Geologic factors like history and environment affect mineral content in coal. Miocene coals have less minerals (3.9%) than Eocene coals (6.7%) due to different peat accumulation rates. Miocene coals are brighter and have less minerals, while Eocene coals are duller and have more minerals. Clay minerals like kaolinite formed in fresh water during peat formation. Most minerals are from the same time as the coal, but some are from later. Quartz and pyrite are common minerals in the coal, with calcite found in fractures.