Liroconite

Liroconite is a complex mineral: Hydrated copper aluminium arsenate hydroxide, with the formula Cu₂Al[(OH)₄|AsO₄]·4(H₂O). It is a vitreous monoclinic mineral, colored bright blue to green, often associated with malachite, azurite, olivenite, and clinoclase. It is quite soft, with a Mohs hardness of 2–2.5, and has a specific gravity of 2.9–3.0.

Vugs in gossan lined with lustrous, blue-green liroconite blades and sparkly, dark blue clinoclase microcrystals from Wheal Gorland (size: 3.4 × 3.0 × 2.0 cm)

Liroconite
General
Category	Arsenate minerals
Formula	Cu2Al[(OH)4|AsO4]·4(H2O)
IMA symbol	Lro[1]
Strunz classification	8.DF.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	I2/a
Unit cell	a = 12.66, b = 7.57 c = 9.89 [Å]; β = 91.25°; Z = 4
Identification
Color	Bright blue to green
Crystal habit	Typically as striated flattened octahedral or lenticular crystals, also massive to granular
Cleavage	Indistinct on {110} and {011}
Fracture	Irregular/uneven, conchoidal
Mohs scale hardness	2–2+1⁄2
Luster	Vitreous to resinous
Streak	Light blue
Diaphaneity	Transparent, translucent
Specific gravity	2.9–3
Optical properties	Biaxial (−)
Refractive index	nα = 1.612 nβ = 1.652 nγ = 1.675
Birefringence	δ = 0.063
2V angle	Measured: 67°
References	[2][3][4]

It was first identified in 1825 in the tin and copper mines of Devon and Cornwall, England. Although it remains quite rare it has subsequently been identified in a variety of locations including France, Germany, Australia, New Jersey and California.^[2]

The type locality for liroconite is Wheal Gorland in St Day, Cornwall in the United Kingdom.^[2] The largest crystal specimen on public display is in the Royal Cornwall Museum in Truro.^[5]

It occurs as a secondary mineral in copper deposits in association with olivenite, chalcophyllite, clinoclase, cornwallite, strashimirite, malachite, cuprite and limonite.^[4]

Structure

Liroconite crystallizes in the monoclinic crystal system.^{[4]: 1 [6]} The crystal structure consists of a framework of AsO₄ tetrahedra, Jahn-Teller-distorted [CuO₂(OH)₂(H₂O)₂] octahedra and [AlO₂(OH)₄] octahedra.^[7]

See also

• Kernowite – an isostructural mineral with iron in place of aluminium