Thursday, November 11, 2010

Origin & Host Rocks for Diamonds

There are literally thousands of known kimberlites and many hundreds of lamproites and lamprophyres but only a handful contain commercial amounts of diamond. One estimate made many years ago suggested that less than 1% of all kimberlites are commercially mineralized (Lampietti and Sutherland, 1978). Although, many hundreds of new discoveries have been made since that paper was published, this statistic remains essentially valid.

Diamondiferous kimberlites and lamproites are essentially restricted to cratons and cratonized terrains. These include stable Archean cratonic cores (known as Archons) as well as cratonized Proterozoic margins (referred to as Protons) (Figure 4). Some unconventional diamondiferous host rocks have also been identified in cratons as well as outside cratonic terrains within tectonically active regions along the margins of cratons. Because high ore grades have been detected in some of these, unconventional commercial host rocks are anticipated to be found in the future (Erlich and Hausel, 2002). Presently, diamond exploration programs are designed to search for conventional host rocks (i.e., kimberlite and lamproite) or for placers presumably derived from these.

The North American Craton showing
regions of favorability for conventional
host rocks.. The diamond shapes are
locations for reported diamond finds
From Hausel, 1998.
Most diamonds are considered xenocrysts that separated from disaggregated mantle peridotite and eclogite during transportation to the earth’s surface in kimberlitic, lamproitic and some lamprophyric magmas.  Kimberlites, lamproites, and lamprophyres tend to occur in clusters of a few to more than 100 occurrences.  Structural control is thought to be important in the emplacement of these, and several structural orientations are often recognized within each district.

The majority of diamond mines are developed in kimberlite such as the Wesselton, DeBeers, Kimberley, Dutoitspan and Ekati, or in placers, particularly beach placers along the west coast of Africa.  Lampietti and Sutherland (1978) reported only about 10% of the known kimberlites were mineralized with diamond. This statistic may no longer be valid in that as many as 50% of kimberlites found in Canada and Wyoming in recent years, and possibly as many as 90% in Colorado have yielded diamond. Even so, only a very small portion are commercially mineralized.  When economic, kimberlites may contain hundreds of millions to billions of dollars worth of stones, thus kimberlite should be a priority target in any exploration program.

Kimberlites are essentially carbonated alkali peridotites that exsolve CO2 during ascent to the surface from the earth’s upper mantle (according to the EPA, such a volcano would be poluting our atmosphere and require taxation under tax and trade) resulting in diatremes with considerable brecciation and dissolution-rounded xenoliths and cognate nodules.  The diatremes appear as sub-vertical to vertical pipes that taper down at depth forming steeply inclined cylindrical bodies. The average angle of inclination of the walls of various pipes in the Kimberley region of South Africa (Wesselton, DeBeers, Kimberley and Dutoitspan) is 82° to 85°.  Ideally, the pipes have rounded to ellipsoidal horizontal cross sections filled with kimberlitic tuff or tuff-breccia.  Many continue from the surface to depths of 1 to 1.5 miles (2-2.4 km), where they pinch down to narrow root zones emanating from a feeder dike.

The Kimberley pipe, which was mined out by 1915 (about 20 years after discovery), contracted sharply at depth.  At the lowest level of mining (3,465 feet), it was no longer pipe shaped but rather had the appearance of three intersecting dikes (Kennedy and Nordlie 1968). Combined with the estimated 5,100 feet (1,600 m) of erosion since the time of emplacement, the depth to the original point of expansion was probably 1.5 miles (2.4 km).

Kimberlitic magmas are interpreted to originate from depths as great as 120 miles (200 km) and travel to the Earth’s surface in a matter of hours (O’Hara and others, 1971). The magma is thought to rise rapidly, possibly 6 to 18 mi/hr (10 to 30 km/hr), in order to transport high-density ultramafic xenoliths.  Within the last few miles of the surface, emplacement rates are thought to increase dramatically to several hundred miles per hour. Such velocities could bring diamonds from the mantle to the surface in less than a day. McGretchin (1968) estimated that the speed of the fluidized material near the surface increased to as much as 870 mi/hr (400 m/s), or about the speed of sound (Mach 1 or 331 m/s).  Some estimates have even suggested kimberlite emplacement at the Earth’s surface may have achieved velocities exceeding Mach 3 (Hughes 1982)! 

