Monday, June 11, 2012

Origin of Peridot

Basalt filled with peridotite nodules from the earth's mantle
Much peridot that is recovered and faceted by rockhounds and lapidaries, is recovered from the erosion of olivine basalts. Some of these basalts are rich in olivine particularly where basalts have preserved many peridotite nodules such as lherzolite and dunite from the earth's upper mantle.

It is interpreted that the basalt is actually a melted by-product of the peridotite. As the magma was created at depth, the peridotite slowly melts producing the black basalt that has less density than the mantle, and will rise to the surface of the earth with pieces of the original mantle rock and erupt in a volcano, if the magma can find a path to the surface by way of a fracture. If the basaltic magma cools rapidly at the surface, some of the original peridotite may be preserved.

Through time, this basalt will erode and the olivine (peridot) will concentrate in streams, along beaches, or may be accumulated by ants to armor their hills from blowing winds. Here, the gemstones can be collected by prospectors and rock hounds. It also means that in some of these localities that the soil between the old volcano and the peridot concentrates likely contain millions of carats of the gemstone that likely could be recovered through digging and sieving.
Raw and faceted peridot


Thursday, February 11, 2010

Geology of Peridot Gemstones

Many have seen peridot in jewelry stores - a beautiful, light, olive-green gem with extraordinary sparkle that really sets the soul on fire! I don’t even wear jewelry, but I fell in love with this gem the first time I saw it. But, where does it come from and how does one prospect for the gem? First, there are lots of discoveries to be made. Just use scientific principals, learn a little about geology, and ignore anyone who tells you that everything has already been found – I’ve heard this so many times and it is so far from the truth. When I discovered a new peridot deposit in Wyoming in 1997, no one had any idea that the gemstone existed in the state. After I collected the material, I showed a gemologist/mineral collector this material as well as the director of the Wyoming Geological Survey and was told it was not gem-quality. You be the judge – take a look at the above brilliant cut peridot that was faceted from this deposit.

Left - Metabasalt from the Rattlesnake Hills & above, one of the first gemstones cut from olivine from the Leucite Hills. Prior to 1997, this gemstone (peridot) was unheard of in Wyoming even though several professional reports, dissertations and thesis were written on this area.

Let’s first look at some characteristics of the mineral. Peridot is a jeweler’s term used for gem-quality olivine. Mineralogists and geologists refer to this mineral as olivine [(Mg,Fe)2(SiO4)]. It is associated with mafic and ultramafic igneous rocks. Common mafic igneous rocks are basalts and gabbros. In particular, alkalic basalts and gabbros are likely to have olivine. Being mafic simply means the rock has relatively high magnesium and iron and low silica relative to other elements. The mafic minerals are dark in color and include olivine, pyroxene and amphibole. Most mafic rocks are also dark.

Ultramafic rocks are those that are even more enriched in magnesium than mafic rocks. Ultramafic rocks have less silica and typically >90% mafic minerals. The ultramafic rocks are rare and include such rock types as komatiite, kimberlite, olivine lamproite, lamprophyre, dunite, peridotite, anorthosite, troctolite and serpentinite.

Left - large peridotite xenolith in black rock lamproite, Leucite Hills and above right, metakomatiite (ultramafic) from Seminoe Mountains, Wyoming.

Where found, olivine is usually olive green, although varieties also come in yellowish-green, reddish-green and brownish-green. In nature, olivine is almost always rounded with a hardness of 6.5 to 7 making it a durable gemstone. With a specific gravity similar to garnet (3.2 to 4.3), it can be panned from streams: it will concentrate in black sands near a source rock. However, it does not transport far in streams because it is friable and chemically reactive. Olivine has imperfect cleavage, produces conchoidal fracture and has vitreous luster (like glass). The double refraction of olivine is high and stronger than any other mineral except zircon.

Geothermometry
Mafic and ultramafic igneous rocks are magmas that erupt at very high temperatures. When these magmas cool, the first mineral to crystallize from the hot magma is magnesium-rich olivine (forsterite) at 3,435oF. Olivine continues to crystallize until iron-rich olivine (fayalite) crystallizes at lower temperatures of 2,200oF (almost as hot as Phoenix in the summer). Lower temperature minerals such as pyroxene and feldspar, will crystallize as the magma cools further.

