Monday, February 8, 2010

CHROMIAN DIOPSIDE - Emerald of Diamond Pipes

Chromian diopside in matrix of a Colorado kimberlite.
Chromian diopside is a rare, emerald-green, mineral with hardness similar to opal: it does not quite match the hardness of emerald, which it tends to mimic in jewelry, when faceted. And chromian diopside can have a deep, satiated green color that would make any emerald pale in comparison. Even though it is found in many places in North America and elsewhere in the world, it is extremely rare because of being found almost exclusively in rare mafic and ultramafic igneous and metamorphic rocks (dark colored rocks with high chromium, magnesium and potassium and low silica) such as some serpentines in northern California, as well as in kimberlites and some lamprophyres in the Wyoming Craton, and the North American Craton. But similar rocks are found in other Cratons around the world. 
Artist's cartoon of a kimberlite
pipe in cross section. When such
volcanoes contain diamond, they
can be worth $billions of dollars.
More than 100 such pipes and dikes
have been found in Colorado &
Wyoming with diamonds and
chromian diopside.

One of the principal host rocks is kimberlite. The GemHunter describes chromian diopside in many kimberlites in the US, in some lamprophyres in Montana and Wyoming, in anthills in Wyoming, and in serpentinites in northern California. Kimberlite is considered as one of the rarest rock types on the surface of the earth by volume: possibly only lamproite and some varieties of lamprophyre are rarer. When found, kimberlite occurs in small, carrot-shaped pipes (maar volcanoes) with small surface expressions that are generally less than 3,000 feet across (most are smaller ranging from narrow dikes to elongate pipes a couple of hundred feet across). In these pipes, chromian diopside is found as disseminated mineral grains, megacrysts (large crystals) and in rare mafic and ultramafic mantle-derived nodules and xenoliths. In such pipes, chromian diopside is typically more abundant than diamond, and because of its distinctive color, it is readily visible (unlike diamond). However, the value of the stone is not high enough to mine kimberlite for chromian diopside, and all diamond mines reject the mineral and only mine for diamond. Chromian diopside and pyrope garnet, both considered gemstones when exhibiting transparency and attractive color, typically report to the tailings at all diamond mines.

Kimberlite eruptions are liken to a shotgun blast from the earth. Many xenoliths and nodules in these volcanoes are plucked from the earth's crust and upper mantle, with some originating from depths as great as 90 to 120 miles. When they arrive at the earth's surface, most of these xenoliths and nodules are rounded. And in some cases, angular blocks and boulders are down-dropped in the magma from the surface in the rapidly cooling magma. In Colorado, some blocks of rock found in the ribs of the Sloan 2 kimberlite exploratory mine tunnel, were the size of golf carts. 

The maar volcano at the Maxwell kimberlite (Colorado) shows a distinct shallow depression with different vegetation than the surrounding granitic country rock.
Imagine standing near this volcano. Hold on to your hat! The magma and gases are under incredible pressure and shoot up from the mantle to the surface in a few hours at most. The eruption does not produce a volcanic cone, instead it forms a maar that looks like an impact crater. As an example, for years, the Winkler kimberlite in northeastern Kansas was interpreted as an impact and named the Winkler crater, but later, it was discovered by Dr. Doug Brookins to be filled with kimberlite. Later, other kimberlites were found nearby along with an olivine lamproite to the south.

If you were standing in the region of a kimberlite eruption, you would hear a sonic boom! The gas from this magma being under incredible pressure, erupts at Mach 2 to Mach 3. Remember those jets in years past, when they often broke the sound barrier! Not only might this destroy your ear drums, if you were near a kimberlite eruption, you would likely suffocate because of all of the carbon dioxide pushing away oxygen - if you were a tree, you would enjoy the treat of the added CO2. Even though CO2 is harmless in normal concentrations, such massive amounts would push all oxygen out of the immediate area leaving you with no air to breath. Now if somehow this didn’t kill you, you would have to avoid cannon balls of mantle nodules and diamonds shot from the volcano – these would be like BBs from a shotgun blast.

