Amphiboles crystallize into two crystal systems, monoclinic and orthorhombic. In total there are seven crystal systems&colon; triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. In total there are six crystal families&colon; triclinic, monoclinic, orthorhombic, tetragonal, hexagonal, and cubic. Two orthorhombic structures and a monoclinic phase are also known. Numerous examples are known with cubic, tetragonal, rhombohedral, and orthorhombic symmetries. The tetragonal and orthorhombic forms of PbO occur naturally as rare minerals. The orthorhombic and tetragonal phases are most common non-cubic variants. Sodium thiocyanate crystallizes in an orthorhombic cell. Crystals of perovskite appear as cubes, but are "pseudocubic" and crystallize in the orthorhombic system. It occurs as prismatic to acicular transparent yellow to white orthorhombic crystals. Black phosphorus has an orthorhombic crystalline structure (CN 3). It has an orthorhombic crystalline structure with a flaky habit. Cerussite crystallizes in the orthorhombic system and is isomorphous with aragonite. Guanidinium chloride crystallizes in orthorhombic space group "Pbca". Niter is a colorless to white mineral crystallizing in the orthorhombic crystal system.