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16.66: Sphene

  • Page ID
    6496
  • 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

    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