Millerite

millerite

chalcopyrite

nickel

sphalerite

Images

Formula: NiS Sulphide
Specific gravity: 5.3
Hardness: 3½
Streak: Dark green
Colour: Brass-yellow
Solubility: Insoluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid
Common impurities: Fe,Co,Cu
Environments:

Hydrothermal environments

Millerite forms in nickel deposits as a low-temperature mineral, often in cavities and as an alteration of other nickel minerals, or as crystal inclusions in other minerals. It occurs in hydrothermal mineral veins, from medium-temperature to lower-temperature Mississippi Valley type associations. It is also developed in certain sedimentary rocks, in particular coal-bearing strata. Mineral associations vary, but typically include other cobalt-nickel sulphides, chalcopyrite, sphalerite and galena (MW).

Localities

At Fall Hill quarry, Ashover, Derbyshire, England, UK, millerite has been found on calcite (RES p104).

At the Linton quarry, Gorsley Common, Herefordshire, England, UK, millerite has been found in dolomite - lined cavities surrounding fossil brachiopods, and in thin calcite - lined fractures. Associated minerals include galena (RES p181, JRS 21.17-20).

At the Clayton adit, Ecton mine, Staffordshire, England, UK, millerite has been found on calcite (RES p308).

At Eaglebrook Mine, Ceulanymaesmawr, Ceredigion, Wales, UK, millerite occurs in two associations: firstly as a microscopic replacement of early siegenite and cobaltpentlandite, and secondly as coarser crystals to 15 mm with ferroan dolomite, quartz, chalcopyrite and siegenite (MW).

At Henfwlch Mine, Ceulanymaesmawr, Ceredigion, Wales, UK, millerite is present in association with chalcopyrite, siegenite, tucekite and galena in quartz and ferroan dolomite (MW).

At Frongoch Mine, Devil's Bridge, Ceredigion, Wales, UK, millerite is very rare, as divergent sprays of slender hairlike crystals to 2 mm in length, in cavities in quartz associated with sphalerite and marcasite (MW).

At Brynyrafr Mine, Ponterwyd, Ceredigion, Wales, UK, millerite forms tangled masses of often twisted needles, generally 5-10 mm in length but exceptionally to 20 mm, spanning quartz cavities and associated with chalcopyrite and occasionally sphalerite. It also occurs embedded in massive quartz (MW).

At Gwaithgoch Mine, Pontrhydygroes, Ceredigion, Wales, UK, millerite has been found as minute golden needles on samples consisting of late ullmannite - chalcopyrite bearing quartz veinstone where the millerite occurs in close association with these sulphides (MW).

At Erglodd Mine, Tal-y-bont, Ceredigion, Wales, UK, millerite occurs rarely as needles to 8 mm embedded in chalcopyrite (MW).

At Esgair Fraith Mine, Tal-y-bont, Ceredigion, Wales, UK, millerite is rare but occurs in association with chalcopyrite, ferroan dolomite and quartz. In polished section, it is also present as intergrowths with siegenite, both occurring embedded in chalcopyrite (MW).

At Esgairhir Mine, Tal-y-bont, Ceredigion, Wales, UK, millerite occurs as a replacement of siegenite; it also forms needles to 2 mm intergrown with siegenite and tucekite (MW).

At Great Orme Copper Mines, Llandudno, Gwynedd, Wales, UK, small inclusions of millerite have been found in galena from a Mississippi Valley-type lead vein occupying a fault-plane. Associated minerals were pyrite, marcasite, chalcopyrite and rare sphalerite (MW).

At Hyddgen Mine, Uwchygarreg, Powys, Wales, UK, millerite occurs rarely with chalcopyrite, galena and tucekite (MW).

At the South Wales Coalfield, Wales, UK, millerite-bearing ironstone nodules are present throughout the coalfield. The nodules contain septarian geodes lined with siderite crystals upon which millerite and a variety of other sulphides (galena, siegenite, sphalerite, chalcopyrite and pyrite) have crystallised. Millerite is most frequently observed in ironstones associated with the bituminous coals but has also been found in the higher-temperature anthracite zone in the northwestern part of the coalfield (MW).

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