OCCURRENCE OF PHOSPHORUS MINERALS IN AUSTRALIAN COAL SEAMS

Although the average proportion of phosphorus in Australian coals is s imilar to that of P in world coal deposits generally (around 0.05% or 500 ppm), a number of individual seams have phosphorus contents above this average. Ply-by-ply studies of selected seams indicate that P is typically abundant in only a few sub-sections or plies; the remaining parts of the seam, even in high-phosphorus coals, commonly have signif icantly lower phosphorus levels. The bulk of the phosphorus in Austral ian coals is present as crystalline mineral particles, although some o f these may be only a few micrometres in diameter and intimately assoc iated with the organic components. X-ray diffraction of low-temperatur e oxygen-plasma ash residues, combined with EDAX investigations of pol ished sections of coal under the scanning electron microscope, show th at the phosphorus usually occurs either as apatite or as a solid-solut ion of Sr, Ba and Ca aluminophosphates that represent minerals of the goyazite-gorceixite-crandallite group. Both apatite and a range of alu minophosphate minerals can occur in the one coal sample, although one phosphate mineral variety is usually the dominant component. The apati te and the aluminophosphates most commonly occur as infillings in the pore spaces and cell cavities of inertinite macerals. They sometimes o ccur alone, but in many cases are intimately associated with kaolinite and possibly quartz. Later-formed veins filled with either apatite or aluminophosphate minerals also occur in some coal seams, particularly in the more vitrinite-rich sub-sections. These typically cross-cut th e pore-filling accumulations. The phosphorus that formed these mineral s was probably derived mainly from phospho-proteins in the organic mat ter of the original peat deposits, although volcanic debris, shells or faecal matter may also have acted as phosphorus sources. The P was ap parently released in a more soluble form during plant decay, and then reprecipitated with other available ions in the pores of appropriate p arts of the peat bed. Factors such as pH and metal availability were p robably critical in determining whether apatite or aluminophosphates w ere formed. Where they occur, phosphate-bearing veins probably represe nt material remobilised from earlier-formed accumulations and precipit ated from groundwaters after the coals had essentially reached their p resent rank.