feldspar
feld.spar also feld.spath.ose \'fel(d)-.spa:r\ \fel(d)-'spath-ik\ \'fel(d)-.spath-.o-s\ n [modif. of obs. G feldspath (now feldspat), fr. G feld field]+ obs. G spath (now spat) spar : any of a group of crystalline minerals that consist of aluminum silicate s with either potassium, sodium, calcium, or barium and that are an essential constituent of nearly all crystalline rocks (hardness 6-6.5, sp. gr. 2.5-2.9) - feld.spath.ic aj