Chromian diopside collected from the Schaffer kimberlite complex
 in Wyoming by the author. Note the characteristic box-like 
crystal habit of diopside and emerald green color of chrome.
Also note the very distinct, box-like diopside at the end of the
arrow that is naturally produced by breaking along cleavage
and parting planes
As CO2 gas expands, it freezes everything in the area! Yep, instead of burning, you would likely catch a cold (if you are still alive). It has been estimated that the magma temperature at eruption would cool to 32oF. This is why there is usually no evidence of baking of the country rock next to kimberlite magma.

In the Colorado-Kansa-Montana-Wyoming region, there are many kimberlites, lamproites and lamprophyres. Nearly all that have been tested contain diamond. Gem diamonds are valuable and some have sold for more than 200,000 times an equivalent weight in gold! 

In the past, research for these kinds of projects at the Wyoming Geological Survey were poorly funded and often measured in the level of a few hundred dollars/year to search for gemstones, precious and base metals as well as conduct regional geological mapping projects, lectures, publish papers, assist prospectors and companies, etc. Yet with this extreme poverty level of funding the WGS found evidence of a major diamond province. This would suggest to me that with a decent budget and a couple of hard-working geologists, many more diamond pipes would be found in Wyoming. In Canada, each kimberlite discovery was estimated to cost a minimum of $1.5 million a few decades ago. In Colorado and Wyoming, more than 100 kimberlites were discovered on a budget of about $30,000 over 30 years! Now imagine what $1.5 million would have done! 
In this aerial photo, large cryptovolcanic structures occur in a field of >50 probable kimberlite
pipes I discovered a few years ago. Several million people drive by these every year not paying 
realizing they are driving next to some probable diamond mines along I-80! The white color in the
 depressions is calcium carbonate. Due to CO2 gas, much of it becomes fixed as calcium carbonate
 upon cooling. Country rock surrounding these depressions is dominated by silicate minerals with
 no obvious source of calcium carbonate. Over the past few years, nine similar districts that include
 a few hundred similar cryptovolcanic structures – nearly all are unexplored were found.

Chromian diopside, a chrome-rich variety of the mineral diopside [CaMg(SiO3)2], typically has an exceptional emerald-green color due to substitution of chrome (1 to 2.8%) for magnesium in its crystal lattice. Generally, the greater amount of chrome in the crystal lattice, the more intense the emerald-green color. The mineral has a hardness of 6 to 6.5; specific gravity of 3.2 to 3.4; and forms monoclinic crystals. In other words, well-formed diopside crystals give the appearance of a box that is compressed in one direction. The mineral has one perfect cleavage {110} which can be a problem in durability of the gemstone. It also has simple and multiple twin planes parallel to the a{100} and  c{001} axes. The cleavage and twin planes are directions of weakness, that need to be protected in jewelry, to insure the gemstone does not cleave when bumped 

The gemstone is referred to by its mineralogical name (chromian diopside), but it has also been referred to as chrome diopside, chromian pyroxene, and 'Cape Emerald', a misnomer after the first cut specimens from Cape Town, South Africa. Since the term "Cape Emerald" was first applied to gem-quality prehnite, to avoid confusion, it would be more appropriate to refer to chromian diopside as ‘Northern Cape Emerald’ based on its type locality at Kimberley in the Northern Cape Province of South Africa, where the gem was recovered from diamondiferous kimberlite along with 'Cape Ruby' (pyrope garnet).

Emerald green chromian diopside surrounded by pyrope garnets. This
material was collected from anthills in Butcherknife Draw south of Green
River, Wyoming and later faceted in Sri Lanka.
Chromian enstatite, a chrome-rich variety of enstatite (MgSiO2), has a hardness of 5.5 to 6 and specific gravity of 3.1 to 3.3, and occurs as orthorhombic crystals with similar emerald-green color as chromian diopside. Both chrome diopside and enstatite have well-developed cleavage and parting, which unfortunately limits the size of facetable material. Enstatite produces crystals with square cross-sections when viewed down the c-axis of the crystal and diopside will have a similar cross-section, but with one pair of crystal surfaces that are inclined to the all other crystal faces

Chromian diopside and enstatite have comparable green color to emerald and tsavorite garnet. The primary source for chromian diopside gemstones is Siberia, where gemstones are recovered during warm summer months. Minor amounts are recovered by collectors and entrepreneurs in Myanmar, Pakistan, South Africa, Brazil, Italy, North America, Sri Lanka and Finland. It is a relatively inexpensive gem due to rarity. Being so rare, there is little effort to market the gemstone - and with gemstones, marketing is everything. Thus, the small amount of chromian diopside and chromian enstatite that make its way to market sell for $50 to $200/carat for faceted stones. And faceted chromian diopside gems that are larger than 2 carats are very rare.

