Cordierite
| Cordierite | |
|---|---|
Cordierite from Italy
|
|
| General | |
| Category | Cyclosilicate |
|
Formula (repeating unit) |
(Mg,Fe)2Al4Si5O18 |
| 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 |
| 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 |
| 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 | |
"Praseolite" redirects here. For prasiolite, see Vermarine
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, which was discovered in 1813, is named after the French geologist Louis Cordier (1777–1861).
Cordierite typically occurs in contact or regional metamorphism of pelitic rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks. Two common metamorphic mineral assemblages include sillimanite-cordierite-spinel and cordierite-spinel-plagioclase-orthopyroxene. Other associated minerals include garnet (cordierite-garnet-sillimanite gneisses) and anthophyllite. Cordierite also occurs in some granites, pegmatites, and norites in gabbroic magmas. Alteration products include mica, chlorite, and talc. Cordierite occurs, for example, in the granite contact zone at Geevor Tin Mine in Cornwall.
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