Download nature of diamond - Geological Sciences, CMU

NATURE OF DIAMOND
http://www.amnh.org/exhibitions/diamonds/
Diamond is exotic, formed in Earth's interior and shot to the surface by extraordinary
volcanoes. A diamond is likely the oldest thing you will ever own, probably 3 billion
years in age, two thirds the age of the Earth. Diamond is a strategic and high-tech
supermaterial for our technological society.
Diamond is amazingly dense. At 3.51 grams per cubic centimeter, it is vastly more
dense than graphite -- the more common form of the light element carbon -- at 2.20
grams per cubic centimeter. This comparison offers an important clue to diamond's
origin: The fact that diamond is "squeezed" much denser than graphite, which forms
near Earth's surface, implies formation at high pressure. As shown on the graph, this
concept was corroborated by experimental synthesis of diamond at high pressure and
temperature.
This simplified diagram shows the conditions of pressure and temperature where
diamond and graphite will be the stable forms of carbon. The points show the
conditions at which diamonds were first grown by the companies ASEA and General
Electric in the early 1950s. Temperatures are in Kelvin; subtract 273 to convert to
degrees Celsius. This magnitude of high pressure is difficult to comprehend. For
example, to make a diamond at 1400 degrees C (orange hot) would require the
pressure of 55,000 atmospheres!
Experiments and the high density of diamonds tell us that they crystallize at very high
pressures. In nature this means that diamonds are created by geologic processes at
great depth within Earth, generally more than 150 kilometers down, in a region
beneath the crust known as the mantle. Other processes, explored later in this
exhibition, bring diamonds to where people can find them.
This diagram shows the interior structure of Earth. The three concentric layers -- the
core, mantle, and crust -- formed within a few hundred million years of Earth's
coalescence 4.5 billion years ago. The core is primarily an iron-nickel alloy and makes
up a large fraction of the mass of Earth. The vast mantle is sandwiched between the
core and the thin crust and is composed predominantly of magnesium and iron
silicate minerals. Our planet's crust is a thin, rocky skin. Diamonds can form in most
of Earth's interior but not near its surface, where graphite is the stable form of
carbon. Indeed, diamonds only survive at Earth's surface because great heat is
required to break down the diamond structure.
The upper mantle is slightly plastic, which allows it to circulate slowly in a creeping,
convective flow that helps drive the surface motion of Earth known as "plate
tectonics."
The cross section shown here provides a closer look at Earth's crust and underlying
mantle. The crust can be divided into ocean basins, underlain by a thin layer of dense,
basaltic rock, and continents, formed of a much thicker but less-dense layer of granitic
rocks. Just below the crust is the portion of the mantle called the lithosphere, which is
rigid and acts like rock. Below this is the asthenosphere, a more plastic, flowing region
that enables the overlying crustal plates to move in what is known as plate tectonics.
The plot of pressure and temperature shows the conditions at which either diamond
or graphite exist. The general conditions present in the Earth are described by curved
lines called geotherms. Note that there are two geotherms: Because the continental
crust is old and thick, conditions are somewhat colder in and beneath it than beneath
the much younger ocean basins. Diamonds can form at depths as shallow as 150
kilometers beneath the continental crust, while beneath oceans they need depths of at
least 200 kilometers, as shown by the diamond boundary on the cross-section.
The search for diamonds has determined that most are derived from kimberlite pipes
in the oldest, nuclear portions of the continents, where the basement rocks are older
than 1.5 billion years.
The oldest parts of continents are called cratons, and can be divided into two
terranes: Archean-age archons, which are older than 2,500 million years, and
Proterozoic-age protons, which are 1,600 -- 2,500 million years old. The distribution
of these terranes is shown on the map. Kimberlite pipes occur in many parts of the
continental crust, but most diamond-rich ones are found in archons. This fact
suggests that most diamonds were formed and stored deep below the cratons, in the
area shown in the lower figure, and were later transported to the surface by kimberlite
and lamproite magmas that extracted them and other samples from the mantle.
The complex volcanic magmas that solidify into kimberlite and lamproite are not the
source of diamonds, only the elevators that bring them with other minerals and
mantle rocks to Earth's surface. Although rising from much greater depths than other
magmas, these pipes and volcanic cones are relatively small and rare, but they erupt in
extraordinary supersonic explosions.
