These large crystals are minerals in the feldspar family, namely the alkali feldspars orthoclase and microcline.
British Dictionary definitions for microcline
a white, creamy yellow, red, or green mineral of the feldspar group, found in igneous, sedimentary, and metamorphic rocks: used in the manufacture of glass and ceramics. Composition: potassium aluminium silicate. Formula: KAlSi3O8. Crystal structure: triclinic
C19: from German Mikroklin, from mikro-micro- + Greek klinein to lean; so called because its cleavage plane is slightly different from 90°
(mī'krō-klīn') A white, pink, red-brown, or green type of potassium feldspar. It is dimorphous with orthoclase feldspar, differing from it in shape and in the fact that it forms at lower temperatures.Chemical formula: KAlSi3O8.