Red mudstone is typically altered to indurated, bluish-gray hornfels with clots or crystals of tourmaline or cordierite.
cordierite and anthophyllite in footwall of metamorphosed deposits, graphitic schist in hanging wall.
British Dictionary definitions for cordierite
a grey or violet-blue dichroic mineral that consists of magnesium aluminium iron silicate in orthorhombic crystalline form and is found in metamorphic rocks. Formula: (Mg,Fe)2AL4Si5O18.nH2O Also called dichroite, iolite
C19: named after Pierre L. A. Cordier (1777–1861), French geologist who described it
(kôr'dē-ə-rīt') A light-blue to dark-blue or gray orthorhombic mineral. Cordierite is a silicate of magnesium, aluminum, and sometimes iron, and is found in granites and in metamorphic rocks that form under relatively low-pressure conditions. Chemical formula: (Mg,Fe)2Al4Si5O18.