Optical Mineralogy Properties

Cleavage

A series of fine, parallel, dark or bright lines

Pleochroism

When a mineral changes colour or hue as the microscopic stage is rotated

Relief

How sharply a mineral grain stands out against its surrounding medium or adjacent minerals when viewed in PPL

The greater the difference between the refractive index of a mineral and its surrounding material the greater its relief

Protolith

The original parent rock from which a metamorphic rock is formed

e.g. the protolith of marble is limestone

Extinction angles

The position during stage rotation where an anisotropic mineral goes completely dark

Twinning

A structural intergrowth where different parts of the same crystal reflect light at different orientations

Zoning

Changes in a crystal between its core and its outer rim

Interference Colours

The distinct colours visible in XPL produced by the phase shift (retardation) between the split light rays passing through the crystal

Isotropic vs. Anisotropic Behaviour

Isotropic minerals rotate light uniformly, causing them to block all light and remain completely black when rotated between crossed polars.

Anisotropic minerals split light into two rays, allowing light to pass through.

Crystal faces

Euhedral - well-formed faces

Anhedral - irregular shapes

Subhedral - show some faces but others are flat or irregular