[Chemical composition] Mg and Fe are completely similar to the same image, but most cordierite is rich in magnesium , because in the crystal structure of cordierite, Mg and Fe are four-coordinated, and Mg 2+ is higher than Fe. The radius of 2+ is small and it is more stable in the tetrahedron. Al 3+ outside the backbone can be replaced by Fe 3+ . In addition, the components often contain H 2 O, K, Na, etc., in the large pores in the structure.
[Crystal structure] orthorhombic system; D202h-Cccm; a 0 = 1.713 to 1.707 nm, b 0 = 0.980 to 0.973 nm, c 0 = 0.935 to 0.929 nm; Z = 4. It has the same structure as beryl, but there is Al→Si in the hexagonal ring, and thus the symmetry decreases. It is worth noting that Mg 2+ and Fe 2+ generally have six coordination positions, but four coordination positions in cordierite.
[Form] intact crystals do not appear often, sometimes as fake hexagonal prism crystals.
Figure G-23 Crystal form of cordierite
[Physical properties] colorless, or light blue, light yellow; glass luster; transparent to translucent. Cleavage {010} medium; shell-like fracture. Hardness 7 to 7.5. The relative density is 2.53 to 2.78.
[genesis and occurrence] is a typical metamorphic mineral produced in gneiss , chip-bearing rocks and altered igneous rocks.
[Main use] The biggest characteristic of cordierite is its small coefficient of thermal expansion. Therefore, it is widely used in ceramics and glass industries to improve its ability to resist rapid cooling and rapid heat.
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