Twinning contact plane

Cassiterite (tetragonal). Knee-twin . Contact type Twin plane is (Oil)... 

Twins are intergrown crystals such that the crystallographic directions in one part are related to those in another part by reflection, rotation, or inversion through a center of symmetry across a twin boundary. Twinned crystals are often prized mineralogical specimens. When twins are in contact across a well-defined plane (which is not always so), the boundary is generally called the composition plane. The only twins that are considered here will be reflection twins, where the two related parts of the crystal are mirror images (Fig. 3.22). The mirror plane that relates the two components is called the twin plane. This is frequently, but not always, identical to the plane along which the two mirror-related parts of the crystal join, that is, the composition plane. Repeated parallel composition planes make up a polysynthetic twin (Fig. 3.23). 

If two individuals conjugate on an r or a z face, a nucleus of a Japanese twin with 1122 as the composition plane is formed. This indicates that a Japanese twin is not a penetration twin (i.e. not the upper half of an X-shape), but a contact twin (i.e. the upper half of a Y-shape). Horizontal banding in geode agate (see Section 10.9) appears through grain size variation due to gravitational sedimentation, and con-... 

A detailed treatment of the plan view images of the twin orientation relationship as well as those involving contacts with 001 support planes can be found in (194,391). 

Occasionally, annealing twins appear under the microscope as in Fig. 2-23(a), with one part of a grain (B) twinned with respect to the other part (.4). The two parts are in contact on the composition plane (111) which makes a straight-line trace on the plane of polish. More common, however, is the kind shown in Fig. 2-23(b). The grain shown consists of three parts two parts (Ai and A 2) of identical orientation separated by a third part (B) which is twinned with respect to Ai and A2. B is known as a twin band. 

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