Roger Weller, geology instructor
If this piece of labradorite 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 micro cleavage 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
Photo is copyright free for non-commercial educational uses.
Just credit photo to R.Weller/Cochise College.