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Peridot

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Green, sparkly peridot has been prized for jewelry-making since at least ancient Egyptian times. The primary source of peridot was an island in the Red Sea (now part of Egypt) called in ancient times Topazios, now known as Zabargad. In ancient times there weren’t a lot of professional mineralogists or gemologists to set the record straight on mineral identification, so many green gemmy minerals were misidentified (like emerald, green tourmaline, and peridot) and the names used were not consistent (one of peridot’s early names was “topaz”).

Mineral formula: Mg2SiO4

Mineral group: member of the olivine group

Crystal system: orthorhombic

Crystal habit: Euhedral to subhedral crystals

Cleavage: perfect on {010}

Fracture: conchoidal

Color: green, yellow-green, to white

Luster: vitreous

Diaphaneity: transparent to translucent

Moh’s scale hardness: 7

Streak: white

Specific gravity: 3.275

Named after: forsterite was most likely mistaken for other green, gemmy minerals through ancient times. The general name “olivine” was given in 1789 by Abraham Werner, based on the yellowish green color of most species of the mineral. The pure Mg end-menber was named forsterite in 1824 by Serve-Dieu Abailard "Armand" Lévy in honor of Adolarius Jacob Forster, a German mineral collector and mineral dealer, who variously resided in England and Russia.

Type locality:  Mount Somma, Somma-Vesuvius Complex, Naples, Campania, Italy 

Geological occurrence: In mafic and ultramafic igneous rocks, and in metamorphosed impure dolomites

Energetic properties: visit the blog Working with Peridot Energy

About Peridot

Peridot is the gemmy variety of the mineral olivine, with a characteristically pale to dark “olive” green color. Olivine has the formula (Mg,Fe)2SiO4, meaning variable amounts of magnesium and iron can be present.  Pure Mg olivine is called forsterite, pure Fe olivine is called fayalite. The magnesium-rich varieties are more abundant than the iron-rich ones, and most peridot is of the magnesium-rich type. Pure Mg-olivine is supposed to be clear. Traces of Fe impart the green color; too much Fe gives a brownish hue. It is easily identified as the only common green-colored igneous mineral, transparent to translucent with a glassy luster, hardness between 6.5 to 7, and conchoidal fracture 

Geological Occurrence

Olivine (and peridot) typically occurs as crystals within mafic and ultramafic igneous rocks, like basalt and gabbro. Olivine is one of the first minerals to crystallize from a magma and one of the first minerals to be altered by weathering – it is unstable in Earth’s surface conditions.

Olivine is thought to be the most abundant mineral in Earth’s mantle. Some occurrences of olivine on Earth’s surface are places where plate tectonic activity has shoved slivers of mantle rocks up to the crust. In some lava flows, the olivine crystals occur in nodules (called xenoliths), thought to represent chunks of mantle rock that were carried up with the magma then erupted. An example of this is San Carlos (Arizona), and Kilbourne Hole (New Mexico).

Mg-rich olivine has been found in meteorites (chondrites and pallasites), on the Moon and on Mars. It has also been found in the tails of comets.

Olivine / Peridot in New Mexico

The most famous occurrence of olivine in New Mexico is Kilbourne Hole, located in the Organ Mountains of Doña Ana County. It is not presently open for collecting. Click HERE to visit the Taos Rockers blog on Dona Ana County for more information.

Kilbourne Hole in south-central New Mexico is a classic example of a maar crater that formed as a result of the explosive interaction of hot basaltic magma with groundwater during a volcanic eruption.

Kilbourne Hole is unique because of the remarkable abundance of both crustal and mantle (peridotite/olivine-bearing) xenoliths that are in basalt bombs ejected during the eruption. Xenoliths are inclusions of pre-existing rock derived from country rocks, in this case, pieces of mantle and crust, that were incorporated into the mafic magma as it moved from a depth of about 40 miles (60 km) to the surface. 

Uses of Peridot / Olivine

  • Non-gemmy olivine has a few industrial uses.
  • It can be used as a substitute for dolomite in steel making
  • It is used for sand-casting aluminum
  • It is used as a rock in sauna stoves in Finland (with a great source in nearby Norway)
  • A new use is for sequestering CO2 crushed olivine weathers (decomposes) readily in the presence of CO2, converting the olivine to silicon dioxide, magnesium carbonate and iron oxide. About 1 liter of crushed olivine can sequester all the CO2 produced by burning 1 liter of oil.
  • Gemmy olivine, called peridot, is extensively used for making jewelry and decorative objects.
  • Synthetic olivine is not produced for industrial or gemological uses. Imitation peridot is typically green glass.

Peridot Trivia

  • The largest peridot gemstone (faceted) is 311.78 carats (62.35 grams) from Zagbargad Island, Egypt, and is now located in the Smithsonian Museums, USA.
  • The fabulous green 200 carat gemstones in the shrine of the Three Holy Kings in Germany’s Cologne Cathedral were thought to be emeralds – nope, they’re peridot.
  • Peridot ranges in price from about $50–80/carat for well-cut gems in the 1–2 ct. size, up to  $400–450 per carat for large gems with excellent color.
  • The oldest olivine crystals known on Earth are very old – about 4.5 billion years old! They come from meteorites (technically, pallasites) that were formed in the earliest stages of Earth’s solar system and later crashed onto Earth.  Olivine has also been found in comet dust.

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