Sapphirine

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Template:Sapphirine

From Kolonne,Sri lanka

Sapphirine image gallery

Sapphirine is a rare mineral, a silicate of magnesium and aluminium with the chemical formula (Mg,Al)8(Al,Si)6O20 (with iron as a major impurity). Named for its sapphire-like colour, sapphirine is primarily of interest to researchers and collectors: well-formed crystals are treasured and occasionally cut into gemstones. Sapphirine has also been synthesized for experimental purposes via a hydrothermal process.

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

Dark blue, bluish to brownish green,green,gray

Diaphaneity

Transparent to opaque.

Refractometer

1.701 to 1.718 with a Birefringence 0.006 to 0.007

Specific gravity

The specif gravity of Sapphirine is 3.54–3.51

Polariscope

Common spinel is isotrope and will remain dark under crossed polars.
Verneuil type synthetic spinel will always (maybe with the exception of red) show strong anomalous birefringence due to excess Al2O3 (see synthetics). This anomalous extinction (as it is currently named) can be seen as "tabby" extinction, resembling the color distribution of a cat's fur, or/and as an Andreas cross caused by pseudo-birefringence.


Spectra

Spectrum natural blue spinel iron.jpg

Spectrum of a natural blue spinel, colored by iron and minor traces of cobalt.

Phenomena

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

Occurrence

Large crystals of fine clarity and colour are known from very few locales: The Central Province (Hakurutale and Munwatte) of Sri Lanka has long been known as a source of facetable greenish blue to dark blue material, and crystals up to 30 mm or more in size have been found in Fianarantsoa (Betroka District) and Toliara Province (Androy and Anosy regions), southern Madagascar. Sapphirine's type locality is Fiskenaesset (Fiskenaes), Nuuk region, western Greenland, which is where the mineral was discovered in 1819.

Synthetics

Sources

  • Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster (6th ed.)
  • Edelsteinkuntliches Praktikum - Ulrich Henn, Gemmologie Jahrgang 44 / Heft 4 / Dezember 1995, Spinell pp.54-62
  • Diploma course notes 1987 - Gem-A
  • Über die Eigenshaften von im Flussmittelverfahren hergestellten synthetischen roten und blauen Spinellen aus Russland - U. Henn/H. Bank, Gemmologie Jahrgang 41 / Heft 1 / April 1992, pp1-7
  • Black Opaque Gem Minerals Associated with Corundum in the Alluvial Deposits of Thailand. Australian Gemmologist, 2008, Vol. 23, No. 6, pp. 242-253. Dr. Seriwat Saminpanya