Feldspar Photos  
Cochise College               
Photos of Minerals
Geology Home Page

Roger Weller, geology instructor

[email protected]     copyright 2016-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.