OPTICAL PROPERTIES OF BIAXIAL MINERALS

Pleochroism

To completely describe the pleochroism of biaxial minerals it is necessary to specify 3 colours, each of which corresponds to the light vibrating parallel to one indicatrix axis.

For example in hornblende the pleochroism may be described as:

The steps to be followed for determining the pleochroic scheme for biaxial minerals are outlined on p. 101 of Nesse. To record the pleochroic scheme for a biaxial mineral requires that you obtain two centred interference figures, e.g. a Bxa and an Optic Normal, and determine the vibration directions in each and then the associated colour for each indicatrix axis.


Extinction

The type of extinction (parallel, inclined symmetrical) observed for biaxial minerals is a function of cleavage and crystal habit for the minerals.

We will examine the types of extinction for various minerals when discussing one presenting characteristics later.


Sign of Elongation

The sign of elongation is dependent on which indicatrix axis is closer to the long dimension of the elongate mineral, grain or fragment.

  1. if the X indicatrix axis is parallel to the length - the mineral is length fast
  2. if the Z indicatrix axis is parallel to the length - the mineral is length slow
  3. if the Y indicatrix axis is parallel to the length, the mineral is either length fast or length slow depending on whether X or Z is in the plane of the section with Y.


Indices of Refraction

To determine the indices of refraction for biaxial minerals you must measure three different indices (nalpha, nbeta, ngamma) to fully describe a biaxial minerals.

This is done using grain mounts, and a procedure similar to that used for isotropic and uniaxial minerals, but is very time consuming.

It is not possible to measure the indices of refraction in a thin section, but comparison of the unknown mineral to other known minerals, whose incices are known, will provide an estimate of the relative indices for the unknown.


Dispersion in Biaxial Minerals

The refractve indices, nalpha, nbeta, and ngamma, for biaxial minerals vary for different wavelengths of light. As a result the value for the 2V angle and the orientation of the indicatrix for a given mineral will vary with the wavelength of light.

The variation in the size of the 2V angle is called optic axis dispersion.

The variation in the orientation of the indicatrix is referred to as indicatrix or bisectrix dispersion.

Dispersion is visible as colour fringes developed along the isogyres of the interference figure. Depending on the intensity of the colour fringes, the dispersion is weak, moderate or strong.

Read section on Dispersion in Orthorhombic, Monoclinic and Triclinic Minerals, p. 106.


We know want to examine the optical properties for the major minerals or groups of minerals which you will be looking at in the remainder of the labs. The minerals are:

Olivine Orthopyroxene Clinopyroxene
Hornblende Tremolite-Actinolite Plagioclase
Biotite Muscovite Chlorite
Microcline Orthoclase Sanidine