Conichalcite

(repeating unit)CaCu(AsO₄)(OH)IMA symbolCon^[1]Strunz classification8.BH.35Dana classification41.5.1.2Crystal systemOrthorhombicCrystal classDisphenoidal (222)
(same H-M symbol)Space groupP2₁2₁2₁Unit cella = 7.39(1) Å,
b = 9.23(1) Å,
c = 5.83(1) Å;
V = 397.66 Å³; Z = 4IdentificationColorGrass-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}CleavageAbsentFractureUnevenTenacityBrittleMohs scale hardness4.5LusterVitreous, greasyStreakGreenDiaphaneityTranslucentSpecific gravity4.3Optical propertiesBiaxial (+/−)Refractive indexn_α = 1.778 – 1.800 n_β = 1.795 – 1.831 n_γ = 1.801 – 1.846Birefringenceδ = 0.023 – 0.046PleochroismVisibleDispersionStrong r < v to r < v moderateReferences^[2][3][4]

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.^[3][4]

Formation

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.^[3]

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.^[4]

References