The mineralogy of some synthetic sulphosalts

The mineralogy of the sulphosalts is not well understood, principally because of the very wide variations in chemical composition of the natural examples and their common admixture with other sulphide mineral species. The synthesis of pure sulphosalt compositions is considered to be a valuable means of solving the problems involved. Changes in characteristic physical properties can then be measured as elements are substituted systematically into the structure. A typical sulphosalt, tetrahedrite, general formula (CuAg)(_10)(ZnFeCu)(_2)(SbAs)(_4)S(_13) was selected for study by synthesis in sealed evacuated silica glass capsules. The phase relations of the Cu-Sb-S: system were studied. Phases obtained were: tetrahedrite, Cu(_12)Sb(_4)S(_13); non-stoichiometric tetrahedrite, Cu(_13.04)Sb(_3.86)S(_13), which has high and low temperature cubic polymorphs; stibioluzonite, Cu(_3)SbS(_4); chalcostibite, CuSbS(_2); and Gu(_3)SbS(_3). Substitution of As for Sb in tetrahedrite (the tetrahedrite-tennantite series) was found to result in a decrease in cubic cell edge, increase in microhardness, decrease in reflectivity and slight changes in quantitative colour measurements. Non-stoichiometry of the series also affected these properties. Substitution of Zn and Fe for Cu in tetrahedrite resulted in an increase in cubic cell edge, increase in microhardness and increase in reflectivity. Substitution of Ag for Cu resulted in an increase in unit cell edge, decrease in microhardness and decrease in reflectivity. Only Ag substitution proved to be temperature dependant, the low temperature breakdown products of Ag-tetrahedrite being (CuAg)(_3)SbS(_4) and (AgCu)(_3)SbS(_3). Chemical bonding in the phases synthesised is discussed in detail. Substitution and the resultant effect on physical properties are discussed in relation to the atomic properties of the substituting element.