Overall, the value and difficulty of recovering chromian diopside and pyrope garnet from kimberlite rock assures that these two minerals will be deposited in mine waste piles. Currently, diamond mines only recover diamonds.

Parcel of gemstones from the Sloan 1 and 2 kimberlites (Colorado) that includes pyrope garnet 
(purple to red) spessartine garnet (orange) and high-quality transparent emerald-green 
chromian diopside. 

Because of relatively high specific gravity, chromian diopside can be panned using a gold pan where it will be recovered with black sands. During the 1980s, the Wyoming Geological Survey received a  Federal grant to search for hidden kimberlites. The grant was partially used to collect more than 1,600 panned samples. About 300 'kimberlitic indicator mineral anomalies' were identified in the Laramie and Medicine Bow Mountains (Wyoming), some of which contained chromian diopside along with pyrope garnet and picroilmenite. This was a 20% success rate and indicates that a major swarm of undiscovered kimberlites occurs in these areas. Essentially all of these remain unevaluated because of the lack of funding by the state legislature. In addition to all of these anomalies, mining companies also found hundreds of anomalies in the region including Cominco American, Superior Minerals, DiamonEx Ltd, and many others. 

Kyanite eclogite nodule from the Aultman 2 kimberlite, Wyoming. Note
the green chrome diopside, gem-quality blue kyanite, and glassy garnet.
What makes chromian diopside so rare is that it is formed under great pressure and temperature within the earth’s upper mantle (similar to diamond). To get these minerals to the surface, unusual geological event(s) must have occurred. Host fractures for kimberlite and similar magmas run from the mantle at depths of 120 miles, but are only a few feet wide and at the most 1 to 2 miles long on the surface! Other chromian-diopside bearing rocks, like those found in California by the GemHunter, are likely thrust up from the mantle to the surface along rock slices in subduction (benioff) zones.

Age dates on the intrusives (and diamondiferous host rocks) in the Wyoming Craton tell us that kimberlite (and related magmas) erupted in multiple episodes that included: (1) prior to 2 billion years ago, (2) at the end of the Precambrian (~ 600 million years ago), (3) in Late Silurian to Early Devonian (400 to 420 million years ago), (4) the Cretaceous (140 to 70 million years ago), (5) the Tertiary (60 to 30 million years ago) and the (6) Pleistocene-Quaternary (3 to 1 million years ago).

A 2-inch long chromian diopside megacryst found in Sloan 2 kimberlite in Colorado by the author.

The kimberlitic magmas (as well as some lamproites and lamprophyres) acted as transporting medium for mantle rocks containing chrome diopside and individual chrome diopside megacrysts that were trapped in the magma. Such mantle rocks as pyroxenite, dunite, eclogite, lherzolite, wehrlite and harzburgite were trapped in the kimberlites – these are known as nodules or ‘xenoliths’ (foreign rock fragments).

Some of the cognate crystals and xenocrysts found in kimberlite, lamproite and lamprophyre include diamond and other rare minerals. Since these are associated with kimberlite (kimberlite is one a few magmas that originates deep enough to sample these) they have become known as ‘kimberlitic indicator minerals’ in the diamond exploration industry. The kimberlitic indicator mineral suite includes chromian diopside, chromian enstatite, pyrope garnet, picroilmenite, chromite and diamond. When kimberlites are found with sufficient quantities of diamond, it may be mined for diamond. The other gemstones, chromian diopside and pyrope garnet that could provide value-added gems to the mine, are in nearly every case ignored by mine operators due to their lower value and possibly due to ignorance. However, recovering these gems and marketing them effectively would provide added value. As it stands, only collectors who gain access to mine tailings, recover these other gemstones usually by hand sorting from the mine tailings.

