Bixbyite
Typically black and metallic in appearance we have here a manganese iron oxide, though the proportions of each are very variable. Crystallising in the cubic system it takes on the characteristic shapes associated with it, the octahedron, the dodecahedron and most frequently the cube. The name has been confused with Bixbite which is a form of red beryl (see http://bit.ly/2AgL9ZJ), and it came from its discoverer, an American mineralogist and dealer from Utah who described it in 1897. It forms in veins and cavities during silica rich rhyolitic volcanism or high temperature metamorphism of manganese deposits, often precipitating from hot fluids but occasionally from mineralised steamy gases pervading the rocks.
Crystals are mostly small, maxing out at a centimetre or so though much material is massive, and a variety of manganese ore. Like many metal ores it is dense, some 5 times more than water, and relatively hard weighing in at 6.5 on Mohs scale, between feldspar and quartz. It is found in India, the Thomas range of Utah, Mexico, Northern Patagonia, Spain, Sweden and South Africa (in the Kalahari Manganese Field, fount of many lovely mineral specimens). Their beauty and bright metallic lustre makes this uncommon mineral very popular amongst collectors despite the absence of strong bright colours or limpid transparency, especially when contrasted on a matrix of bright white or grey rhyolitic lava. The 5.5 x 3.8 x 3.3 cm specimen in the photo is from Utah, and has attached to it a lovely bonus doubly terminated white topaz crystal (see http://bit.ly/2jQ7sB0) that reflect beautifully in the Bixbyite.
Loz
Image credit: Rob Lavinsky/iRocks.com
https://www.mindat.org/min-691.html http://www.minerals.net/mineral/bixbyite.aspx http://www.galleries.com/Bixbyite http://bit.ly/2n50xWa
