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Spinel
Current inventory:  0 gems
 

Chemistry:  MgAl₂O₄  [Magnesium Aluminum Oxide]

Discovered in 1546;   IMA status: Valid (pre-IMA; Grandfathered).
The name Spinel is of uncertain origin, possibly coming from the Latin word spina (or spinella), meaning little thorn, referring to the spine-shaped octahedral Spinel crystals.

Classification

Mineral Classification:

Oxides

Strunz 8th Ed. ID:

4/B.01-10

Nickel-Strunz 10th Ed. ID:

4.BB.05

4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
B : Metal: Oxygen = 3:4 and similar
B : With only medium-sized cations

Related to:

Spinel Group. Gahnite - Spinel Series. Spinel - Hercynite Series. Magnesiochromite - Spinel Series

Crystal Data

Crystallography:

Isometric - Hexoctahedral

Crystal Habit:

Commonly as octahedra, to 30 cm, may show dodecahedron or cube faces, rarely as dodecahedra; coarse granular, rounded, massive.

Twinning:

On [111] as both twin and composition plane, the spinel law, penetration or contact, may be repeated as sixlings.

Physical Properties

Cleavage:

None observed; Parting on [111]

Fracture:

Conchoidal, Irregular/Uneven to Splintery

Tenacity:

Brittle

Hardness (Mohs):

7.5 - 8.0

Density:

3.58 - 4.10 (g/cm³) (increasing with Fe and Zn content)

Luminescence:

Reds and Pinks: crimson in LW UV and SW UV, red in X-rays, no phosphorescence. Blue: inert in UV. Deep Purple: red in LW UV, essentially inert in SW UV, lilac in X-rays. Pale Blue and Violet: green in LW UV and X-rays, essentially inert in SW UV.

Radioactivity:

Not Radioactive

Optical Properties

Color:

Colorless, Red, Blue, Green, Brown, Black

Transparency:

Transparent to Translucent to Opaque

Luster:

Vitreous

Refractive Index:

1.72 - 1.80  Isotropic

Birefringence:

0.000 - Isotropic minerals have no birefringence

Dispersion:

0.020

Pleochroism:

Usually None; anomalous in some blue zincian varieties.

Occurances

Geological Setting:

A common mineral, formed at high-temperatures as an accessory in igneous rocks, principally basalts, kimberlites, peridotites, and in xenoliths; in regionally metamorphosed aluminum-rich schists; in regionally and contact metamorphosed limestones; a detrital mineral.

Common Associations:

Andalusite, Chondrodite, Corundum, Forsterite, Phlogopite, Scapolite, Sillimanite

Common Impurities:

Ti, Fe, Zn, Mn, Ca

Type Locality:

n/a

Year Discovered:

1546

View mineral photos:

Spinel Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

The Spinel Group of minerals includes Spinel, Gahnite, Galaxite, Hercynite and Magnesiochromite. Although Spinel is most famous for its red variety, it is also found in other colors such as green, blue, purple, brown and black. The blue Spinel is colored by cobalt and is quite beautiful. Star Spinels and color-change Spinels are also (rarely) found.

Spinel is a historically important gem because it has been mistaken for Ruby in some very famous crown jewel collections. Perhaps the most famous of these is the Black Princes's Ruby set in the British Imperial State Crown (Crown Jewels of England). It is estimated to be 170 carats. Originally thought to be a large Ruby, it was later proven to be a fine Spinel. Although Spinel gems are usually small, 3 carats or less, very large gems have been cut. The largest is probably the 520 carat gem found at the Banque Markazi in Teheran, Iran.
 

  
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