86 New Data on Minerals. 2011. Vol. 46 EXPERIMENTAL STUDY OF CRYSTALLIZATION PRODUCTS OF CНALCOPYRITE SOLID SOLUTION Tatyana A. Kravchenko Institute of Mineralogy and Petrography Siberian Branch RAS, Novosibirsk, [email protected] In order to understand the conditions of formation of cubanite СuFe2S3, talnakhite Cu9Fe8S16, mooihoekite Cu9Fe9S16 and haycockite Cu4Fe5S8 in magmatic Cu-Fe ores of the Norilsk type the method of melt cooling from 1150–1100°C up to room temperature and subsequent annealing at 600 and 800°C phase associations of the cen- tral part of Cu-Fe-S system have been synthesized: 50 at.% of S, Cu/Fe = 1.22–0.25, 47 at.% S, Cu/Fe = 1.12–0.63 and 45 at.% S, Cu/Fe = 1.44–0.69. According to the received results, cubic cubanite enriched in cop- per (Cu/Fe і 0.5) crystallizes in associations with tetragonal chalcopyrite Cu1xFe1+xS2 and cubic talnakhite. The new data concerning steady phase equillibriums of mooihoekite with bornite Cu5FeS4 and cubic pc phase of the haycockite composition with cubic cubanite enriched in iron (Cu/Fe Ј 0.5) bornite and pyrrhotite Fe1xS are received. 1 figure, 1 table, 20 references. Key words: Cu-Fe-S system, chalcopyrite solid solution, crystallization of melt. Introduction system for determination of phase composi- tions and phase equillibriums in the sphere of Cubic talnakhite Cu9Fe8S16 (Bud`ko, Kula - chalcopyrite solid solution. gov, 1963; Cabri, 1967), tetragonal mooihoekite Cu9Fe9S16 (Cabri, Hall, 1972; Muraviova et al., Methodics 1972), rhombic haycockite Cu4Fe5S8(Cabri, Hall, 1972) and cubic cubanite CuFe2S3 (Cabri, 1973) Up to now the most complete experimental are attributed to the products of crystallization of research of phases from the sphere of chalcopy- chalcopyrite (Yund, Kullerud, 1966) intermedi- rite solid solution is the work by L.J. Cabri (Cabri, ate (Merwin, Lombard, 1937) solid solution 1973). The scheme of phase relations of the cen- established experimentally in the central part of tral part of the Cu-Fe-S system at 600°C con- the Cu-Fe-S system at 800–300°С. Together structed by Cabri agrees with the corresponding with chalcopyrite CuFeS2 these minerals are scheme of R.A. Jund and G. Kullerud at 700°C the basic components of the magmatic Cu-Fe (Yund, Kullerud, 1966) and is an evident illustra- ores of the Norilsk type. Close of compositions tion of high-temperature and possible low-tem- and complex intergrowths with each other and perature phase associations in relation to the for- other sulfides complicate diagnostics of natural mation of tetragonal chalcopyrite (557°C, Yund, minerals and do not allow to determine unam- Kullerud, 1966). Hence, the scheme of Cabri biguously their age relations and to connect (black hatch lines on the Fig.) has been used as a features of their composition and structure basis for a choice of initial compositions of sam- with definite conditions of crystallization. The ples synthesized in the presented work. As evi- basic experimental researches of the Cu-Fe-S dent from the Figure, initial compositions of the system (Yund, Kullerud, 1966; Kullerud et al., synthesized samples correspond to compositions 1969; Cabri, 1973; Barton, 1973; Likhachiov, of possible phase associations with the listed 1973; Sugaki et al., 1975; Vaughan, Craig, 1981; above products of crystallization of the chal- Vaughan, Craig, 1997; Tsujmura, Kitakaze, copyrite solid solution (iss on the Figure). 2004) are connected with the sphere of exis- Synthesis of samples is performed from the tence of chalcopyrite solution at 800—300°С. following elements: carbonyl iron A-2, copper B3 Conceptions about phase equillibriums at low and ultrapure sulfur additionnally dehydrated by temperatures are not clear and inconsistent, melting in vacuum. All samples have been syn- because they are based on the results of inves- thesized in evacuated quartz ampoules by a tigation of natural phase associations and method of melt cooling from 1150—1100°C up to results of extrapolation of isolated experimen- room temperature. The temperature of melt tal data in the sphere of low temperatures (» 1070°C) is determined according to the results (Vaughan, Craig, 1981; Vaughan, Craig, 1997). of the thermal analysis of the most infusible ini- The purpose of the presented work is syn- tial samples. The mode of cooling has been cho- thesis of steady at room temperature phase sen experimentally in view of the data on tem- associations of the central part of the Cu-Fe-S peratures of the iss crystallization. Cooling was Experimental study of crystallization products of cнalcopyrite solid solution 87 Fig. 1. The scheme of relationships of the phases synthesized in the presented work (continuous blue lines) on the scheme of phase relations of the central part of system Cu-Fe-S from Cabri (Cabri, 1973) at 600°C (hatch black lines, Cabri, 1973). 