Silicates — olivine group · Sourced PROFILE
Forsterite
Mg₂SiO₄
The magnesium end-member of olivine, present in the unusual ultra-magnesian metamorphic assemblages at Kuh-i-Lal.
AT A GLANCE
- Formula
- Mg₂SiO₄
- Crystal system
- Orthorhombic
- Space group
- Pbnm
- Mohs hardness
- 7
- Density
- 3.275 g/cm³
- Optical character
- Biaxial (+)
CRYSTALLOGRAPHY
Crystal form & structure
Euhedral to subhedral thick crystals with wedge-shaped terminations; commonly granular or compact massive.
HAND-SAMPLE PROPERTIES
Physical properties
- Cleavage
- Imperfect on {010} and {100}
- Fracture
- Conchoidal
- Tenacity
- Brittle
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Observed colours
- Green, yellowish, lemon-yellow, white, greyish or blue-grey
LIGHT & OPTICS
Optical properties
- Optical class
- Biaxial (+)
- Refractive index
- nα 1.635; nβ 1.651; nγ 1.670
- Fluorescence & electrical behaviour
- No diagnostic fluorescence is reported in the cited profile.
GEOLOGICAL SETTING
Formation
Forms in mafic and ultramafic igneous rocks and in thermally metamorphosed, magnesium-rich carbonate rocks.
MINERAL COMPANY
Common associations
Enstatite, plagioclase, phlogopite, magnetite, chromite, dolomite, brucite, diopside, corundum, calcite and spinel
FIELD NOTES
How to identify it
Hardness 7, conchoidal fracture, granular habit and olive-green colour are useful field clues. Optical properties and chemistry distinguish magnesium-rich forsterite from iron-rich olivine members.
NAME & HISTORY
Where the name comes from
Named for the English mineral collector and dealer Adolarius Jacob Forster.
SCIENTIFIC SOURCE
Handbook of Mineralogy — Forsterite
External reference; factual profile paraphrased and attributed. Copyright remains with the publisher.
PROVENANCE
Documented occurrences
Kuh-i-Lal district, Ishkoshim · Forsterite is documented in sapphirine-bearing assemblages of an ultra-magnesian metamorphic lens at Kuh-i-Lal, together with spinel and humite-group minerals. This is not collecting guidance.
Sourced · District precisionSapphirine + Forsterite and Sapphirine + Humite-Group Minerals in an Ultra-Magnesian Lens from Kuhi-lal, SW Pamirs, Tajikistan