Fornacite

fornacite

shattuckite

willemite

chrysocolla

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Formula: CuPb2(CrO4)(AsO4)(OH)
Compound arsenate, brackebuschite group, forms a series with vauquelinite
Specific gravity: 6.27
Hardness: 2 to 3
Streak: Olive-green
Colour: Deep olive-green, golden yellow in transmitted light (small grains)
Solubility: Totally soluble in hydrochloric acid
Environments

Hydrothermal environments

Fornacite is a rare mineral in the oxidised zones of some hydrothermal base-metal deposits. Associated minerals include dioptase, wulfenite, hemihedrite, phoenicochroite, duftite, mimetite, shattuckite, chrysocolla, hemimorphite, willemite and fluorite (HOM, Mindat).

Localities

At the type locality, the Renéville Mine, Renéville, Kindanba District, Pool Department, Republic of the Congo, fornacite occurs as confused groups of small, prismatic crystals associated with dioptase (Mindat).

At the Oumlil East Mine, Oumlil, Agdz Cercle, Zagora Province, Drâa-Tafilalet Region, Morocco, fornacite occurs as crystals less than 0.2 mm encrusting altered galena, and associated with cerussite, litharge and mottramite. The fornacite has a very low VO4 content (MinRec 38.5.375).

Torr Works Quarry (Merehead Quarry), Cranmore, Mendip, Somerset, England, UK, is the only fornacite locality in the UK (as of December 2020) and the mineral is extremely rare here. It has been found as minute crystals and crystal rosettes, up to 0.5 mm across but mostly less than 0.1 mm. Fornacite appears to have been one of the last minerals formed; crystals occur on the surface of cerussite crystals, that in turn encrust calcite crystals which line the original cavity.
Botryoidal aggregates of malachite occur in close proximity to the fornacite. Cavities containing mereheadite, plumbonacrite and rickturnerite occur adjacent to those containing fornacite, and yellow prisms of mimetite are present in some cavities.
Specimens have been found with fornacite occurring in a manganese oxide cavity containing hydrocerussite, mendipite and crednerite. Other associated minerals present in smaller quantities include malachite, calcite, mimetite, cerussite and a few single flecks of parkinsonite included in the hydrocerussite (JRS 13.35-36).

At the Gallagher Vanadium & Rare Minerals Corporation Mine, Cochise County, Arizona, USA, fornacite is a rare and unlikely mineral, given that the Gallagher mine has a generally low arsenic and chromium content. It has been identified in three vugs in a sample of vein quartz as small olive-green crystals (R&M 90-4.343).

At the Reward and Brown Monster mines, Reward, Russ Mining District, Inyo Mts, Inyo county, California, USA, fornacite is relatively uncommon at the Brown Monster mine and is found as groups of crystals on drusy quartz or lining fractures in marble. At the Reward mine fornacite occurs with mottramite, mimetite, vanadinite and wulfenite on fractures in marble. It has also been found as floaters with minute sprays of pyromorphite, hemimorphite and pseudomorphs of chrysocolla after malachite (Min Rec 41-2.188).

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