OBSIDIAN (a rock, not a mineral)

By Dr. Ray Grant and Chris Whitney-Smith

The “Mineral of the Month” for February is not a mineral but a rock, obsidian. Obsidian forms when silica rich magma cools very fast, so fast that crystals can’t form. This could happen if the volcanic eruption is under water or ice.  Millions of years ago there was more obsidian in Arizona, but it is unstable and over time it changes from glass to form perlite or breaks down and is eroded.

The best-known Arizona occurrence of obsidian is the Apache tears location near Superior.  This obsidian formed from a volcanic eruption about 15 million years ago.  Since then the hydration process has changed most of the obsidian to perlite and only small round remnants of the obsidian are left, the Apache tears.  The perlite is being mined there for a lightweight aggregate material.

A website, Sources of Archaeological Obsidian in the Greater American Southwest (www.swxrflab.net/swobsrcs.htm) has an interactive map of the obsidian locations in the southwest. The map has nine locations in Arizona and a couple on the New Mexico - Arizona border, and many others in the surrounding states. There are also a couple of additional Arizona locations in the text. You click on the location on the map and a detailed description of each of the locations comes up. There is township, range, and section data given for each locality, a fairly detailed description of what was found there and in some cases photographs and maps. There is a lot of chemical data for the obsidian as the goal was to identify the sources of obsidian used by prehistoric people. The obsidian from each of the various localities has a unique chemical composition. For example 220 samples of obsidian were analyzed from Pueblo Grande in Phoenix.  Four were from the Sand Tanks location, 67 were from the Sauceda Mountains locality and so forth.  Obsidian was important to the native people and collected and exchanged all over the southwest.

A location that caught my attention was Topaz Basin in Yavapai County. It is not on the website map, but in the text. I checked the USGS Arizona Place Names Gazetter and there is an official place, Topaz Basin.  It is close to Interstate 17, by the route 169 turnoff.  The website states the archaeological investigators found no topaz and that perhaps the small clear obsidian nodules they found were misidentified as topaz.  Sounds like a place to visit.

Show & Tell or Just Show: Members are invited to bring (ONE) sample from their collection of the mineral of the month and give a brief story about where they collected it or something about the specimen. 

Unknown minerals for identification can still be brought to the meetings.

IMAGE CAPTIONS:

1) OBSIDIAN Var. FIRE, Cabochon, 2.7cm, Oregon, USA. Tom Dodge Collection, Jeff Scovil photo.

2) OBSIDIAN Var. SILVER SHEEN, Silverback Gorilla Carving by Gerd Dreher, 21.2cm, Russia. Houston Museum of Natural Science Collection, Jeff Scovil photo.