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Lead Occurrence

Lead rarely occurs native. Its most important ore is the sulphide, galena, PbS, with which are occasionally associated the selenide and telluride. The common occurrence of galena is to be attributed to its relative insolubility in water, the readiness with which it crystallises, and the fact that it is formed from other compounds by various reactions, both wet and dry. Probably its natural formation is commonly due to the action of hydrogen sulphide on other lead compounds in presence of water. It is found associated with quartz, fluorspar, calcspar, and barytes in various geological strata, is widely distributed throughout the world, and occurs in veins or flat beds between the strata. In Cornwall and Devon, galena lies in veins called " killas " within an argillaceous schist of the Devonian formation; in and around Alston Moor, at the junction of Northumberland, Cumberland, and Durham, the same ore is found in flat veins or " flats " in carboniferous limestone; in Shropshire, parts of Wales and Scotland, and the Isle of Man, the ore occurs in rocks of the Silurian formation; whilst in parts of Ireland it is found in granite.

Though of little practical importance, the complex sulphides in which lead is associated with other metals are numerous and interesting, and include thioarsenides, thioantimonides, thiobismuthides, and thiostannides. Typical minerals of this kind are: sartorite, PbAs2S4; zinkenite, PbSb2S4; bournonite, CuPbSbS3. Zinkenite has been produced artificially by fusing together galena and stibnite in the proper proportions. If the sulphide is regarded as the fundamental lead mineral, other minerals are derived from it by oxidation, carbonation, etc. The oxides are rarely found, but occur as massicot, PbO; minium, Pb3O4; and plattnerite, PbO2.

Cerussite, lead carbonate, PbCO3, ranks next in importance to galena as a lead ore. Its formation is attributed to the action of carbonated waters on other lead compounds. It is found in Devon and Cornwall, in Yorkshire, at Leadhills in Scotland, and in County Wicklow. A basic carbonate, known as hydrocerussite, is Pb3(OH)2(CO3)2.

Lead chloride, PbCl2, occurs as the rare mineral cotunnite, produced as a sublimate in volcanic action; the basic chlorides, matlockite, Pb2OCl2, and mendipite, Pb3O2Cl2, are better known, but the chloro-carbonate, phosgenite, Pb2Cl2CO3, is rare.

Anglesite, lead sulphate, PbSO4, is a mineral of some importance, occurring at Leadhills and elsewhere; it may have been formed as an oxidation product of galena, or by the action upon this mineral of acid ferrous sulphate solutions formed by the oxidation of pyrites. Basic lead sulphate, Pb2SO5, occurs as the rare mineral lanarkite, whilst leadhillite is PbSO4.2PbCO3.Pb(OH)2.

Lead chromate, PbCrO4, occurs as crocoite or crocoisite, the molybdate PbMoO4, as wulfenite, the tungstate PbWO4, as stolzite.

Lastly there are three minerals which are interesting on account of their isomorphism: the chlorophosphate pyromorphite Pb5(PO4)3Cl, the chloroarsenate mimetite or mimetesite Pb5(AsO4)3Cl, and the chlorovanadate vanadinite Pb5(VO4)3Cl.

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