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Kyanite is a typically blue aluminosilicate mineral, found in aluminium-rich metamorphic pegmatites and sedimentary rock. It is the high pressure polymorph of andalusite and sillimanite, and the presence of kyanite in metamorphic rocks generally indicates metamorphism deep in the Earth's crust. Kyanite is also known as disthene or cyanite. [5]
Sillimanite or fibrolite is one of three aluminosilicate polymorphs, the other two being andalusite and kyanite. A common variety of sillimanite is known as fibrolite, so named because the mineral appears like a bunch of fibres twisted together when viewed in thin section or even by the naked eye. Both the fibrous and traditional forms of ...
Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock. [5]
Andalusite, kyanite, and sillimanite are the principal aluminium silicate minerals. [2] [3] [4] The triple point of the three polymorphs is located at a temperature of 500 °C (932 °F) and a pressure of 0.4 GPa (58,000 psi). These three minerals are commonly used as index minerals in metamorphic rocks.
cordierite + quartz + sillimanite + K-feldspar (orthoclase) ± biotite ± garnet (If the temperature is below 750 °C there will be andalusite instead of sillimanite) cordierite + orthopyroxene + plagioclase ± garnet, spinel; In carbonate rocks: calcite + forsterite ± diopside, periclase; diopside + grossular + wollastonite ± vesuvianite
An example is provided by the aluminium silicate minerals, kyanite, andalusite, and sillimanite. All three have the identical composition, Al 2 SiO 5. Kyanite is stable at surface conditions. However, at atmospheric pressure, kyanite transforms to andalusite at a temperature of about 190 °C (374 °F).
Examples include sillimanite, kyanite, staurolite, andalusite, and some garnet. [15] Other minerals, such as olivines, pyroxenes, hornblende, micas, feldspars, and quartz, may be found in metamorphic rocks but are not necessarily the result of the process of metamorphism. These minerals can also form during the crystallization of igneous rocks ...
The aluminosilicates –bkyanite, andalusite, and sillimanite, all Al 2 SiO 5 – are structurally composed of one [SiO 4] 4− tetrahedron, and one Al 3+ in octahedral coordination. The remaining Al 3+ can be in six-fold coordination (kyanite), five-fold (andalusite) or four-fold (sillimanite); which mineral forms in a given environment is ...