#sperrylite

Sweet Notes is looking at the expensive stuff on display in The Vault at the Natural History Museum.

In London, England.

A bit more about the display here:

Sperrylite

Exceptional minerals require exceptional circumstances to paraphrase an old scientific dictum, and the platinum arsenide in the pictures is only found in appreciable quantities in 3 places on Earth, each with a wild and unique geological back story. It is opaque and shiny, forming mainly cubic and octahedral crystals when it isn't found in massive aggregate form. Like many similar minerals it is very dense in the hand (over 10x more than water) and since it contains platinum is it also quite hard, weighing in at 6-7 on Mohs scale, roughly between feldspar and quartz. It was first discovered by a chemist named Sperry at the Sudbury impact crater in Canada (see http://bit.ly/2wTXlOk and http://bit.ly/2x3rwV8 for further details), and formed when the heat released by the impact melted the rocks onto which the bolide crashed, which then segregated by density into layers, forming new minerals within. It is also found in the various ore bearing layers (eg the Merensky Reef) of the Bushveld igneous intrusion, one of the founts of South Africa's mineral wealth and one of the largest chunks of mantle magma that ever solidified in the crust (some 2bn years ago).

The 4.0 x 3.2 x 2.1 cm specimen in the photo comes from the third location, indirectly associated with the largest mass extinction in Earth's history at the end Permian some 251 million years back. Massive amounts of melted mantle rose into and through the continental crust of what is now Siberia, forming layered trapps of lava on the surface, and baking coal where they stalled in the crust, releasing gases that caused a sudden and catastrophic (for life at least) climate change. Some of the magma that stalled in the crust settled in layers of different minerals as the magma's chemistry changed as crystallisation removed certain elements preferentially, just like other magmas did at the Bushveld layered intrusion or the Sudbury impact crater at other times.

One of them is particularly rich in platinum, copper and nickel, and is found in the region around the mining city of Norilsk, whose human history could be argued to be one of the grimmest ever told. Sperrylite is the most common platinum mineral and since it is relatively tough and resistant to erosion it is also found in rivers downstream of all the localities as placer deposits (from the Spanish for pleasure, since it can be mined with a shovel and a pan rather than digging through hard rock).

Loz

Image credit: Joe Budd/Rob Lavinski/iRocks.com