Acta Musei Moraviae

Pyromorphite from Jihlava ore district (Czech Republic)

The supergene mineral assemblage of the Jihlava district, where Pb-Zn-Ag ores were exploited in the medieval age, is characteristic by pyromorphite occurrences. Pyromorphite forms yellow-green to green hexagonal crystals up to 1 cm in size and sometimes also brown or white, needle-like aggregates overgrowing goethite in quartz or baryte gangue material. It commonly forms oscillatory zoned crystals, with strong variations in Ca content (0.01–0.95 apfu), which is low in brown (< 0.09 apfu) and high in white (<0.83 apfu) varieties. Some zones already exhibit composition of phosphohedyphane. Major part of the data correspond to almost pure end member, with trace amounts of Al³⁺ (<0.03), As⁵⁺ (<0.05), Ba²⁺ (< 0.02), Fe³⁺ (< 0.04), Si⁴⁺ (<0.03), S⁶⁺ (<0.01), V⁵⁺ (<0.02), Zn²⁺ (<0.04) and (OH^–) <0.09 (all in atom per formula unit). Besides pyromorphite and goethite, the supergene mineral assemblage also contains rare kintoreite, plumbogummite and acanthite. Kintoreite crystallised during dissolutionreprecipitation reactions of pyromorphite in conditions of high Fe activity, while plumbogummite is related to Al-rich solutions. Pyromorphite formed in deeper parts of the oxidation zone due to alteration of galena-bearing ore veins extremely poor in pyrite or pyrrhotite.