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16.60: Scapolite

  • Page ID
    6488
  •  

    Scapolite
    Chemical composition Complex sodium aluminium silicate
    Crystal system Tetragonal
    Habit Prismatic and massive
    Cleavage Distinct to perfect (depending on direction)
    Fracture Conchoidal
    Hardness 6
    Optic nature Uniaxial -
    Refractive index 1.545-1.580 (depending on color)
    Birefringence 0.016-0.02
    Dispersion Low, 0.017
    Specific gravity 2.634-2.74 (depending on color)
    Lustre Vitreous
    Pleochroism Medium to strong (depending on color)

    Diagnostics

     

    Polariscope

    Scapolite can be easily confused with quartz - in particular the violet version with amethyst - due to overlapping refractive indices. The optic character for scapolite is uniaxial with a negative sign while quartz is uniaxial with a positive sign. In addition; quartz will usually show a bull's-eye. Separating the two is, in general, an easy task (30 seconds) when one is familiar with the conoscope addition to the polariscope.

     

    Tenebrescence

    File:Tenebrescent scapolite.jpeg

    Figure \(\PageIndex{1}\): Tenebrescent scapolite before and after exposure to UV light.
    Photo courtesy of Scott Davies, americanthai.com

    A fairly recent find (2005) in Badakhshan, Afghanistan is tenebrescent scapolite. This colorless to silvery material is unearthed near the hackmanite deposits and shows an aquamarine color after exposure to SWUV light. The intensity of this color (blue) depends on the time it has been exposed to the UV lighting. Exposure to a UVP UVG4 SWUV lamp for 15 minutes triggered an almost Santa Maria aquamarine blue color that faded gradually during the following 2-3 minutes in natural daylight.