Contact between kimberlite & granite
shows no evidence of baking. The
contact is knife sharp and the adjacent
granitic rocks show no alteration. This
could only happen if the kimberlitic
magma was cool upon emplacement.
The temperature of the magma at the point of eruption is relatively cool (Figure 5).  Watson (1967) indicated a magma temperature of less than 600°C on the basis of the coking effects on coal intruded by kimberlite.  A low temperature of emplacement is also supported by the absence of any visible thermal effects on country rock adjacent to most kimberlite contacts.  Davidson (1967) suggested the temperature of emplacement may have been as low as 200°C based on the retention of argon.  Hughes (1982) pointed out that the near-surface temperatures of the gas-charged kimberlite melt may be as low as 0°C owing to the adiabatic expansion of CO2 gas as kimberlite erupts at the surface.

Kimberlites typically transport xenoliths and xenocrysts to the surface.  Many of these are derived from mantle depths and some form a distinct suite of minerals that are referred to as kimberlitic indicator minerals. The traditional indicator minerals used to explore for kimberlite include pyrope garnet, chromian diopside, chromian enstatite, picroilmenite, chromite, and diamond.

Serious interest in lamproite intensified following the discovery of a world-class diamond deposit in olivine lamproite in 1979 in the Kimberley region at Argyle, Western Australia. Prior to this discovery, geologists around the world were focused only on kimberlite and argued that kimberlite was the only source rock for diamonds. It was essentially impossible to get exploration funding for projects other than those associated with kimberlite. This closed minded philosophy would have continued to the present if it were not for the fact that the drainage adjacent to Argyle had been filled with diamonds. The discovery led to the recognition of other diamondiferous lamproites in Australia, Brazil, China, Gabon, Zambia, Ivory Coast, India, Russia, and the US and also led to recognition that previous diamond producers in Arkansas and India were actually in olivine lamproite rather than kimberlite..

Scott-Smith (1996) subdivides lamproites into two general groups: phlogopite-leucite lamproites (~60% SiO2) and olivine lamproites (>20% MgO, 35–45% SiO2 and 7% K2O) with abundant serpentine pseudomorphs after olivine. Instead of pipes with steep walls that slowly diminish in diameter with increasing depth, lamproites are characterized by “champagne-glass” vents filled by tuffaceous rocks, often with massive volcanic rocks in the core.

Lamproites appear to have formed under variable thermal gradients originating from depth and in some cases extending into the diamond stability field (Nixon 1995). A qualitative correlation between diamond and olivine in lamproite is confirmed in both the Ellendale and Kapamba provinces, in which diamond grades are consistently higher in olivine lamproites than leucite lamproites (Figure 6). When found, diamonds occur primarily in pyroclastic rocks; the magmatic phases are notoriously diamond poor owing to the high temperatures sustained in the flows during eruption (Scott-Smith, 1986).

Ellendale 7 diamondiferous lamproite
exposed in dozer trench. This commercial
pipe was hidden under a few feet of soil.

Where vents flare out, a potential for substantial tonnages exist in larger craters.  At Argyle, Western Australia, past reserve estimates of 94 million tons of ore at an average grade of 750 carats/100 tonnes led to its classification as a world-class deposit. Some of the richer portions of this deposit yielded grades as high as 2,000 carats/100 tonnes. However, large numbers of the Argyle diamonds are graphitized and partially resorbed; more than 60% are irregular in shape and include macles, polycrystalline forms, and rounded dodecahedrons. The largest Argyle diamond weighed 42.6 carats with the overall size of diamonds being quite small (average < 0.1 ct).  Nearly 80% are brown with the remaining stones dominantly yellow or colorless. Very significant are rare but economically important pink to red diamonds that bring Argyle fame.

At one point, Argyle mined nearly 40% of the world’s annual diamonds: by the end of 2000, the mine had produced an extraordinary 558,400,000 carats (Shigley and others, 2001).

Argyle diamond mine in 1986 shortly
after discovery. This became the
largest producing diamond mine in
the world.
Many lamproitic diamonds are relatively small and include common “fancy” yellow to brown stones. For example, macro diamonds (>1 mm) from the Ellendale field in Western Australia are dominantly yellow dodecahedra with many micro diamonds being colorless or pale-brown, frosted, step-layered octahedral (Shigley and others, 2001).

Because of a relatively high specific gravity (3.5), diamonds are often found in secondary stream or marine placers with other minerals of relatively high specific gravity such as magnetite, spinel, ilmenite, rutile, garnet, gold, etc.  Historically, there have been many reports of gold prospectors finding diamonds while searching for placer gold.  Examples include California, Colorado, Georgia, North Carolina and Wyoming in the United States and New South Wales in Australia (Hausel, 1998). 

