Magnesiocummingtonite: Properties and Occurrences

Magnesiocummingtonite is a common component of some regional metamorphic rocks. It is the magnesium-rich endmember of the cummingtonite-grunerite series of the amphibole group of minerals, with the following composition: (Mg)7Si8O22(OH)2. If more intense metamorphism were to occur, the minerals hypersthene, enstatite or olivine are produced at the expense of cummingtonite.

General information

  • Formula: Mg)7Si8O22(OH)2
  • Transparency: Crystals are translucent to opaque.
  • Crystal System: monoclinic; 2/m.
  • Cleavage: is good in two directions at 56 and 124-degree angles.
  • Fracture: splintery.

Properties

Magnesiocummingtonite is considered to be an aptly named magnesium-rich variety of the mineral cummingtonite which has Fe2+ substituting for some of the Mg. Divalent manganese may also substitute for magnesium in the crystal structure.

  • Cleavage: {110} Good
  • Color: Brown, Gray-green, White, Light Gray, Dark green.
  • Density: 3.13
  • Hardness: 5-6 – Between Apatite and Orthoclase
  • Luster: Silky
  • Streak: white

Occurrence: In metamorphosed ultrama¯c rocks of the amphibolite facies; as exsolution lamellae in magnesio-arfvedsonite.

It may be difficult to distinguish cummingtonite from the Mg-rich variety in hand specimens. However, the refractive index, specific gravity, and degree of attraction to a magnetic field increase with more Fe content.

Association: Anthophyllite, tremolite, chlorite, magnesite, talc, enstatite (Cima di Gagnone, Switzerland).

 

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