Figure F52. Photomicrographs (both in cross-polarized light) depicting an apparent alkali feldspar phenocryst from Unit 10 in Sample 183-1139A-65R-3, 16-18 cm. A. A large feldspar phenocryst, displaying only Carlsbad twinning, that is altered along fractures. Box A is the area shown in part B of this figure. Box B highlights another feldspar crystal in the groundmass that also only displays Carlsbad twinning, but the twin plane is more diffuse. This is interpreted to represent secondary alteration of plagioclase to alkali feldspar. B. A close-up of the base of the phenocryst in (A) showing zonation that has inhibited the development of characteristic albite twinning.