Trigonal Crystal System

A trigonal crystal system is characterized by four axes, where the longer vertical axis (c) is perpendicular to the three axes (a₁, a₂, a₃) in horizontal plane, which the positive half-axis (+a₁, +a₂, +a₃) are intersected at at 120° (trigonal section, 3-fold axis of rotational symmetry in vertical axis c). Mineral examples include e.g., calcite, corundum, quartz, magnetite, siderite, tourmaline and hematite.