Description: Olivine, one of earth’s most common minerals, describes a chemical series with two end members: Fayalite is the iron-rich version, Forsterite the magnesium. Olivines are important rock-forming minerals in mafic and ultra-mafic igneous rocks such as basalt, gabbro and peridotite. The distinctive olive-green color, translucent luster and hardness (greater than glass) are usually adequate to distinguish olivine from other minerals. Olivine is found in a large number of stony meteorites (minerals, out of this world :).
Chemical Formula: (Mg,Fe)2SiO4
Crystal Form: Massive - Granular, peridot crystals can be dipyramidal
Crystal System: Orthorhombic
Color: Usually olive green, can be yellow- to bright-green; iron-rich specimens are brownish green to brown
Streak: Colorless
Luster: Vitreous, transparent to translucent
Fracture: Brittle, conchoidal - Very brittle fracture produces small, conchoidal fragments
Cleavage: 001} Good, {010} Distinct
Hardness: 6.5 - 7 on Moh’s scale
Density/Specific Gravity: 3.27 - 3.37 g/cm3 !!
Magnetism: N/A
Taste: N/A
Hydrochloric acid: N/A
Radioactivity: N/A
Fluorescence: Mg-rich Forsterite fluoresces light yellow under both short and long UV
Distinguishing Physical Properties:
Color: Olive, apple or yellow-green color
Hardness: 7 (often difficult to determine)
Fracture: Conchoidal
Cleavage: none
Photograph Attribution: Olivine specimen photograph by Sean C. Murphy, 2020.
2nd Photograph Attribution: Olivine and Chromite specimen photograph by Sean C. Murphy, 2020.