ANGLESITE

anglesite, a mineral consisting of lead sulphate, pbso_{4}, crystallizing in the orthorhombic system, and isomorphous with barytes and celestite. it was first recognized as a mineral species by dr. withering in 1783, who discovered it in the parys copper-mine in anglesey; the name anglesite, from this locality, was given by f.s. beudant in 1832. the crystals from anglesey, which were formerly found abundantly on a matrix of dull limonite, are small in size and simple in form, being usually bounded by four faces of a prism and four faces of a dome; they are brownish-yellow in colour owing to a stain of limonite. crystals from some other localities, notably from monteponi in sardinia, are transparent and colourless, possessed of a brilliant adamantine lustre, and usually modified by numerous bright faces. the variety of combinations and habits presented by the crystals is very extensive, nearly two hundred distinct forms being figured by v. von lang in his monograph of the species; without measurement of the angles the crystals are frequently difficult to decipher. the hardness is 3 and the specific gravity 6.3. there are distinct cleavages parallel to the faces of the prism (110) and the basal plane (001), but these are not so well developed as in the isomorphous minerals barytes and celestite. [illustration: anglesite specimen.] anglesite is a mineral of secondary origin, having been formed by the oxidation of galena in the upper parts of mineral lodes where these have been affected by weathering processes. at monteponi the crystals encrust cavities in glistening granular galena; and from leadhills, in scotland, pseudomorphs of anglesite after galena are known. at most localities it is found as isolated crystals in the lead-bearing lodes, but at some places, in australia and mexico, it occurs as large masses, and is then mined as an ore of lead, of which the pure mineral contains 68%.