Page 33 - A Practical Introduction to Optical Mineralogy

THE MICROSCOPIC STUDY OF MINERALS SYSTEMATIC DESCRIPTION OF MINERALS

Internal reflections
R(%) Grey scale Light may pass through the polished surface of a mineral and be
reflected back from below. Internal reflections are therefore shown by
0-10 dark grey
10-20 grey all transparent minerals. When one is looking for internal reflections,
particular care should be paid to minerals of low to moderate reflectance
20-40 light grey
40-60 white (semi-opaque minerals), for which internal reflections might only be
60-100 bright ~hite detected with difficulty and only near grain boundaries or fractures.
Cinnabar, unlike hematite which is otherwise similar, shows spectacular
red internal reflections.
Bireflectance
This is a quantitative value, and for an anisotropic grain is a measure of 1. 6.3 The external nature of grains
the difference between the maximum and minimum reflectance values.
However, bireflectance is usually assessed qualitatively, e.g. Minerals have their grain shapes determined by complex variables act-
ing during deposition and crystallisation and subsequent recrystallisa-
Weak bireflectance: observed with difficulty, t!.R. < 5% (e.g. hematite) tion, replacement or alteration. Idiomorphic (a term used by reflected-
Distinct bireflectance: easily observed, t!.R. > 5% (e.g. stibnite) light microscopists for well shaped or euhedral) grains are unusual, but
some minerals in a polished section will be found to have a greater
tendency towards a regular grain shape than others. In the ore mineral
Pleochroism and bireflectance are closely related properties; the term
descriptions in Chapter 3, the information given under the heading
pleochroism is used to describe change in tint or colour intensity,
'crystals' is intended to be an aid to recognising minerals on the basis of
whereas bireflectance is used for a change in brightness.
grain shape. Textural relationships are sometimes also given.
1.6.2 Properties observed using crossed polars 1.6.4 Internal properties of grains
The analyser is inserted into the optical path to give a dark image. Twinning
This is best observed using crossed polars, and is recognised when areas
with differing extinction orientations have planar contacts within a
Anisotropy single grain. Cassiterite is commonly twinned.
This property varies markedly with crystallographic orientation of a
section of a non-cubic mineral. Anisotropy is assessed as follows: Cleavage
This is more difficult to observe in reflected light than transmitted light,
(a) Isotropic mineral: all grains remain dark on rotation of the stage, and is usually indicated by discontinuous alignments of regularly shaped
e.g. magnetite. or rounded pits. Galena is characterised by its triangular cleavage pits.
(b) Weakly anisotropic mineral: slight change on rotation, only seen Scratches sometimes resemble cleavage traces. Further information on
on careful examination using slightly uncrossed polars, e.g. twinning and cleavage is given under the heading of 'crystals' in the
ilmenite. descriptions in Chapter 3.
(c) Strongly anisotropic mineral: pronounced change in brightness
and possible colour seen on rotating the stage when using exactly Zoning
crossed polars, e.g. hematite. Compositional zoning of chemically complex minerals such as tetrahed-
rite is probably very common but rarely gives observable effects such as
Remember that some cubic minerals (e.g. pyrite) can appear to be colour banding. Zoning of micro-inclusions is more common.
anisotropic, and weakly anisotropic minerals (e.g. chalcopyrite) may
appear to be isotropic. Anisotropy and bireflectance are related proper- Inclusions
ties; an anistropic grain is necessarily bireflecting, but the bireflectance The identity and nature of inclusions commonly observed in the mineral
in PPL is always much more difficult to detect than the anisotropy in i given, as this knowledge can be an aid to identification. Pyrrhotite, for
crossed polars. example, often contains lamellar inclusions of pentlandite.
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