IDIOMORPHIC AND XENOMORPHIC CRYSTALS
Minerals which have been allowed to develop their own crystalline shape are called euhedral or idiomorphic. Those minerals which crystallised in the spaces between earlier formations and are bounded not by their own but by pre-existing surfaces, are said to be anhedral or allotriomorphic. Yet others which have replaced a pre-existing crystal and thus taken on its shape, are termed xenomorphic. Completely euhedral crystal forms are not common, as most crystals grew up from a base and so are only partially idiomorphic (Plate 6).