Cordierite
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.
| Cordierite | |
|---|---|
| General | |
| Category | Cyclosilicate |
| Formula (repeating unit) | (Mg,Fe)2Al4Si5O18 |
| IMA symbol | Crd |
| 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 | |
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