[Chemical composition] The composition is relatively stable, and there are often a small amount of similar substances like Fe 3+ instead of Al, and sometimes a small amount of mixed substances such as Ti, Ca, Mg and alkali.
[Crystal structure] orthorhombic system; 
a 0 = 0.743 nm, b 0 = 0.758 nm, c 0 = 0.574 nm; Z = 4. In the crystal structure, there are double-chain [AlSiO 5 ] 3- which are alternately arranged along the c-axis of [SiO4] and [AlO 4 ].
[Form] The crystal is long column or needle (Figure G-38). There are streaks on the cylinder of the [001] crystal ribbon. The aggregate is radial or fibrous. Sometimes it is hairy and exists as an inclusion in quartz and feldspar crystals. The hairy ceremonial stone is called a thin stone. This needle-like crystal form of the sillimanite is related to the presence of [SiO 4 ] and [AlO 4 ] double chains and [AlO 6 ] octahedral chains in its structure.
[Physical properties] white, gray or light green, light brown, etc.; glass luster. {010} Cleavage completely, this cleavage plane doubles in a parallel structure. Hardness 6.5 ~ 7.5. The relative density is 3.23 to 3.27. Thermal analysis: heating to 1545 ° C, the sillimanite is converted to mullite and quartz. Mullite is an important ceramic material. Its structure is the same as that of the stellite stone, but there is excess Al→Si entering the tetrahedral double chain. In order to balance the electricity price, some O 2- - absent, ie mullite The chemical formula is: Al 4+2x Si 2-2x O 10-x , and x is the number of O 2- absent.
Figure G-38 sillimanite crystal aggregate
[Origin] and occurrence metamorphic minerals, aluminum contact metamorphic rocks of a high temperature zone in output. For example, in the northwest of Beijing Zhoukoudian, the muddy cement of the Permian Hongmiaoling sandstone formed a sillimanite stone after thermal deterioration with the granite . In the regional metamorphism, as an early mineral formation, the sillimanite is also found in the chip rock and gneiss .
[Identification characteristics] Rod-shaped, needle-shaped crystal form, produced in contact metamorphic zone and metamorphic rock.
[Uses] is the manufacturing of high alumina refractories and acid resistant material, a silica-alumina ceramic alloy technology, the internal combustion engine and the spark plug insulator aircraft, automobiles, ships parts used.
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