Morphological, chemical and mineralogical speciation of a low-Ca, high-Fe fly ash from a bituminous coal has been investigated by examination of size, density and magnetic fractions. Fractionation by size revealed little information as to speciation among particle types. However, separation of the ash into eight density fractions and into magnetic and non-magnetic components showed major differences in particle properties. It was found that glasscontaining particles can be divided into three general types: Type 1, being low-Fe content, low-density hollow spheres comprising aluminosilicate/mullite glass ceramics; Type 2, of intermediate density, being ferroaluminosilicate/ mullite glass ceramics; and Type 3, high density composite particles of spinel/hematite crystals embedded in an iron-substituted glass. It is proposed that Type 1 and Type 2 particles are derived from thermal decomposition of clay minerals with a range of Fe contents. Type 3 particles are considered to arise from thermal decomposition of pyrite in the presence of small quantities of aluminosilicate minerals. Two general types of glass were distinguished: Glass I(f), being largely a low-iron aluminosilicate; and Glass II(f), being a ferroaluminosilicate of high Fe-content. XRD and vibrational spectroscopic evidence suggest that, in both glass types, Fe is substituted for Al in an aluminosilicate-type structure.