OLIVINE GROUP
CHEMISTRY - The Olivine Group consists of
Fayalite Fe2SiO4
Fosterite Mg2SiO4
CRYSTALLOGRAPHY Orthorhombic
CRYSTAL GROWTH AND HABITS Both Fayalite and Forsterite produce tabular crystals with wedge shaped termination. They can also be granular or compact massive.
COLOR AND OTHER OPTICAL PROPERTIES Both Fayalite and Forsterite range in color from greenish yellow through various shades of green to browns. They are commonly transparent to translucent.
HARDNESS 6.5 - 7
SPECIFIC GRAVITY Fayalite 4.4 Forsterite 3.3
LUSTER Vitreous to resinous
STREAK White
BREAKABILITY Two imperfect cleavages and conchoidal fracture
OCCURRENCE Both Fayalite and Forsterite are found in mafic and ultra mafic igneous rocks like peridotite and dunite, and in metamorphosed impure limestones. They are also found in stony-iron meteorites.
ASSOCIATED MINERALS Plagioclase, phlogopite, magnetite, chromite, corundum, spinel, augite
NAME Forsterite was named in honor of Adolarius J. Forster and English mineral collector (1739-1806).
Fayalite was named after the island Fayal (Faial) in the Azores. Specimens of the mineral were found there and studied. It is now believed, however, that the mineral was dumped in the area by ships jettisoning there slag ballast.