Abstract
ABSTRACf The systematics of the columbite group have been studied to quantify variations in composition and structure. Multiple regression methods involving 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 results are: aD= 14.258 + 0.166Mn/(Mn + Fe) + O.0072Ta/(Ta + Nb) - 0.06Ti 0.02Sn + 0.05Sc; ho = 5.7296 + 0.03 1Mn/(Mn + Fe) + 0.0024Ta/(Ta + Nb) - 0.024Ti - 0.009Sn + 0.02Sc; Co= 5.0495 + 0.033Mn/(Mn + Fe) + O.OllTa/(Ta + Nb) 0.004Ti, where aD,ho, and Coare the cell parameters (A) calculated from unit-cell concentrations of elements. With these equations, crystal-chemical trends, the effects of heating experiments, the degree of cation order, and the structural effects of heterovalent cation substitution can be predicted for samples of columbite-group minerals.
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