Feldspar Photos  
Cochise College               
Photos of Minerals
Virtual Geology Museum      Hall of Minerals

Geology Home Page

Roger Weller, geology instructor

wellerr@cochise.edu     copyright 2008-R.Weller

 
labradorite-plagioclase twinning
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 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.
 

feldspar
Photo is copyright free for non-commercial educational uses.
Just credit photo to R.Weller/Cochise College.