Placer diamond deposits formed throughout geological history as is evident by diamonds in ancient Proterozoic paleoplacers in the Witwatersrand metaconglomerates of South Africa and the Snowy Range Group in the Wyoming Province, United States, as well as modern placers along the Atlantic Coast of Africa and Smoke Creek near the Argyle mine, Australia. 

Because of extreme hardness, diamonds can transport great distances in fluvial systems with little to no evidence of wear. Some of the more productive deposits include stream and marine placers where a large percentage of diamonds are gem-quality owing to fracturing and disaggregation of imperfect industrial diamonds during stream transport. Considerable numbers of diamonds have been mined from stream sediments along the Orange River basin in southern Africa and continuing in beach sands down current from the mouth of the Orange River along the Atlantic Coast.

Wednesday, November 10, 2010

Diamonds - Mineralogy, Identification & Geology

Recently, I've received inquiries from prospectors wanting to find out how to identify raw diamonds. One of my favorite stories concerned a rock hound who used his truck's windshield to test diamonds. In this method, all diamonds were thought to scratch his windshield. I suspect after not being able to see out his window, he finally contacted me to find out why he was finding so many diamonds. Unfortunately, windshield glass has a hardness of 5.5 to 6.5 which means that most silicates (including quartz) will scratch glass. This is why people in dusty regions of the country have to replace their car windows so often. So this method is useless.

For the amateur, I recommend using a diamond detector or diamond detective. These simple instruments test the surface conductivity of a mineral and either reads as "diamond" or as "synthetic". Most diamonds will read as diamond and all other minerals should read as synthetic. This will save most prospectors several years of college courses in mineralogy, etc.

Diamonds are extraordinary minerals with extreme hardness and inherent beauty often sought for personal adornment and industrial use. Because the genesis of this unique mineral requires extreme temperature and pressure, natural diamond is rare.  So rare that some diamonds are the most valuable commodity on earth based on weight.

Diamonds are mined on several continents. The value of the raw production has resulted in a multi-billion dollar industry. Natural diamond production averages more than 110 million carats annually valued at more than $7 billion for raw stones. Diamond values dramatically increase after faceting and the value again dramatically increases with dressing in jewelry, such that diamond jewelry typically sells for 10 or more times the value of the raw stone. Industrial diamonds, which are of considerably lower value, includes synthetic industrial diamonds. Synthetic industrial diamond production has an average annual value of around $1 billion per year.

Diamond consists simply of carbon.  In nature, native carbon may occur as one of the following polymorphs: diamond, graphite or lonsdaleite (Erlich and Hausel, 2002).  The physical differences between these polymorphs are due to different bonds between the carbon atoms in the crystal structure. In diamond, the coordination of the carbon atoms is tetrahedral with each atom held to four others by strong covalent bonds resulting in a mineral with extreme hardness.

In contrast, graphite has six-member hexagonal carbon rings which resonate between single- and double-shared electron bonds.  These graphite sheets are very strong, but the hexagonal rings are stacked and do not share electrons between adjacent sheets, only a residual electrical charge – thus no chemical bonds occur between the sheets, resulting in graphite being soft, and the sheets easily separated.

The hexagonal modification of diamond, known as lonsdaleite, has a closer-packed arrangement of atoms than diamond or graphite resulting in a rare mineral of extreme hardness (Lonsdale, 1971).  Lonsdaleite was initially synthesized at temperatures greater than 1,000°C under static pressures exceeding 130 kbar (Bundy and Kasper, 1967).  DuPont deNemours and Co. obtained the same transformation by intense shock compression and thermal quenching.  Lonsdaleite has since been identified in meteorites and in rare unconventional host rocks: the most notable being the Popigay Depression in Siberia (Erlich and Hausel, 2002). The extreme hardness of lonsdaleite (about 35% greater than diamond) makes it ideal for industrial grinding, but its rarity makes it unattractive for commercial use.

Diamonds are isometric and have high symmetry and cubic, octahedral, hexoctohedral, dodecahedral, trisoctahedral and related habits. Twinning along the octahedral {111} plane is common and often are flattened parallel to this plane producing a crystal habit that appears as flatten, triangular-shaped diamond known as a macle. 

Cubic diamond - note the greasy luster

Cube. Cubes are a relatively uncommon habit for diamond, and when found are primarily frosted industrial stones. Many have been found in placers in Brazil, and a significant percentage of diamonds in the Snap Lake kimberlites of Canada have cubic habit (Pokhilenko and others, 2003). Crystal faces of a cube often exhibit square-shaped pyramidal depressions rotated 45° diagonally to the edge of the crystal face.  The cube may also include scattered trigons mixed with pyramidal and other depressions of hexagonal morphology visible with a microscope.

