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Formula: CuPb2(CrO4)(AsO4)(OH)
Compound arsenate, brackebuschite group, forms a series with
vauquelinite
Crystal System: Monoclinic
Specific gravity: 6.27 measured, 6.30 calculated
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
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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