Powellite, sitting atop Stilbite
This molybdenum ore got its name from an ex director of the USGS, and is one of the small number of common minerals that this metal forms, the other one being Wulfenite (see http://bit.ly/2vF1aoK). It forms in hydrothermal veins, in which hot waters circulating in the depths of the Earth are heated and circulate, picking up and reprecipitating elements as they encounter varied chemical and pressure/temperature combinations, often acting like a giant still.
In this case the fluids encounter the reduced silvery sulphide Molybdenite and oxidise it (see http://bit.ly/1I4XWKt for an explanation), often retaining the characteristic shape of the original mineral rather than its own, a process known as pseudomorphism after the Greek for 'false shape'. It often forms a solid solution series (in which one element substitutes for another) with the calcium tungstate mineral Scheelite (see http://bit.ly/2vlH4AR).
Colour ranges from colourless to yellow and brown through to green and blue or black, while the characteristic crystal habit consist of a four sided pyramid. The mineral is quite dense, and relatively soft, weighing in at 3.5-4 on Mohs scale. Famous locations include various mines in the USA, Nasik in India (born in the Deccan Trapp lavas that covered vast spaces around the time of the death of the dinosaurs), Turkey, Russia, Scotland, Chile, Panama and Morocco. It has been faceted but suitable material is as rare as hen's teeth.
The 3cm gemmy brown crystal in question is from India, and grew on a zeolite mineral which we also covered in the distant [past (see http://bit.ly/2fbCj7n). The specimen measures 6.2 x 4.2 x 2.9 cm
Image credit: Rob Lavinsky/iRocks.com
http://bit.ly/2feN6vV http://www.galleries.com/Powellite http://www.mindat.org/min-3275.html http://www.gemdat.org/gem-3275.html http://www.minerals.net/mineral/powellite.aspx
