Page 226 - A Practical Introduction to Optical Mineralogy

REFLECTED-LIGHT THEORY ISOTROPIC AND ANISOTROPIC SECTIONS
N 5.3.4 Exercise on rotation after reflection
analyser
This exercise demonstrates the rotation of polarised light on reflection
from an anisotropic grain, e.g. ilmenite, hematite (Fig. 5.8).

(1) Select a single optically homogeneous grain which exhibits distinct
bireflectance and distinct anisotropy. Sketch the grain, positioned
A in one of the four extinction positions (found using exactly crossed
polars), and indicate the reflectance values Rmax and Rmin of the

trace of crystallographic
axis

s
Figure 5. 7 This figure illustrates the geometry of reflection of normal incident
linearly polarised monochromatic light from a bireflecting (hkl) grain of a uniax-
ial transparent mineral turned to 45" from extinction. The incident light vibrating
in the plane of the polariser (E-W) is resolved, on the polished surface, along the grain in 45° orientation.
grain in brightest
two principal axes Rmax and Rmin corresponding to nmax and nmln· This results in position (PPL) and Polarisation colour seen
rotation of the plane of polarisation (i.e. the azimuth of vibration) through the extinction (x-polars). in x-polars.
angle A r so that the reflected light is linearly polarised but vibrating parallel to Reflected-light vibrates
OA. E-W.
the combination of two components of different magnitude and differ-
ent phase. It is the difference in absorption which 'slows down' one
component relative to the other and gives a phase difference. Some light
will now pass through the analyser because of ellipticity as well as
rotation. Dispersion of the degree of ellipticity contributes to colour
effects.
So far only uniaxial minerals have been considered. The theory of
reflection as outlined above only applies to lower symmetry minerals for
sections normal to a symmetry plane; only these sections will contain
two principal vibration directions that reflect linearly polarised light.
analyser rotated anti-
General sections through low symmetry minerals reflect elliptically
clockwise through A r
polarised light even from the section's principal vibration directions. degrees to restore
Because of the possible differing crystallographic orientation of the extinction in x-polars
and proving that reflected
three refractive indices and the three absorption coefficients of low
light now vibrates parallel
symmetry absorbing minerals, and the dispersion of their orientation,
toOA.
the concept of optic axes and isotropic sections of biaxial transparent
minerals does not have a simple analogy in the case of absorbing Figure 5.8 Exercise to demonstrate the rotation of polarised light on reflection from an anisotropic
minerals (see Galopin & Henry 1972, p. 88). grain, e.g. ilmenite, hematite.
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