Ottrelite: Properties and Occurrences

Ottrelite is a form of chloritoid. Its empirical formula is (Mn, Fe, Mg)2Al4Si2O10(OH)4. It was originally described as a Mn-rich, but not necessarily Mn-dominant chloritoid, consequently many “ottrélites” are in reality Mn-rich chloritoids. It is a brittle mica containing manganese and is similar to margarite in physical and chemical properties. It typically occurs in low- and medium-grade metamorphic rocks.

General Information

  • Category: Nesosilicate
  • Formula (repeating unit): (Mn, Fe, Mg)2Al4Si2O10(OH)4
  • Strunz classification    09.AF.85
  • Formula mass: 490.11
  • Color: Pistachio green

Properties

Ottrelite is a gray to black mineral occurring in small scales in certain schists and being a variety of chloritoid. It is a manganese-rich mineral. It is a brittle mica containing manganese and similar to margarite in physical and chemical properties. It is not radioactive.

  • Crystal habit: Granular
  • Mohs scale hardness: 6-7
  • Luster: Vitreous to nearly adamantine on cleavages
  • Streak: Greenish, grayish
  • Diaphaneity: Translucent
  • Specific gravity: 3.50
  • Density: 3.52

Occurrence: Formed under nonshearing stress conditions in veins and cavities within low- to medium-grade metamorphic rocks; also a medium-temperature hydrothermal alteration mineral.

Ottrelite slate is a low-grade metamorphic rock generally formed by the metamorphosis of mudstone, shale, or sometimes volcanic ash, under relatively low pressure and temperature conditions (prehnite-pumpellyite facies). With increasing, metamorphic grade increases in muscovite grain size transform slate into phyllite.

Dark slates usually owe their color to carbonaceous material or to finely divided iron sulfide. Reddish and purple varieties owe their color to the presence of hematite (iron oxide), and green varieties owe theirs to the presence of much chlorite, a green micaceous clay mineral.

Association: Dickite, kaolinite, rutile, andalusite, chlorite, pyrophyllite, davreuxite, quartz.