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16.27: Clinohumite

  • Page ID
    4363
  • Clinohumite
    Chemical composition Magnesium silicate
    Hardness 6
    Optic nature Biaxial +
    Refractive index 1.628 - 1.674
    Birefringence 0.024 - 0.041
    Specific gravity ~ 3.21
    Lustre Vitreous
    Pleochroism Strongly trichroic
    Fluorescence

    SWUV: strong orange yellow
    LWUV: generally inert

    Figure \(\PageIndex{1}\): A 9.45 ct brownish orange clinohumite tested at the Gem Testing Laboratory, Jaipur.

    Clinohumite is a member of the humite group. As a gemstone it is rare, but some cut stones are reported. The given below details are (largely) abstracted from Journal of Gemmology, 2007, 30, 5/6, 303-306 "A remarkably large clinohumite".

    Crystallography

    Monoclinic

    Diagnostics

    Color

    Bright yellow-orange color resembling some hessonite and spessartine garnets.
    Cause of color: Mn

    Diaphaneity

    Transparent

    Refractometer

    Clinohumite has a biaxial optic character with a positive optic sign.
    The refractive index range of clinohumite: nα = 1.628 - 1.638, nβ = 1.641 - 1.654, nγ = 1.662 - 1.674.
    The maximum birefringence ranges from 0.024 to 0.041.

    For a specific stone tested (9.45 ct. brownish orange clinohumite tested at the Gem Testing Laboratory, Jaipur): R.I.: 1.646-1.670, birefringence: 0.024.

    Pleochroism

    Strong trichroism: yellow, orange and brownish yellow.

    Specific gravity

    S.G.: ~ 3.21

    Spectrum

    Figure \(\PageIndex{2}\): Diffraction spectrum


    Brownish-orange clinohumite.
    General cut-off till 430nm (from 400nm).

    Luminescence

    SWUV: strong orange-yellow;
    LWUV: generally inert

    Magnification

    Inclusions: Strong color and growth zoning, flat parallel reflective planes, fluid-like inclusions similar to trichites in tourmaline. Such liquid inclusions along with doubling appear very much like tourmaline; tube like inclusions; sometimes swirly zones.

    Occurrence

    Geographical occurrence: Gem quality Clinohumite is known to occur in two important localities - The Pamir mountains (at Kukh-i-lal, Sumdzin and Changin); in Tajikistan in association with spinel and in the Taymyr region (Basin of Kotui river) in Siberia; Also in Mahenge, Tanzania.

    References

    • G.Choudhary and Chaman Golecha, A remarkably large Clinohumite, Journal of Gemmology, 2007, 30, 5/6, pp 303-306
    • Henn, U., Hyrsl, J., and Milisenda, C.C., Gem Quality clinohumite from Tajikistan and the Taymyr region, Northern Siberia. Journal of Gemmology, 27(6), 335-9
    • Hyrsl, J., 2001. Gem News International. Spinel with clinohumite from Mahenge, Tanzania, Gems and Gemology, 37(2), 144-5
    • Laurs, B.M., and Quinn, E.P., 2004. Gem News International. Clinohumite from Mahenge, Tanzania. Gems and Gemology, 40(4), 337-8