Kimberlite and lamproite are similar mixtures of rock material. Their important
constituents include fragments of rock from Earth's mantle, large crystals, and the
crystallized magma that glues the mixture together. The magmas are very rich in
magnesium and volatile compounds such as water and carbon dioxide. As the
volatiles dissolved in the magma change to gas near Earth's surface, explosive
eruptions create the characteristic carrot- or bowl-shaped pipes.
Kimberlite magma rises through Earth's crust in networks of cracks or dikes. The
pipes only form near Earth's surface. This cross-section of a kimberlite pipe shows
the carrot-shaped profile produced by explosive eruption. The root zone starts in
fissures, where gases are released from the rising magma and drive the eruption; they
blow out the fragment-laden kimberlite to form the volcano's tuff ring and fill the
pipe. Depth measurements show the level of erosion for various kimberlite pipes in
South Africa. Adapted from Hawthorne (1975).
These drawings illustrate the formation and filling of the typical champagne-glass
shape of a lamproite pipe. The initial stage of the eruption, powered by gases either
from the lamproite magma or from boiling ground water, corrodes the hosting rock
to form the champagne-glass shape (top). The eruption then produces particles of
ash, lapilli, and pumice that partially fill the crater and form a tuff ring (middle).
Finally, the crater fills with a lava pond from the degassed lamproite magma (bottom).
Adapted from a sketch by Barbara Scott-Smith
Kimberlite magmas carry foreign rocks -- xenoliths -- from Earth's mantle to the
surface. Xenoliths are geologists' only samples from the deep Earth, and carry
information about diamond growth conditions. The 2 most common types of
xenoliths are peridotites and eclogites. Peridotite is the main constituent of the mantle
beneath the crust and consists primarily of olivine -- the gem variety is peridot.
Eclogite, consisting primarily of garnet and a green pyroxene, is formed by plate
tectonics when basalt of the ocean crust founders into the mantle. Certain kinds of
xenoliths contain diamonds.
These diagrams show the compositions of mantle xenoliths. Lherzolite is a variety of
peridotite thought to form most of the upper mantle.
Harzburgite is another kind of peridotite with less clinopyroxene. Garnet harzburgites
contain red garnet and, occasionally, diamonds.
Eclogite, a very different rock, consists of garnet and sodium-rich pyroxene; some
also contains diamonds.
Diamonds with inclusions are like little space capsules from the mantle: pristine
mineral samples are protected by the diamond's indomitable embrace and transported
to the surface by a volcanic rocket. Inclusions capture a picture of the rock and
environment in which diamonds grow and indicate that garnet harzburgite (a type of
peridotite) and eclogite are the most common rocks in which diamonds have grown.
A single mineral inclusion rarely defines a specific rock, but two or more minerals may
enable interpretation of rock associations and origin. Some inclusion minerals are
virtually unique to diamond sources and are thus sought in the exploration for
diamonds.
A purple pyrope garnet (a.), an indicator of garnet harzburgite, in a brownish diamond
octahedron from the Udachnaya pipe, Sakha Republic, Russia (about 0.8 mm across).
Orange "G5" garnet (b.), typical of diamond eclogite, showing the conspicuous
octahedral shape imposed by the enclosing diamond (about 0.5 mm across).
Red chromian pyrope and green chromian diopside, indicators of a peridotite, in a
diamond octahedron from the Mir pipe, Sakha Republic, Russia (each about 0.2 mm
across).
Most diamonds consist of primeval carbon from Earth's mantle, but those from
eclogites probably contain carbon recycled from the ocean crust by plate tectonics -the carbon of microorganisms. How do we know? Carbon atoms occur in three
different masses, or isotopes. Unlike high-temperature processes in deep Earth, lowtemperature, biological processes, such as photosynthesis, are sensitive to the
differences in mass, and actively sort different carbon isotopes. Thus, the ratios of
carbon isotopes in organic materials -- plants, animals, and shells -- vary, and also
differ from those in the carbon dioxide of the atmosphere and the oceans.
Geochemists "read" the carbon isotopes in samples to interpret nature's record.
