Tuesday, January 29, 2013

Green Rocks and Minerals

This past fall, I was working on a gold project near Cheyenne, Wyoming and met two old friends for dinner. One had an affection for green minerals and rocks, so I thought I would tell you a little about some green rocks in my back yard.  One specimen has a lot of lumps and kind of reminds me of brains. So I call it jaded brains. It has a pleasing light-green color and was picked up in the jade fields of Wyoming.

Light green jade
The geological and geochemical processes that produced jade are intriguing. The jade fluids produced under great pressure (and temperature) tended to replace much of the original rocks and minerals to produce jade. Each atom of the original mineral or rock was mobilized and replaced by the atoms that make up jade. We don't know if this was a rapid process or a process that took place over millions of years.  But we can see the results and sometimes, much of the original rock matrix was replaced by jade leaving feldspar phenocrysts (larger crystals in a fine-grain groundmass or matrix) somewhat in tact.  In other specimens, we find quartz crystals that are preserved and untouched, but in others, even the quartz is not safe, as we find evidence of quartz (hexagonal) being replaced by jade.

Jade porphyry. Much of the original rock matrix was replaced by jade leaving some greenish
white feldspar in place that are also partially replaced.


Jade specimen with quartz crystal preserved.