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Formula: FeAsS
Oxidation states: Fe2+As1-S1- (AM 87.1692-1698)
Danaite is a cobalt-bearing variety of arsenopyrite
Sulphide, arsenopyrite group,
arsenic-bearing mineral
Crystal System: Monoclinic
Specific gravity: 6.07 measured, 6.18 calculated
Hardness: 5½ to 6
Streak: Black
Colour: Tin white to steel white
Solubility: Arsenopyrite is moderately soluble in nitric acid but insoluble in hydrochloric and sulphuric acids
Common impurities: Ag,Au,Co,Sn,Ni,Sb,Bi,Cu,Pb
Environments:
Pegmatites (sparingly)
Metamorphic environments
Hydrothermal environments
Arsenopyrite is the most common arsenic-bearing mineral. It occurs with
tin and tungsten ores in
high-temperature
hydrothermal deposits, associated with silver and
copper
ores, galena,
sphalerite,
pyrite and
chalcopyrite. It is frequently
associated with gold. It is found in
contact metamorphic deposits,
and disseminated in
limestone.
Localities
At Devil's Peak, Sai Kung District, New Territories, Hong Kong, China, the mineralisation occurred in
quartz veins in the contact zone between a
granite intrusion and acid volcanic rocks. The mine is now closed,
and inaccessible for collecting.
Chalcopyrite occurred in the
quartz
veins with
beryl, wolframite,
molybdenite, pyrite and
arsenopyrite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Needle Hill Mine, Needle Hill, Sha Tin District, New Territories, Hong Kong, China, is a tungsten mine, abandoned
in 1967. The principal ore is wolframite, and the principal gangue mineral
is
quartz. Molybdenum also occurs. The
mineralisation consists of a series of parallel fissure veins that cut through
granite. Wolframite and
quartz are the main minerals, but galena,
sphalerite, pyrite,
molybdenite and fluorite have also
been found here
(Geological Society of Hong Kong Newsletter 9.3.29-40).
The quartz-wolframite veins are of
high-temperature hydrothermal formation,
and grade into wolframite-bearing
pegmatites.
Wolframite is almost always associated with
molybdenite. Other
associated minerals found occasionally include pyrite,
chalcopyrite, arsenopyrite,
sphalerite, bismuth,
fluorite, topaz and
cassiterite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
At the Huangshaping Mine, Huangshaping Pb-Zn-polymetallic ore field, Guiyang Co., Chenzhou, Hunan, China,
arsenopyrite occurs as well formed crystals with a bright metallic lustre, up to 4 mm across, on a dark matrix
(AESS).
Arsenopyrite from Huangshaping - Image
At the Yaogangxian Mine, Yaogangxian W-Sn ore field, Yizhang Co., Chenzhou, Hunan, China, arsenopyrite has been
found associated with quartz and with
fluorite
(AESS)
Arsenopyrite from Yaogangxian - Image
At Charcas, Charcas Municipality, San Luis Potosí, Mexico,
the primary minerals are
sphalerite, galena,
chalcopyrite, bornite,
tetrahedrite,
arsenopyrite, pyrite and
silver minerals such as jalpaite,
diaphorite and acanthite. In
the host rock, as metamorphic or alteration minerals, danburite,
datolite, hedenbergite,
epidote, chlorite,
andradite, actinolite
and wollastonite have been reported.
Quartz, calcite and
danburite crystallised during the entire life of the systems, throughout
the intrusive emplacement, metamorphism, and mineralising events. With depth, both
sphalerite and galena decrease
while chalcopyrite increases.
Secondary sulphides formed include
bornite, covellite,
digenite and chalcocite.
Native silver, native gold,
hematite and goethite were
deposited after the sulphides
(Minrec 55.6.727-728).
Arsenopyrite is common as part of the ore mineral assemblage and can be expected in all mines. In hand
specimens, granular aggregates and larger crystals of stubby, prismatic habit and a brilliant metallic luster can
reach 2 cm on edge. Aggregates of stubby, mushroom-shaped crystals are known as large as 20 cm across.
Arsenopyrite precipitated earlier than the other sulphides such as
sphalerite, chalcopyrite,
tetrahedrite and galena.
Arsenopyrite has been found deposited over danburite, indicating
a very early precipitation of the danburite
(Minrec 55.6.732-735).
Arsenopyrite from Charcas - Image
Dry Gill Mine, Caldbeck, Allerdale, Cumbria, England, UK. Arsenic is
of widespread occurrence in the vicinity of the mine. Large quantities of arsenopyrite occur in the
tungsten-bearing quartz
veins in the Carrock tungsten mine, and arsenopyrite also
occurs in the lead and baryte
mines to the north and east of the Dry Gill mine, sometimes in large quantities. The weathering of
arsenopyrite near the surface has given rise to a number of
arsenic-bearing alteration products
(Minrec 54.5.609-611).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, arsenopyrite may occur in crystals to 2 mm coating
löllingite masses. The Emmons pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.503).
Alteration
chalcopyrite, arsenopyrite, CO2
and O2 to Fe-tennantite, siderite and
sulphur
10CuFeS2 + 4FeAsS + 4CO2 + 8O2 = Cu10Fe2As4S13 +
4Fe(CO3) + 11/2S2
(CM 28.725-738)
chalcopyrite, arsenopyrite and
pyrite to Fe-tennantite and
troilite
10CuFeS2 + 4FeAsS + FeS2 = Cu10Fe2As4S13 + 13FeS
(CM 28.725-738)
chalcopyrite, arsenopyrite and
sulphur to Fe-tennantite and
pyrite
10CuFeS2 + 4FeAsS + 13/2S2 = Cu10Fe2As4S13 + 12FeS2
This reaction occurs at a comparatively low temperature
(CM 28.725-738).
chalcopyrite, arsenopyrite and
sulphur to Fe-tennantite and
troilite
l0CuFeS2 + 4FeAsS + l/2S2 = Cu10Fe2As4S13 + l2FeS
(CM 28.725-738)
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