Iron axinite, formally ferroaxinite, is the iron-dominant member of the axinite mineral group, a small family of complex calcium-aluminium boro-silicates that includes manganaxinite, magnesioaxinite, and tinzenite. Ferroaxinite is by some margin the most common axinite encountered in gem use and is the species responsible for the deep clove-brown to violet-brown crystals that have made axinite a connoisseur's stone since the nineteenth century.

Composition and crystallography

The chemical formula is Ca2FeAl2BSi4O15(OH), with iron occupying the divalent metal site that admits manganese or magnesium in the other species. Axinite crystallises in the triclinic system, the lowest-symmetry crystal system, and its crystals show a characteristic flattened, axe-head shape that gives the group its name, from the Greek axine for axe. This bladed habit, with sharply oblique faces, is unusual enough that axinite is recognisable by morphology alone in many specimens.

The hardness is 6.5 to 7 on the Mohs scale, the specific gravity is 3.27 to 3.35, and the refractive indices run from approximately 1.674 to 1.688 with a biaxial negative optic character. Birefringence is moderate at 0.010 to 0.012, sufficient to produce noticeable doubling of inclusions or back facets in well-cut stones.

Colour and pleochroism

Ferroaxinite's distinguishing visual feature is its strong trichroic pleochroism, with three observable colours along its three optical directions: typically a clove or cinnamon brown, a violet to purple, and an olive to yellow-green. The dichroscope readily separates these colours and is the simplest gemmological test for the species. The body colour seen in normal viewing is a blend dominated by the brown and violet components, often described as plum-brown.

Iron is the principal chromophore, and the colour is intrinsic to the crystal lattice rather than the result of any treatment. No commercial enhancements are routinely applied to ferroaxinite.

Sources

The species takes its name from the Bourg-d'Oisans area of the French Alps, where it has been mined since the eighteenth century, although gem-quality material is now scarce there. Mexico's Baja California, particularly the Casa de Yeso and surrounding skarn deposits, has yielded large fine crystals. Brazil's Minas Gerais, the Russian Urals, the United States (notably New Jersey and California), Tanzania, and Pakistan all produce gem ferroaxinite in modest quantities. The faceted material rarely exceeds five carats, and stones above ten carats are exceptional.

Cutting and care

The marked pleochroism imposes a real demand on the cutter, who must orient the rough so that the table face shows the most attractive colour blend. Poorly oriented stones can appear muddy or olive; correctly oriented ones glow with a violet-tinged plum. Step cuts and emerald cuts often suit the elongated rough shapes, although mixed and brilliant cuts are made when material allows.

Ferroaxinite is durable enough for occasional-wear rings and well suited to earrings, pendants, and brooches. Its lower toughness than ruby or sapphire warrants protective settings in ring use, and ultrasonic and steam cleaning are best avoided in favour of warm soapy water and a soft brush. The mineral is collected as much for its crystal aesthetics as for cut stones, and exceptional Bourg-d'Oisans or Baja specimens command serious prices among mineralogists.