Plagioclase feldspar (pronounced pla-geo-klays feld-spar)

Plagioclase feldspar is a common member of the the feldspar group of minerals. It contains both sodium (Na) and calcium (Ca) ions that freely substitute for one another in the silicate crystal structure during crystalization.

The color of plagoclase ranges from white, to gray, to an iridescent purple in a variety called labradorite. A good way to distinguish between the feldspars is to look for striations. Striations are found on many clevage planes of Plagioclase feldspar, but are not present on potassium feldspar.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu