Pleochroism

Some minerals are pleochroic. When rotated under plane polarised light they change colour. Biotite and the amphibole hornblende are pleochroic.

Pleochroic haloes are tiny spheres of dark discolouration within crystal grains. They are attributed to the presence of zircons trapped within crystals of other minerals, such as biotite mica. Zircon (Zr[SiO4]) is a common accessory mineral of many igneous rocks. It often crystallises early, and becomes trapped within grains of later-crystallising phases. Zircons contain a high proportion of radioactive elements. When these decay, they damage the crystal structure of the surrounding grain, producing a pleochroic halo.

Crystals which are very thin and sheet-like are described as having a platy form. They are thinner than tabular crystals and less elongate than bladed crystals.

Compare with macrocryst and porphyroblast.

The crystallisation of one mineral on the surface of another, producing a rim or part of a rim.

Sub-spherical sedimentary grains comprising a nucleus surrounded by concentric layers of carbonate precipitate. Ooids form abiogenically. They are sometimes referred to as ooliths.