Octahedron. The octahedron is an eight-sided crystal that has the appearance of two four-sided pyramids attached at a common base. Each pyramid contains four equilateral triangles known as octahedral faces. In nature, an octahedral face will often have positive or negative trigons: small equilateral triangles visible under a microscope. These are growths or etches on the crystal surface that represent a product of disequilibrium during transport to the earth's surface from the initial stable conditions at depth within the mantle.

 Partial resorption of the octahedron will result in different crystal habits including a rounded dodecahedron (12-sided) with rhombic faces. Further resorption may result in ridges on the rhombic faces yielding a 24-sided crystal known as a trishexahedron. Many diamonds from Argyle, Australia, Murfreesburo, Arkansas, and the Colorado-Wyoming State Line district exhibit resorbed crystal habits. Four-sided tetrahedral diamonds are sometimes encountered that are distorted octahedrons (Bruton 1979; Orlov 1977). 

Diamonds commonly enclose mineral inclusions along cleavage planes. These tiny inclusions provide important data on the origin of diamond and may be used to determine the age of the stone or to identify the unique chemistry associated with the genesis of diamond.

Bort. Bort is poor grade diamond used as an industrial abrasive. It forms rounded grains with a rough exterior and has a radiating crystal habit.  The term is also applied to diamonds of inferior quality as well as to small diamond fragments. 

Carbonado is a black to grayish, opaque, fine-grained aggregate of microscopic diamond, graphite, and amorphous carbon with or without accessory minerals. The material is hard, occurs mainly as irregular porous concretions and dendritic aggregates of minute octahedra, and sometimes forms regular, globular concretions. Carbonado is characterized by large aggregates (averaging 8 to 12 mm in diameter) that commonly weigh as much as 20 carats.  Specimens of several hundred carats are not uncommon.  The density for carbonado is less than that for diamond, and varies from 3.13 to 3.46 gm/cubic cm. Although carbonado had been found in placers in Brazil and Russia, it was not until the 1990s that it was found in situ. Twenty-six grains of carbonado ranging in size from 0.1 to 1 mm were recovered from a 330-lb sample taken from avachite (a specific type of basalt from the Avacha volcano of eastern Kamchatka) (Erlich and Hausel, 2002).

Physical Properties of Diamond
Diamond exhibits perfect octahedral cleavage with conchoidal fracture. The mineral is brittle and will easily break with a strike of a hammer. Even so, it is the hardest of all naturally occurring minerals and assigned a hardness of 10 on Mohs scale and nearly 8000 kg/mm squared on the Knoop scale. Corundum, the next hardest naturally occurring mineral has a Mohs hardness of 9.  Even so, corundum only has a Knoop scale hardness of 1370 kg/mm2.  Because of diamond’s extreme hardness as well as excellent transparency, diamond is extensively used in jewelry and has a variety of industrial uses. Diamond’s hardness varies in different crystallographic directions. This allows for the mineral to be polished with less difficulty in specific directions using diamond powder. For example, it is less difficult to grind the octahedral corners off the diamond, whereas grinding parallel to the octahedral face is nearly impossible. 

With perfect cleavage in four directions parallel to the octahedral faces, an octahedron can be fashioned from an irregular diamond by cleaving (Orlov 1977). The specific gravity of diamond (3.516 to 3.525) is high enough that the gem will concentrate in placers with “black sand”. This density is surprisingly high given the fact that it is composed of such a light element. Compared to graphite, diamond is twice as dense due to the close packing of atoms. 

Color. Diamonds occur in a variety of colors including white to colorless and in shades of yellow, red, pink, orange, green, blue, brown, gray and black (Figure 3). Those that are strongly colored are termed fancies. Colored diamonds have included some spectacular stones. For example, at the 1989 Christie's auction in New York, a 3.14-carat Argyle pink sold for $1.5 million. More recently, a 0.95-carat fancy purplish red Argyle diamond sold for nearly $1 million (US). The world’s largest faceted diamond, a yellow-brown fancy known as the 545.7-carat Golden Jubilee (Harlow, 1998), is considered priceless. Possibly the most famous diamond in the world, the 45-carat Hope, is a blue fancy.

