Page 170 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES OXIDES
Figure 3.10 Figure 3.11
Solid solution in Cassiterite
spinels. crystals.

cassiterite twin
on (011)
giving both contact and penetration twins. There is a poor { 100} cleav-
age.
1.990-2.010
Thinsection n 0
ne = 2.093-2.100
0 = 0.096-0.098
Uniaxial +ve (crystals are length slow)
Fe 3 0 4
D = 6.98-7.02 H = 6- 7
Transparent spinels have high relief (n > 1.7) in thin section and are
isot~opic. ~n octahedral habit, sometimes with twinning on { 111}, helps cowuR Colourless or slightly red or brown.
to dtstmgmsh them from the garnets. Opaque spinels are isotropic; they *PLEOCHROISM Occasionally present in coloured varieties with o pale colours and e dark
differ in their reflectance values. yellow, brown, reddish.
There is considerable variation in the chemical composition of natural C LEAVAGE { 100} and { 010} prismatic cleavages parallel to length of mineral.
spinels, and this leads to a variety of colours and degrees of opacity *RELIEF Extremely high.
(Table 3.2). Magnetic (ferrimagnetic) spinels also exist, the best known *BIREFRINGENCE Very high, but interference colours are often masked by the colour of
being magnetite. The spinels can often be identified on the basis of their mineral.
mode of occurrence, textural relations and associated phases, but TWINNING Simple and repeated, common on { 011}.
chemical analysis is usually required for satisfactory identification. ZONING Variation in iron content leads to colour banding.
Polished Cassiterite is grey, sometimes appearing slightly brownish. With
Cassiterite SnO,
section R = 11 % andRe = 13%, bireflectance is weak but usually visible in
Cassiterite may contain minor amounts of Fe, Nb, Ta, Ti, W or Si. granular aggregates and twinned grains. Cassiterite is darker than sphal-
0
Crystals Cassiterite is tetragonal with a: c = 1 : 0.672. Crystals are usually short
erite and only slightly brigher than gangue minerals. Anisotropy is
prisi?s [001] with {110} and {100} prominent (Fig. 3.11). Faces in
distinct in greys but internal reflections, which are common and colour-
[ 101] and [ 001] are often striated. Twinning on { 011} is very common
less to brown, often mask the anisotropy.
Cassiterite occurs as isolated prismatic to rounded crystals, geniculate
Table 3.2 Spinels.
(knee-like) twins or granular aggregates. Colloform aggregates contain-
Spinel Composition Structural Colour/opacity ing colloidal hematite are known as 'wood tin' . Twinning is common and
type cleavage traces are often present. Zonation of iron content may be seen
in crossed polars because Fe absorbs the light in the internal reflections.
spinel MgAI,0 4 normal n = 1.719, colourless or red, VHN = 1027-1075.
blue brown etc. Occurrence Cassiterite is mainly found with wolframite, tourmaline, topaz,
hercynite FeAI,O. normal n = 1.835, dark green to black
arsenopyrite, molybdenite, pyrrhotite and bismuthian minerals in high
magnetite Fe,o. inverse opaque
temperature hydrothermal veins, pegmatites, greisens, stockworks and
maghemite Fe,O, inverse opaque (metastable with respect
disseminations associated with acid igneous rocks. It is found as a
to hematite)
ulvospinel Fe,TiO. inverse opaque detrital heavy mineral in sediments (such as the commercial placer
chromite FeCr,O. normal opaque, dark brown on edges deposits of Malaysia) and in gossans over stanniferous sulphide
deposits. Wood tin is found in the secondary oxidation zone.
Two common intermediate varietes are:
Distinguishing Compared with cassiterite, sphalerite is brighter, isotropic and softer;
normal green to blue green
pleonaste (Mg,Fe)AI 2 0 4
features wolframite is slightly brighter and has fewer internal reflections; and
picotite (Fe,Mg)(Al,Cr),O.
rutile is brighter.
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