A New Study of Twinning in Neptunite.

Penetration Twins Penetration twins of the sort seen in neptunite are composed of two individuals that are identical (ideally) and cross at an angle that is determined by the twin law and described by the Miller indices of the twin law. Comparison with figure 11 shows that the correct twin plane is (10 Graph HT <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mover accent="true"><mn>9</mn><mo stretchy="true"></mo></mover></mrow></math> ht ), not (90 Graph HT <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mover accent="true"><mn>2</mn><mo stretchy="true"></mo></mover></mrow></math> ht ). One of these planes is the true twin plane - it has the same Miller indices as the twin law; the other one, at right angles, has different and in most cases irrational indices and is referred to as a contact plane but not a twin plane. The crystallographic "law" of simple rational indices states that major planes on crystals, including both faces and twin planes, typically have Miller indices involving small integer values (usually 5 or less). [Extracted from the article]