Olivine-rich magmas originate from the earth’s mantle. One rock that is relatively common in Wyoming and on the West Coast is serpentinite. This rock is composed entirely of serpentine and brucite and is the result of alteration of olivine. It is produced from an olivine-rich rock, such as dunite (see above photo of black basalt with large yellowish-green dunite nodule) that reacts with water vapor at high temperature (750oF) to produce serpentine and brucite (a hydrated magnesium oxide). Serpentinization is a common process because most olivine in igneous rock will react with water vapor at high temperature, or interact with groundwater during eruption, or interact with water vapor produced during regional metamorphism.

Gemology
Gem quality olivine is referred to as peridot. Olivine has also been referred to as chrysolite, noble chrysolite and evening emerald, terms now considered archaic. Popular cuts for peridot include table and step cuts as well as brilliant and rose cuts in gold jewelry. Most peridot on the market is less than 1 carat in weight. The largest known faceted peridot was a 319-carat stone from the Zabargad Island in Egypt.

Right- A marquise cut of flawless peridot from Leucite Hills, western Wyoming.

Peridot is a low-cost gemstone. For instance, the gem typically ranges in price for faceted gems of $20 to $100/carat. Higher prices are reserved for larger stones that exhibit excellent clarity.

Geology & Genesis
Olivine occurs as tiny grains when found in place. Large grains are rare except as mantle xenocrysts, megacrysts, xenoliths and nodules trapped in host igneous rocks such as lamproite, kimberlite, lamprophyre and some basalts. Although of limited supply, some meteorites have yielded peridot that is ‘out of this world’.

Basalts are periodically good sources for olivine. Basalts may contain small grains of olivine known as phenocrysts and/or may have small to large nodules of dunite or peridotite composed almost entirely of olivine. A dunite is simply a rock formed almost entirely of olivine. Peridotite is similar but contains minor amounts of pyroxene and sometimes garnet. Continental and oceanic olivine basalts with fresh peridot are typically Tertiary to Quaternary in age.

Left: garnet peridotite with red gem-quality garnet and green gem-quality olivine that is partially serpentinized.

Such xenoliths and nodules are often accompanied by olivine xenocrysts and phenocrysts that weather out of the hard basalt as detrital grains in soils and anthills. Some of the better known deposits are found at San Carlos (Peridot), Arizona; Kilbourne Hole, New Mexico; Chihuahua, Mexico; Hebei, China; Eifel and Dreiser Weiher, Germany; Lansarote, Canary Islands; Sidamo Province, Ethiopia; Queensland, Australia; Ross Island, Antarctica; Hawaii; and the Leucite Hills, Wyoming (Kievlenko 2003; Hausel and Sutherland, 2006; Hausel, 2009).

Peridot placers are uncommon simply because the gem is relatively unstable and susceptible to chemical weathering. This results in decomposition of peridot in creeks and rivers. The mineral is also friable resulting in size reduction during stream transport. Thus most peridot is recovered from eluvial deposits where peridot grains are found in soil adjacent to the source rock.

Deposits
Zabargad Island (formerly St. Johns Island) is located in the Red Sea. This deposit was the source of peridot from Biblical times (Hurlbut and Switzer 1979). The island is inhospitable and located 50 miles from the Egyptian port of Berenice. Much of the olivine formed by hydrothermal alteration of peridotite. The peridot is found in open fissures and cavities and in vein-like deposits in brecciated serpentinite (Sinkankas 1964).

The Zabargad peridot deposits were worked by Egyptian slaves for the benefit of the Pharaohs as early as 1500 BC. The island was later invaded by Crusaders during the Middle Ages and the 2 mi2 island was renamed St. John’s Island. However, the location of the peridot deposit was lost, and for centuries remained unknown until it was rediscovered in the early part of the 20th century (Keller 1990). Zabargad (which means peridot in Arabic) forms a small island within the Red Sea rift. The island is covered by an inhospitable desert with no source for fresh water and only scattered desert vegetation. The host rock is hydrothermally altered serpentinized breccia (Sinkankas 1964; Kievlenko 2003). The gems occur in open cavities in veins and as overgrowths of flat brown olivine crystals. In some cases, peridot up to 8 inches long, has been found. The gems are pristine with flattened tabular form.

In the United States, peridot is reported in Arizona, California, Colorado, Hawaii, New Mexico and Wyoming.

Right - view of Boars Tusk lamproite volcanic neck, Leucite Hills, Wyoming.

