Identification Tables for Common Minerals in Thin Section

John Faithfull 1998
Identification Tables for Common Minerals in Thin Section
These tables provide a concise summary of the properties of a range of common minerals. Within the tables, minerals are arranged by colour so as to
help with identification. If a mineral commonly has a range of colours, it will appear once for each colour.


To identify an unknown mineral, start by answering the following questions:


(1) What colour is the mineral?
(2) What is the relief of the mineral?
(3) Do you think you are looking at an igneous, metamorphic or sedimentary rock?


Go to the chart, and scan the properties. Within each colour group, minerals are arranged in order of increasing refractive index (which more or less
corresponds to relief). This should at once limit you to only a few minerals. By looking at the chart, see which properties might help you distinguish
between the possibilities. Then, look at the mineral again, and check these further details.


Notes:
(i) Name: names listed here may be strict mineral names (e.g., andalusite), or group names (e.g., chlorite), or distinctive variety names (e.g., titanian augite). These tables contain a
personal selection of some of the more common minerals. Remember that there are nearly 4000 minerals, although 95% of these are rare or very rare. The minerals in here probably
make up 95% of medium and coarse-grained rocks in the crust.


(ii) IMS: this gives a simple assessment of whether the mineral is common in igneous (I), metamorphic (M) or sedimentary (S) rocks. These are not infallible guides - in particular
many igneous and metamorphic minerals can occur occasionally in sediments. Bear this in mind, even if minerals are not marked as being common in sediments.


(iii) Colour in TS etc: the range of colours for each mineral is given, together with a description of any pleochroism. Note that these are colours seen in thin-section, not handspecimen.
The latter will always be much darker and more intense than thin section colours.


(iv) RI: the total range of refractive index shown by the mineral with this coulour is shown: This covers any range due to compositional variation by solid solution, as well as the two
or three refractive indices of anisotropic minerals.


(v) Relief : is described verbally, followed by a sign indicating whether the relief is positive or negative (ie greater or less than the mounting medium of the thin-section - 1.54).
Minerals with refractive indices close to 1.54 have low relief, those with much higher or lower refractive indexes will have high relief.


(vi) Extinction: angles are only given where minerals usually show a linear feature such as a cleavage and/or long crystal faces. For plagioclase feldspars (stippled) the extinction
angles given are those determined by the Michel-Levy method (see a textbook for details).


(vi) Int. Figure: this gives details of the interference figure. Any numbers given refer to the value of 2V (normally a range is given), followed by the optic sign. For uniaxial minerals
the word "Uni" is given, followed by the sign. Your course may or may not have covered interference figures. If not, ignore this section!


(vii) Birefr: Birefringence is described verbally, In some cases the maximum is given as a colour, in other cases you will need to cross-refer to an interference colour chart.


(viii) Twinning etc.: a few notes about twinning, or other internal features of crystals may be given. If no twinning is mentioned, then the phenomenon is not common in thin section,
but this does not mean that it NEVER occurs.


(ix) Notes: general tips on appearance, occurrence and distinguishing features. May include indication of whether the mineral is length fast or slow - again a feature not covered in all
courses - but a useful and easily-determined property.


By:ESA