Page 173 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES
OXIDES
*CLEAVAGE None; basal parting present.
Chromite FeCr,0 4
Crystals
Usually containing Mg and AI, chromite may also contain Zn y Mn *RELIEF High (about the same as garnet).
Thin section Chromite is cubic and a member of the spinel group. Crystals' ar~ rar~ ALTERATION Corundum can alter to Al,Si0 5 minerals during metamorphism, by
bu~occur as octahedra modified by { 001} faces. There is no cleavage. addition of silica, or to muscovite if water and potassium are also
D- 5.1.
available.
Chr_omite is opaque except in very thin grain margins which are brown- BIREFRINGENCE Low, common interference colours are first order greys and whites.
Ish m colour. *TWINNING Lamellar twinning is commonly seen on { 1 Oi 1}. Simple twins can occur
Polished
Ch~omite is grey, sometimes appearing slightly brownish. R = 12 % but with { 0001} as the twin plane.
section
vanes with chemical co~posi_tion. This reflectance value is significantly DISTINGUISHING Corundum has low birefringence, high relief, no cleavage and lamellar
less than that of magnetite. H1gh Fe and Cr values increase R but AI and FEATURES twinning. Apatite has lower relief and still lower birefringence.
Mg decrease R . Although cubic and usually isotropic, chromite some- *OCCURRENCE Corundum occurs in silica-poor rocks such as nepheline-syenites, and
times shows weak anisotropy. Iron-poor chromite may have scarce other alkali igneous undersaturated rocks. It may occur in contact
reddish brown Internal reflections. aureoles in thermally altered aluminous shales, and in aluminous
. Chr~mite occurs as rounded octahedral grains resembling droplets, xenoliths found within high temperature basic igneous plutonic and
m~erstltially in silic~tes,_ or as granular aggregates. It is an accessory hypabyssal rocks. In these aluminous xenoliths, corundum is frequently
mme_ral In most pendot1tes and derived serpentinites. Cataclastic tex- found in association with spinel, orthopyroxene and cordierite. Cor-
ture IS common. A zonation in reflectance related to chemical zonation undum occurs in metamorphosed bauxite deposits, and also in emery
may be observed. Marginal discoloration and alteration may occur. deposits. It can occur as a detrital mineral in sediments.
InclusiOns of Fe + T1 + 0 phases, e.g. rutile, may be present. VHN
= 1195-1210.
Occurrence
Chromite is only abundant in certain mafic igneous rocks especially Hematite Fe,0 3
large layered intrusions (e.g. the Bushveldt lopolith) as c~mulates or Hematite is often titaniferous, i.e. there is a hematite-ilmenite solid
possibly oxide-liquid segregations. It is found as podiform concen- solution. See Section 3.4.
tratiOns, poss_ibly originally cumulates, in Alpine-type serpentinites and Crystals Hematite is hexagonal, a :c = 1: 1.3652 and usually occurs as tabular
also as~ detntal heavy mineral in sedimentary and metamorphic rocks. crystals { 0001} often in subparallel growths (Fig. 3.12). Penetration
Chro~1te may occur as cores within magnetite grains. Iron-rich rims of twinning occurs on { 0001} and lamellar twinning on { 1 Oi 1}. There is no
chrom1tes, commonly observed in serpentinites, are known as ferrit- cleavage. D = 5.2.
chromit. The rims have a slightly higher reflectance than the chromite Thin section Hematite is opaque but deep red in very thin plates. It is uniaxial -ve
cores and are magnetic. with absorption o > e.
Distinguishing Polished Hematite is light grey and only weakly bireflecting, with R 0 = 30% and
Co~pared with chromite, magnetite is brighter. The two minerals are
features
similar unless direct comparison of brightness can be made. However section R. = 25 %. It is much brighter than magnetite and ilmenite. Anisotropy
remember that magnetite is magnetic! ' is strong in bluish and brownish greys. The deep red internal reflections
are scarce except in very thin plates. Hematite coatings give a red
colouration to internal reflections of transparent grains such as quartz.
Corundum Al,O, Hematite occurs as idiomorphic tabular crystals and fibrous radiating
trigonal cia 1.364
aggregates. It is also found as microcystalline colloform masses. It is
1.768-1.772
often intergrown with other Fe + Ti + 0 minerals and occurs as lamel-
1.760-1.763
0.008-0.009 lae in ilmenite. Hematite may contain lamellae of ilmenite or rutile.
Lamellar twinning is common and a pseudo-cleavage consisting of elon-
Uniaxial - ve (crystal hexagonal, rarely prismatic) gate pits may be present. VHN = 920-1062.
D = 3.98-4.02 *H = 9
COLOUR
Colourless, but gem quality corundum is often coloured blue (sapphire)
or red (ruby) m hand specimen.
PLEOCHROISM
Normal c_orundu_m is not pleochroic but gem quality minerals are weakly
Figure3.12
pleochroic, particularly sapphire with e blue and 0 light blue.
Typical hematite
HABIT Rarely euhedral, usually as small rounded crystals.
crystals.
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