Arizona. According to Sinkankas (1959) the most productive peridot locality in the US is 2.5 miles southwest of the San Carlos Indian village in Gila County southeastern Arizona. Here peridot is found in the top flows of a basaltic mesa near Peridot, Arizona. Larger stones are recovered in a canyon below the mesa where mined by bulldozer, sledgehammer and pry bar. Unfortunately, much of the peridot has an undesirable brownish component. Even so, specimens with better colors and sizes (5 to 35 carats) are sometimes recovered (Kiovula and others 1992). Sinkankas (1959) reported the peridot to be found embedded in basalt flows near Peridot Mesa and much of the productive material to be found as detrital grains eroded from the basalt in adjacent soils and drainages. This peridot typically is about the size of sand grains, but larger specimens (0.25 to 0.5 inches) are found, including one specimen that measured 1.5 inches across.

Left - view of Black Rock lamproite in background where more than 13,000 carats of gem-quality peridot was discovered in two anthills by W. Dan Hausel. Gems ranging from a millimeter to 0.5 inch in diameter were found in the soils adjacent to Black Rock and in place in the lamproite flow.

To the north of Peridot, Arizona, gem peridot and pyrope garnet is found in the four corners region of Arizona, New Mexico and Utah. In the Arizona portion of this field, peridot is found at Buell Park where it is found in a serpentinized breccia pipe 10 miles north of Ft. Defiance, Apache County, Arizona. Buell Park forms a circular depression about 2 miles in diameter. The host rock is a breccia pipe underlying the topographic basin and bounded on the southeast by Peridot Ridge. Peridot Ridge forms a narrow ridge of volcanic rock. The namesake of this ridge comes from the abundant disseminated and granular olivine in the host rock and soil. Peridot is extracted from debris on the slopes of the ridge and from adjacent areas along with pyrope garnet (‘Arizona Ruby’), chromian diopside, ilmenite, and enstatite. Pyrope and peridot are also present in a small hill in the center of the basin. The peridot is typically about 0.2 inch across with specimens ranging up to 1 inch across. The material is flawed and has a yellowish-to olive green color (Kievlenko 2003). The nearby Garnet Ridge intrusive consists of four, small, intrusives within 3 miles of one another. The largest is about 1,000 feet across and consists of tuff and lapilli tuff with chromian diopside and pyrope garnet.

The rocks in the Navajo volcanic field were the source of some of the pyrope garnet used to salt of a sandstone outcrop in northwestern Colorado to promote diamonds. This fraud later became known as the '1872 Great Diamond Hoax' (Hausel and Stahl 1995). Garnets from the Mules Ear area, in particular, exhibit geochemistry similar to garnets at the diamond hoax site (McCandless and others 1995).

New Mexico. The Navajo volcanic field continues east into New Mexico, where scattered nodules and cognate xenocrysts with peridot are found in breccia pipes. At the Greens Knob intrusive in San Juan County in the northwestern corner of New Mexico, lapilli tuff occurs in an 0.5 mile area. This rock was initially classified as kimberlite, but more recently has been reinterpreted as a rock known as minette (a lamprophyre) (Mitchell 1986). Minettes have also been identified at nearby Fluted Rock, Outlet Neck, the Beast, Beelzebub, and the Black Rock diatreme.

In addition to the 4 corners area, peridot is found to the south near the Mexican border. Kilbourne Hole in southern New Mexico, 20 miles southwest of Las Cruces, is a late Pleistocene volcanic maar consisting of a prominent crater with basalt, basalt breccia, pyroclastics and abundant mantle xenoliths with lower and upper crustal nodules. Many of the mantle xenoliths are olivine-rich dunites and peridotites. Fuhrbach (1992) reports that many nodules contain gem-quality peridot.

The crater is elliptical in plan and was formed by explosive eruption. The crater occurs in the Camp Rice Formation (Pleistocene) which is overlain by the Afton Basalt. Along the south end of the maar and on top of these two units is a prominent tuff ring ejecta rim. The tuff ring breccia contains angular blocks of Afton basalt in a matrix of unstratified pyroclastics with lower and upper crustal xenoliths.

Peridot is abundant and found with augite, diopside and occasionally enstatite. A significant percentage of the gem material from the maar will produce faceted gems <0.5 carats. Some larger peridot found in the area includes unfaceted specimens up to 126 carats. The stones are very bright with excellent clarity; however, only about 15 to 20% is facetable due to abundant fractures (Fuhrbach 1992).

Utah. In Utah, three ultramafic diatremes are found in the Colorado Plateau of southern Utah, southeast of Mexican Hat in San Juan County. These are the Mule Ear, Moses Rock, and Cane Valley intrusives. The rocks are thought to be minettes (Mitchell 1986; Erlich and Hausel, 2002). The Moses Rock dike is exposed over 3.8 miles and has a maximum width of 950 feet. The dike is brecciated and intrudes Permian sandstones and siltstones of the Cutler Formation. The dike is dated at 30 million years old. The breccia includes a small number of eclogite, pyroxenite, and rare peridotite nodules.

