Geology Home Page
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 sharp lines.
Photo is copyright free for non-commercial educational uses.
Just credit photo to R.Weller/Cochise College.