Surface composition of silica particles embedded in an Australian bituminous coal

The composition and structure of the surface layers of a series of silica particles (10-20 microns across), embedded in a bituminous coal from the Whybrow seam, Sydney Basin, Australia, have been characterized in situ using time-of-flight secondary ion mass spectrometry (TOFSIMS), ion imaging, and depth profiling. The silica particles investigated are typically encased in a multilayered shell, the composition of which differs from average composition of both the silica and the bulk coal. The analysis directly demonstrates the presence of a silanol-rich (Si-OH) interfacial layer 3 nm in thickness. This silanol-rich region separates the bulk silica and a complex non-silica layer encasing the particles. The interfacial region also shows significant lithium enrichment (approximately fivefold over bulk) which implies diffusion and precipitation of lithium-containing species during the authigenetic formation of the surface layers of the silica grains. The outer layer encasing the silica particles is 10 nm in thickness and is composed of clays and carbonates, and, in some cases, includes organic material. The elemental constituents of this layer include aluminium, sodium, potassium, magnesium, iron, and lesser amounts of titanium and copper. The variation in the aluminium concentration from the outermost surface to the deeper layers is less than that of other non-silica species. A relatively high amount of calcium is found associated with the silica bulk. Although only non-respirable-sized silica particles are examined in this work, the methods of analysis developed have potential in providing an insight into the surface composition of respirable particles and in further studies of the surface bioavailability of silica species.