Cordierite

Cordierite
General
Category	Cyclosilicate
Formula (repeating unit)	(Mg,Fe)2Al4Si5O18
IMA symbol	Crd[1]
Strunz classification	9.CJ.10
Dana classification	61.02.01.01 Cordierite group
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Cccm
Unit cell	a = 17.079 Å, b = 9.730 Å, c = 9.356 Å; Z = 4
Identification
Color	Blue, smoky blue, bluish violet; greenish, yellowish brown, gray; colorless to very pale blue in thin section in transmitted light
Crystal habit	Pseudo-hexagonal prismatic twins, as imbedded grains, and massive
Twinning	Common on {110}, {130}, simple, lamellar, cyclical
Cleavage	Fair on {100}, poor on {001} and {010}
Fracture	Subconchoidal
Tenacity	Brittle
Mohs scale hardness	7–7.5
Luster	Greasy or vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.57–2.66
Optical properties	Usually optically (-), sometimes (+); 2V = 0–90°
Refractive index	nα = 1.527 – 1.560 nβ = 1.532 – 1.574 nγ = 1.538 – 1.578 Indices increase with Fe content.
Pleochroism	X = pale yellow, green; Y = violet, blue-violet; Z = pale blue
Fusibility	on thin edges
Diagnostic features	Resembles quartz, can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness
References	[2][3][4][5]

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).^[3] A high-temperature polymorph exists, indialite, which is isostructural with beryl and has a random distribution of Al in the (Si,Al)₆O₁₈ rings.^[4] Cordierite is also synthesized and used in high temperature applications such as catalytic converters and pizza stones.