Lead Phosphates (Pyromorphite and Phosphohedyphane) from the Oxidation Zone of Baryte-Lead Ores in the Ushkatyn-III Deposit, Central Kazakhstan

The paper describes Ca-Pb phosphates—pyromorphite and phosphohediphane from the oxidation zone of barite-lead (calcite-barite-galena) ores of the Ushkatyn-III deposit in Central Kazakhstan. Both minerals occur equally frequently within the deposit. Lead phosphates coexist with minerals as almost unaffected by surface changes in ores (galena, pyrite, barite, calcite, rhodochrosite, chamosite, and others), so with minerals of highly oxidized ores (cerussite, montmorillonite, kaolinite, goethite, and others). As the processes of supergene processes develop (during the transition from weakly to strongly oxidized ores), the compositions of newly formed phosphates change regularly in the following consequence: phosphohedifane → rhythmically zonal Ca-Pb phosphate → pyromorphite. At the same time, the habit (and appearance) of mineral crystals changes from dipyramidal-prismatic through elongated prismatic (barrel-shaped) to pinacoidal-prismatic (short-columnar). Crystallization of pyromorphite is possible already at very low concentrations of lead, phosphorus and chlorine in solution. The main source of chlorine is groundwater, and phosphorus is coming from the organic matter of the soil cover overlying ore-bearing deposits. Peculiarities of the chemical composition and crystal structure of Ca-Pb phosphates, as well as the nature of their mineral associations, suggest the presence of a discontinuity in isomorphic miscibility in the series pyrofmorphite–phosphohedifane and phosphohedifane–chlorapatite.