Figure F19. Photomicrograph in cross-polarized light of a plagioclase phenocryst completely replaced by fine-grained illite (birefringent mineral) and possible halloysite (gray) (interval 193-1188F-14Z-1, 102-105 cm; width of view = 0.7 mm. Photomicrograph ID# 1188F_80, thin section 80).