16.66: Sphene

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Sphene

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

File:Green sphene.jpeg

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

n_α = 1.843 - 1.950, n_β = 1.870 - 2.034, n_γ = 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