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Olivine Mineral Facts:

Chemical Formula: (Fe,Mg)₂SiO₄  Known primarily as Olivine, Gemstone form known as Peridot.

Colors: Green
Shades of green, pale green, olive green, greyish-green, brownish, rarely yellow.

Hardness: 6.5 to 7

Density: 3.27 to 3.37

Cleavage: The mineral has no cleavage.

Crystallography: Orthorhombic
Crystals are prismatic usually modified with domes and pinacoids; also as grains, massive, and compact. Single crystals are rare.

Luster:. Vitreous luster.

Optics: (Refractive Index): = a = 1.6674, b=1.6862, y=1.7053

Composition, Structure and Associated Minerals:
Olivine is the essential constituent of the peridotites, dunite being a pure olivine rock. It also occurs in dark colored ferro-magnesium  basic and ultra-basic igneous rocks, varying from an accessory character to that of a main constituent of the rock. Such rocks include basalt, diabase (dolerite), picrite, and in the olivine gabbros. Olivine is also formed by the dedolomitisation of an impure dolomite, the variety forsterite being formed, which by alteration produces serpentine. In fact his mineral is very readily altered to serpentine; often  magnesium carbonates, iron ore, etc., may form at the same time.

Olivine derives its name from the usual olive-green color of the mineral, and is the term usually given to the species when speaking of it as a rock-making mineral. Peridot is an old name for the species. Olivine is also found at times as glassy grains in some meteorites (pallasites). Occasionally found in crystalline dolomitic limestones. Associated often with pyroxene, the plagioclase feldspars, magnetite, corundum, chromite, serpentine, etc. The transparent green variety, known as peridot, and used as a gem material, was found in ancient times in the East, the exact locality for the stones not being known.

Identification and Diagnostics
The mineral is characterized by its glassy luster, green color, granular structure and solubility in acids. Olivine is rather slowly soluble in hydrochloric acid and yields gelatinous silica upon evaporation. After evaporation to dryness, take up residue in water with nitric acid, filter off silica, add ammonia in excess to precipitate ferric hydroxide, filter, add ammonium oxalate to prove absence of calcium, add sodium phosphate and obtain precipitate of ammonium-magnesium phosphate (test for magnesium).

Occurrence, Localities and Origins:
Olivine occurs as an original constituent of basic igneous rocks and as a metamorphic product in dolomitic limestones.  Members of the olivine group occur in the basaltic lavas of many volcanoes as those of the Hawaiian Islands; in the limestone inclusions in the lava of Mt. Somma, near Naples; Crystals are found in the lavas of Vesuvius. Larger crystals, altered to serpentine, come from Snarum, Norway. Olivine is also found in various basic rocks in Vermont and New Hampshire and at Webster, N. C. At the latter place granular aggregates of almost pure olivine constitute great rock masses known as dunite. The only member of the group that is of any economic importance is a pale yellowish green transparent olivine, which is used as jewelry under the name of "peridot."

Gem material is found near the communities of San Carlos and Peridot in Gila and Graham counties of Arizona, scattered loose in the soil. The gem grains came from a basic volcanic rock.  Gem material is also mined in several east African countries including Kenya and Tanzania. Asia also produces gems from both Myanmar (Burma) and Sri Lanka. At present, fine crystals of peridot are occasionally found in Upper Egypt, near the Red Sea, and in rounded grains associated with pyrope garnet in certain igneous rocks of Arizona and New Mexico.

For more information on gem Peridot, see:  Peridot - Bright Greens

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