Free sulphur may also result from the decomposition of pyrites, as in pyritic shales and lignites, or from the alteration of galena: thus crystals of sulphur occur, with anglesite, in cavities in galena at Monteponi near Iglesias in Sardinia; whilst the pyrites of Rio Tinto in Spain sometimes yield sulphur on weathering.
Crystals of barytes are orthorhombic and isomorphous with the strontium and lead sulphates (celestite and anglesite); they are usually very perfectly developed and present great variety of form.
The chief lead ores are galena and cerussite; of minor importance are anglesite, pyromorphite and mimetesite (qq.v.).
Anglesite, or lead sulphate, PbSO 4, is poor in silver, and is only exceptionally mined by itself; it occurs in quantity in France, Spain, Sardinia and Australia.
Lead sulphate, PbSO 4, occurs in nature as the mineral anglesite (q.v.), and may be prepared by the addition of sulphuric acid to solutions of lead salts, as a white precipitate almost insoluble in water (1 in 21,739), less soluble still in dilute sulphuric acid (1 in 36,504) and insoluble in alcohol.
It is often associated with blende and pyrites, and with calcite, fluorspar, quartz, barytes, chalybite and pearlspar as gangue minerals; in the upper oxidized parts of the deposits, cerussite and anglesite occur as alteration products.
The normal sulphates are the more important, and occur widely and abundantly distributed in the mineral kingdom; anhydrite, gypsum, anglesite, barytes, celestite and kieserite are among the commonest species.