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Formula: NiS
Sulphide
Crystal System: Trigonal
Specific gravity: 5.3 to 5.5 measured, 5.374 calculated
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:
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 Brownley Hill Mine, Nenthead, Alston Moor, Eden, Cumbria, England, UK, millerite has been found as sparse
isolated green needles on a matrix of ankerite crystals with differing colour
due to iron discolouration
(AESS)
Millerite from Brownley Hill - Image
At Fall Hill quarry, Ashover, Derbyshire, England, UK, millerite has been found on
calcite
(RES p104).
Millerite from Fall Hill - Image
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 as a single brassy
metallic needle 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).
Millerite from the Eaglebrook Mine - Image
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 Bryn-yr-Afr 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).
Millerite from Bryn-yr-Afr - Image
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 Esgair Hir 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).
Millarite from Esgair Hir and Esgair Fraith mines -
Image
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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