Page 156 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES SULPHIDES
Covellite is hexagonal, a: c = 1: 1.43026. It occurs as platy { 0001}
Cinnabar HgS Crystals
Crystals Cinnabar is trigonal, a: c = 1: 2.2905, and occurs as thick tabular crystals with a perfect { 0001} basal cleavage. D = 4.6.
{ 0001} or P!ismatic [ 10il] crystals. There is a { 0001} twin plane and Thin section Covellite is greenish in very thin flakes.
Covellite is blue and strongly pleochroic from blue to bluish light grey,
perfect { 1011} cleavage. D = 8.09. Polished
Thin section Cinnabar is deep red. Refractive index values (A. = 598 nm) are section except in basal sections which remain blue. Ro = 7 and R. = 22%.
Anisotropy is very strong with bright ' fiery' orange colours.
n 0 = 2.905 and n. = 3.256.
Covellite occurs as idiomorphic platy crystals and flakes as well as rather
Polished Cinnabar is light grey to bluish light grey, weakly pleochroic, with
'micaceous' aggregates. The good basal cleavage, parallel to the length
section Ro = 28% and R. = 29%. Anisotropy is moderate with greenish grey
of grains, is often deformed. VHN = 69-78.
tints, but these are often masked by abundant deep red internal
Covellite
reflections.
Cinnabar occurs as granular aggregates and idiomorphic crystals. basal section (B) and cross
sections of covellite showing
Deformation multiple twinning may be present. As a result of variation cleavage traces (C): chalcoctte
in polishing hardness with orientation, granular aggregates may re- (Ch) is replacing the covellite
semble a two phase intergrowth at first glance.
Occurrence Cinnabar is rare, occurring in low temperature hydrothermal veins,
impregnations and replacement deposits often associated with recent
volcanics. It often replaces quartz and sulphides and is associated with
native mercury, mercurian tetrahedrite-tennantite, stibnite, pyrite and
marcasite in siliceous gangue. VHN = 51-98.
Distinguishing Compared with cinnabar, hematite is brighter, harder and has very rare
features internal reflections; pyrargyrite is very similar but with less intense
500 IJ.m I PPL
internal reflections; and cuprite Cu 2 0 is bluish grey, harder and usually
associated with native copper.
Notes Metacinnabarite is a high temperature cubic polymorph of HgS. It
Covellite commonly occurs as a 'secondary' mineral after Cu + Fe + S
occurs as grains within cinnabar and is slightly darker, isotropic, lacks Occurrence
internal reflections and is softer than cinnabar. minerals, often in the zone of secondary enrichment.
Distinguishing Covellite is easy to identify. Digenite is blue but neither pleochroic nor
Cobaltite CoAsS features anisotropic.
Cobaltite may contain significant amounts of Fe and Ni in solid solution. Notes Blaubleibender covellite Cu,+xS occurs with covellite and is identical in
Crystals Cobaltite is orthorhombic (pseudo-cubic). It commonly occurs in cubes appearance except under oil immersion, when: R. = bluish grey
or pyritohedrons but may be octahedral. There is a perfect { 001} covellite R 0 = reddish purple R. = bluish grey
cleavage. D = 6.0-6.3. blaubleibender covellite R 0 = deep blue
Polished Cobaltite is pinkish white with R = 53 %, similar to pyrite. Both
section bireflectance and anisotropy, with brownish to bluish tints, are weak. Galena PbS
Galena may possibly contain some Se, Te, Ag, Sb, Bi, As in solid
Cobaltite is often idiomorphic and of 'cubic' morphology. It may be
solution but usually only in trace amounts.
granular or skeletal. Colour zonation has been observed and complex The crystallographic symmetry of galena is cubic and crystals are com-
fine lamellar twinning and cleavage traces may be present. Crystals
monly cubic, cuba-octahedral and (less often) octahedral in shape
VHN = 1176-1226.
(Fig. 3.5). Twinning on { 111} is common and lamellar twinning may
Occurrence It is associated with Cu + Fe + Sand Co + Ni + As minerals in high to
medium temperature deposits in veins and as disseminations. occur on {114}. There is a perfect {001} cleavage. D = 7.58.
Distinguishing Compared with cobaltite, pyrite is yellowish and harder.
features
Covellite CuS Figure3.5
Covelline is an alternative name recommended by the International Typical galena
Mineralogical Association. crystals.
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