Cyclosilicate: Axinite

On an excursion in the Alps in 1797, mineralogist, R. J. Hauy discovered some highly vitreous, piezio-electric wedged-shaped crystals that resembled schorl, the dark variety of tourmaline. The incorrect designation, “vitreous schorl”, was used even after axinite (Ca,Mn,Fe,Mg)3Al2BSi4O15(OH) was identified as a complicated borate silicate group of minerals. The characteristic axe shape of its crystals gave it its name.

If magnesium is the predominant element of the incorporated ions into its four-member tetrahedral “ring” silicate structure, then it is magnesioaxinite. In ferroaxinite, the iron content is greater than manganese. Manganaxinite is the result if manganese exceeds the iron content. In tinzenite, manganese exceeds the iron content, and calcium is less than 1.5. Since axinite develops in the triclinic crystal system with its low degree of symmetry, and its angles are very acute. Its habit is beautifully illustrated in a picture of rich brown crystals in matrix on page 67 of the Color Treasury of Crystals by Vincenzo De Michele of the Natural History Museum of Milan, Italy.

Massive material in lamellar to granular form is found. Homogeneous growth of the crystals almost always causes feathers and zoning. An excellent photograph of zoning and large two-phase inclusions in a yellow-brown Sri Lankan stone can be found on page 415 of The Photo Atlas of Inclusions in Gemstones by E.J. Gubelin and J. I. Koivula.

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Axinite, a metasomatic (replacement) mineral, occurs in contact metamorphic areas in basic rocks and veins. Gem quality material is found in several counties in California, Sri Lanka, and in the French Alps. Deposits of massive material are located in Luning, Nevada, Pennsylvania, and New Jersey. Other sources are Cornwall, England, Finland, Norway, Germany, Japan, Russia, Tasmania, and Mexico. Tinzenite is found in Switzerland and magnesioaxinite in Tanzania.

Crystals large enough to cut gems over ten carats in size are extremely rare. Dr. Joel Arem states in his Color Encyclopedia of Gemstones that “clean stones over 5 carats are —- worthy of museum display”. Despite being a bit brittle, its brilliance and sumptuous colors make it a unique and exquisite gemstone.

Gemstone Properties

Specie Axinite
Composition: (Ca,Mn,Fe,Mg) 3Al2BSi4O15 (OH).
Class: Silicate {Cyclosilicate}.
Group: Axinite.
Species: Magnesioaxinite, Ferroaxinite, Manganaxinite, Tinzenite.
Crystal System: Triclinic.
Varieties: By color, species names & crystallography
Colors: Colorless, yellow, olive green, cinnamon brown, violet brown, pale violet to reddish violet, and blue.
Phenomena: None.
Streak: White.
Diaphaneity: Transparent and translucent
Habit: Distinctive wedge-shape crystals, crystalline aggregate, & massive.
Cleavage: One perfect and some poor.
Fracture: Conchoidal and brittle.
Fracture Lustre: Vitreous.
Lustre: Highly vitreous.
Specific Gravity Varies between 3.26 to 3.36; Magnesioaxinite is 3.18
Hardness 6.5 to 7.0
Toughness: Fair
Refractive Index Alpha at 1.674 to 1.693; Beta at 1.681 to 1.701; Gamma at 1.68
Birefringence: 0.010 to 0.012
Optic Character Biaxial negative; positive if high in magnesium.
Dispersion: High.
Pleochroism Usually strong trichroic in reddish-brown, yellow-brown, olive green, yellow, & colorless; In Luning, Nevada material: reddish-brown, deep brown, pale brown, or colorless; In Sri Lankan material: reddish brown, deep violet, & colorless to pale yellow.
Ultraviolet Flourescence In Franklin, New Jersey material: SW=red; LW=inert; In magnesioaxinite from Tanzania: SW=dull red; LW=orange-red.
Spectra broad lines at 4150, 4660, & 4920; narrow line at 5120; possible lines at 4440 & 5320.
Color Filter no information
Solubility dissolves slowly in HF; use HCl with caution.
Thermal Traits avoid thermal shock; fusible with intumescence (swelling) at 2.5 to 3.0.
Treatments none known
Inclusions is seldom flawless; has color zones, feathers, & rare two-phase inclusions.
By Edna B. Anthony, Gemologist P.O.# 62653; Colorado Springs, CO. 80962
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