Prospecting for Perlite May Lead to Tears EarthCache
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Prospecting for Perlite May Lead to Tears
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Find out the origin and uses of the porous gray-white rock and rounded blebs of translucent volcanic glass in this area.
BLM LINK:
http://www.blm.gov/pgdata/etc/medialib/blm/nv/information.Par.77566.File.dat/collecting_on_publiclands.pdf
This area has been prospected in the past for the industrial commodity perlite, which here also contains small "Apache tears."
Perlite is a hydrated (contains water) volcanic glass with a pearly luster and concentric "onion-skin" fractures. The "Apache tears" are unhydrated cores of obsidian (volcanic glass) occurring at the center of a group of concentric fractures in the perlite. Perlite often occurs at the chilled margins of rhyolitic (light-colored, silica-rich) volcanic flows or shallow intrusive rocks, where the volcanic glass has fractured and allowed meteoric water to enter the glass structure. The extent of hydration of the glass increases with temperature; thus the unhydrated "Apache tear" cores represent the point at which the volcanic rock cooled below the optimum temperature for hydration to occur.
The light grayish white rock in the cut wall and the "sand" eroding from it are composed of perlite, which typically contains 2%-6% water. Companies that mine perlite process it by heating the perlite to high temperatures at which the water in the glass vaporizes, causing the perlite to "pop" like popcorn. This produces a product called expanded perlite, with up to 20 times the volume of the original perlite.
Most expanded perlite is used to make lightweight construction materials such as concrete and plaster aggregate, acoustical ceiling tile, and insulation board, but because it is very porous, it is also used to filter industrial effluent, fruit juices, and oils. It is used as a soil conditioner to increase drainage and water retention, and as a filler in paints and an extender in plastics. The United States is the world's foremost producer of perlite; most of the mined deposits are located in New Mexico and other states west of the Rocky Mountains, including Nevada. There was probably too much unhydrated glass (obsidian) in this perlite to make it a good marketable source of perlite, which is why it was not developed any further than the dozer cut you see.
According to one legend, the origin of the term "Apache Tears" for these small obsidian nodules is as follows.
In July 1870, a band of about 75 Apaches responsible for an earlier raid was surprised by US Cavalry troops on a rugged mountain top overlooking what is now Superior, Arizona. The outnumbered Apache warriors who escaped the initial volley of gunfire deliberately jumped off the steep cliff face of the mountain to their deaths. The bereaved families and friends of the Apache braves later gathered at the base of the cliffs and mourned for days following the tragedy. Legend is that their tears turned into stone upon hitting the ground and the translucent gray to black nodules of obsidian found nearby are commonly called Apache Tears. The place in Arizona where this occurred is now known as Apache Leap Mountain.
To log this Earthcache, visit the site and answer the following questions, and email them to the Earthcache owner in a separate e-mail with the heading PROSPECTING FOR PERLITE EARTHCACHE ANSWERS.
1. LOOK AT THE CUT ROCK FACE OF THE PROSPECT. DESCRIBE THE GRAY-WHITE PERLITE ROCK - IS IT EASILY ERODED OR VERY RESISTANT TO EROSION? (TRY MANUALLY "ERODING" IT YOURSELF AS A TEST)
2. WHAT ARE THE GENERAL SHAPES AND SIZES OF THE APACHE TEARS WEATHERING OUT OF THE PERLITE?
3. CAN YOU APPROXIMATE THE DIAMETER OF THE LARGEST APACHE TEAR YOU SEE ON THE GROUND AROUND THE PROSPECT AREA? (FOR INSTANCE IS IT THE SIZE OF A RAISIN? AN OLIVE? A GRAPE? AND EGG? OTHER?)
4. WHEN YOU LOOK AT THE CUT ROCK FACE OF THE PROSPECT, ESTIMATE THE NUMBER OF DARK GRAY/BLACK APACHE TEARS YOU CAN COVER WITH YOUR HAND.
5. WHAT OTHER KINDS OF ROCKS DO YOU FIND IN THE IMMEDIATE VICINITY OF THE PROSPECT. DO THEY APPEAR TO BE PREDOMINANTLY SEDIMENTARY, IGNEOUS, OR METAMORPHIC ROCKS AND HOW CAN YOU TELL?
THIS ABANDONED PROSPECT IS LOCATED ON PUBLICLY ACCESSIBLE BLM LAND AND IS NOT PRIVATE PROPERTY. BLM REGULATIONS AND GUIDELINES ON ON COLLECTING SMALL AMOUNTS OF ROCK FOR PERSONAL USE (NOT FOR SALE) ARE DESCRIBED ON THE BLM LNK PROVIDED.
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