A ternary solvus involving the orthorhombic and monoclinic forms of this mineral group is used to determine the temperature of formation of high-grade metamorphic rocks. The most important members of this mineral group for that purpose are pigeonite, augite, and enstatite. These minerals are mostly of the form X, Y, Si-2, O-6, where X is sodium or calcium and Y is magnesium, iron, or aluminum. They are single chain inosilicates, as opposed to the hydroxyl-containing, double chain inosilicates of the amphibole group. Peridotite, the dominant rock in the mantle, contains a high proportion of the "ortho" and "clino" forms of this mineral group. For 10 points, name this mineral group that, with olivine, constitutes most of the mantle and is als
A ternary solvus involving the orthorhombic and monoclinic forms of this mineral group is used to determine the temperature of formation of high-grade metamorphic rocks. The most important members of this mineral group for that purpose are pigeonite, augite, and enstatite. These minerals are mostly of the form X, Y, Si-2, O-6, where X is sodium or calcium and Y is magnesium, iron, or aluminum. They are single chain inosilicates, as opposed to the hydroxyl-containing, double chain inosilicates of the amphibole group. Peridotite, the dominant rock in the mantle, contains a high proportion of the "ortho" and "clino" forms of this mineral group. For 10 points, name this mineral group that, with olivine, constitutes most of the mantle and is also common in basalt and gabbro.