1–16 – initial com- positions of the synthesized samples. iss, bnss and po – areas chalcopyrite, bornite and pyrrhotite solid solutions. o – stoichiomet- ric compositions of minerals: tetragonal chalcopyrite CuFeS2 (cp), bornite Cu5FeS4 (bn), pyrite FeS2 (py), troilite FeS and products of crystallization of chalcopyrite solid solution (iss): talnakhite Cu9Fe8S16 (tal), cubanite CuFe2S3 (cb), mooihoekite Cu9Fe9S16 (mh) and haycockite Cu4Fe5S8 (hc). On the Figure by hatch blue lines equillibriums with participation of phases present in insignificant quan- tities (designated by asterisk in the Table) are designated. These phases are formed after iss crystallization (Cabri, 1973). carried out in two stages. The first stage – fast according to 2dcriterion at a significance level cooling (with a rate of 50° per hour) up to 1000, of 99%. Cmin is as follows (in weight %): Cu – 900, 850 or 800°C and keeping at these tempera- 0.04; Fe – 0.03; S – 0.01. X-ray phase analysis tures from several hours up to 10 days. The sec- was performed with the help of diffractometer ond stage – slow cooling (with a rate of 60° per DRON-3. a day) up to 300°C, keeping at 300°C from sever- al hours up to 3 months, further cooling up to Results room temperature with the switched off furnace. Fast cooling was performed for determination of For the phases synthesized in the presented the iss composition at temperatures of melt crys- work the following standard names of their natu- tallization (1000— 850°C, Yund, Kullerud, 1966) ral analogues of stoichiometric composition and in view of the data on the melt existence at 800°C corresponding structure are used: cubic tal- (Tsujmura, Kitakaze, 2004). It is a mode I. Further nakhite Cu9Fe8S16, tetragonal bornite Cu5FeS4, representative parts of the synthesized samples chalcopyrite CuFeS2 and mooihoekite Cu9Fe9S16, annealed at 600°C within 1.5 months, and at rhombic cubanite CuFe2S3 and haycockite 800°C – within 20 days with the subsequent Cu4Fe5S8. Thus in the presented work, as well as cooling in cold water. It is a mode II. in the work by Cabri (Cabri, 1973), for the syn- Synthesized samples have been investigated thesized phases with cubanite and haycockite by methods of optical microscopy and X-ray compositions the cubic structure (fcc and pc, analysis. Polished sections are prepared from respectively) is established. half of each sample (section along the center Results of synthesis of phase associations of from top- down). The chemical composition of the central part of the Cu-Fe-S system (samples phases is determined by microprobe analysis with the following compositions: 50 at.% of S, with the help of microanalyzer "Camebax- Cu/Fe = 1.22—0.25; 47 at.% of S, Cu/Fe = Micro". Chalcopyrite CuFeS2 is used as a stan- 1.12—0.63 and 45 at.% of S, Cu/Fe = 1.44—0.69) dard. Accelerating voltage – 20 кV, current of containing the listed above products of iss crys- absorbed electrons – 40 nA, angle of sampling tallization are shown in the Table and on the – 40 °, time of account – 10 seconds on each Figure. For the samples synthesized by cooling of analytical line, diameter of probe – 2—3 the melt from 1150°C up to room temperature microns. Precision of determination of all com- (modes I), composition of all synthesized phases ponents is within the limits of 2 relative %. is presented. For the samples synthesized accord- Detection limit of elements Cmin is calculated ing to the mode I with subsequent annealing at 88 New Data on Minerals. 2011. Vol. 46 Table 1. Results of synthesis of the samples belonging to the central part of system Cu-Fe-S N of sample Synthesis according to modes I Synthesis according to modes II Initial composition: Phases Composition of phases, at.%, Total, Composition of phases, at.%, Total, S, Cu, Fe, at.% weight % weight % weight % weight % Cu/Fe Сu Fe S Сu Fe S 1 cp 25.28 24.86 49.86 25.29 24.98 49.73 50, 27.5, 22.5 34.90 30.17 34.72 99.79 34.86 30.26 34.59 99.71 1.22 bn 47.90 10.79 41.31 30.41 23.38 46.21 60.78 12.04 26.46 99.28 40.72 27.52 31.22 99.46 py* bn, bn 1а tal 27.64 23.34 49.02 26.92 24.63 48.45 50, 27.5, 22.5 38.00 28.19 33.99 99.79 36.64 29.45 33.27 99.36 1.22 iss 32.70 20.51 46.79 32.31 21.33 46.36 43.76 24.13 31.59 99.48 43.08 25.00 31.19 99.27 29.62 23.34 47.04 39.80 27.58 31.92 99.30 bn, bn 5 tal + cb 25.54 25.81 48.65 27.72 24.53 47.75 50, 17.5, 32.5 34.97 31.07 33.61 99.65 37.68 29.30 32.74 99.72 0.54 8 tal 27.04 24.12 48.84 26.29 24.63 49.08 47, 28, 25 36.68 28.75 33.42 98.85 35.80 29.48 33.52 98.80 1.12 cp 25.43 24.71 49.86 bn 34.86 29.76 34.48 99.10 bn 48.32 11.59 40.09 60.69 12.80 25.41 98.90 2 cp 24.36 25.83 49.81 24.81 25.69 49.50 50, 25, 25 33.43 31.15 34.48 99.06 33.94 30.90 34.16 99.00