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.