Virtually all carbon atoms, the ones in a diamond or a tree or you, came from the
stars. Particularly at Earth's surface the proportions of 12C and 13C (the carbon
isotopes of mass 12 and 13) get redistributed. Expressed as simple numbers in 13C
notation -- in which larger numbers mean more 13C -- organic carbon has large
negative values, average Earth has a mildly negative value, and the carbon in shells is
near zero.
The narrow range of 13C values for harzburgitic diamonds in the histogram on the
top resembles the range of average Earth, indicating that the mantle is the likely
carbon source. The large range for eclogites suggests mixing of organic carbon (the
strongly negative numbers), mantle carbon (mildly negative numbers), and shell-like
carbon (values near zero). These data support recycling of once-living carbon from
Earth's surface deep into the mantle to form diamond.
When ocean floor slides into the mantle, the basaltic rock becomes eclogite, and
organic carbon in sediments may become diamond.
Kimberlites are generally much younger than the diamonds they bring to Earth's
surface. Kimberlites and lamproites have been dated between 50 and 1,600 million
years old. Diamonds associated with harzburgites are about 3.3 billion years old -more than two thirds the age of Earth itself, and those from eclogites generally range
from 3 billion to less than 1 billion years old. These age differences help clarify a
picture of diamonds having crystallized and been stored beneath the ancient
continental cratons and only later being lifted to Earth's surface by kimberlites.
Since inclusion minerals crystallized simultaneously with their diamond host, the age
of the inclusions gives the age of the diamond. The ancient age of peridotite
diamonds suggests that the formation of ancient Archean continental cores (archons)
included diamond crystallization in the underlying mantle lithosphere. A relatively
cool, rigid, deep keel beneath these continental nuclei provided a stable environment
in which diamonds crystallized and were stored. Subsequently, oceanic crust diving
into the mantle was metamorphosed into eclogite and pasted onto this keel. Much
later passage of kimberlite magmas through the keel dislodged diamonds from both
peridotite and eclogite and sent them to Earth's surface.
This cross-section of continental crust shows the 200-km-thick cool keel (part of the
mantle lithosphere) that provided a stable environment for diamond crystallization
and preservation. Kimberlites centered over the keel are likely to yield harzburgitehosted diamonds from the storage zone (marked with diamonds). Kimberlites near
the edge of the keel are more likely to contain eclogite-hosted diamonds, while those
off the keel are likely to be barren of diamonds.
In the last 20 years scientists have discovered new sources of diamond. Continental
collisions -- a result of plate tectonics -- can subject slices of a crust to immense burial
and uplift. In Kazakhstan, for example, diamonds formed in buried crust that
returned to Earth's surface. Meteor impacts produce immense pressures, and
diamonds can be formed and sprayed among the impact debris. Meteorites also
experience impacts themselves and can contain diamonds. And the most ancient
meteorite material contains star dust, the remnants of the death of stars. Some of this
star dust is extremely tiny bits of diamond, just big enough to be crystals and older
than the solar system itself.
Very small "microdiamonds," averaging only 12 micrometers across, were discovered
during diamond exploration in a region called the Kokchetav Massif, in northern
Kazakhstan, in large slices of metamorphic rock that must have been pushed at least
120 kilometers deep into Earth and returned. Discovery of this process, termed
ultrahigh pressure (UHP) metamorphism, has revolutionized ideas about and interest
in what can happen to Earth's crust. Recently scientists have found traces of diamond
around meteor impacts. At the 35-million-year-old Popigai crater in Siberia, graphite
transformed into microdiamond aggregates up to 1 centimeter across. It is now
suspected that diamonds form in most major impacts, becoming a new indicator of
ancient cosmic collisions. In 1987, microscopically small fragments of diamond, called
"nanodiamonds," were recovered from meteorites that predate the solar system. New
studies indicate that they formed more than 5 billion years ago in flashes of radiation
from dying red-giant stars into surrounding clouds of methane-rich gas. The process
is essentially the same as the new process for growing synthetic diamond called CVD
-- chemical vapor deposition.
Cartoon of the formation of a UHP terrane that can yield diamonds. At top, the
down-going subducted ocean crust (green) has a thin covering of sediment (gray) that
is sheared off and driven upward (inset), apparently caused by the continental
collision (middle) that squeezes the diamond-bearing metamorphic rocks back into
the crust (bottom).
