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Spinel

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Spinel is the name of both an individual mineral and of a group of metal-oxide minerals that share the same crystal structure. Minerals in this group include gahnite, franklinite, and chromite. It is found as glassy, hard octahedra, or as grains or masses. Although familiar as a blue, purple, red, or pink gemstone, also occurs in other colors. Red spinel is called ruby spinel; its blood-red color is due to the presence of chromium. A minor constituent of peridotites, kimberlites, basalts, and other igneous rocks, also forms in aluminumrich schists and metamorphosed limestones. Water-worn crystals come from stream deposits. The earliest known spinel dates back to 100 BCE and was discovered near Kabul, Afghanistan.

Name: Perhaps from the Latin spinella, for little thorn, in allusion to the spine-shaped octahedral crystals.

Association: Forsterite, chondrodite, scapolite, phlogopite, corundum, sillimanite, andalusite.

Mineral Group: Spinel group.

Polymorphism & Series: Forms three series, with magnesiochromite, with gahnite, and with hercynite.

Crystallography. Isometric; hexoctahedral. Usually in octahedral crystals. In “twinned octahedrons (spinel twins). Dodecahedron may be present as small truncations. Other forms rare.

Composition: MgAI20 4. MgO = 28.2 percent, A120 3 = 71.8 percent. The magnesium may be replaced, in part, by ferrous iron or manganese, and the aluminum by ferric iron and chromium. The clear red, nearly pure, magnesium spinel is known as ruby spinel. Pleonaste is the iron spinel, dark green to black; and picotite is the chrome spinel, yellowish to greenish brown

Diagnostic Features: Recognized by its hardness (8), its octahedral crystals, and its vitreous luster. The iron spinel can be distinguished from magnetite by its nonmagnetic character and white streak.

Chemical Properties

Chemical Classification Oxide minerals
Chemical Composition MgAl2O4

Spinel Physical Properties

Color Various; red, pink, blue, lavender/violet, dark green, brown, black, colourless
Streak White
Luster Vitreous
Cleavage None
Diaphaneity Transparent to Opaque
Mohs Hardness 7.5–8.0
Specific Gravity Zn-rich can be as high as 4.40, otherwise it averages from 3.58-3.61
Diagnostic Properties Hardness, octahedral crystals, vitreous luster
Crystal System Cubic
Tenacity Brittle
Parting Separation plane {111} indistinct and probably represents parting rather than cleavage.
Fracture Irregular/Uneven, Splintery, Conchoidal
Density 3.6 – 4.1 g/cm3 (Measured)    3.578 g/cm3 (Calculated)

Spinel Optical Properties

Type Isotropic
Color / Pleochroism Non-pleochroic
Twinning common
Birefringence Isotropic minerals have no birefringence

Occurrence

It is a common metamorphic mineral occurring imbedded in crystalline limestone, gneisses, and serpentine. Occurs also as an accessory mineral in many basic igneous rocks. It is frequently formed as a contact metamorphic mineral associated with phlogopite, pyrrhotite, chondrodite, and graphite. Found frequently as rolled pebbles in stream sands, where it has been preserved because of its resistant physical and chemical properties. The ruby spinels are found in this way, often associated with the gem corundum, in the sands of Ceylon, Siam, Upper Burma, Madagascar. Ordinary spinel is found in various localities in New York and New Jersey.

Spinel Uses Area

  • It used as a gemstone.
  • Synthetic spinel, which looks like a glass but has a significantly higher strength against pressure, may also have applications in military and commercial use.

Distribution

Many localities. Some for good specimens include:

  • gem crystals from around Mogok, Myanmar (Burma).
  • In Sri Lanka, in the Ratnapura and Elahera districts, and elsewhere.
  • In the USA, at many places from Amity and Edenville, Orange Co., New York to Andover, Allegany Co., New Jersey; fine crystals from Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey.
  • From near Midland, and at the Crestmore quarry, Riverside Co., California.
  • In Canada, at Burgess, Ontario, and Wakefield, Quebec.
  • In the Eifel district, and at Bodenmais, Bavaria, Germany.
  • On Monte Somma and Vesuvius, Campania, Italy.
  • From Andrahomana, Ambinda, Besakoa, and many other places in Madagascar. Large crystals from the Emeldjak and Katalakhu deposits, in the Aldan Shield, Sakha, Russia.
  • At Gorondarinskoye, Pamir Mountains, Tajikistan.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Dana, J. D. (1864). Manual of Mineralogy… Wiley.
  • Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
  • Mindat.org. (2019). Spinel: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/min-727.html [Accessed 4 Mar. 2019].
Cite this article as: Geology Science. (2019). Spinel. [online] Available at: http://geologyscience.com/minerals/spinel/ [9th December 2019 ]
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