Alunite

alunite, also called Alum Stone, a widespread rock-forming sulfate mineral that occupies pockets or seams in volcanic rocks such as rhyolites, trachytes, and andesites, where it presumably formed through their chemical reaction with escaping sulfurous vapours. It has been used as a source of potash (during World War I) and as a source of alumina (during World War II); in Europe it was once used extensively to make potash alum, and it has been mined for this purpose since the 15th century. Large deposits exist near Beregovo, Ukraine; Almería, Spain; and Bulahdelah in New South Wales, Australia. For chemical formula and physical properties, see sulfate mineral (table).