Page 332 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN007E-968 June 30, 2001 17:35

378 High-Pressure Synthesis (Chemistry)

Up to where these materials dissociate, the obvious ef- Brazil, and Russia. The simplest diamonds to understand
fect of high pressures (depth in the earth) is increased are the small, dark fine-grained fragments, found in a few
density, which is accomplished structurally by atomic re- meteorites, which undoubtedly formed from graphite by
arrangements in the crystal lattice. The principal coordi- shock compression and heating during impact.
nation changes for aluminum and silicon (both with four Most natural diamonds are dark or flawed. Especially
to six nearest oxygen neighbors) have been mentioned for puzzling are red and brown hues. Even the colorless crys-
important minerals such as aluminum silicates and quartz. tals, when sectioned and examined by fluorescence, etch-
Other very important phases are represented by pyroxene ing, and other techniques, reveal many layers of growth.
(MgSiO 3 ) and forsterite (Mg 2 SiO 4 ), both of which are Isotopic dating methods indicate that most diamonds are
common in the basic igneous rocks of the upper mantle several thousand million years old.
and crust. Changes in forsterite include transformation to Another characteristic of natural diamond is its nitro-
a spinel phase of the same composition and then dispro- gen content. Most, called type Ia, have many parts per
portionation to MgSiO 3 and MgO at about 700 km. The million of nitrogen in the form of coalesced groups of
MgSiO 3 phase transforms to an ilmenite structure and then nitrogen atoms. They produce an infrared absorption at
to a perovskite lattice without composition change. This 1280 cm −1 but are inactive in electron paramagnetic reso-
means a change in the coordination number of silicon from nance (EPR). The more rare type Ib diamonds are yellow
4 in the 1-atm pyroxene form to 6 in the other forms. The due to isolated nitrogen atoms that replace carbon atoms.
magnesium coordination number also increases as these They absorb light at 1130 and 1343 cm −1 in the infrared
structural changes take place. Seismic velocity changes and show an EPR spectrum. After an hour in the labora-
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would be expected at the zone boundaries representing tory at 1800–1900 C and 6 GPa, the type Ib were largely
these transitions, but the demarcation may be fuzzy be- transformed to type Ia; most of the nitrogen atoms had
cause of composition gradients and substitution of other coalesced. Synthesized type Ib diamonds behaved simi-
ions in these structures (e.g., Fe for Mg). larly. Evidently natural type Ib diamonds did not experi-
Forsterite (Mg 2 SiO 4 ) is a constituent of a most inter- ence temperatures above about 1500 C for more than a
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esting and mysterious rock, kimberlite, which is the host year.
of natural terrestrial diamond, although only a small per- Most of the kinds of natural diamond have not yet been
centage of kimberlites contain diamond and fewer yet duplicated in the laboratory. In fact, an active area of re-
in amounts warranting mining. It is still controversial search is the high-pressure, high-temperature chemistry
whether diamonds are formed in the kimberlite or are of carbon in rocks at depth in the earth and how it got
simply carried into their present locations by this igneous together to form diamond crystals. The studies seem to
rock. In any case, diamonds in kimberlite often contain center on the system C H Si O with the possibility of
inclusions of the following minerals: forsterite (a form of species such as CH 4 , CO, and CO 2 .
olivine), pyroxene, garnet, the coesite form of SiO 2 (with- With increasing depth in Earth, the oxide compounds
out the stishovite form), and others. This obviously means tend to dissociate to simpler oxides and finally only metal
these phases are present as small crystals simultaneously alloys are stable at the core. The metal–rock boundary at
with the growing diamond. By determining the pressure the core is quite distinct. The density of Earth’s metallic
and temperature conditions for their stability, it is pos- core at the pressures known to exist there indicate that
sible to bracket the conditions for diamond synthesis in it contains a significant fraction of elements lighter than
Earth’s mantle. Thus, from laboratory studies, diamonds iron. If diamond anvils can be improved, some incremen-
are apparently formed at depths of about 100 to 300 km tal progress in the observation and interpretation of these
(about3.5to10GPa)andtemperaturesabove1000 C.The trends may be expected.
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upper pressure limit is based on the fact that coesite but
not stishovite is found in kimberlites. These limits are sur-
prisingly close to those found in the metal–carbon systems SEE ALSO THE FOLLOWING ARTICLES
from which diamond is manufactured (e.g., 4 to 6 GPa).
This agreement is a bit surprising because, while metal CHEMICAL THERMODYNAMICS • EARTH’S CORE • HIGH-
inclusions are common in manufactured diamonds, there PRESSURE RESEARCH • SUPERCONDUCTIVITY
is no evidence of elemental metal as an inclusion inside
natural diamonds from kimberlites, so the chemistries of BIBLIOGRAPHY
the two growth systems differ.
Several polycrystalline varieties of diamond exist, rang-
Ahrens, T. J. (1980). “Dynamic compression of earth materials,” Science
ing from somewhat porous or contaminated masses, such
207, 1035–1040.
as framesite or carbonado, to ballas, which is essentially Anthony, T. R. et al. (1990). “Carbon-12 enriched diamond with high
pure carbon. Ballas is found only in northwestern Africa, thermal conductivity,” Phys. Rev. B 142, 1104.