Spinel Details

Spinel

Spinel is a mineral that has the chemical formula MgAl2O4. It is commonly found in octahedra in the cubic crystal system, though some specimens may show dodecahedron or cube faces. Twinning on {111} is frequent, and spinel-law, penetration-, or contact-group types have been recorded. This direction sometimes shows parting, but there is no cleavage. The hardness is 7.5–8, and the SG is 3.5–4.1, increasing with the iron or zinc content. Spinel may be transparent to nearly opaque, and colors include brown, red, black, orange, yellow, green, blue, indigo, and violet. Colorless spinel, at least of any size, will almost certainly be artificial. The single RI is 1.718. Spinel is formed at high temperatures as an accessory in igneous rocks, in regionally and contact metamorphosed limestones, and in other contexts. Spinel forms three series, with magnesiochromite, with hercynite, and with gahnite, the latter itself providing the occasional dark blue zinc-rich gemstone.

Varieties

Spinel forms in many different colors, and some of the most well-known varieties include:

• Red spinel
• Blue spinel
• Black spinel
• Green spinel
• Yellow spinel

Physical Properties

Some of the key physical properties of spinel include:

• Hardness: 7.5-8
• Specific Gravity: 3.5-4.1
• Color: Brown, red, black, orange, yellow, green, blue, indigo, and violet
• Luster: Vitreous to dull
• Transparency: Transparent to opaque
• Fracture: Conchoidal
• Cleavage: None

Chemical Properties

Spinel is an oxide mineral that contains magnesium, aluminum, and oxygen. Its chemical formula is MgAl2O4, and it has a Mohs hardness of 7.5-8. Spinel is insoluble in water and acids, but it can be attacked by hydrofluoric acid. Its specific gravity ranges from 3.5 to 4.1, and its refractive index is 1.718.

Occurrence

Spinel is found in a variety of environments, including:

• Igneous rocks
• Regionally and contact metamorphosed limestones
• Other contexts

Uses

Spinel has a number of uses, including:

• As a gemstone
• In ceramic pigments and glazes
• In refractory materials
• In electronics and optoelectronics
• In high-performance coatings
• In nuclear reactors as a structural material and fuel

 

Inclusions in Natural Spinel