16.66: Sphene
- Page ID
- 6496
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| Sphene | |
|---|---|
| Chemical composition | CaTiOSiO4 |
| Crystal system | Monoclinic |
| Habit | Wedge shaped, massive |
| Cleavage | Distinct, prismatic |
| Fracture | Conchoidal |
| Hardness | 5.0-5.5 |
| Optic nature | Biaxial + |
| Refractive index | 1.880-2.099 |
| Birefringence | 0.100-0.135 |
| Dispersion | High, 0.051 |
| Specific gravity | 3.45-3.55 |
| Lustre | Resinous to sub-adamantine |
| Pleochroism | Distinct to strong |
Figure \(\PageIndex{1}\): Green sphene from Madagascar.
Photo courtesy of Scott Davies, americanthai.com
| Sphene image gallery |
Sphene is the older name of this mineral which refers to its crystal shape. Titanite is the universal name amongst mineralogists today and refers to its content of titanium. Greenovite is the name given to red or pink sphene. Gemologists use the name sphene for gem material titanite.
Sphene is isostructural with tilasite, malayaite and fersmantite.
Habit
Wedge-shaped crystals that may show parting due to twinning. Less common massive or lamellar. Sphene is normally fine-grained but occasionally forms large crystals.
Sphene may show some degree of metamictization.
Diagnostics
Color
Yellow, brown, green, reddish.
Color is caused by rare earth elements (didymium) for yellow stones and chromium for the intense green chrome sphene.
Streak
White.
Diaphaneity
Transparent to opaque.
Refractometer
nα = 1.843 - 1.950, nβ = 1.870 - 2.034, nγ = 1.943 - 2.110 with a birefringence of 0.100 - 0.192.
Optical nature: biaxial positive.
A strong doubling of facets is seen in cut stones.
Polariscope
Due to its small to moderate 2V value (17-40°) a clear biaxial interference pattern may be seen (two melatopes visible in one image).
Pleochroism
Moderate to strong trichroism.
Yellow to brown stones: colorless, greenish-yellow, reddish.
Spectroscope
Sometimes a spectrum can be seen.
Mean absorption lines: 586, 582. Sphene may show (weak) rare earth spectra due to didymium.
Fluorescence
None (probably due to iron content).
Varieties
- Chrome sphene, intense green (color caused by chromium).
- Greenovite, a reddish variety owning its color to manganese (MnO) impurities.
Treatments
Sphene is changed to red or orange through heating.
Durability
Sphene's relative low hardness makes it vulnerable to abrasion.
Occurrence
Sphene is a common and widespread mineral in many igneous rocks and metamorphic rocks. Associated minerals are pyroxene, amphibole, feldspar and quartz.
Facet-grade sphene was found in Mulla Ghani Baba in Mohmand Agency, NW of Peshawar, Pakistan, in 2004. The color of this sphene is medium to dark brown, and a significant 'red flash' was notable in the faceted gems. In Badadkshan, Afghanistan, sphene was also found in 2004. The color of Afghan sphene is greenish-yellow. While India also produces fine sphene, Madagascar remains the main source for facet-grade sphene. While brown stones from Madagascar can be large, the green stones are most prized by gem collectors and connoisseurs.
Other deposits:
Myanmar, Sri Lanka, Brazil, Mexico, Russia and Austria
Sources
- Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster (6th ed.)
- Gemstones of the world, 3rd rev and exp edition (2006) - Walter Schuman ISBN 1402740166
- Mineralogy second edition (2002) - Dexter Perkins ISBN 0130620998
- Introduction to Optical Mineralogy (2004) -
- Gemmology 3rd edition (2005) - Peter Read
- Gem Reference Guide (1995) - GIA ISBN 0873110293