fornacite

{{Short description|Rare lead, copper chromate arsenate hydroxide mineral}}

{{Infobox mineral

| name = Fornacite

| image = Dioptasereneville2.jpg

| imagesize = 260px

| alt =

| caption = Dioptase (blue green), cerussite (light pink) and fornacite (green) from Renéville, Djoué, Brazzaville Region, Republic of Congo

| category = Arsenate mineral

| formula = Pb₂Cu(CrO₄)(AsO₄)(OH)

| IMAsymbol = For{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| molweight =

| strunz = 7.FC.10

| dana = 43.4.3.2

| system = Monoclinic

| class = Prismatic (2/m)
(same H-M symbol)

| symmetry = P2₁/c

| unit cell = a = 8.101(2),
b = 5.893(11),
c = 17.547(9) [Å];
β = 110.00(4)°; Z = 4

| color = Deep olive-green

| colour =

| habit = Aggregates of steep pyramidal to bladed, rounded crystals

| twinning =

| cleavage =

| fracture = Irregular/uneven, conchoidal, sub-conchoidal

| tenacity = Brittle

| mohs = 2–3

| luster = Resinous, waxy, greasy

| streak = Olive green

| diaphaneity = Transparent

| gravity =

| density = 6.27 g/cm³

| polish =

| opticalprop = Biaxial (+)

| refractive = n_α = 2.142 n_γ = 2.242

| birefringence = δ = 0.100

| pleochroism =

| 2V = Large

| dispersion =

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| prop1 =

| prop1text =

| references = [http://rruff.info/doclib/hom/fornacite.pdf Handbook of Mineralogy][http://www.mindat.org/min-1583.html Mindat with locations][http://www.webmineral.com/data/Fornacite.shtml Webmineral data]

}}

Fornacite is a rare lead, copper chromate arsenate hydroxide mineral with the formula: Pb₂Cu(CrO₄)(AsO₄)(OH). It forms a series with the phosphate mineral vauquelinite. It forms variably green to yellow, translucent to transparent crystals in the monoclinic – prismatic crystal system. It has a Mohs hardness of 2.3 and a specific gravity of 6.27.

It was first described in 1915 and named after Lucien Lewis Forneau (1867–1930) the governor of the French Congo. Its type locality is in Reneville, Republic of Congo.

It occurs in the oxidized zone of ore deposits and is associated with dioptase, wulfenite, hemihedrite, phoenicochroite, duftite, mimetite, shattuckite, chrysocolla, hemimorphite, willemite and fluorite.