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16.20: Axinite

  • Page ID
    4356
  • Axinite
    Chemical composition Complex borosilicate
    Crystal system Triclinic
    Habit Axe shaped.
    Cleavage Perfect {100}
    Fracture Uneven to conchoidal, brittle
    Hardness 6.5 - 7
    Optic nature Biaxial - (or +)
    Refractive index 1.674 - 1.706
    Birefringence 0.010 - 0.012
    Dispersion Low, 0.018 - 0.020
    Specific gravity 3.27-3.29
    Lustre Bright vitreous
    Pleochroism Moderate to Strong

    File:Axinitesm2.jpg

    Figure \(\PageIndex{1}\): A pinkish/greenish brown axinite.
    Photo courtesy of Chaman Golecha, Gem Testing Laboratory, Jaipur.

    Chemical composition

    Ca3Al2BSi4O15(OH). Much variation in composition due to replacement of Ca by Fe, Mg, Mn. Therefore the composition may be written as (Ca, Mn, Fe, Mg)3Al2BSi4O15(OH)

    • +Mg - magnesioaxinite
    • If Fe more than Mn - Ferroaxinite
    • If Mn more than Fe - Manganaxinite
    • If Mn more than Fe and Ca less than 1.5 - tinzenite

    Crystallography

    Triclinic.
    Habit: broad and acute edged usually (axe shaped); varied habit; Also massive, lamellar, granular, curved. Generally in clusters, also tabular.

    Diagnostics

    Color

    Clove Brown, rarely honey yellow or plum blue. Also pearly gray, greenish yellow; violet-brown; colorless-yellowish(Mn); pale violet/reddish (Mn), blue (Mg).

    Diaphaneity

    Transparent to semi transparent.

    Hardness

    7-6.5 (directional)

    Specific gravity

    3.27-3.29 [Ferroaxinite:3.17-3.31; Magnesioaxinite:3.18; range 3.26-3.36]

    Refractometer

    Optic Character: anisotropic; Biaxial Negative (If high in Mg may turn +ve).
    R.I.: nα = 1.674-1.693, nβ = 1.680-1.701 and nγ = 1.684-1.706.
    Generally R.I.:1.67 to 1.68
    Maximum birefringence: 0.010 - 0.012

    Spectrum

    Strong bands at 512, 494, 466 and sometimes 415nm; strong lines at 523, 444.

    Luminescence

    UV fluorescence: May show red-orange under SWUV

    Pleochroism

    Strong depending on body colour and source. Generally olive green/violet blue and cinnamon brown.

    Simulants

    • Andalusite: Lower S.G., spectrum, R.I. etc
    • Barite: Higher heft, lower hardness
    • Smoky Quartz: Lower Heft (floats 2.88), bulls-eye optic figure, R.I. etc
    • Sphene: Doubling, dispersion, spectrum

    Occurrence

    Geological occurrence:
    Occurs in cavities of granite or diabase; contact zones

    Geographical locations:
    Good crystals from St. Cristophe, France; various American localities; Bahia, Brazil; Cornwall, England; Mexico; Also in California; Sri Lanka; Nevada (masses); New Jersey; Germany; Norway; Finland; USSR; Japan; Tasmania; Switzerland (Tinzenite); Tanzania (Magnesoaxinite); Sri Lanka (ferro axinite-cinnamon brown)