Spinel

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Spinel
Chemical composition Mg(Al2O4) Magnesium aluminum oxide
Crystal system Cubic
Habit Octahedral, contact twins
Cleavage Imperfect
Fracture Conchoidal, uneven
Hardness 8
Optic nature Isotropic
Refractive index 1.712 - 1.736
Birefringence None
Dispersion 0.026
Specific gravity 3.58 - 3.61
Lustre Vitreous
Pleochroism None
Badakshan spinel, Afghanistan

Spinel image gallery

Spinel is a mineral species. For many centuries, most gem spinels were misidentified as sapphire or ruby because they have similar properties and occur in the same geological deposits. The historically significant 5.08 centimeter "Black Prince Ruby" in the center of the British Imperial Crown was only recently identified as a spinel. This stone is irregular in shape and has a somewhat squareish outline. Additionally, it was not faceted, merely polished. Spinels also occur in a vast array of colors. They are slightly softer than sapphires but still very durable.
The earliest known use of spinels was as ornaments found in Buddhist tombs in Afghanistan. Blue spinels have been found in England, dating back to the Roman occupation (51 BC to 400 AD).


Chemical composition

Common spinel belongs to the "spinel series", which belongs to the "spinel group".
The general formula for the spinel group is A2+B3+2O4. The 3 series of the spinel group are defined by the B3+ cation.
The spinel group is made up of 3 isomorphous series.

The isomorphous series:

  • Spinel series (aluminum)
    • Spinel - MgAl2O4 (n = 1.719, sg ~ 3.60)
    • Hercynite - FeAl2O4
    • Gahnite - ZnAl2O4 (n = 1.805, sg = 4.62)
    • Galaxite - MnAl2O4
  • Magnetite series (ferric iron)
    • Magnetite - FeFe2O4
    • Magnesioferrite - MgFe2O4
    • Ulvöspinel - FeFeTiO4
    • Franklinite - ZnFe2O4
    • Jacobsite - MnFe2O4
    • Trevorite - NiFe2O4
  • Chromite series (chrome)
    • Chromite - FeCr2O4
    • Magnesiochromite - MgCr2O4

Most of the above series members are rare in nature with the exception of the members of the spinel series, magnetite and chromite. To gemologists common spinel and gahnite are of most interest.

When gemologist refer to "spinel", we usually imply common spinel, that is the spinel that belongs to the spinel series of the spinel group.

Diagnostics

Spinel can be confused with many stones by appearance alone, yet optical properties usually rule out most of them.
As spinel belongs to an isomorhous series, the optical and physical properties may vary.

Color

Spinel: colorless, green, blue, red, black.
Gahnite: blue-green, yellow, brown.

Varieties:

  • Pleonaste (also named ceylonite) - (Mg,Fe)Al204 - dark green to blue-green
  • Gahnospinel - (Mg,Zn)Al204 - pale to dark blue

Diaphaneity

Transparent to opaque.

Refractometer

Spinel is isotropic and the refractive index of common spinel is generally around 1.712 to 1.720. Red spinel can have a refractive index up to 1.74.
The only other isotropic gemstone that falls within this range is grossular garnet, but it will usually be higher and the color is also different.
Other members of the spinel series, such as gahnite will have higher refractive indices.

All other gemstones can easily be seperated from spinel by their optic nature.

Synthetic spinel has a usual refractive index of 1.726.
Gahnospinels have a refractive index between 1.725 and 1.753, while pleonaste (ceylonite) has an R.I. range of 1.77 to 1.80.

Specific gravity

As with the refractive indices, the specif gravity of spinel can vary due to isomorphous replacement.
The values for most gem grade material lies between 3.58 and 3.61. Pleonaste has a S.G. between 3.63 and 3.90.
Gahnospinels may have a specific gravity up to 4.06.

Polariscope

Common spinel is isotrope and will remain dark under crossed polars.
Verneuil type synthetic spinel will always (maybe with the exception of red and blue) show anomalous birefringence due to excess Al2O3 (see synthetics).

Spectra

Spectrum natural blue spinel iron.jpg

Spectrum of a natural blue spinel, colored by iron.

Synthetics

Spinel is synthesized by the Verneuil (flame-fusion) process and the flux-melt method, although the first process does not render a true synthetic in most cases.

It was found that while trying to synthesize spinel through the Verneuil process, the resulting boules would easily fracture and no reasonably sized gemstones could be cut from them.
The ratio MgO to Al2O3 is 1:1 for common spinel and by changing that ratio (adding Al2O3) the boules became stable. Because that alters the chemical formula of the synthetic, it is not a true synthetic (but accepted as such).

For the creation of red stones, this alteration was no option and, usually, no red synthetic spinel boules created by the flame-fusion process result in large stones (but are known to excist). The larger sized red synthetic spinels are created with the flux-melt method.

As a result in the changing of the MgO:Al2O3 ratio, the flame fusion synthetics have higher refractive indices (usually stable at 1.726) and a higher specific gravity (3.64).

Blue synthetics have also been produced by the flux-melt process.

A recent development (2007) is the procuction of red to pink synthetic spinels by the Czochralski pulling method.

Phenomena

  • Asterism (4 and 6-pointed stars)
  • Color change (rare)

Inclusion images

Apatite inclusions and their star-like outgrowths along the 60 degree hexagonal plane - looking much like stellate dislocation systems, in a cobalt blue spinel.
Photo courtesy of John Huff, gemcollections.com
Octahedral spinel inclusions with "Saturn-ring" stress fractures in a Burma red spinel.
Photo courtesy of John Huff, gemcollections.com
Octahedral inclusions in a Burma red spinel.
Photo courtesy of John Huff, gemcollections.com


Spinel inclusions gallery

Sources

  • Introduction to Optical Mineralogy 2004 - William D. Nesse ISBN 0195149106
  • Gemmology 3rd edition (2005) - Peter G. Read ISBN 0750664495
  • Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)