Download or here in RTF format

Looking at the mineralogy of
an unknown thin-section
(1) Examine by holding up to light. Look for large-scale banding, compositional
variation, xenoliths, clasts, or other structures which may not be so obvious down the
microscope. For many coarse-grained rocks, this is the easiest way to estimate
average grain-size.
(2) If large grains of opaque minerals are present, examine these in reflected light,
using a hand lens if necessary. Note whether these are black (probably Fe-Ti-Cr
oxides), or greyish (probably pyrite or other sulphides).
(3) Examine the thin section under the microscope using the low-power lens. Look
rapidly at as much of the section as you can - taking special care to cover any different
areas identified in stage (1) above. Adjust the diaphragm to get the best illumination
you can. Switch back and forth between PPL and cross-polarized light. You may
decide to describe the rock as a whole, or to separately describe markedly different
parts of the section. If the rock is very fine-grained you may need to use a higher power
objective lens.
Describe the overall texture of the specimen, and try to decide if it's an igneous,
metamorphic or sedimentary rock.
eg "a foliated metamorphic rock with 2-3mm porphyroblasts and tight small-scale
folds"
(4) Decide how many different phases are present, and estimate the volume % of
each using the standard views on your grain-size chart. Note that a phase may be a
distinct mineral, or it may be any distinctive substance present eg glassy, or ultra-fine
grained materials. Remember that many minerals will show a range of properties
depending on their crystal orientation in the section.
(5) For each mineral, systematically record as many properties as you can. Often, a
drawing will help explanation and description, esp. of textural features. You may be
able to determine some or all of the following:
A. % abundance in the section.
B. Grain shape and size, orientation, etc., and textural relationship to other
phases present. In metamorphic rocks look for evidence about whether crystals
grew before or after structures such as folds or foliation.
C. Relief in PPL. Note the relief relative to the mounting medium at the edge of
the slide, but it may also be useful to know relief relative to other minerals,
particularly if you know what these are eg "relief high, but lower than garnet"
D. Colour in PPL. noting any pleochroism, and its orientation (eg "pleochroic
from pale yellowish-brown to deep brown; long crystals darkest brown when
oriented N-S etc). Note whether grains are clear or turbid, or rich in inclusions .
E. The presence, shape and orientation of any cracks, if present. These may be
cleavages, and may show a clear relationship to the shape of crystals, or to one
another - show this on a sketch.
F. Under crossed polars find the maximum birefringence. It's probably easiest
to use terms like isotropic (black eg garnet); low (greys and whites eg quartz),
moderate (up to first order purple eg sillimanite), high (second and third order
bright colours eg clinopyroxene, olivine, muscovite), and extreme (washed out
buff-pink colours eg carbonates). Note the orientation of grains showing the
maximum and minimum colours if appropriate. Note any anomalous
birefringence colours.
G. Look for any twinning picked out by different birefringence or extinction
position. If possible, note its orientation relative to crystal or grain shape.
H. Look for any zoning indicated by differences in colour, birefringence or
extinction between the cores and margins of grains. It may be simple, or
osscilatory.
I. If there are long crystals, or linear cleavage or twin planes, note the extinction
angle relative to the feature. Record any irregular extinction which may indicate
deformation or lattice strain.
J. Note any evidence of reaction between minerals in the section, or alteration.
If you can't identify fine-grained, cruddy alteration, note its colour, and whether
cores or margins of grains are preferentially attacked, or whether alteration is
patchy, or along cracks, or veins etc.
K. Any associated minerals you can definitely identify. Knowing common
associations of minerals is a very useful identification tool.
L. Any other odd or notable features.
M. On the basis of the properties measured, and anything you know about the
rock type, identify the mineral. Identification tables for common minerals can be
downloaded and printed from
http://www.hmag.gla.ac.uk/john/teaching/mintable.rtf
This may seem a lot to do, but with practice, you should only need to spend a few
seconds to a minute or two determining each property.
NOTE : This list of properties to record is not exhaustive. There are many other useful
properties not covered in this course which can greatly help identification, including
relative relief, and refractive index determined using the Becke Line, interference
figures and optic sign, and the orientation of fast and slow vibration directions. If you
like microscopy, why not get a good optical mineralogy book and try to learn some of
these for yourself?
John Faithfull October 2004