Boleite One of the more beautiful silver minerals has the common habit (as the typical shape of a cr

BoleiteOne of the more beautiful silver minerals has the common habit (as the typical shape of a crystal is called) of forming lovely dark blue cubes. Its composition bears testimony to the main manner in which metals (including precious ones) travel and are concentrated from the depths of the crust, moving in solution in hot caustic fluids of various origins, from volcanism and metamorphism to dewatering of sediments deep within a basin under the pressure of overlying rocks. In all cases the metals are mobilised and concentrated from large diffuse areas of rock, and then precipitate out in various forms when a change in chemical or pressure/temperature conditions are encountered.In these hot subterranean brines, the metals commonly travel tied to and ion in solution. Common ligand ions (as the transport ones are known) include sulphur from volcanic gases and chlorine from salt in the brine, ferrying dissolved metals around in the depths of the earth. The cargo then precipitates out in native form, or tied to other ions in the form of primary minerals, often as sulphides. These can then be further transformed into secondary minerals, often when encountering oxygenated and carbonated waters percolating down from the surface.Boleite has a complex formula (KPb26Ag9Cu24(OH)48Cl62 ) , reflecting a polymetallic solution that abruptly dumped its load, with many metals precipitating simultaneously and forming primary sulphides. At some later point in geological time, chlorine rich waters passed through, altering the primary sulphides (and probably carrying them off as sulphates) and remobilising the metals into this mineral grab bag. It has also turned up in furnace slag that has been immersed in and leached by the sea.It is a minor ore of all three metals, named after its type copper mining locality at Boleo in Mexico. Each cube is actually 3 crystals oriented at right angles to each other, twinned to form semeing Platonic perfection. It has occasionally been faceted for collectors, though its perfect cleavage (a line of weakness where there are less bonds holding the crystal lattice together) makes it tricky to cut and its softness (3 on Mohs scale) make it unsuitable for jewellery use. Its heavy metallic content makes it dense, some 5 times as much as water, giving it a distinctive heft in the hand. As well as Baja California, other localities include the complex polymetallic deposit at Broken Hill in New South Wales, Arizona, Russia, Chile and the Mendip Hills of England.Our first sample (0.9 x 0.9 x 0.8 cm) is a lovely solitary cube, while our second (6.5 x 4 x 3.5 cm) is a clump of them on the original matrix, while the third (3.5x4.4cm) is an exceptional cube on matrix measuring a centimetre across. All three come from the type locality on the Baja California peninsula.LozImage credit: 1&2: Rob Laavinsky/iRocks.com 3: Exceptional mineralshttp://www.mindat.org/min-712.htmlhttp://www.galleries.com/Boleitehttp://bit.ly/1ZTViBW -- source link

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