Hemihedrite

chromate

galena

Formula: ZnPb10(CrO4)6(SiO4)2(OH)2
Compound chromate
Specific gravity: 6.42
Hardness: 3
Streak: Saffron yellow
Colour: Bright orange to henna-brown to nearly black
Environments:

Hydrothermal environments

Hemihedrite is formed through oxidation of galena, sphalerite and pyrite in the oxide zone of hydrothermal lead-bearing veins.

At the Rat Tail claim, Maricopa county, Arizona, USA, hemihedrite is formed by the oxidation of galena, sphalerite and pyrite in quartz veins cutting andesite. The assemblage consists largely of hemihedrite, willemite, cerussite and phoenicochroite, with minor amounts of mimetite and vauquelinite (AM 55.1088-1102).

At the type locality, the Florence Mine, Pinal county, Arizona, USA, hemihedrite occurs in the oxidised zone of lead-bearing veins that cut limestone intruded by an altered porphyry (rock with coarse phenocrysts in a finer groundmass) and quartzite (Mindat), associated with cerussite, phoenicochroite, vauquelinite, willemite, galena, sphalerite, pyrite, tennantite and chalcopyrite (HOM, AM 53.1421-1427).

The sequence of formation is cerussite first, followed by hemihedrite and wulfenite, with wulfenite continuing to crystallise after hemihedrite, and vauquelinite last (AM.55.1088-1102).

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