Page 191 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES NATIVE ELEMENTS
Graphite
It is associated with cuprite Cu,O and Cu + Fe + S minerals, often in
flakes of graphite showing
deposits associated with basic extrusives. Copper is common in the 'buckled' cleavage traces
oxidation zone, where it results from the oxidation of copper sulphides.
Distinguishing Compared with copper, gold is brighter and coloured yellow or white.
features
Gold Au
Gold may contain Ag, Cu, Pd or Rh.
Crystals Gold is cubic and occurs as cubic, dodecahedral or octahedral crystals,
but repeated twinning on { 111} often gives reticulated and dendritic
aggregates. D = 19.3. L_ __ z_oo_ .... _m _ __.JI XPOLS
Polished Gold is bright yellow. Argentiferous gold is whiter and cupriferous gold
section is pinker. R = 74 %, making gold much brighter than pyrite and
chalcopyrite. It is isotropic but with incomplete extinction when a green-
ish colour is observed. Gold does not tarnish, but large grains scratch Graphite is common in metasediments, where it forms from or~an~c
easily and may be difficult to polish. Occurrence
material; when abundant, a graphitic schist results. Such graph1te IS
Gold occurs as irregular grains, blebs or veinlets, often in sulphides
indicative of reducing conditions, and pyrite is usually also present.
(e.g. pyrite, arsenopyrite). The various varieties of gold are often inter-
Graphite also occurs in vein-like deposits and large masses, some of
grown with each other or with Au + Bi + Te and Sb + As-containing
which are of uncertain origin.
minerals. Gold occurs as very fine coatings which can easily be lost on
polishing. VHN = 50-52. Distinguishing Compared with graphite, molybdenite is texturally similar but much
feaures brighter. .
Occurrence Gold is found in hydrothermal deposits, often associated with igneous
rocks; in placer deposits, where it appears to be chemically mobile, Note Small flakes of graphite in metamorphic rocks are much more ev1dent
resulting in nugget growth; and in auriferous quartz veins. It seems to be using oil immersion.
present throughout the temperature range of vein mineralisation. Gold
often occurs as very small grains, even in economic gold deposits. Silver Ag
Distinguishing Compared with gold, chalcopyrite is less yellow, darker and weakly Silver may contain minor amounts of Au, Hg, As, Sb, Pt, Ni, Pb or Fe.
features anisotropic. Crystals Silver is cubic. D = 10.5.
Notes Electrum (Au,Ag) contains 30 to 45 % A g. It is brighter (R = 83%) and Silver is white but it soon tarnishes. With R = 95% it is much brighter
Polished
softer (VHN = 34-44) than pure gold. section than the common ore minerals. It is isotropic, but false anisotropy may
result from fine polishing scratches.
Graphite C
Silver occurs in dendritic or irregular masses and as inclusions, often
Crystals Graphite is hexagonal, a :c = 1 : 1.27522. The layered structure results in silver-bearing sulphides or sulphur-poor minerals. VHN ~ 46-11.8.
in a perfect { 0001} cleavage. Crystals are hexagonal tablets { 0001}. Occurrence Silver is found with Co + Ni + Fe arsenides, usually assoc1ated w1th
D = 2.1. basic igneous rocks. It also occurs in the oxidised zones of galen~-
Thin section In very thin flakes graphite is deep blue and uniaxial -ve. bearing veins. Many veins recorded as silver vems are m fact argentl-
Polished Graphite is brownish grey with a marked pleochroism from R 0 = 16% ferous galena veins, the silver being produced as a by-product of.lead
section (grey) toR.= 6% (dark brownish grey). It appears slightly brighter recovery. Silver is associated with native copper and it is often assoctated
than gangue minerals. The anisotropy is strong in yellowish greys. with carbonate.
Extinction is parallel to the cleavage (corresponding to the grey of R 0 in
PPL) but deformation commonly results in undulose extinction.
Graphite occurs as flakes, tabular crystals, aggregates or botryoidal
masses. Flakes are sometimes very long and broken or buckled. The
cleavage is usually evident and often deformed. In fact graphite is rather
difficult to polish, and surfaces of large grains are often damaged.
VHN = 12.
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