When limestone gets baked Many rare calcium rich minerals form…

When limestone gets baked Many rare calcium rich minerals form…

When limestone gets baked

Many rare calcium rich minerals form in odd rocks called skarns, which one could describe as a geological stew. Granites and other intrusive magmas that stall in the crust on the way up form for a variety of reasons, ranging from meteorite impact or mountain building adding thermal energy to the crust and melting it (in the case of mountains by stacking slices of crust atop each other until the heat and pressure at the bottom melt the lower layers) to the underplating of the bottom of the crust by hot mantle magmas. In every case they rise, stop and freeze somewhere along the way, and altering the rocks within which they rest in a process called contact metamorphism, which leaves an aureole of different layers of mineralogy around the newly emplaced granite.

The heat given off by crystallisation starts to bake the surrounding country rock, and as freezing proceeds and the residual melt gets more hydrous these fluids are often expelled into the baked rock, carrying with them the elements that didn’t fit into the crystal structures of the standarg granitic minerals. These mix with elements from the baked rock creating a wide variety of exotic minerals of which this amphibole (a large family of hydrous magmatic minerals, see http://bit.ly/2h3fXUP for another example) called Richterite is one. It has also been found on occasion in manganese ore deposits, metasomatised rocks that have been transformed by circulating metamorphic fluids during mountain building and occasional calcium rich igneous rocks such as alkali magmas and carbonatites.

Crystals take the form of prisms, and vary from brown through yellow to green, and more rarely rosy red. With a Mohs hardness of 5-6 it could be used in jewellery, but faceted stones are usually found in mineral collections rather than rings. Famous localities are found in Canada, Sweden (the type locality), Afghanistan, Madagascar, Burma, Western Australia and the USA and it was named in 1865 after a German mineralogist who co discovered Indium. The 2.1 x 1.6 x 0.6 cm crystal in the photo comes from a single pocket find a decade or so ago in the Badakhshan region of Afghanistan.

Loz

Image credit: Rob Lavinsky/iRocks.com

https://www.mindat.org/min-3416.html

http://bit.ly/2w4unxl

http://www.galleries.com/Richterite