Series Mineral Formula Pyralspite Pyrope Almandine Spessartine Mg3Al2Si3O12 Fe3Al2Si3O12 Mn3Al2Si3O12 Ugrandite Uvarovite Grossular Andradite Ca3Cr2Si3O12 Ca3Al2Si3O12 Ca3Fe2Si3O12 The structure of garnets consists of a continuous network of SiO4 tetrahedra bonded to BO6 octahedra at the apices. Sorosilicates Sorosilicates are fairly rare. The most important group of minerals in this class is the epidote group. Group Mineral Formula Epidote Epidote Clinozoisite Allanite Ca2(Fe3+, Al)Al2O(SiO4)(Si2O7)(OH) Ca2Al3O(SiO4)(Si2O7)(OH) X2Y3O(SiO4)(Si2O7)(OH) Single Chain Inosilicates Pyroxenes are an important group of minerals in the single chain inosilicate class. They have the general formula XYZ2O6, where X and Y represent octahedral sites and Z represents the tetrahedral sites occupied by Si4+ or Al3+. The most common pyroxenes can be represented as part of the ternary system whose endmembers are wollastonite (CaSiO3), enstatite (MgSiO3), and ferrosilite (FeSiO3). The structure of pyroxenes consists of chains of tetrahedra and octahedra that run parallel to the c axis. One chain of octahedra is sandwiched between two chains of tetrahedra. This arrangement of chains can be visualized as I-beams. Pyroxenes differ in the direction of their I-beams. When the I-beams all have the same direction, the pyroxene is a clinopyroxene. When the I-beams alternate directions, the pyroxene is a protopyroxene. When two I-beams pointing in the same direction are followed by another two I beams pointing in the opposite direction—and so on—the pyroxene is an orthopyroxene. Direction of I beams + + + + + + clino- + + + proto- + + + + ortho- The arrangement of I-beams in pyroxene accounts for the 90° cleavage. Double Chain Inosilicates Amphiboles are an important group of minerals in the double-chain inosilicate class. They have the general formula W0-1X2Y5Z8O22(OH, F)2. W represents 10- to 12-coordinated sites occupied by Na+ or K+; X represents 6- to 8-coordinated sites occupied by Ca2+, Fe2+, Mg2+, Mn2+, Na+, or Li+; Y represents 6coordinated sites occupied by Fe2+, Fe3+, Mg2+, Mn2+, Al3+, or Ti4+; and Z represents tetrahedral sites occupied by Al3+ or Si4+. The most common amphiboles can be represented as part of the quaternary system whose endmembers are anthophylite (Mg7Si8O22(OH)2), tremolite (Ca2Mg5Si8O22(OH)2), ferroactinolite (Ca2Fe5Si8O22(OH)2), and grunerite (Fe7Si8O22(OH)2). The structure of amphibole is very similar to the structure of pyroxene. It is based on chains of tetrahedra and octahedra that run parallel to the c-axis and can be visualized as I-beams. Just as in the structure of pyroxene, the directionality of the I-beams distinguishes different types of amphibole. Their arrangement accounts for cleavage.