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(also called disthene), a silicate mineral with the chemical composition Al2[SiO4]O. Kyanite contains up to 63.1 percent Al2O3 and sometimes up to 2 percent admixtures of iron oxide or chromium oxide. It crystallizes in the triclinic system, forming long columnar or tabular crystals and their aggregates. Its color ranges from greenish blue to dark blue-green and yellow; sometimes it is colorless. The mineral is characterized by a sharp anisotrophy in hardness. In the direction parallel to the length of the crystal the hardness equals 4.5, and in the direction at right angles, it is up to 7.5. The density ranges from 3,560 to 3,680 kg/m3. Kyanite is formed with the plutonic metamorphism of clay-rich sedimentary rocks; it is also encountered in quartz veins and contact zones of pegmatites. It is used as a valuable high-alumina raw material for refractory and acidresistant articles and for spark plug insulators for internal combustion engines.