CORDIERITE, a silicate of magnesia, alumina and ferrous oxide (2 [Mg,Fe0]•2A1,03.5Si02) occasionally cut as a gem stone, and named in 1813 by flatly in honour of P. L. Cordier, who dis covered its remarkable pleochroism, or exhibition of varying col ours viewed from different directions. The natural mineral com monly contains o.5%-3% of water, and is sometimes referred to as iolite ciov, violet ; XtOos, stone) in allusion to the violet colour of many specimens, or dichroite in allusion to its dichroism (pleochroism).
Cordierite crystallizes in the orthorhombic system, well devel oped crystals being usually short prisms with large basal planes. In rocks cyclic twins are common, giving crystals a pseudo-hex agonal habit. The pleochroism of the mineral is often strongly marked, the colours being dark blue or violet, pale blue and yel lowish. These colours can be frequently seen without the dichro scope. In refraction, density and hardness, it much resembles quartz, but in thin sections it is distinguished by its biaxial char acter and common alteration. The pure magnesian cordierite has been prepared artificially. Through formation of solid solution with silica the artificial cordierite shows a range of composition from to It melts incongru ently with formation of mullite and glass. By crys tallization of glasses of the composition of cordierite at 900° C, an unstable modification (U. cordierite) has been obtained. This passes at 925°-I,15o° into the normal a cordierite, but the reverse change has not been effected.
Cordierite occurs in igneous rocks only as a result of contam ination of the magma by aluminous sediment. As such it is known in granites, norites, rhyolites, andesites and lamprophyres. The true home of cordierite is in the thermally altered argillaceous sediments surrounding igneous intrusions and in the paragneisses and schists. In the former it is the commonest constituent of the spots of spotted slates (spilosites, desmosites). In the hornfelses it is frequently associated with andalusite, sillimanite, spinel and corundum. The common alteration product of cordierite—of ten developed as pseudomorphs—is known as pinite, a variable mix ture of chlorite and sericite. Fine, well-developed crystals of cor dierite are rare. (C. E. T.)