ON THE COMPOSITIONAL FIELD AND ISOMORPHIC FEATURES OF ISOCUBANITE

The treatment of microprobe data of oceanic isocubanite (85 analyses i ncluding authors data) and its synthetic equivalent ''iss'' phase (44 analyses) is presented. It is shown that the natural mineral is charac terised with the extensive compositional field (Cu from 19,0 up to 34, 0 mass %, Fe from 31,2 up to 45,8 mass %) corresponding to the low cop per range of synthetic solid solution (''iss'' phase) at 300 and 400 d egrees C. A formula with variable coefficients ascribing the compositi onal field of natural isocubanite is presented. Strong reciprocal corr elation of Cu and Fe along with the absence of any correlation in Cu-Z n, Fe-Zn and Cu-S in isocubanite is corresponding to the similar prope rties of low copper compositional field of ''iss'' phase at 300 and 40 0 degrees C. Statistical treatment of published data on ''iss'' phase composition indicated that two types of solid solutions should be reve aled: solid solutions of I type (substitution) in the low copper field (Cu < 1 formula unit) and those of II type (subtraction) within the h igh copper field (Cu > 1 formula init), A formula for II type of solid solution accounting the change of copper valent state is proposed.