Mineral microspherules in Chinese coal and their geological and environmental significance

► Various mineral microspherules have been identified in coal. ► Most microspherules, such as pyrite, glauconite, hematite, calcium oxalate and allophane, are colloidal in origin. ► Some microspherules, such as garnet and sphene, are mainly derived from sediment-source region. ► A few microspherules are probably from volcanogenic sources or from cosmic dust. ► Fine-grained minerals may play an important role in the formation of ultrafine fly ash particles.