Conichalcite

Conichalcite
A thin crust of conichalcite on a rock.
General
Category	Arsenate minerals
Formula (repeating unit)	CaCu(AsO4)(OH)
IMA symbol	Con
Strunz classification	8.BH.35
Dana classification	41.5.1.2
Crystal system	Orthorhombic
Crystal class	Disphenoidal (222) (same H-M symbol)
Space group	P212121
Unit cell	a = 7.39(1) Å, b = 9.23(1) Å, c = 5.83(1) Å; V = 397.66 Å³; Z = 4
Identification
Color	Grass-green to yellowish green, pistachio-green, emerald-green; may be zoned; light green to yellowish green in transmitted light.
Crystal habit	Crusts of acicular to almost fibrous crystals. Also as botryoidal masses and compact crusts.
Twinning	Rare on {001}
Cleavage	Absent
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	4.5
Luster	Vitreous, greasy
Streak	Green
Diaphaneity	Translucent
Specific gravity	4.3
Optical properties	Biaxial (+/−)
Refractive index	nα = 1.778 – 1.800 nβ = 1.795 – 1.831 nγ = 1.801 – 1.846
Birefringence	δ = 0.023 – 0.046
Pleochroism	Visible
Dispersion	Strong r < v to r < v moderate
References

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.

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 reacts 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.

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.