COMPOSITIONAL AND STRUCTURAL SYSTEMATICS OF THE COLUMBITE GROUP

The systematics of the columbite group have been studied to quantify v ariations in composition and structure. Multiple regression methods in volving 89 heated samples and five synthetic equivalents of columbite- group minerals give equations that permit prediction of the effects of composition on unit-cell parameters for fully ordered samples. The re sults are: a(0) = 14.258 + 0.166Mn/(Mn + Fe) + 0.0072Ta/(Ta + Nb) - 0. 06Ti - 0.02Sn + 0.05Sc; b(0) = 5.7296 + 0.031Mn/(Mn + Fe) + 0.0024Ta/( Ta + Nb) - 0.024Ti - 0.009Sn + 0.02Sc; c(0) = 5.0495 + 0.033Mn/(Mn + F e) + 0.011Ta/(Ta + Nb) - 0.004Ti, where a(0), b(0), and c(0) are the c ell parameters (Angstrom) calculated from unit-cell concentrations of elements. With these equations, crystal-chemical trends, the effects o f heating experiments, the degree of cation order, and the structural effects of heterovalent cation substitution can be predicted for sampl es of columbite-group minerals.