guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Optical properties: basic principles
4.1.2 Isotropy and anisotropy
Isotropic materials
Optically isotropic substances such as gases, liquids, glasses and the highly symmetric cubic
minerals show optical behaviour that is independent of the direction of light propagation.
This means, their optical properties (light velocity, refractive index and colour) are identical
in all directions.
The three-dimensional propagation of light in an isotropic material can be presented
graphically as 3-D models for wave or ray velocity and for refractive index, both of which are
spheres (Fig. 4-3). The model preferred by mineralogists is the optical indicatrix, which
describes the refractive index as a direction-dependent variable.
Figure 4-3. Isotropic substances; 3D surfaces of v and n
Raith, Raase & Reinhardt – February 2012
A. Ray velocity surface: For each ray direction, the velocity value is represented by a specific distance
from a chosen origin (v = 0). The geometric form representing all directions of ray propagation is a
sphere with radius v.
B. Indicatrix: For each ray direction, the refractive index is represented by a specific distance from a
chosen origin and is marked off parallel to the vibration direction and perpendicular to the ray
propagation direction. The geometric form representing the refractive index for all ray propagation
directions is a sphere with radius n. Each ray propagation direction has an infinite number of potential
vibration directions.
In thin section microscopy, glasses and cubic minerals generally show a single specific
refractive index and colour independent of orientation (Fig. 4-4). An indication of crystal
orientation in thin section can therefore only be derived from morphological properties, for
example, crystal outlines or cleavage (Fig. 3-9). Another important characteristic of optically
isotropic materials is that light waves do not experience any change in vibration direction.
This means that E-W vibrating plane-polarized light waves maintain their E-W orientation
after passing through the isotropic materials (glass, mineral). Therefore, they are blocked by
the analyzer, which is a N-S oriented polarizer (Fig. 4-4 A,E).
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