By this point in the course you are familiar with the following minerals which make up the majority of those present in igneous, metamorphic and sedimentary rocks.
The last half of the text is devoted to the description of the common minerals (200 or ~10% of those known minerals) found in most rocks. However, over 3,000 different minerals have been identified and any one or more of these may be present in a given thin section in amounts from 1-2% to a single grain.
|acicular||needle-like grains (actinolite, tremolite)|
|bladed||elongate, slender (hornblende)|
|columnar||elongate with equidimensional cross sections (qtz, pyroxenes)|
|equant||equidimensional grains (quartz, olivine)|
|fibrous||grains form long slender fibers (asbestos, sillimanite)|
|lathlike||flat elongate grains (plagioclase)|
|prismatic||crystal faces defined by prism (apatite)|
|tabular||book shape (plagioclase)|
|euhedral||grains have well formed crystal faces|
|subhedral||poorly formed and/or irregular crystal faces|
|anhedral||no regular crystal faces|
In grain mounts, straight edges on grains generally indicate the presence of cleavage. In thin section, cleavage is difficult to recognize in minerals with a low relief, under normal circumstances, by closing the aperture diaphragm cleavage is more readily observed.
Grains at the edge of the thin section may exhibit cleavage more so than grains in the centre, as these grains will readily break along the cleavage planes as the thin section is being made.
The angle between cleavages is useful for identifying the broad mineral group to which the unknown belongs, e.g. if the grain exhibits two cleavages that intersect at 56 - 124° then the unknown belongs to the amphibole group.
The type of alteration product observed within an individual grain may be used as an aid in identifying that grain, e.g.:
Knowing the common mineral associations in a variety of rock types may aid in making an "educated guess" as to the identity of an unknown mineral.
Knowing the association for a given rock may aid in the identification of a specific mineral which might otherwise have been overlooked.
On Pages 113-144 of Nesse there is a list of common igneous, metamorphic and sedimentary rocks and the minerals which might be expected to occur in each.