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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. |
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. |