Copiapite

Copiapite
Copiapite from the Bolesław Mine, Kłodzko District, Lower Silesia, Poland
General
Category	Sulfate minerals
Formula (repeating unit)	Fe2+Fe3+4(SO4)6(OH)2·20(H2O)
IMA symbol	Cpi[1]
Strunz classification	7.DB.35
Crystal system	Triclinic
Crystal class	Pinacoidal (1) (same H-M symbol)
Space group	P1
Unit cell	a = 7.337 Å, b = 18.76 Å, c = 7.379 Å; α = 91.47°, β = 102.18°, γ = 98.95°; Z = 1
Identification
Color	Sulfur-yellow to orange when crystalline, greenish-yellow to olive-green when massive
Crystal habit	Tabular pseudo-orthorhombic platy crystals, typically in scaly incrustations or granular pulverulent aggregates
Twinning	Contact twins
Cleavage	Perfect on {010}, imperfect on {101}
Fracture	Irregular/uneven, micaceous
Tenacity	Fragile
Mohs scale hardness	2.5–3
Luster	Pearly on {010}
Diaphaneity	Transparent to translucent
Specific gravity	2.04–2.17
Optical properties	Biaxial (+)
Refractive index	nα = 1.506 – 1.540 nβ = 1.528 – 1.549 nγ = 1.575 – 1.600
Birefringence	δ = 0.069
Pleochroism	X = Y = pale yellow to colorless; Z = sulfur-yellow
2V angle	Measured: 45° to 74°, Calculated: 48° to 72°
Solubility	Soluble in water
References	[2][3][4][5]

Copiapite is a hydrated iron sulfate mineral with formula: Fe²⁺Fe³⁺₄(SO₄)₆(OH)₂·20(H₂O).^[3] Copiapite can also refer to a mineral group, the copiapite group.

Copiapite is strictly a secondary mineral forming from the weathering or oxidation of iron sulfide minerals or sulfide-rich coal. Its most common occurrence is as the end member mineral from the rapid oxidation of pyrite. It also occurs rarely with fumaroles. It occurs with melanterite, alunogen, fibroferrite, halotrichite, botryogen, butlerite and amarantite.^[3] It is by far the most common mineral in the copiapite group.

It rarely occurs as single crystals, is in the triclinic crystal system, and is pale to bright yellow. It is soluble in water, changing the water color to deep orange or orangish-red. In solution copiapite is very acidic. In high concentrations a negative pH can occur, as reported in waters draining from Richmond Mine at Iron Mountain, California.^[4] Copiapite can easily be distinguished from native sulfur because it does not give off an odor when dissolved in water. It can be distinguished from similar appearing uranium minerals, such as carnotite, by its lack of radioactivity. The only way to differentiate between the minerals in the copiapite group is by X-ray diffraction.

Copiapite was first described in 1833 for an occurrence near Copiapó, Atacama, Chile.^[5] It is sometimes known as yellow copperas. Other occurrences are in California, Nevada, and in the filled paleo sinkholes and caves of Missouri.