Mineralogical composition of the Ashibetsu coals in the Ishikari coal-field:2. Sulfide and sulfate minerals
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Sulfide and sulfate minerals in the seven Paleogene coal seams of the Ashibetsu colliery in the Ishikari coalfield, Hokkaido, were investigated by X-ray diffraction of low temperature ash, coal, shale and tuff. The distribution of total sulfur in the coal seams is compared with the occurrence of these minerals and the morphological change of pyrite.<BR>Most of the sulfide minerals found in the Ashibetsu coal seams are pyrite. Marcasite is rare. Pyrite tends to appear near the roof or the floor of the coal seams, and the sulfide and sulfate minerals were not detected in the main part of the coal seams save one, the Torashita-sanshaku seam. In that case, pyrite and gypsum occurred throughout the seam. Total sulfur content is also very low in the main parts of all the coal seams except the Torashita-sanshaku seam. However, total sulfur content is still low in the main part of the Torashita-sanshaku seam at a different location. This evidence suggests that most of the coal seams were formed in a fresh water environment, though a few areas were formed in a brackish water environment. This suggestion is consistent with the depositional environment inferred from the clay mineral composition.<BR>Pyrite exists as very small, microscopically observable crystals even in the coals containing less than 0.5% total sulfur (S). Framboids are rarely present in such low sulfur coals. The number of isolated, small crystals and of framboids increases as S approaches 1%. Bigger framboids and clusters of framboids appear in coals containing more than 1% S. Pyrite spheres, formed from infilled framboids, and larger pyrite spheres or larger, irregular pyrite nodules, formed from infilled clusters of infilled framboids, are often observed in coals containing more than 2% S. All the pyrites of different shapes found in the Ashibetsu coals are syngenetically formed as the result of the activity of sulfur reducing bacteria in different depositional environments.<BR>Gypsum, szomolnokite, conquimbite, roemerite, melanterite, and hydronium jarosite were identified in some coal samples. These sulfate minerals are weathering products from pyrite. Gypsum appears in the early stages of weathering in calcite-rich coal. Iron sulfate minerals appear in calcite-poor coals from an early stage. A coal sample, rich in iron sulfate minerals, was exposed to the laboratory atmosphere for about two and half years, resulting in the appearance of copiapite and paraconquimbite. This indicates that promotion of weathering may cause the iron sulfate mineral phase to be more varied.
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