Page 155 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES
SULPHIDES
Bornite
Distinguishing Compared with chalcocite, djurleite Cu~, 96 S (orthorhombic) is very
myrmekitic intergrowth of features similar; sphalerite is slightly darker, isotropic, and often shows internal
bornite (B) and reflections; and tetrahedrites are less blue, harder and isotropic.
chalcopyrite (C)
Notes Copper sulphide minerals are complex owing to the variation in crystal-
lographic and optical properties with slight-changes in Cu: S ratio. Their
colour changes readily owing to surface damage during polishing as well
c as to tarnishing.
Chalcopyrite CuFeS z
Incorporation of many other elements (e.g. Ni, Zn, Sn) is possible at
high temperatures in the cubic polymorph, which has a range in compo-
300 ~J.m PPL sition in the Cu + Fe + S system. Unmixing occurs on cooling, resulting
in inclusions in chalcopyrite.
Crystals Chalcopyrite is tetragonal , a : c = 1 : 1. 9705. Crystals are commonly
Occurrence Bornite is usually associated with other Cu + Fe + S minerals in the scalenohedral or tetrahedral in appearance (Fig. 3.4). Twinning is
'secondary environment'. common on {112} and {012} and cleavage is {011}. D = 4.28.
It can result from unmixing of high temperature Cu + Fe + S solid Thin section Chalcopyrite is opaque but alteration leads to associated blue green
solutions on cooling. staining or associated secondary hydrous copper carbonates which are
Distinguishing Compared with bornite, pyrrhotite is lighter brown and distinctly aniso- blue to green in colour.
features tropic; they rarely occur together. Polished Chalcopyrite is yellow and tarnishes to brownish yellow. R = 42-46 %,
section slightly less than pyrite and similar to galena. Anisotropy is weak with
Chalcocite Cu 2 S
dark brown and greenish grey tints, and is often not visible.
Chalcopyrite usually occurs as irregular or rounded grains. It is common
Digenite Cu9S 5
Ramdohr (1969) states that 'what has hitherto been considered as as rounded inclusions or in fractures in other sulphides, especially pyrite
"chalcocite" with the formula Cu 2 S is a great number of semi- and sphalerite. Colloform masses of chalcopyrite have been reported.
independent minerals and solid solutions, whose relationships are not Simple and multiple twinning is common and cleavage traces are some-
yet fully understood and for which there are diverse interpretations'. times observed. Several phases may be present in chalcopyrite as ex-
Care must therefore be taken when examining samples reportedly con- solved blebs, lamellae or stars (e.g. ZnS) and indicate a high tempera-
taining chalcocite.
ture origin. VHN = 186-219.
Crystals Chalcocite CuzS is orthorhombic, a :.b :c = 0.5822: 1:0.9701. Dige- Occurrence Chalcopyrite is a common accessory mineral in most types of ore deposit
nite Cu9Ss is cubic. Both minerals are usually massive. D = 5.77. as well as in igneous and metamorphic rocks. It is the major primary
Polished ~halcocite appears bluish light grey with R = 32%, whereas digenite is copper mineral in prophyry copper deposits and it occurs, with ·bornite,
section hg~t grey. to .bluis.h l!ght grey with R = 22 %. Chalcocite is weakly in the stratiform sulphide deposits of the Copperbelt. Chalcopyrite
~ntsotr?ptc wtth pmktsh, greenish grey or brownish tints. Digenite is appears to be a relatively mobile mineral in ore deposits and commonly
ISOtrOpiC.
replaces and veins other minerals, especially pyrite.
Both minerals occur in granular aggregates and are commonly in Distinguishing Compared with chalcopyrite, pyrite is white, much harder and com-
intergrowths with each other or other Cu + Fe + S minerals. Lancet features monly idiomorphic, and gold is much brighter but may be yellower or
shaped inversion twinning indicates cooling from the high temperature whiter. Small isolated grains of pyrite and chalcopyrite can be very
hexagonal polymorph through 103 octo the orthorhombic polymorph. similar in appearance.
Cleavage traces may be observed and are enhanced on weathering.
VHN: 68-98 chalcocite, 56-67 digenite.
Occurrence Digenite is indicative of higher temperatures and higher sulphur activity
than chalcocite. Both minerals are associated with other copper and iron
Figure3.4
sulphides, especially covellite, in low temperature hydrothermal veins
Typical
and. in the 'secondary environment'. They occur in cupriferous, red-bed
chalcopyrite
sedimentary rocks and are widespread as replacement minerals. crystal.
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