MICROANALYSIS OF A TONSTEIN UNCOVERS A MINERALOGICAL MYSTERY: THE FIRST REPORTED OCCURRENCE OF FERROAN MAGNESITE IN COAL

Altered volcanic ash is commonly found in coals throughout the world, typically preserved as thinly-bedded, kaolinite-rich layers called tonsteins. Several volcanic ash layers from the Minusinsk coal basin in southern Siberia, Russia were selected for microanalysis by scanning electron microscopy with an energy dispersive detector and X-ray diffraction of the low temperature ash. In addition to commonly the occurring minerals found in tonsteins such as kaolinite, K-feldspar, quartz, apatite, zircon, and sulfides such as galena, sphalerite, and pyrite, the mineralogical profile of this tonstein includes a unique groundmass component: ferroan magnesite, whose identity was confirmed by X-ray diffraction. A ferroan magnesite of similar composition was briefly mentioned in one paper as a rare mineral found with brackish-marine siderites in clastic rocks associated with coal but has never before been reported in coal itself. The relative abundances of the volcanogenic phenocrysts of K-feldspar, apatite, and quartz are consistent with a parent magma of felsic to intermediate composition, which makes the primary ash sediment an unlikely source of Mg for the precipitation of ferroan magnesite. The Mg-rich carbonate in the peat swamp may best be explained by the migration and mixing of Mg-rich pore waters but more evidence is needed to confirm the source.