Glossary (103 total)

Acicular means needle-shaped. The term is usually applied to crystal form.

Minerals displaying a diamond-like lustre are described as adamantine. These minerals are transparent or translucent and have very high refractive indices, producing the high degree of…

Relating to or arising from the action of the wind; aeolian sediments are wind-blown. Also spelt eolian. Have a look at the ferruginous arenite sandstone (L326) in the 1A Reference Series.

See lamellar twinning.

Rocks undergo mineralogical change at low pressures, produced by reactions with circulating fluids. The replacement of the original minerals in the rock tends to take place primarily along…

Amorphous materials are non-crystalline solids. That is to say that they do not display long-range order in their structures. Glass is amorphous. Have a look at the rhyolite glass (L113) in…

A vesicle (necessarily in an igneous rock), which has been filled in by a mineral crystallising from late-stage fluids. Amygdales are commonly produced by the crystallisation of quartz,…

A crystal displaying none of its characteristic crystal faces is called anhedral.

The speed of light through any medium is inversely proportional to the refractive index of the medium. The refractive index of a mineral is determined by the arrangement and type of atoms…

Igneous rocks made up of grains too small to be seen with the naked eye have an aphanitic texture. These rocks do not contain phenocrysts. They are non-porphyritic.