The amphiboles have a molecular structure made of double chains of silica (SiO4) tetrahedra, surrounded by metal and hydroxyl ions. They form at high temperatures in igneous and metamorphic rocks that contain water. Their dry cousins, the pyroxenes, do not have hydroxyl and consist of single silica chains. Both mineral groups tend to crystallize in long prisms or needles, but amphiboles have a lozenge or diamond shaped cross-section with corner angles of 124 and 56 degrees whereas pyroxenes are nearly square in cross section at 87 and 93 degrees.
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