Page 18 - A Practical Introduction to Optical Mineralogy

THE MICROSCOPIC STUDY OF MINERALS SYSTEMATIC DESCRIPTION OF MINERALS
Pleochroism is due to the unequal absorption of light by the mineral in made up of tiny elevations and depressions which reflect and refract
different orientations. For example, in a longitudinal section of biotite, the light.
when p!ane polarised light from the polariser enters the mineral which If the Rls of the minerai- and resin are similar the surface appears
has its cleavages parallel to the vibration direction of the light, consider- smooth. Thus, for example, the surfaces of garnet and olivine which
able absorption of light occurs and the biotite appears dark brown. If the have much higher Rls than the resin appear rough whereas the surface
mineral section is then rotated through 90° so that the plane polarised of quartz, which has the same RI as the resin (1.54) is smooth and
light from the polariser enters the mineral with its cleavages now at right virtually impossible to detect.
angles to the vibration direction, much less absorption of light occurs To obtain a more accurate estimate of the RI of a mineral (compared
and the biotite appears pale yellow. to 1.54) a mineral grain should be found at the edge of the thin section,
where its edge is against the cement. The diaphragm of the microscope
Habit should be adjusted until the edge of the mineral is clearly defined by a
This refers to the shape that a particular mineral exhibits in different thin, bright band of light which is exactly parallel to the mineral bound-
rock types. A mineral may appear euhedral, with well defined crystal ary. The microscope tube is then carefully racked upwards (or the stage
faces, or anhedral, where the crystal has no crystal faces present, such as lowered), and this thin band of light - the Becke line - will appear to
when it crystallises into gaps left between crystals formed earlier. Other move towards the medium with the higher RI. For example, if Rlmineral is
descriptive terms include prismatic, when the crystal is elongate in one greater than Ricement the Becke line will appear to move into the mineral
direction, or acicular, when the crystal is needle like, or fibrous, when when the microscope tube is slowly racked upwards. If the RI of a
the crystals resemble fibres. Flat, thin crystals are termed tabular or mineral is close to that of the cement then the mineral surface will
platy. appear smooth and dispersion of the refractive index may result in
slightly coloured Becke lines appearing in both media. The greater
Cleavage the difference between a mineral's RI and that of the enclosing cement,
Most minerals can be cleaved along certain specific crystallographic the rougher the surface of the mineral appears. An arbitrary scheme
directions which are related to planes of weakness in the mineral's used in the section of mineral descriptions is as follows:
atomic structure. These planes or cleavages which are straight, parallel
and evenly spaced in the mineral are denoted by Miller's indices, which RI Description of relief
indicate their crystallographic orientation. Some minerals such as quartz
and garnet possess no cleavages, whereas others may have one, two, 1.40- 1.50 moderate
three or four cleavages. When a cleavage is poorly developed it is called 1.50-1.58 low
a parting. Partings are usually straight and parallel but not evenly 1.58-1.67 moderate
1.67-1.76 high
spaced. The number of cleavages seen depends upon the orientation of
> 1.76 very high
the mineral section. Thus, for example, a prismatic mineral with a square
cross section may have two prismatic cleavages. These cleavages are
seen to intersect in a mineral section cut at right angles to the prism zone, The refractive indices of adjacent minerals in the thin section may be
but in a section cut parallel to the prism zone the traces of the two compared using the Becke line as explained.
cleavages are parallel to each other and the mineral appears to possess
only one cleavage (e.g. pyroxenes, andalusite). Alteration
The most common cause of alteration is by water or C0 2 coming into
Relief contact with a mineral, chemically reacting with some of its elements,
All rock thin sections are trapped between two thin layers of resin (or and producing a new, stable mineral phase( s). For example, water reacts
cementing material) to which the glass slide and the cover slip are with the feldspars and produces clay minerals. In thin section this
attached. The refractive index (RI) of the resin is 1.54. The surface relief alteration appears as an area of cloudiness within the transparent feld-
of a mineral is essentially constant (except for carbonate minerals), and spar grain. The alteration may be so advanced that the mineral is
depends on the difference between the RI of the mineral and the RI of completely replaced by a new mineral phase. For example, crystals of
the enclosing resin. The greater the difference between the RI of the olivine may have completely altered to serpentine, but the area occupied
mineral and the resin, the rougher the appearance of the surface of the by the serpentine still has the configuration of the original olivine crystal.
mineral. This is because the surfaces of the mineral in thin section are The olivine is said to be pseudomorphed by serpentine.
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