Some minerals show a kind of twinning that is very characteristic of them, and their presence is used to identify these minerals.
As we already know, twins are a regular growth of two or more crystals from the same mineral which have different orientations. These various orientations of the twin components are apparent because each component will present a different interference colour and/or a different extinction position.
Twins may be composed of two or more individuals. The contact surface between the individuals is flat, and one or more planes may exist.
The geometric relationship between the individuals of a twin crystal can be described in terms of twin axes or twin planes.
The most characteristic twins are those of the feldespars.
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