Certain minerals are present in the rocks from the upper mantle that occur with
diamonds in kimberlite and lamproite pipes, as seen in nearby cases of xenoliths and
diamond inclusions. Some of these minerals, being resistant to weathering and denser
than quartz sand, concentrate in channel bottoms. Because they occur in far greater
abundance than diamond, exploration geologists look for these "indicators" among
the gravel of regions they suspect may host diamond-bearing pipes.
Indicator minerals for diamond include, in order of decreasing significance: garnet,
chromite, ilmenite, clinopyroxene, olivine, and zircon. But the order of persistence in
streams is zircon, ilmenite, chromite, garnet, chromian diopside, and olivine.
Diamond itself is obviously a most important indicator.
Most indicator minerals have a distinctive color. Seen here are red pyrope garnets,
green chromian clinopyroxene, black ilmenite and chromite, and yellowish-green
olivine.
The best way to see a kimberlite pipe is first hand, like a miner or geologist, in the
tunnels that provide access to the pipe in an underground mine. The tunnel recreated
in the exhibition goes from the local bedrock, through a boundary zone that is highly
fragmented, and into the kimberlite, with its inclusions of mantle rocks and diamonds.
Today diamonds are mined in about 25 countries, on every continent but Europe and
Antarctica. However, only a few diamond deposits were known until the 20th century,
when scientific understanding and technology extended diamond exploration and
mining around the globe. For 1,000 years, starting in roughly the 4th century BCE,
India was the only source of diamonds. In 1725, important sources were discovered in
Brazil, and in the 1870s major finds in South Africa marked a dramatic increase in the
diamond supply. Additional major producers now include several African countries,
Siberian Russia, and Australia.
It is a modern misconception that the world's diamonds come primarily from South
Africa: diamonds are a world-wide resource. The common characteristic of primary
diamond deposits is the ancient terrain that hosts the kimberlite and lamproite pipes
that bring diamonds to Earth's surface.
This map shows both the major deposits and the ancient bedrock, both the 2,500million-year-old archons and less productive 1,600 to 2,500-million-year-old protons,
that contain the diamond pipes. The diamonds in secondary deposits have been
moved by erosion away from the pipes.
The monumental increase in diamond production in the 20th century is shown on this
graph. India's maximum production, perhaps 50,000 to 100,000 carats annually in the
16th century, is very small by modern standards. Brazil and Venezuela are barely
discernible compared to South African production following discoveries in 1867. For
the most part, except for major wars and economic recessions, diamond production
has been steadily increasing since then, with non-African sources growing in relative
proportion. Major production is now dominated by Australia, Botswana, Russia, and
Congo Republic (Zaire), but South Africa is still a major producer, in both volume
and value.
Eighty percent of the diamonds mined annually are used in industry; 4 times that
production is grown synthetically for industry - that's a total of over 500 million carats
or 100 metric tons. Diamond is a fundamental industrial material that affects our daily
lives. Because diamond is the hardest substance, it is used to cut, grind, and polish
most hard substances. It fashions stones, ceramics, metals, and concrete, as well as
eyeglasses, gems, and computer chips. Its growing specialty-uses include blades, some
used in critical surgery; specialty windows; and heat spreaders. And of course
diamond phonograph needles reproduced music for 50 years.
Diamond has three primary roles in industry: it is used as a cutting tool, it is imbedded
in another material and used as a tool or abrasive, and it is turned to powder or paste
for grinding and polishing. Diamond is selected for such use where its hardness and
resistance to abrasion - its long working life and fast cutting action - outweigh its
costs. Moreover, diamond's resistance to wear enables it to cut reproducibly time after
time, a requirement of automated production. Diamond machining tools for turning,
milling, and boring are preferred where finely finished surfaces of high precision are
needed. Diamond is used for machining a wide variety of plastics, glasses, and metals,
shaping products such as the drums for copying machines, polygon mirrors in laser
printers, and aluminum-alloy pistons in automobile engines. However, diamond
cannot be used for machining alloys of iron. Under intense machining conditions the
diamond abrades very quickly against some materials, apparently because of a hightemperature reaction between iron and carbon to yield iron carbide.
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