Leightonite

Leightonite is a rare sulfate mineral with formula of K₂Ca₂Cu(SO₄)₄•2H₂O.

Leightonite
General
Category	Sulfate minerals
Formula (repeating unit)	K2Ca2Cu(SO4)4•2H2O
IMA symbol	Lgh
Strunz classification	7.CC.70
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Identification
Color	Pale blue, greenish-blue; pale blue in transmitted light.
Crystal habit	Lathlike crystals, flattened, elongated or equant; pseudo-orthorhombic
Twinning	Lamellar twinning on (100) and (010) produces pseudo-orthorhombic symmetry
Cleavage	None observed
Mohs scale hardness	3
Luster	Vitreous
Diaphaneity	Transparent, translucent
Specific gravity	2.95
Optical properties	Biaxial (-)
Refractive index	nα = 1.578 nβ = 1.587 nγ = 1.595
Birefringence	δ = 0.017
References

Crystal class

Leightonite forms flattened to elongated bladed crystals of variously interpreted crystal structure. Its crystal system is reported as triclinic morphologically, but also as pseudo-orthorhombic due to intricate lamellar twinning that mimics orthorhombic symmetry. Because it is triclinic, the crystal is represented by a system of three unequal vectors with corresponding unequal angles between them.

Optical class

Leightonite is anisotropic, meaning it has more than one refractive index, in this case three as it is biaxial. The mineral can split polarized light into two rays with different direction and velocity, resulting in the appearance of interference colors when recombined and viewed under polarized light.

Discovery and occurrence

It was first described in 1938 for an occurrence in the Chuquicamata Mine, Colama, El Loa Province, Antofagasta Region, Chile, and named in honor of Tomas Leighton Donoso (1896–1967), Professor of Mineralogy at the University of Santiago, Chile.

It occurs in alkali oxidized zones of copper deposits and is associated with natrochalcite, blodite, atacamite, bellingerite, kröhnkite, gypsum and quartz in the discovery location at Chuquicamata, Chile, along with chalcanthite, anhydrite and lammerite in Tsumeb, Namibia. It has also been reported from the Schwaz area of North Tyrol, Austria, and the Visdalen Soapstone Quarry, Lom, Norway.

At the mining site of Chuquicamata, Chile, Leightonite is not found in rich ore; rather it only appears in borderland material within 50 m (160 ft) of the surface, acting as a cement between rock fragments as it fills in cracks and cross-fiber veins in surfaces as a network of crystals. Although a hydrous sulfate of copper, it is not a major source of the element. Because of its rare nature, it is valued by mineral collectors.