#arsenates

[mineralogasm]

Scorodite (FeAsO4*2H2O) with quartz (SiO2) from Tsumeb, Namibia.

[mineralogue-blog]

Annabergite, also known as nickel bloom and historically known as nickel ochre, is the nickel analogue to erythrite (cobalt bloom), with the chemical formula Ni3(AsO4)2 · 8H2O. Where cobalt bloom is a distinctive crimson/purple, annabergite is apple-green in colour. Like erythrite it has a hardness of 1.5-2.5, with the hardness differing on different cleavage faces. It occurs in series with erythrite, and when the ratio of Co:Ni is about 1:1, it is light pink, becoming white/grey to pale green to apple-green as the ratio of nickel to cobalt occurs.

Crystals are rare, but are generally flattened prismatic to acicular when formed; annabergite more commonly occurs as a green coating on nickel minerals. See also erythrite.

Sources:

Mindat

Wikipedia

Image 1: Annabergite crystals in a 1.9cm vug from Lavrion, Greece. irocks.com  Image 2: Annabergite with cuprian smithsonite from Km-3 mine, Attica Prefecture, Greece. irocks.com

[mineralogue-blog]

Bayldonite is a relatively rare green secondary arsenate mineral. It has the chemical composition PbCu3(AsO4)2(OH)2 and occurs in oxidised zones of copper- and lead-bearing deposits. It occurs in various shades of green, and has an apple-green streak.

Sources:

Mindat

Webmineral

Image 1:  Bayldonite pseudomorph after mimetite from Tsumeb, Namibia. (c) Rob Lavinsky and irocks.com  Image 2: Acicular bayldonite in bayldonite vug from Tsumeb, Namibia. (c) Rob Lavinsky and irocks.com

[mineralogue-blog]

Erythrite is a distinctive cobalt arsenate with the formula Co3(AsO4)2 · 8H2O. Its name originates from the Greek word erythros, “red”. It has a Mohs hardness of 1.5-2.5, with differing hardnesses on different cleavage faces. Its colour is crimson to pink or purpleish, and single crystals sometimes exhibit colour banding. It has a pale red to pink streak.

Erythrite forms prismatic crystals, although it is rare to find well-formed crystals. It generally occurs as small crystal aggregates or a crust or druse on other mineral surfaces, particularly on cobalt deposits, where it is called cobalt bloom. Its bright, distinctive colour serves as an indicator of cobalt ore to prospectors, although erythrite has no economic value of its own.

It forms a series with annabergite (nickel bloom), where nickel replaces cobalt. 

Sources:

Wikipedia

Mindat

Webmineral

Image sources: 1, 2