POLYMORPHISM, CRYSTALLOGRAPHIC ORIEN- TATION AND AXIAL RATIO OF CORDIERITE-

Cordierite was first defined as a separate mineral in the beginning of the 19th century. After initially being classified as hexagonal, its orthorhombic symmetry was established in 1824. About a century later it was found that a hexagonal modification of cordierite could exist after all: it could be synthesized in the laboratory. In 1955 the only natural occurence of hexagonal cordierite found hitherto was described from a buchite from India. Orthorhombic cordierites appear not in one specific structure: a series of structures is possible, defined by a varying distortion from the hexagonal structure. This structural variation is dependent on a two-step ordering process of (Al, Si), and possibly (Al, Mg). Some confusion prevails as to the crystallographic orientation of cordierite. Both a> b and b> a have been used. It is proposed to use b.> a> c exclusively. Attention is drawn to the fact that two series of axial ratios are currently used, which differ appreciably. It is proposed to use those based on X-ray data