Structural and Compositional Variations in Some Natural Dolomites 论文

摘要

Dolomite concentrates from sedimentary rocks ranging in age from Eocene to Ordovician have been examined by powder and single-crystal X-ray methods. They not infrequently contain about 5 mol per cent excess $$CaCO_{3}$$, as determined by a powder X-ray method, and have weakened order reflections relative to those of dolomite of 1: 1 molar $$CaCO_{3}:MgCO_{3}$$ composition. Basal reflections often have attenuated or even bimodal intensity distributions. The changes in cell size are much greater than could be produced by the $$FeCO_{3}$$ and $$MnCO_{3}$$ percentages shown in spectrographic analyses of thirteen of the samples. These compositional and structural features, similar to those of synthetic protodolomites, are found in sedimentary dolomites of widely varying particle size that make up from a few per cent to virtually the entire rock being examined. They were not observed in the metamorphic or hydrothermal dolomites examined or in most of the samples from evaporite sequences. The decreased intensity of order reflections observed is only partially accounted for by solid solution of $$FeCO_{3}$$, $$MnCO_{3}$$, and excess $$CaCO_{3}$$. This fact, together with the sharpness of reflections from sets of planes parallel to |c] in single-crystal photographs of some materials relative to the doubled reflections from basal planes, suggests that mixed layering occurs. The amount of excess $$CaCO_{3}$$ in these dolomites is clearly metastable in sedimentary environments. The $$CaCO_{3}$$-rich dolomites lose $$CaCO_{3}$$ and approach the appropriate equilibrium composition if held at a high temperature in the presence of a flux.