J. Buhl, A. Willgallis
Mar 25, 1985
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Journal
Chemical Geology
Abstract
Abstract Wolframite was synthesized hydrothermally at 1 kbar between temperatures of 200° and 400°C. The pH-values and periods of cooling were varied. The crystallographic constitution and the chemical composition were investigated by electron probe microanalysis. No simple relationship between composition and temperature in the 250–400°C interval could be established. From 300°C upwards the positive correlation postulated by W.O. Oelsner appears to hold whilst at lower temperatures of formation, e.g. 250°C, the opposite obtains. In this low-temperature region the compound Na2O(WO3·H2H2O)4–10 coexists with wolframite. More or less pronounced fluctuations in the composition of the all-mixed crystals could be identified by analysis of micro-dimensions, although they were the largest in ferberite-rich samples. Rapidly cooled crystals were more homogeneous than those subjected to prolonged cooling. At the temperature of 300°C wolframites of all compositions crystallize so that complete miscibility of hubnerite and ferberite exists. Variation of the pH in the weakly acid to weakly alkaline pH region does not appear to influence the composition of the products of crystallisation. The properties described here indicate that the use of compositional features of wolframites as thermogenetic indicators is highly problematical.