In
geology (and
mineralogy in particular), lustre is defined as being the ability of a
mineral to
reflect light.
The nature, intensity and quality of this light varies greatly with mineral structure and therefore makes a useful diagnostic tool.
When describing the lustre of a mineral, the following qualifiers can be used:
- Adamantine lustre
A brilliant, sparkling lustre occuring in minerals which are transparent and have a high refractive index.
Minerals which exhibit adamantine lustre: Diamond, Cerussite.
- Silky lustre
A lustre with a silk-like appearance.
This is due to the reflection of light from a fine aggregate of parallel fibres, not unlike the sheen present on real silk whereby light is reflected from the fine weave of the fabric.
Minerals which exhibit silky lustre: Satin spar.
- Pearly lustre
An iridescent, pearl-like lustre.
This is the result of the reflection of light from a series of parallel surfaces, usually the cleavage surfaces of a sheet silicate.
Minerals which exhibit pearly lustre: Talc, Gypsum.
- Greasy lustre
A lustre in which a mineral appears to have an oily coating.
This greasy sheen is caused by the scatter of light on a microscopically fine surface.
Minerals which exhibt a greasy lustre: Antigorite, Gangue.
- Metallic lustre
A lustre similar in appearance to polished steel.
This trait is seen in some opaque minerals, as well as the obvious group of native metals, e.g. Copper, Chalcopyrite.
- Resinous lustre
A lustre like that of a resin. Typically, this means a mineral with a colour ranging from translucent yellowish to brown.
Minerals which exhibit resinous lustre: Sulphur, Sphalerite.
Further qualification of lustre is provided by using the terms
shiny,
dull or
splendant to describe the intensity.
For example,
talc can be described as having a
pearly to greasy, shiny lustre.