Anorthite megacrysts from Tonosawa, Hakone Volcano, Japan, were studied by electron probe microanalyzer, X-ray powder diffractometry, X-ray single-crystal analysis, infrared spectrometry, and inductively coupled plasma analysis. The pyroxene-andesite lava series contain anorthite megacrysts with the composition Ab4An94Ot2 (Ab: albite; An: anorthite; Ot: other minor end-members). Ideal anorthite is CaAl2Si2O8; however, the present colourless and yellow anorthite megacrysts can also host minor amounts of Mn, Zn, and –(CH2)n-. Yellow anorthite is a new variant for feldspar groups. Moreover, the crystal structure establishes excess Si and Al cations as □Si4O8 and AlAl3SiO8 endmembers, respectively. Chromium-fused salites (clinopyroxenes) rich in Al2O3 are observed as inclusions tens of micrometers in size encompassed by yellow anorthite megacrysts. This additional solubility of Al2O3 points to crystallization of the present clinopyroxene under high-pressure conditions. The natural formation and observation of the present anorthites clarifies that metallogenesis related to hydrocarbons maybe contributed to the far-reaching effects of the mixing of basaltic magma with seawater and slab sediments on the crystallization of these megacrysts from the melt under high-pressure conditions.