CHROMIAN DIOPSIDE - Emerald of Diamond Pipes

INTRODUCTION

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 faceted jewelry. And chromian diopside can have a deep, satiated green color that would make any emerald pale in comparison. 

Even though, 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 igneous rocks with high chromium, magnesium, potassium and low silica) such as rare serpentines in northern California, and peridotite in Norway, as well as in kimberlites and some lamprophyres in the Wyoming Craton, and the North American Craton. Similar rocks are found in other Cratons of the world. 

One of the principal host rocks is kimberlite. Chromian diopside occurs in most kimberlites in the US within mantle-derived nodules, megacrysts and xenocrysts trapped within the kimberlite, in some lamprophyres in Montana and Wyoming, in scattered anthills in southwestern 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 is found 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 occurs 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 and it is unknown if any metallurgical research was ever conducted for extraction of these unique gemstones. If the price of the gems rose considerably through marketing strategy, these gems would be considered for extraction.

Cartoon of a kimberlite pipe in
cross section. Such pipes typically
form maar volcanoes at the
surface that project down into the
earth as carrot-shaped pipes with
branching dikes and sills, all 
connecting at greater depth to a
feeder dike that originated into
the earth's upper mantle. The
depth to the feeder dike from the
surface may be only a few
 thousand feet.

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 trapped xenoliths and nodules are rounded. In some cases, angular blocks and boulders are down-dropped into the rapidly cooling magma from surface strata. In Colorado, some large blocks of crustal rock were found in the mine ribs of the Sloan 2 kimberlite exploratory tunnel. 

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 at eruption. 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 forms a maar that looks like an impact crater. For example, the Winkler crater in northeastern Kansas was originally interpreted as an impact structure. Later, it was discovered by Dr. Doug Brookins that the crater was filled with kimberlite, and other kimberlites were found nearby as well as an olivine lamproite to the south.

If you are 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 broke the sound barrier! Not only might this destroy your ear drums, if you were near a kimberlite eruption, you likely would suffocate because of all of the carbon dioxide pushing away oxygen (sort of like exercising while wearing a Fauci-style mask: if it is not perforated, you could faint from a lack of oxygen; but if it is perforated, the mask is useless as gases and all other tiny particles would readily pass through). 

So, if you are a tree, you would enjoy a treat of massive CO2. Such massive amounts of CO2 would push all oxygen out of the immediate area leaving no air to breath. If somehow this didn’t kill you, you would need to avoid cannon balls of mantle nodules and diamonds shot from the volcano (along with both hot and cold kimberlite magma and ash) – 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 in the crystal.

And, as CO2 gas expands, it freezes everything in the area! Yep, instead of burning, you could 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 much of the 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 with a couple of hard-working geologists could lead to discovery of many more diamond pipes in Wyoming. In Canada, each kimberlite discovery is estimated to cost a minimum of $1.5 million (pre-Biden money values) 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 per kimberlite, would have done! Wyoming potentially could be a diamond producer - especially since the WGS, DiamonEx Ltd, and the author found evidence for hundreds of undiscovered kimberlites in Wyoming.

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.

MINERALOGY and CHEMISTRY
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

MINING CHROME DIOPSIDE
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).

Serpentinized garnet peridotite, mantle-derived, nodule collected from the Aultman 2 kimberlite,
State Line district, Wyoming. Not only is chromian-diopside found in the matrix of the kimberlite 
magma, it also is found as large megacrysts, and found in a variety of mantle nodules such as
this serpentinized garnet peridotite that was trapped in the kimberlite magma as it began its 
eruption in the upper mantle (90 to 120 miles deep) to the earth's surface.

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 with planes of cleavage and parting,
projecting through the gemstone, Colorado-Wyoming
State Line diamond district. 

While searching serpentines in the Hayfork area of northern California, the author discovered 
high-pressure serpentinized peridotite (possibly obducted mantle slices) containing chromian
diopside-rich layers. Not far from these serpentines, prospectors found some diamonds
 in stream placers.

Not all diopside contains emerald green color - in fact the emerald green color - due to the presence of chromium, is uncommon in most diopsides. In conclusion, we thank you for reading our blog. At this point, we once listed our facebook page, but due to a plethora of erroneous and bias fact checks, as well as censoring facts and opinions, we will no longer support such activity.

Very large diopside megacryst from a Wyoming kimberlite collected by the author. Note the
presence of parting (cleavage) planes. Also note a lack of emerald-green color. Such color
in diopside is a result of small amounts of chromium chromophore. Typical chromian-diopside 
described in kimberlite may have 1 to 3.5% Cr2O3. However, ureyite pyroxene recovered from
jadeites in places like California and elsewhere, are reported to contain as much as 27.6% Cr2O3.

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