Roger Weller, geology instructor
email@example.com copyright 2010-R.Weller
albite showing plagioclase twinning-2
If this piece of albite is turned to just right to the correct angle of a light source,
light is reflected off of a series of narrow strips on a cleavage surface. This
phenomenon, which is common for plagioclase feldspars, is called "plagioclase
twinning" or "polysynthetic twinning". The cleavage surface consists of two
slightly different crystal orientations which formed as the plagioclase cooled.
Light is being reflected off one set of microcleavage surfaces of one of the
crystalline orientations. If the crystal is tipped again slightly with respect to the
light source, the other set of reflecting cleavage surfaces will reflect light.
The reflections are very sharp lines.
Photo is copyright free only for non-commercial educational uses.
Just credit photo to R.Weller/Cochise College.