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| Formula | (Fe,Mg,Mn)Al2SiO5(OH)2 |
Mg-rich variety known as Sismondine |
| Crystal System | Monoclinic or Triclinic | 2/m |
| Crystal Habit | Foliated plates or massive Pseudohexagonal tabulate crystals are rare |
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| Cleavage | {001} perfect |
Not visible in thin section. Twinning common on {001}. |
| Color/Pleochroism | (x) colorless, olive green, gray |
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| Optic Sign | Biaxial (+) , (-) | Most are optically positive but negative minerals have been found |
| 2V | 45° to 68° with some outliers |
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| Optic Orientation | X or Y = b (monoclinic); Z-c angle= 2°-30°; Y = b (triclinic) |
Two polymorphs, one monoclinic, the other, triclinic. |
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Refractive Indices | 1.713-1.730 |
Values decrease in relationship with the replacement of Fe by Mg. |
| Max Birefringence | 0.006-0.022 | High Positive Relief |
| Elongation | yes | |
| Extinction | inclined | |
| Dispersion | r > v | |
| Distinguishing Features | Named for its superficial resemblance to chlorite, chloritoid is characterized by its dark green to gray-green color. It has a hardness of 6.5 and exhibits many optical qualities such as pleochroism, high relief and frequent lamellar twinning. | |
| Occurrence | Results from low to medium grade regional metamorphism as well as in hydrothermal environments. Common associative minerals are muscovite, chlorite, garnet, staurolite and kyanite. | |
| Editors | Sue Scheufele ('05), Louisa Hall ('15) Data Sources: Deer, W. A., R. A. Howie, and Jack Zussman. Rock-forming Minerals: Volume 1. Ortho-and Ring Silicates. Longmans, 1962. | |
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A cluster of chloritoid crystals in thin section (Sample CH-8b). |
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This rollover shows chloritoid, the elongate mineral, in both plane polarized light and cross polarized light. Note the lamellar twinning present in xpl. The width of the field of view is 7.75 mm (Sample DR-2b). |