Elsewhere in Utah, olivine lamproite (Miocene-age) (see Geological Time Scale) known as the Robber’s Roost, occurs as a narrow northwest-trending dike cropping out over a strike length of 1.6 miles in the Colorado Plateau near Hanksville.

Lamproites and mica peridotites are also found 40 miles east of Salt Lake City near Kamas. These are the Moon Canyon lamproites (40 Ma) southeast of Kamas that consist of hypabyssal olivine-sanidine-diopside-richterite-phlogopite lamproite flows and sills. The Whites Creek lamproite (13 million years old) to the northeast of Kamas is a series of small orendite dikes that penetrated upper Cretaceous shale 2 miles northwest of the Uinta North Flank Fault Zone. These have some chrome spinel, phlogopite and olivine. It is not known if any of the olivine is gem-quality.

Colorado. Gem-quality olivine is found near Salida in central Colorado. The peridot is within the 39-Mile volcanic field. Here peridot occurs in basalt. Some of the soils in the field is enriched with peridot such that small and limited recovery operations have produced as much as 12,000 carats/day, with the largest piece of rough weighing 5.8 carats. Elsewhere in Colorado, olivine is found in kimberlite in the Colorado-Wyoming state line district north of Ft. Collins (Hausel, 1998). Nearly all of the olivine in these kimberlites is serpentinized and it is rare to find primary olivine. No peridot gemstones have been reported in these kimberlites to date, although most contain gem-quality diamonds, pyrope and chromian diopside.

Thin section of kimberlite rock from the Nix pipe in Colorado in plane polarized light. The rounded blue to violet colored minerals are olivine grains partially replaced by serpentine.

California. Sinkankas (1964) reported olivine, up to 1 inch across to be found in marble quarries at Riverside, California.

Hawaii. Nodules of dunite up to 6 inches across contain gemmy grains of peridot in basalt in Hawaii. Peridot is also found in black sand beaches.

Wyoming. For over a hundred years, olivine was known in the Leucite Hills of southwestern Wyoming. The Leucite Hills is a lamproitic volcanic field that has been studied by numerous researchers all the way back to the late 1800s. Most researchers mentioned the presence of olivine in this area, but the quality of the material for some reason was ignored by all. Elsewhere, some olivine is reported in basalt in the Absaroka mountains near Yellowstone.

Gem-quality peridot found in Leucite Hills
The discovery of gem-quality olivine (peridot) did not occur until the author began a mapping and diamond research project in this area in 1997 (Hausel, 1998). This area was considered to have high potential for diamonds since the mid-1980s by the author, because of similarities of the lamproites to kimberlite (Hausel, 2006). We now know that lamproites enriched in olivine (known as olivine lamproites) are likely to contain diamond. However, most olivine lamproites are serpentinized which results in a relatively soft rock that erodes easily. As such, all of the rich diamondiferous olivine lamproites have been found hidden under thin layers of soil, such as the diamond pipes in Murfreesboro Arkansas; Argyle, northern Australia; and Ellendale, Western Australia.

Thus when I began to search the Leucite Hills for diamond, I began searching for olivine. In addition, we recovered chromite from rocks at Endlich Hill and Black Rock that had geochemistry equivalent to diamond-stability chromites found in diamond pipes elsewhere in the world. This means that some lamproites in the Leucite Hills had to originate from depths great enough where diamond is stable. This strongly supports that diamonds will someday be found in the Leucite Hills volcanic field, but the better localities are likely hidden under thin soil cover and/or sand dunes.

During my search for diamonds, I came across two anthills that were solid green from all of the peridot. These were found on the west side of Black Rock.
A portion of the 13,000 carats of peridot recovered from two anthills along the flank of Black Rock.

I recovered the two anthills and we examined them for diamond and peridot. Most of the peridot in these anthills was very high-quality (Hausel, 2001). More impressively, the two anthills yielded >13,000 carats of olivine ranging in size from 1 to 12 mm (0.04 – 0.5 in); and much of the material was gem-quality (Hausel, 2006). Most of the material, in spite of size, was facetable, even though small. The source of the olivine was Black Rock which contains xenoliths of peridot as large as 0.5 in across with some small nodules of peridotite and dunite. Peridot is also found in other lamproites in the volcanic field including Endlich Hill, Hatcher Mesa, South and North Table Mountain and Wortman dike. The soils adjacent to these lamproites have not been examined, but sieving soil near the lamproites is guaranteed to yield peridot.

