Axinite
Contents
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.
Luster
Bright vitreous
Hardness
7-6.5 (directional)
Specific gravity
3.27-3.29 [Ferroaxinite:3.17-3.31; Magnesioaxinite:3.18; range 3.26-3.36]
Cleavage
Distinct one directional
Fracture
uneven to conchoidal; brittle stone
Refractometer
Optic Character: anisotropic; Biaxial Negative (If high in Mg may turn +ve) R.I.: alpha:1.674-1.693 Beta: 1.680-1.701 and gamma:1.684-1.701; Generally R.I.:1.67 to 1.68 D.R.: 0.010 - 0.012
Dispersion
Dispersion: low (0.018 to 0.020)
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 cinammon 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)