More than sixty years ago V.M.Goldschmidt analysed the geochemistry of alkali metals in magmatic rocks, sea water and sediments (Goldschmidt et al., 1933). Over the last decades a spectrum of alkali metals sulfides became
a subject of research. There are the natural K-bearing
(9-18 weight %) sulfides - djerfisherite [K6(Cu,Fe,Ni)24S26Cl], rasvumite [KFe2S3], bartonite [K3Fe10S14], murunskite [K2Cu3FeS4], the Na-bearing sulfides - erdite [NaFeS2 · 1.5-2H2O], chvilevite [Na(Cu,Fe,Zn)2S2] (Czamanske et al., 1979; Dobrovolskaya et al., 1982; Kachalovskaya et al., 1988). The djerficherite-like sulfide with lithium [K5.5Li0.6Fe24.0S25.9Cl1.0] received by experiment (Tani, 1977) shows a possible existence of other alkali metals sulfides.
Natural sulfides were found in different geological environments. Djerfisherite, rasvumite, erdite, bartonite occur mainly in alkalic rocks and pegmatites (Khibina, Lovozero massifs, Russia; Coyote Peak California, USA). Djerfisherite and murunskite occur in ultraalkalic rocks of Murunsky massif, Siberia. Djerfisherite was also found in Cu-Ni deposits of the Norilsk region, in kimberlites of South Africa and Yakutia, in peridotites of Inaglinsky massif, Siberia. Chvilevite is known in Pb-Zn ores of the Akatuy deposit, Siberia. Comparison of the chemical compositions of sulfides from different environments serve to reveal their geochemical features, especially the isomorphism.
Isomorfism may be shown on the example of djerfisherite, which occurs widely and differs by the chemical composition in different occurences. More than 150 analyses of djerfisherite showed the isomorphic replacements of Fe by Cu and Ni; there are also limited replacement of Cu by Ni, of K by Na, of K by Li. The presence of Cl in djerfisherite indicates an isomorphism between sulfur and chlorine. Accumulation of data on chemical composition of djerfisherite from different environments and meteorites permited to reveal three variaties of this sulfide: the first enriched in Fe (up to 50 weight %) in meteorites, pegmatites of the Khibina massif and CU-Ni deposits of the Norilsk region; the second enriched in Cu (up to 23 weight %) in alcalic rocks; the third enriched in Ni (up to 20 weight %) in kimberlites.
Other alkali metals sulfides do not demonstrate the isomorphism, perhaps because there are few their findings, but also due to an unstability proper at least for some of them. This is confirmed by destruction of murunskite grains, transformation of rasvumite, formation of K-Cu-Fe-bearing sulphides which differ by chemical composition, but occur in association with djerfisherite and other sulphides.
The study of phase relations in (K,Na)-(Fe,Ni)-Cu-S-H2O system showed that sulfides of alkali metals group have been crystallized in non-equilibrium conditions. The different activity of K, Na, S determined changes of sulfides parageneses (Dobrovolskaya and Nekrasov, 1992).
What are the sources of alkali metals in sulfides? V.M. Goldshmidt pointed out that high concentrations of alkali metals are proper to residual magmas. Study of mineral parageneses indicates that the formation of K-Na sulfides was connected with postmagmatic processes: either with the potassium or sodium metasomatism (massifs of ultra-alcalic rocks), or with the enhanced role of volatile components under the high pressures and temperatures (kimberlites, Cu-Ni deposits). In both cases the sulfides of alkali metals reflect reactive relations between fluids and rocks.
Czamanske, G.K., Erd, R.C., Sokolova, M.N., Dobrovolskaya, M.G. & Dmitrieva, M.T., Am. Mineral. 64, 776-778 (1979).
Dobrovolskaya, M.G., Sokolova, M.N., Tsepin, A.I., Malov, V.S. & Rogova, V.O., Proceedings of the 13th general meeting of the International mineralogical association, Varna, September 19th-25th 1982, 261-269 (1982).
Dobrovolskaya, M.G. & Nekrasov, I.Ya., Doklady Akademii nauk SSSR 322, 2, 373-378, in Russian (1992)
Goldschmidt, V.M., Berman, H., Hauptmann, H. & Peters, Cl., Zur Geochemie der Alkalimetalle. Nachr. von der Gesellschaft der Wissenschaften zu Gottingen (1933).
Kachalovskaya, V.M., Osipov, B.S., Nazarenko, N.G., Kukoev, V.A., Mazmanyan, A.O., Egorov, I.N. & Kaplunnik, L.N., Zapiski Vsesoyuznogo Mineralogicheskogo Obshchestva 117 (2), 204-207, in Russian (1988).
Tani, B.S., Am. Mineral. 62, 819-823 (1977).