Saturday, September 2, 2017

GemHunter's Guide to Finding Peridot

One of many public field trips scheduled to teach people where and how to find gemstones and gold. On this trip, the author discussed pyrope garnet and chromian diopside in anthills at Butcherknife Draw in southwestern Wyoming. Hausel, a Hall-of-Fame geologist formally at the Wyoming Geological Survey at University of Wyoming (1977 to 2007); and a Hall-of-Fame martial arts instructor for the Departments of Physical Education, Kinesiology, Club Sports and Extended Studies (1977-2010); and author of more than 1,000 publications, was the most awarded geologist and martial arts instructor in the history of the University of Wyoming and Wyoming Geological Survey. Dozens of awards were presented to Hausel by international, national and local associations- these included the Education Award from the Rockhound Hall-of-Fame, the Thayer Lindsey Award from the PDAC in Canada, and the Wyoming Geological Association Distinguished Service Award.

Peridot is a beautiful gemstone sometimes found in anthills associated with nearby olivine basalts (picrobasalt) and alkali olivine basalts. It is also recovered from some lamproites and basalts that trapped peridotite nodules (xenoliths) in magma and transported it to the earth's surface from depth. It occurs in some lunar basalts and is found in rare meteorites. When the author worked at the University of Utah in the 1970s, samples of lunar basalt, anorthosite, and breccia were tested for chemistry and mineralogy. Peridot is also present in rare terrestrial rocks known as ultramafic komatiites and olivine lamproites and notably derived from peridotite xenoliths. The peridot color in these rocks is typically olive green, but can also be grass-green, lime-green, yellow-green, and even orange to brown.

Peridot, also known as olivine, forms a solid-solution that ranges from a magnesium-rich end member - forsterite (Mg2SiO4) to the iron-rich end member - fayalite (Fe2SiO4). The orthorhombic crystals of the solid-solution mineral [(Mg,Fe)2SiO4] is typically called olivine by mineralogists. When gem quality, the mineral is referred to as peridot or chrysolite by gemologists. 

Typical of minerals derived from the upper mantle, peridot has a relatively high specific gravity ranging from 3.2 to 4.3. It is glassy (vitreous) and has a favorable hardness (H=6.5 to 7) for gemstone. The largest known faceted peridot resides in the Smithsonian collection and reported to weigh 310-carats. While at the University of Wyoming, the author ordered gems faceted from Wyoming peridot that was only 8 millimeters to 1 millimeter across. These were cut in Sri Lanka, a place of many gem-cutters whom specialize in small stones.

Part of the more than 13,000 carats of peridot gemstones
discovered by Hausel in the Leucite Hills of
Wyoming.
Some of the better places to collect peridot in the United States are: (1) the green sands of Mahana Beach on Oahu Hawaii (also known as Papakolea Beach), (2) Peridot Mesa and Buell Park on the San Carlos Indian Reservation in Arizona. The Arizona peridot includes many beautiful gems ranging from millimeter size up to 5-carats in weight. (3) In New Mexico, peridot is found in anthills surrounding maar volcanoes known as Kilbourne Hole and Hunts Hole. These maar volcanoes are on BLM public lands in an area adjacent to the Mexican border frequented by teams of heavily armed cartels transporting people and drugs across the border.

Although olivine was known in Wyoming since the late 19th century, no one had bothered to examine the clarity of the calcium-magnesium-silicate until the author followed up on reports of olivine in the Leucite Hills in 1997. And, much of the olivine was gem-quality. During  reconnaissance of the lamproite field in southwestern Wyoming in a search for diamonds, the author recovered more than 13,000-carats of gem-quality peridot in two anthills near Black Rock. The anthills were collected and taken back to the Wyoming Geological Survey to be processed. The quality of the majority of the stones was very high and the anthills and ranged in size from 1 mm to  12 mm long, while larger gems were found in soils between anthills and the Black Rock lamproite. Later (2005), the author conducted field investigations in the Leucite Hills volcanic field search for diamonds, since peridot is derived from mantle xenoliths and xenocrysts eroded from the lamproites. Worldwide, peridot is used as a tracer mineral to search for diamondiferous lamproite, similar to the classical diamond indicator minerals associated with kimberliteLamproite is only one of two host rocks mined for commercial amounts of diamonds in the world. To verify the quality of the Wyoming peridot gems, many were cut in Sri Lanka and sent back to the Geological Survey in Laramie where the faceted stones were displayed in the foyer at the Dr. Daniel N. Miller building on the UW campus.

One of several peridot gemstones faceted from the Wyoming
olivine
 rough collected by Hausel in 1997. One can easily see
small threads in the background paper by looking through the
gem. The
 quality of most peridot from the Leucite Hills is 
very high.
Olivine has also been identified in thin-sections of kimberlite from the State Line district south of Laramie, in kimberlite in the Iron Mountain district northeast of Laramie, in kimberlite in the Indian Guide (Sheep Rock) district north of Laramie, as well as in kimberlites in Kansas and Montana by various researchers. However, the olivine in these kimberlites are mostly to entirely replaced by serpentine and smaller than those found in the Leucite Hills, and are not gem-quality. The author also identified serpentine pseudomorphs after olivine in serpentinite (ultramafic komatiite) at South Pass of Wyoming - a prominent gold district in Wyoming.

The author (aka GemHunter) searching for diamonds and colored gemstones in the Leucite Hills in 1997.  

The author at work at the University of Wyoming teaching
students how to break rocks
.

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