Coal

The identification of coal and coal ash in thin section is relatively simple but has hardly ever occurred in archaeological micromorphology. Coal ash is composed chiefly of silica and alumina with smaller quantities of iron and calcium. The silica occurs mostly in the form of plant-derived opal with variable amounts of quartz and rock fragments as well. This chapter discusses types of coal, including brown coal or lignite, cannel coal, bituminous coal, and anthracite. A thin section of brown coal is deep reddish orange to opaque in PPL, with plant-growth rings and transverse rays still traced out in dark and light orange patterns. A thin section of cannel coal shows very dark, often opaque, areas, occasional quartz grains and lightorange pores with complex extinction patterns in XPL. Bituminous coal is mostly opaque in thin section but punctuated by laminae of deep orange organic material in PPL, showing extinctions occurring in complex waves under XPL.