Axinite

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

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.: n_α = 1.674-1.693, n_β = 1.680-1.701 and n_γ = 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 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)