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.

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