Peridot - Part 2: From the Depths to the Surface

Greetings Gemstone Fans:

I mentioned in my post Peridot - Part 1 that the mineral olivine, known in gem-quality as peridot, can be brought up from depth by volcanism.  I visited one of these sites in New Mexico - Kilbourne Hole, a volcanic explosion crater - and got to see the results for myself.

Such craters are called "maar" craters, and are made by a shallow underground volcanic explosion. They were once confused with impact craters, which are caused by chunks of rock or metal from space striking the planet's surface. In fact, the craters on the Moon were thought by many to have been caused by volcanism, and were only shown to certainly be of impact origin after we visited there in person.

Kilbourne Hole is off the beaten path in southern New Mexico, but not so remote that it can't be found by dedicated stone hunters. (As it happens, I do not visit sites to hunt rocks, I visit them as a geoscientist, and was there to learn about the geology.) This maar is found on federal BLM land, not a national or state park, so is not protected in the same way. Larger crystals of olivine have long since been carried off by rock hounds, but there are patches of sand-sized green crystals remaining. Many of these were produced by rock hunters shattering thousands of olivine xenoliths looking for gem quality stones. There are only a handful of places on the Earth where olivine crystals are found on the surface like this, and so such sites are very popular.

Rarely at this site, one might find a volcanic rock, dark basalt, with splashes or crystals of bright green glassy material still clinging to it. The picture here shows just such a rock, encrusted with forsterite crystals. Forsterite is the name for olivine crystals that have a lot of magnesium in them, as opposed to fayalite, which is the more iron-rich end of the olivine spectrum. Such chemical differences are important to geologists who are trying to understand the volcanic history of the area.

It really is an amazing thought to look at this material and ponder how deeply in the planet it may have started its journey before making it to the surface. Not to mention the dramatic way it finally arrived.

Jennifer

Image Credits:

Kilbourne Hole - Bureau of Land Management, www.blm.com

Forsterite Crystals from Kilbourne Hole - Wikimedia Commons, Rob Lavinsky, iRocks.com – CC-BY-SA-3.0