1971 Volume 66 Issue 6 Pages 237-246
The so called parallel growth and co-axial twin are described on the basis of newly proposed ‘twin point groups’. It is shown that the composition methods of these twins can be consistently classified in accordance with the new description law of twin morphology.
As examples of parallel growth, orthorombic sulfur and co-axial parallel growth between zilcon and xenotime are given. Their symmetries are expressed respectively as tm 2.4/mtm by Hermann-Mauguin's nomenclature. As to the co-axial twin, calcite and quartz are mentioned as examples.
In the case of quartz twins with parallel axes, which consist of both left and right hand quartz, the ‘twinned point groups’ can not rule their symmetries.
An expression, ‘twin joint’ is proposed to describe the domains at the twin boundary and the types of twin joints are classified systematicaly using alphabetical representations.
The essential difference between twins and single crystals is discussed.