Page 229 - A Practical Introduction to Optical Mineralogy

REFLECTED-liGHT THEORY
ISOTROPIC AND ANISOTROPIC SECTIONS
two vibration directions. Since the grain is distinctly bireflecting it
Using slightly uncrossed polars the polarisation colours obtained will
should be possible to determine whether Rmax lies E-W (grain at its
brightest in PPL) or N-S (grain at its darkest in PPL). usually be sufficiently characteristic of the mineral to be useful in
identification. A mineral showing four good extinction positions at 90°
(2) Rotate the grain exactly 45° from extinction so that Rmax is directed
NE-SW. Sketch the grain, again showing Rmax and Rm 1 n, using a and the same tint 45° either side of an extinction position is probably
longer line for Rmax to signify the greater percentage of light uniaxial. If most sections show poor extinction and colours cannot be
reflected. balanced about the 'best' extinction position the mineral is probably of
lower symmetry.
(3) On your sketch, complete the rectangle to show the approximate
vibration direction of the reflected light (OA in the figure). The eye is best trained in the study of anisotropy by examining
(4) To prove that the light is in fact vibrating in this direction, push in polished minerals of varying anisotropy and comparing observations
the analyser and slowly rotate it a few degrees counter-clockwise with those given in standard tables.
(or rotate the polariser clockwise, oo ~ 90°) until a position of
darkness is obtained. This rotation causes the vibration direction Answer to problem in Section 5.2.2
of the analyser to become normal to the vibration direction of OA,
Plot the mineral B on to the diagram using its chromaticity co-ordinates
so resulting in extinction. The 'apparent angle of rotation' Ar
x andy. Draw a straight line from A through B to the spectral locus. All
cannot be measured with sufficient accuracy to be of much use in
three minerals should lie on this line, and they have a dominant
identification using most student microscopes.
wavelength of 486 ± 4 nm. This means that B is bluish in colour and the
(5) Minerals showing strong pleochroism in PPL or vivid polarisation
colours (e.g. covellite) display dispersion of the angle of rotation; hue (shade) of blue is exactly the same as covellite. The distance of a
mineral from A towards the spectral locus indicates the purity (satura-
on rotation of the analyser colours are obtained rather than a
simple position of darkness. This display of colours is explained by tion or depth) of the colour. As sphalerite is essentially colourless and
extinction of some wavelengths of light at a given angle of rotation covellite is distinctly blue, we can say that B will be slightly bluish. The
while others are transmitted to varying degrees. Slight movement Y% values (brightness) approximate toR % for white light. Covellite is
of the analyser changes the distribution of extinguish~d and trans- dark, sphalerite grey and mineral B is slightly brighter than sphalerite.
mitted wavelengths. In a simple example, blue colours result from In summary, mineral B will appear in polished section as a slightly
extinction of red light and vice versa. bluish light grey mineral ; it will be slightly brighter than sphalerite and
(6) The origin of polarisation colours, as seen using exactly crossed the blue colour will be of the same hue as the covellite basal section.
polars, is explained in Section 5.3.3.

5.3.5 Detailed observation of anisotropy
Using exactly crossed polars, the 'strength' of anisotropy may be esti-
mated from the amount of light reaching the eye with the section in the
45° orientation. If a mineral is strongly anisotropic then the anisotropy
will be immediately evident if a group of grains of the mineral are
examined and the stage rotated. The grain showing the strongest aniso-
tropy can then be studied further to obtain additional information. It is
important to ensure that the group of grains does represent one mineral!
The actual 'tints' seen with the polars exactly crossed should be noted.
The vividness of the colours, i.e. how colourful they are, is an indication
of the dispersion of the rotation angle and degree of ellipticity. Two
examples may help in explanation: a bright grey colour represents
strong anisotropy but small dispersion; a dark blue colour represents
weak anisotropy but strong dispersion. ' Distinct' is a useful term to use
because it indicates how easy it is to see the anisotropy.
216
217