Twinning

Twinned Andalusite (left), Diopside (right) and Calcite (right) Both of these are XP views. The left hand view shows andalusite that has been twinned to give a cross (called chaistolite). Twins of this sort, where different crystals seem to grow into each other, termed pentration twins, are rarely seen in thin section. More commonly twins are seen within a single grain. The view on the right has a large diopside grain in the center. Faint stripes can be seen running across the grain. These are twins -- and the stripes show because the different domains in the grain have slightly different optical orientations. The mineral around the diopside is calcite. It, too, is twinned. The twins show as pastel stripes in some places.

Two XP Views Showing Plagioclase The striped mineral in these two XP views is plagioclase. Plagioclase is generally a nondescript clear mineral in PP light. When viewed in crossed polars (XP) it often shows twinning as alternating bands of white, black and gray. Rotating the stage causes the different bands to go extinct (turn black) at different times. The different domains in the grains are all plagioclase, but they have slightly different optical orientations, thus giving rise to the striped appearance. Common plagioclase just shows twinning in one direction (view on left), but sometimes (view on right) two sets of twins are present.

Microcline (left) and Cordierite (right) Twins These two XP views show examples of twinning in microcline (left) and cordierite (right). Microcline often displays two sets of twins, more or less perpendicular to each other. Cordierite twins in a manner similar to plagioclase, but the twins often pinch out as can be seen here.