Conichalcite

Conichalcite, CaCu(AsO4)(OH), is a relatively common arsenate mineral related to duftite (PbCu(AsO4)(OH)). It is green, often botryoidal, and occurs in the oxidation zone of some metal deposits. It occurs with limonite, malachite, beudantite, adamite, cuproadamite, olivenite and smithsonite.[2][3]

Conichalcite
A thin crust of conichalcite on a rock.
General
CategoryArsenate minerals
Formula
(repeating unit)		CaCu(AsO4)(OH)
Strunz classification8.BH.35
Dana classification41.5.1.2
Crystal systemOrthorhombic
Crystal classDisphenoidal (222)
(same H-M symbol)
Space groupP212121
Unit cella = 7.39(1) Å,
b = 9.23(1) Å,
c = 5.83(1) Å;
V = 397.66 Å³; Z = 4
Identification
ColorGrass-green to yellowish green, pistachio-green, emerald-green; may be zoned; light green to yellowish green in transmitted light.
Crystal habitCrusts of acicular to almost fibrous crystals. Also as botryoidal masses and compact crusts.
TwinningRare on {001}
CleavageAbsent
FractureUneven
TenacityBrittle
Mohs scale hardness4.5
LusterVitreous, greasy
StreakGreen
DiaphaneityTranslucent
Specific gravity4.3
Optical propertiesBiaxial (+/-)
Refractive indexnα = 1.778 - 1.800 nβ = 1.795 - 1.831 nγ = 1.801 - 1.846
Birefringenceδ = 0.023 - 0.046
PleochroismVisible
DispersionStrong r < v to r < v moderate
References[1][2][3]

Formation
Mat of conichalcite spheres on limonite base from the Ojuela Mine, Mapimí, Durango, Mexico (size: 10.4 x 8.9 x 4.2 cm)

Conichalcite forms in the oxidation zones of copper orebodies. Here groundwater enriched with oxygen react with copper sulfide and copper oxide to produce an array of minerals such as malachite, azurite and linarite. Conichalcite is often found encrusted on to limonitic rocks that have yellow to red colors.

Conichalcite will also form a solid solution series with the mineral calciovolborthite. When these two minerals form a solid solution series, the two interchanging elements are arsenic and vanadium. Conichalcite is the arsenic rich end member of the series and calciovolborthite is the vanadium rich end member.[2]

Notable occurrences of conichalcite include Juab Co., Utah; Lincoln and Lyon counties of Nevada and Bisbee, Arizona, in the US; Durango, Mexico; Collahuasi, Tarapaca, Chile; Calstock, Cornwall and Caldbeck Fells, Cumbria, England; Andalusia, Spain; and Tsumeb, Namibia.[3]

References
  1. Mineralienatlas
  2. Mindat.org
  3. Handbook of Mineralogy
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