Twinning

 Most minerals occur as random aggregates of grains in the rocks of the Earth . These grains are generally anhedral (lacking external faces) , but being crystalline , they posses an internal order evidenced by cleavage , optical properties , and X-ray diffraction . 

There are , however , some relatively common intergrowth patterns of well-formed crystals (as well as anhedral grains) that are not random in nature . Such as parallel growths of the same crystalline substance , and crystallographically controlled (nonrandom) intergrowths of two or more crystals of the same substance that are related by a symmetry element (that is not normally present in either individual crystal . Such crystallographically controlled intergrowths are called twins or twinned crystals .

An aggregate of similar crystals with their crystallographic axes and faces parallel is called a parallel growth . Such aggregates , although they may at first appear to represent several crystals , are a single crystal because the internal (atomic) structure remains unchanged in orientation throughout the specimen . 

A twin is a symmetrical intergrowth of two (or more) crystals of the same substance . Such crystallographic intergrowths are called twinned crystals . The two or more individuals of the twinned aggregate are related by a symmetry element that is absent in the original (untwinned) crystal . Twinning is common in crystals , and the size of the twinned units can range from an almost atomic scale (with twin lamellae or twin domains on the order of tens to hundreds of angstroms in size) to such a large scale that the individuals are easily seen by the naked eye . Generally the twins that are easiest to recognise in hand specimen are contact twins or penetration twins . Although the twinned relationship in such symmetric intergrowths is easily recognised , the twin law that underlines the twinned relationship may not be so obvious .

A twin relationship that is more subtle in its appearance is polysynthetic twinning . In a polysynthetic twin the successive composition planes of the twins are parallel to each other . When a large number of individuals in a polysynthetic twin are closely spaced , crystal faces or cleavage surfaces cutting across the composition planes show striations owing to the reversed positions of adjacent individuals . A highly diagnostic polysynthetic twin is albite twinning in the plagioclase feldspar series . The individual twin lamellae that can be seen by the naked eye are commonly quite thin , ranging from 0.1 to several mm in thickness . This twin is evidenced by parallel lines or striations seen on cleavage directions . 

Striations, however , as seen on crystal faces , are by no means , always the result of polysynthetic twinning . Pyrite cubes typically show striations that are the result of successive combinations of other faces or of another form (called a pyritohedron) in narrow lines with the cube .