pleochroism of Cordierite - Same specimen. Left image in polarized light. Locality : Coimbatore District, Chennai (Madras), Tamil Nadu, India Size: 1.38 x 1.01 — Photo: Didier Descouens · CC BY-SA 3.0
Cordierite
Also known as: Iolite, Dichroite.
Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present, and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: (Mg,Fe)2Al3(Si5AlO18) to (Fe,Mg)2Al3(Si5AlO18). A high-temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)6O18 rings. Cordierite is also synthesized and used in high temperature applications such as catalytic converters and pizza stones.
Intro adapted from Wikipedia (CC BY-SA).
Cordierite is an IMA-recognized mineral species in the silicate class, with the chemical formula Mg₂Al₄Si₅O₁₈.
It crystallizes in the orthorhombic crystal system (space group Cccm). It is a member of the Beryl structural group.
Key properties: Mohs hardness 7–7.5; specific gravity 2.6–2.66; typically blue or gray or brown or purple; white streak; Vitreous luster; refractive index 1.552; optically biaxial.
IMA symbol Crd, grandfathered status — known before the IMA’s founding, named in 1813.
Found in thermally metamorphosed argillaceous sediments and high-grade regionally metamorphosed schists, gneisses, and granulites; in mafic igneous rocks and granites; detrital.
It is found associated with Sillimanite, potassic feldspar, muscovite, biotite, corundum, spinel, garnet, andalusite.
The mineral was named to honor Pierre Louis A. Cordier (1777–1861), French mining engineer and geologist, who first studied the species.
Collections: Gemstones, Collector Favorites.