Hellyerite

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.

Hellyerite
Zaratite (emerald-green coating), hellyerite (powder-blue) and heazlewoodite (light bronze)
General
CategoryCarbonate mineral
Formula
(repeating unit)		NiCO3·6(H2O)
IMA symbolHy[1]
Strunz classification5.CA.20
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/c
Unit cella = 10.77, b = 7.29
c = 18.68 [Å]; β = 94°: Z = 8
Identification
ColorPale blue
Crystal habitAs crystal fragments and microcrystalline coatings
TwinningFine lamellar parallel to cleavage
CleavageOne perfect, two good at 112° to each other and perpendicular to the perfect cleavage
Mohs scale hardness2.5
LusterVitreous
DiaphaneitySemitransparent
Specific gravity1.97
Optical propertiesBiaxial (-)
Refractive indexnα = 1.455 nβ = 1.503 nγ = 1.549
Birefringenceδ = 0.094
PleochroismX = Y = very pale greenish blue; Z = pale greenish blue
2V angleMeasured: 85°
Alters toDecomposes on air exposure
References[2][3][4]

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
  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Hellyerite at Webmineral.com
  3. Handbook of Mineralogy
  4. Mindat.org
  5. Williams KL,Threadgold IM,Hounslow AW. Hellyerite: A new nickel carbonate from Heazlewoord, Tasmania. The American Mineralogist,1959;44:533-538.
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