Twinning
One consequence of the symmetry of the internal structure of crystals is the possible growth of twinned crystals.

A twinned crystal is a single crystal divided into two (or more) parts in which the crystal lattice of one part is

differently oriented with respect to the next one.

Repeat twinning is a prominent feature of many minerals, particularly the plagioclase feldspars. Plagioclase

feldspar crystal shows this common twinning. The crystal is divided up into narrow lamellae with alternate

orientations. The black and white stripes are caused by lamellae of one orientation being in an extinction

position, while lamellae of the second orientation are not.

In the first half of the class, you have seen many twined minerals. Among these include, Orthoclase, Plagioclase

and Staurolite were very common. Under the microscope, you will see different types of twining; albite, carlsbac,

Baveno, and Manebach. Examples of each of these are givin in the pictures below.