchers propose differ- ent formulas for germanite (Tables 1 and 2). These formulas are characterized by differ ent cation/anion ratios equal to 1 : 1 (Sclar et al.,1957; Levy, 1966); 1 : 0.95 = 1.052 (Springer, 1969); 34 : 32 = 1.062 (Tettenhorst 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 al., 1992). #05_nenash_en_0802:#05_nenash_en_0802.qxd 21.05.2009 20:05 Page 35 On the Chemical Composition of Germanite 35 Table 1. Germanite formulae proposed by different researchers Formula Reference Ме/S Сu3(Fe,Ge)S4 De Jong, 1930 1 Сu3(Fe,Ge,Zn,Ga)(S,As)4 Sclar et al., 1957 1 → + 2+ 3+ 4+ Сu6FeGeS8 Сu 3Сu 3Fe Ge S8 Levy, 1966 1 (Сu,Fe,Zn,W,Mo,V,Ge,As Ga)S0.95 Springer, 1969 1.052 → + 2+ 3+ 4+ Сu26Fe4Ge4S32 Сu 16Сu 10Fe 4Ge 4S32 Tettenhorst et al., 1984 1.062 + 2+ 2+ 3+ Cu 20(Cu ,Fe ,Zn)6Fe 2Ge6S32 Spiridonov, 1987 1.062 + 2+ 2+ 3+ + 2+ 2+ 3+ Cu 22(Cu ,Fe ,Zn)6Fe 2(Ge,As)6S32Cu 22(Cu 2Fe 2Zn2)6Fe 2(Ge4As2)6S32 Spiridonov et al.,, 1992 1.125 + 2+ 3+ 4+ Сu 8Сu 5Fe 2Ge 2S16 Godovikov, 1997 1.062 Table 2. Theoretical composition of germanite (in wt. %), based on formulae proposed by different researchers Authors Cu Cu+ Cu2+ 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 pelements, 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 W are delements; so, it can to that of colusite, namely Cu26Fe4Ge4S32. This be assumed that V, Mo, and W can substitute formula is electrically 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- tallochemical formula of germanite, proposed tion that W and Mo occupy the sites of diva- + 2+ 3+ 4+ by A. Godovikov (1997), Cu 8Cu 5Fe 2 Ge 2S16 lent cations or trivalent Fe (Fig. 1). The sum → + 2+ 3+ 4+ Cu 16Cu 10Fe 4Ge 4S32. A crystallochem- (Cu + As) inversely proportional to the sum ical formula of germanite, proposed by E. (Zn + Ge), which is evidence for the isomor- + 2+ 2+ 3+ 2+ 4+ → + 5+ Spiridonov (1987), Cu 20(Cu ,Fe , Zn)6 Fe 2 phism Zn + Ge Cu + As (Fig. 2). 4+ Ge 6S32, is not electrically neutral. Later, in the work on germanocolusite, E. Spiridonov with coauthors (1992) proposed another crys t allochamical formula for germanite, + 2+ 2+ 3+ Cu 22(Cu ,Fe ,Zn)6Fe 2(Ge,As)6S32. In this case, the formula is electrically neutral, but the sum of atoms in the unit cell is 68 rather than 66 as in colusite whose formula was adopted by E. Spiridonov as the basis for examination of the germanite formula; therefore, the Me/S ratio is atoms equal to 36/32 rather than 34/32. These contradictions prompted us to make an additional analysis of the literature data on germanite. 37 chemical and electron microprobe analyses 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- ered to be electrical neutral if it had the valence balance (±Δ, the absolute value of the deviation 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 #05_nenash_en_0802:#05_nenash_en_0802.qxd 21.05.2009 20:05 Page 36 36 New data on minerals. M.: 2003. Volume 38 Table 3. Electron microprobe and chemical (*) analyses of germanite in wt. % (upper row) and in f.u. (lower row). Analyses 1, 6, 22, 23, 24, and 36 are recalculated based on 64 atoms in the unit cell; analyses 8, 21, and 35, based on 68 atoms; the remainder, based on 66 atoms № os. Cu Fe Zn Ge Ga As V W Mo S Σ 1 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.