The Leucite Hills volcanic field consists of 22 exposed lamproite flows, dikes, necks, plugs, cinder and pumice cones (http://leucitehills.blogspot.com). Radiometric dates indicate volcanic activity commenced at about 3.1 million years ago and continued to 0.9 million years ago (Hausel, 2006). The possibility of hidden diamondiferous olivine lamproites in the field is likely.

Peridot rough and faceted gems from Black Rock.

The olivine grains from nearby Hatcher Mesa are similar to olivine found in lherzolite and harzburgite mantle nodules in kimberlite and suggest a deep seated origin for these rocks.

Left– flawless gem peridot faceted from Black Rock olivine. Below Left – Group of gemstones from Green River Basin anthills include red to pink pyrope garnet and emerald green chromian diopside from the Butcherknife Draw area south of Green River and two peridot from the Leucite Hills north of Rock Springs.






PROSPECTING FOR PERIDOT

The easiest way to prospect for peridot is to look in places where it is already found, or look for places (in geological reports) that mention olivine. Few geologists are taught how to identify gem material and thus much of it has been overlooked during thesis and dissertation research projects as well as by geological studies by geological surveys etc. When I discovered the peridot in the Leucite Hills, I was amazed because of all of the geologists who had looked at these in the past including two very prominent geologists. But over the years, I discovered hundred of deposits and occurrences. Some possibly had never been seen by mankind before I found them and others were just plain overlooked by everyone else.

Alkalic basalt fields are worthy of examination. When you find a field you want to investigate, take a shovel, gold pan and sieve and simply start digging adjacent to basalt flows. Sieve the material into a gold pan and either wet or dry pan the material to look for olivine. This has not been done in the Leucite Hills yet, but it is a given that sieving soil adjacent to Black Rock will yield some fine gemstones as well as gems >1 carat. My ants were limited to the size of material that they could carry, even though I found pieces of peridot in the Black Rock lamproite that were easily 10 to 20 carats or more in weight.

Map of the Leucite Hills volcanic field (left) and the discoverer of the Leucite Hills peridot leads field trip to Leucite Hills (below). Hausel stands in center of photo at Zirkel Mesa.

REFERENCES
Erlich, E.I., and Hausel, W.D., 2002, Diamond Deposits - Origin, Exploration and History of Discovery. Society of SME. 374 p.

Fuhrbach, J.R., 1992, Kilborne Hole peridot: Gems and Gemology, v. 28, p. 16-27.

Hausel, W.D., 1998, Field reconnaissance of the Leucite Hills peridot (olivine) occurrence, Rock Springs uplift, Wyoming: WSGS Mineral Report MR98-2, 6 p.

Hausel, W.D., 1998, Diamonds & Mantle Source Rocks in the Wyoming Craton with Discussions of Other US Occurrences. WSGS Report of Investigations 53, 93 p.

Hausel, W.D., 2006, Geology & Geochemistry of the Leucite Hills Volcanic Field, Wyoming Geological Survey Report of Investigations 56, 71 p.

Hausel, W.D., 2009, Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.

Hausel, W.D., and Stahl, Sandy, 1995, The great diamond hoax of 1872: Wyoming Geological Association Resources of Southwestern Wyoming Guidebook, p. 13-27.

Hausel, W.D., and Sutherland, W.M., 2006, World Gemstones: Geology, Mineralogy, Gemology & Exploration: WSGS Mineral Report MR06-1, 363 p.

Hurlbut, C.S., Jr., and G.S. Switzer. 1979. Gemology. John Wiley and Sons. New York, p. 243.

Keller, P.C. 1990. Gemstones and Their Origins. Van Nostrand Reinhold, New York, 144 p.

Kievlenko, E.Y. 2003. Geology of Gems. Ocean Pictures Ltd., Littleton, CO, 432 p.

Koivula, J.I., Kammerling, R.C., and Fritsch, E., 1992. Gem News. Gems & Gemology, 28:1. p. 60.

McCandless, T.E., Nash, W.P., and Hausel, W.D., 1995, Mantle indicator minerals in ant mounds and conglomerates of the conglomerates of the southern Green River Basin, Wyoming: Wyoming Geological Association Resources of Southwestern Wyoming Guidebook, p. 153-163.

Mitchell, R.H. 1986. Kimberlites: Mineralogy, Geochemistry, and Petrology. New York: Plenum Press. 442.

Sinkankas, J. 1959. Gemstones of North America. Van Nostrand Reinhold Company. New York, NY. 675.

Sinkankas, J. 1964. Mineralogy. Van Nostrand Reinhold Company. New York, NY. 585.

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