KOSMOCHLOR (NaCrSi_2O_6) FROM COASTAL SEDIMENTS OF TERSKY COAST IN THE WHITE SEA, KOLA PENINSULA

摘要

Kosmochlor is an extremely rare mineral firstly found in several iron meteorites and, on the Earth, in a single jade deposit at Burma. By composition, the mineral is a sodium chromium pyroxene NaCrSi_2O_6. The paper displays morphological description, X-ray structural data and chemical composition of the kosmochlor grain for the first time found in Russia: in coastal sediments at the northern - Tersky coast of the White Sea, Kola Peninsula. The finding was made in a schlich sample during the prospecting for diamonds in Quaternary deposits of the region. The isometric grain is as large as 0.17 X 0.15 X 0.15 mm, it has the emerald-green color. The mineral composition (wt percent): SiO_2 -52.31, TiO_2 -0.05, Al_2O_3 -1.86, Cr_2O_3 -25.57, FeO -1.29, MnO -0.01, MgO -2.79, CaO - 4.33, Na_2O - 11.73, V_2O_5 - 0.10. The empirical formula of Tersky kosmochlor is Na_(0.85)Ca_(0.17)Cr_(0.76)Mg_(0.16)Al_(0.05)Fe_(0.04)[Si_(0.97)Al_(0.03)O_6]. Comparative study shows that the mineral is chemically similar to the kosmochlor from meteorites. The grain contains the thin micro-veinlet inclusions of high-chromium diopside (2.8-3.4 percent Cr_2O_3) with chemistry similar to pyroxenes associating diamonds in kimberlites. According to experimental data the mineral association of kosmochlor and chrome-diopside takes place at the pressure as high as 25 kbar, but relatively low temperature (800-850 C). It is assumed that the most possible source of Tersky kosmochlor could be the deep-seated xenoliths from kimberlite and alkaline-ultramafic explosive pipes of Arkhangelsky and/or Kola regions.