Saturday, September 2, 2017

GemHunter's Guide to Finding Peridot

One of more than a hundred field trips led by the GemHunter (Dan Hausel) to teach prospectors and geologists how to
 find gemstone & gold deposits. In this photo, Hausel discusses pyrope and chromian diopside gemstones found in anthills
in the Butcherknife Draw area of southwestern Wyoming.
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 that contain peridotite nodules (xenoliths) trapped in the basalt and lamproite magma and brought to the earth's surface from the earth's upper mantle in the hot magma. It occurs in some lunar basalts recovered by the Apollo program and is also found in rare meteorites. When the GemHunter conducted research at the University of Utah in the 1970s, samples of basalt, anorthosite, and breccia were tested for chemistry and mineralogy. Some lunar basalts contained minor amounts of olivine. Peridot is also present in rare rocks on earth known as ultramafic komatiites and olivine lamproites and is notably derived from xenoliths of peridotite. The color of peridot 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 known as forsterite (Mg2SiO4) to the iron-rich end member known as fayalite (Fe2SiO4). Solid-solutions are essentially mineral mixtures. 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 that ranges from 3.2 to 4.3. It is glassy (vitreous) and has a good hardness (H=6.5 to 7) for a gemstone. The largest known faceted peridot resides in the Smithsonian collection and reported to weigh 310 carats. The smallest gems are tiny. While employed as the Senior Economic Geologist at the Wyoming Geological Survey at the University of Wyoming, the GemHunter had gems faceted from peridot that was only 8 millimeters to 1 millimeter across. These were cut in Sri Lanka, a place where many gem-cutters specialize in small stones.

Part of the more than 13,000 carats of peridot gemstones
discovered by the GemHunter in the Leucite Hills of
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 the maar volcanoes known as Kilbourne Hole and Hunts Hole. These maar volcanoes are on BLM managed public lands in an area near the Mexican border that is apparently frequented by teams of illegal aliens crossing the border transporting illegal drugs.

Peridot was only recently discovered in Wyoming. In 1997, the GemHunter followed up on reports of olivine in the Leucite Hills that had been mentioned in passing by past researchers, only to discover more than 13,000 carats of gemstones in two anthills near Black Rock. The anthills were collected and taken back to the Wyoming Geological Survey to be processed and verify the discovery. The quality of the majority of the stones was very high and the anthills contained gems from 1 mm to  12 mm long, while larger gemstones were found in nearby soils between the anthills and the Black Rock lamproite by sieving the soils. Later (2005), the GemHunter conducted field investigations in the Leucite Hills volcanic field to map the lamproites and search for potential diamonds, since the peridot was derived from mantle xenoliths and xenocrysts eroded from the lamproites. Worldwide, peridot is a tracer mineral used 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 gemstones, 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 the GemHunter in 1997. One can easily see all of the
small threads on the background paper by looking through the gem. The
quality of most peridot from the Leucite Hills was very high.
The GemHunter (W. Dan Hausel) also identified olivine in rock 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 Sheep Rock district north of Laramie, as well as in kimberlites in Kansas and Montana. However, the olivine's in these kimberlites were mostly to entirely replaced by serpentine and considerably smaller than those found in the Leucite Hills and not of gem-quality. Hausel also identified serpentine pseudomorphs after olivine in serpentinite (ultramafic komatiite) in the South Pass greenstone belt of Wyoming - a prominent gold district in Wyoming.

Wyoming Geologist, W. Dan Hausel (aka GemHunter) searching for diamonds and colored gemstones in the Leucite Hills
in 1997.  Hausel discovered peridot in the Leucite Hills and made many other gemstone, gold, mineral discoveries. 

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