Mineralogical and petrological studies of the origin of Mg-Fe hydrate-carbonate-hydroxide minerals

点击翻译按钮获取中文摘要

Pyroaurite-sjogrenite, one of Fe-Mg hydrate-carbonate-hydroxide minerals exhibits five types of mode of occurrence in serpentinites from Yoshikawa, Aichi Prefecture; (1) vein-type, (2) nodule-type, (3) crusty type, (4) platy type, (5) earthy type. The minerals of type(3) to type(5) contain some amounts of Ni besides Mn and Cr. The optical properties of these minerals are variable because of their chemical characteristics; vein-type minerals are extremely pleochroic with high refractive indices and the others are either weakly pleochroic or not pleochroic, with rather low refractive indices. The optical orientation of coalingite was determined for a euhedral flake.A rapid reversible dehydration-hydration process is characteristic of this mineral below about 250゜C. An intermediate dehydrated crystalline phase is formed between 110゜ and 140゜C and continue to exist up to 300゜C, gradually contracting the basal spacing. It should be noted, however, that two dehydrated crystalline phases were identified during the thermal decomposition of coalingite which DTA pattern is very similar to that of pyroaurite-sjogrenite.Pyroaurite-sjogrenite together with coalingite develops directly from brucite in serpentinites in this area. The reaction may take place in the presence of <CO_2> -bearing underground water which was injected into the brucite-serpentinite. <N_1> was derived undoubtedly from olivines in the ultramafic rocks. After the pyroaurite-sjogrenite was formed, the serpentinite body intruded into the country rocks, accompanied by intensely crushing and fracturing. Coalingite may be formed at the ground surface conditions.Thermal behavior of <H_2O> and <CO_2> in the inter-layer through the dehydration-hydration process should be keenly studied in the near future.