In most other gemstones, color is the result of transition element impurities; but, this is not the case for diamond. Color in many diamonds is related to nitrogen and boron impurities or is the result of structural defects. Diamonds with dispersed nitrogen may produce yellow (canary) gemstones. If diamond contains boron it may be blue, such as the Hope diamond. The Hope was found in India; although many natural blue diamonds have come from the Premier mine in South Africa. Blue diamond with trace boron are semiconductors. Natural irradiation may result in blue coloration in some diamonds (Harlow, 1998).

The most common color is brown. Prior to the development of the Argyle mine in Australia in 1986, brown diamonds were considered industrial. But due to Australian marketing strategies, brown diamonds are now highly prized. The lighter brown stones are labeled champagne and darker brown referred to as cognac. Yellow is the second most common color and are referred to as “Cape” diamonds in reference to the Cape Province. When the yellow color is intense, the stone is referred to as “canary”.

Pink, red and purple diamonds are rare. The color in these is due to tiny lamellae (referred to as pink graining) in an otherwise colorless diamond. The color lamellae are thought to be a result of deformation of the diamond structure.

Even though there are many green diamonds, few are faceted, primarily because most have a thin surface covering clear diamond such that if the stone is faceted, the green layer is removed. Faceted green diamonds are so rare that only one is relatively well known (the 41-carat Dresden Green), and is thought to have originated in India or Brazil. The color in most green diamonds is the result of natural irradiation. Other green diamonds may result from hydrogen impurities. Another variety, known as a green transmitter produces strong fluorescence that tends to mask the yellow color of the stone. Other colors include rare orange and violet diamonds (Harlow, 1998).

One of the better-known black diamonds is the 67.5-carat Orlov. Black diamonds are colored by numerous graphite inclusions, which also make the diamond an electrical conductor. These are difficult to polish due to abundant soft graphite, thus black gem diamonds are uncommon. Opalescent, or fancy milky white diamond, is the result of numerous mineral inclusions and possibly nitrogen defects in the crystal (Harlow, 1998).

Diamond has high coefficient of dispersion (0.044): the coefficient being the difference in refractive index of two visible light wavelengths at the opposite ends of the spectrum (one blue-violet and the other red). This results in distinct fire in faceted diamond due to high dispersion. Diamond is completely transparent to a broad segment of the electromagnetic spectrum. It is also transparent to radio and microwaves. Colorless diamonds are transparent to visible light wavelengths extending into the ultraviolet, and a few rare diamonds are transparent over much of the ultraviolet spectrum.

Diamond has a luster described as greasy to adamantine that is related to its high refractive index (IR=2.4195) and density. Such high density greatly diminishes the speed of light. For example, the speed of light in a vacuum is 186,000 mi/sec, but in diamond, it is only 77,000 mi/sec (Harlow, 1998).

Many diamonds are luminescent: approximately one-third of all diamonds luminance blue when placed in ultraviolet light. In most cases, luminescence will stop when the ultraviolet light is turned off (known as fluorescence). Diamonds fluoresce in both long- and short-wave ultraviolet light. The fluorescence is usually greater in long wave and diamond may appear blue, green, yellow or occasionally red. However, fluorescence is generally weak, and it may not be readily apparent to the naked eye. In some cases, light emission is still visible for a brief second after the ultraviolet light source is turned off (known as phosphorescence). Some diamonds may also show brilliant phosphorescence when rubbed or exposed to the electric charge in a vacuum tube; or when exposed to ultraviolet light (Dana and Ford, 1951).

At room temperature, diamond is four times as thermally conductive as copper, even though it is not electrically conductive. Because of the ability to conduct heat, diamond has a tendency to feel cool to the lips when touched, since the gemstone conducts heat away from the lips. This is why diamonds have been referred to as “ice”. GEM testers (about the size of a pen) are designed to identify the unique thermal conductivity of diamond and distinguish it from other gems and imitations.

Diamonds are hydrophobic (non-wetable). Even though diamond is 3.5 times heavier than water, it can be induced to float on water. Because it is hydrophobic, diamond will attract grease, thus providing an efficient method for extracting diamond from ore concentrates (i.e., grease table). Oil, grease, and other hydrocarbons have an affinity for materials that do not contain oxygen (such as diamond).

Diamonds are unaffected by heat except at high temperatures. When heated in oxygen, diamond will burn to carbon dioxide (considered a pollutant by the EPA- for crying out loud, this is nothing more than plant food - the EPA again provides evidence of its lack of credibility). Without oxygen, diamond will transform to graphite at much higher temperatures (1900°C). Diamonds are unaffected by acids.

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