Tinzenite The rare mineral group known as axinite grows in…

Tinzenite The rare mineral group known as axinite grows in…

Tinzenite

The rare mineral group known as axinite grows in beautiful bladed axe like crystals that are named after the Greek for axe. The colours of this calcium iron/magnesium/manganese borosilicate vary from violet to reddish brown or yellow, and its hardness of 6-7 on Mohs scale means that it is sometimes faceted for gem collectors (though the perfect cleavage along lines of weaker bonding within the structure makes this difficult, as entire layers of the crystal can separate off when polishing it on the wheel). Like some members of the garnet group, the three elements separated above by slashes form what is called a solid solution series, in which these elements are mixed, and the type of axinite depends on the proportions of the mixture between them in the crystal structure. This version is Tinzenite, usually yellow to orange in colour and hued by manganese.

Most axinite forms when the heat from granitic magma rising and cooling in the Earth bakes the rocks it pushes through, a process known as contact metamorphism, though a related process called metasomatism, in which hot element rich fluids circulate between the two kinds of juxtaposed rock. Axinite, like demantoid garnet (see http://tinyurl.com/kqop7lh), only forms when the granites intrude limestones forming a type of rocks (that often host mineral deposits) called skarns. It also occurs more rarely in certain kinds of silica poor mafic volcanic rocks and the Tinzenite variety is usually found here, in association with ophiolites, chunks of oceanic crust and upper mantle that have been pushed onto continents in collisions or similar rocks post metamorphism.

Inclusion free specimens are rare, as is common with many metamorphic gems, in which elements that do not enter the crystal structure of the gem in question crystalise as different minerals within the structure of the gem. Axinite displays very strong pleochroism (see our post on Alexandrite for an explanation at http://bit.ly/1PvWSE5), a phenomenon in which the crystal appears differently coloured depending on the direction from which it is viewed. This happens because the crystal structure separates light into different rays, each travelling through the crystal through separate pathways, and having some of their wavelengths absorbed on the way. The different light pathways are not absorbed in the same way, leaving three separate residual colours visible in the crystal.

It is one of the rare minerals with electrical properties, called pyro and piezo electricity. When rapidly heated or cooled, or stresses with pressure, the structure generates an electrical current. Tourmaline is another such mineral, and gems in shop windows often attract dust due to the currents generated during the day/night cycle. It also fluoresces reddish orange in UV light (for an explanation of how minerals do this see our past post on glowing diamonds at http://bit.ly/2uLwzXv). Axinite is found in France, Russia, Brazil, Paistan and Tanzania. It is not treated, and should not be exposed to harsh chemicals, detergents or ultrasonic cleaners. The 2.1 x 2.0 x 0.7 cm specimen in the photo comes from the Kalahari Manganese Field in South Africa.

Loz

Image credit: Rob Lavinsky/iRocks.com

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