Hellyerite
Zaratite (emerald-green coating), hellyerite (powder-blue) and heazlewoodite (light bronze)
General
Category	Carbonate mineral
Formula (repeating unit)	NiCO3·6(H2O)
IMA symbol	Hy[1]
Strunz classification	5.CA.20
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 10.77, b = 7.29 c = 18.68 [Å]; β = 94°: Z = 8
Identification
Color	Pale blue
Crystal habit	As crystal fragments and microcrystalline coatings
Twinning	Fine lamellar parallel to cleavage
Cleavage	One perfect, two good at 112° to each other and perpendicular to the perfect cleavage
Mohs scale hardness	2.5
Luster	Vitreous
Diaphaneity	Semitransparent
Specific gravity	1.97
Optical properties	Biaxial (-)
Refractive index	nα = 1.455 nβ = 1.503 nγ = 1.549
Birefringence	δ = 0.094
Pleochroism	X = Y = very pale greenish blue; Z = pale greenish blue
2V angle	Measured: 85°
Alters to	Decomposes on air exposure
References	[2][3][4]

Hellyerite, NiCO₃·6(H₂O), is an hydrated nickel carbonate mineral. It is light blue to bright green in colour, has a hardness of 2.5, a vitreous luster, a white streak and crystallises in the monoclinic system. The crystal habit is as platy and mammillary encrustations on its matrix.

Occurrence

The environment of formation, associated only with metamorphosed ultramafic rocks, is diagnostic compared with gaspeite, another nickel carbonate which is associated with supergene weathering of nickel sulfides.

Hellyerite is observed forming in shear planes in serpentinite, produced by carbonation of the serpentinite. Hellyerite forms in this environment in nickel rich serpentinites, which are metamorphosed equivalents of ultramafic cumulate rocks such as peridotite and dunite. Peridotite and dunite, when fresh, can contain up to ~4,000 ppm nickel within olivine.

It was first identified in 1958 in the Old Lord Brassy mine,^[5] Tasmania, Australia and named after Henry Hellyer (1791–1832), Chief Surveyor of the Van Diemens Land Company.^[3] It is also reported from the Pafuri nickel deposit in Limpopo Province, South Africa.^[4]

References