Facet-quality chromian diopside, pyrope garnet, almandine garnet  and spessartine garnet collected from anthills in the
the Butcherknife Draw region of the Green River Basin (Wyoming) by the author.

Some specimens collected in the State Line district have included chromian diopside megacrysts up to 2 inches across, impressive pyrope-almandine megacrysts up to 5 inches in diameter, and eclogite and peridotite (lherzolite) cobbles (filled with chromian diopside) up to 1.5 feet in diameter. One eclogite nodule recovered from the Sloan kimberlite contained 20% diamond! At the Iron Mountain district (Wyoming), several kimberlites have chromian diopside, but not in as great quantity as those in the State Line district.

In the southern Green River Basin, southwestern Wyoming, hundreds of anthills were identified armored with chromian diopside and enstatite, as well as beautiful, reddish-purple, reddish-pink, and yellowish-orange, transparent to translucent pyrope and almandine garnet. A few diamonds have also been found in anthills and in nearby diamondiferous lamprophyre breccias. Many of the indicator minerals are found on Cedar Mountain and in Butcherknife Draw south of Interstate 80 to the south of Fort Bridger and Green River. They have also been identified in the Bishop conglomerate along the edges of Cedar Mountain and apparently on top of Diamond Peak to the south in Colorado. This anomaly covers more than 250 square miles.
Chromian diopside and enstatite with garnet collected from Cedar Rim diamondiferous lamprophyres
 by Dr. Richard Kuchera.

Useful maps for the State Line district include: (1) US Geological Survey topographic maps and US Bureau of Land Management Surface and Mineral Management Status maps of Ft. Collins and Laramie (1:100,000 scale); (2) General location maps of known kimberlites (see Hausel, 1998); (3) Preliminary (1:24,000 scale) geological map of the Wyoming portion of the Colorado-Wyoming State Line district (see Hausel and others, 1981). For the Green River Basin, see (1) US Geological Survey topographic maps of Firehole Canyon and Evanston (1:100,000 scale).

Chromian diopside cross with white topaz
Elsewhere in the US, chromian diopside has been found in (1) serpentinized breccia, kimberlite and anthills at Buell Park and Garnet Ridge Arizona, (2) serpentinites in northern California; (3) kimberlites in Estes Park and the City Park of Boulder, Colorado, (4) kimberlite in Middle Sybille Creek, Wyoming, (5) the six pack lamprophyre, Wisconsin, (6) the Homestead and Williams kimberlites, Montana, (7) Green Knobs kimberlite, New Mexico; (8) Cascadilla George kimberlite, New York, (9) Winkler and several other kimberlites in northeastern Kansas, (10) kimberlites in Elliott County, Kentucky; (11) Cane Valley, Mule Ear, Mosses Rock kimberlites and serpentinized breccias, Utah; (12) Mt Horeb kimberlite, Virginia; and (13) Lake Ellen kimberlite, Michigan. And there are several other sites (see Hausel, 1998).

Several years ago, while searching for diamonds in northern California, I was searching for evidence for high pressure rocks and discovered chromian diopside in serpentinites overlying a subduction zone. In this same region, a few relatively large diamonds were found in Hayfork Creek near the Trinity River. The serpentinites were discovered near the towns of Weaverville and Hayfork.

When faceted, chromian diopside is difficult to beat. It is a beautiful stone that is as attractive if not more attractive than emerald. It is just difficult to get a consistent supply simply because no company exclusively mines for chromian diopside. It is a by-product of diamond mining and few diamond miners recover the stone as it usually reports to the mine tailings. If you watch any of the jewelry channels, you will see this stone often as supplies become available.

Chromian diopside showing directions of cleavage and parting,
Colorado-Wyoming State Line diamond district. 

See the following two sites for more photos of chromian diopside: Cash & Treasures

Chromian diopside in limonite-rich seam in a serpentinite from northern California collected in the Hayfork area by
the author.
Not all diopside contains emerald green color - in fact the emerald green color - due to the presence of chromium, is uncommon in most diopsides.

Very large diopside megacryst crystal found in a Wyoming kimberlite by the author. Note the presence of parting (cleavage) planes.
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