Diamonds
George E. Harlow, and Rondi M. Davies – Guest Editors
Table of Contents
Overview
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2005 Topics
Overview
Diamond, the fascinating ultrahard mineral, is the focus of considerable interest and scientific research. Recent advances particularly relevant to geoscientists include: diamond as a recorder of Earth processes from the perspective of inclusions, chemistry, and conditions of formation; syn- thesis for research applications and processing to modify color and physical properties, important to diamond gems and anvils; the implications of nanodia- monds from meteorites.
- Diamonds
- Inclusions in Sublithospheric Diamonds: Glimpses of Deep Earth
- Stable Isotopes and the Origin of Diamond
- Strange Diamonds: The Mysterious Origins of Carbonado and Framesite
- Microdiamonds in Ultrahigh-Pressure Metamorphic Rocks
- Meteoritic Nanodiamonds: Messengers from the Stars
- High-Pressure High Temperature Treatment of Gem Diamonds
- Growing Diamond Crystals by Chemical Vapor Deposition
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Next Issue
v1n3 Genesis: Rocks, Minerals, and the Geochemical Origin of Life
Guest editor: Robert M. Hazen, Geophysical Laboratory
In the beginning, Earth was a blasted, lifeless planet of water, air, and rock. From these raw materials, life emerged through a sequence of geo-chemical processes, including mineral-catalyzed organic synthesis, con-centration of those molecules in mineral surfaces, and assembly of life’s essential macromolecules.
- Genesis: Rocks, Minerals, and the Geochemical Origin of Life Robert M. Hazen (Carnegie Institution of Washington’s Geophysical Laboratory)
- Geochemical Influences on Life’s Origins and Evolution Joseph V. Smith (University of Chicago, USA)
- Mineral Catalysis and Prebiotic Synthesis: Montmorillonite-Catalyzed Formation of RNA James P. Ferris (Director of the New York Center)
- Geochemical Connections to Primitive Metabolism George D. Cody (Carnegie Institution of Washington)
- Sketches for a Mineral Genetic Material A. Graham Cairns-Smith (University of Edinburgh)
2005 Topics
- Fluids in Planetary Systems (January 2005 )
- Diamonds (March 2005)
- Genesis: Rocks, Minerals, and the Geochemical Origin of Life (June 2005)
- Toxic Metals in the Environment: The Role of Surfaces (September 2005)
- Large Igneous Provinces: Origin and Environmental Consequences (December 2005 )
Thematic Articles
Active research on diamond, a carbon mineral with superlative proper- ties, extends into many realms of natural and material sciences. Extreme hardness and transparency make diamond a valuable gem and a high-pressure research tool, as well as a superabrasive. Natural forma- tion at high pressure and resistance to weathering make diamonds our most informative messengers from Earth’s mantle. A review of diamond’s charac- ter and forms leads into the topics of the articles in this issue of Elements.
Diamonds originate in the deep roots of ancient continental blocks (cratons) that extend into the diamond stability field beneath about 140 km. Over the last two decades, rare diamonds derived from even greater depths—the deep upper mantle, the transition zone (410–660 km), and the lower mantle—have been recognized. Inclusions in diamonds from the deep upper mantle and the transition zone document sources of basaltic composition, possibly related to subduction of old oceanic crust back into Earth’s mantle. Diamonds from the lower mantle carry inclusions that largely confirm predictions of the composition and mineralogy of the deep mantle based on a “pyrolite” (primitive peridotitic) composition of silicate Earth. For some inclusions, however, the chemical evidence again points to a connection with subducting oceanic slabs, possibly ponding at the top of the lower mantle.
Most diamonds form in a relatively narrow depth interval of Earth’s subcontinental mantle between 150 and 250 km. From carbon isotope analyses of diamond obtained in the 1970s, it was first proposed that eclogitic diamonds form from crustal carbon recycled into the mantle by subduction and that the more abundant peridotitic diamonds formed from mantle carbon. More recent stable isotope studies using nitro- gen, oxygen, and sulfur, as well as carbon, combined with studies of mineral inclusions within diamonds, have strengthened arguments supporting and opposing the early proposal. The conflicting evidence is reconciled if mantle carbon is introduced via fluid into mantle eclogites and peridotites, some of which represent subducted oceanic crust.
Polycrystalline aggregates of diamond called carbonado and framesite have excited the attention of scientists because their crystallization histories are thought to depart markedly from established modes of diamond genesis. In contrast to kimberlitic diamonds, the geochemical signatures of carbonados are systematically crustal. Since the apparent age of carbonados is Archean (~3.2 Ga), a number of exotic formation theories have been invoked, including metamorphism of the earliest subducted lithosphere, radioactive transformation of mantle hydrocarbon, and mete- orite impact on concentrated biomass. Unlike carbonados, framesites are known to originate in the mantle. They appear to have crystallized very rapidly, shortly before the eruption of the kimberlites that brought them to Earth’s surface, suggesting that old cratonic materials can be remobilized after long-term storage in the lithosphere.
Since the first report of microdiamonds of metamorphic origin in crustal rocks of the Kokchetav Massif, northern Kazakhstan, diamonds have been described from several other ultrahigh-pressure (UHP) metamor- phic terranes. In situ diamond is the best indicator of ultrahigh-pressure conditions (>4 GPa), and testifies to subduction of continental crust to depths within the diamond stability field followed by relatively rapid exhu- mation. In contrast to other UHP terranes, the Kokchetav Massif contains rocks with unusually abundant diamonds, particularly in the Kumdy-Kol region. Kumdy-Kol diamonds exhibit diverse morphologies, dependent upon the host rock. Raman and cathodoluminescence spectra and carbon isotope composi- tions differ between core and rim, indicating two distinct growth stages.
Primitive chondritic meteorites contain up to ~1500 ppm of nanometer- sized diamonds. These nanodiamonds contain isotopically anomalous noble gases, nitrogen, hydrogen, and other elements. The isotopic anomalies indicate that meteoritic nanodiamonds probably formed outside our solar system, prior to the Sun’s formation (they are thus presolar grains), and they carry within them a record of nucleosynthesis in the galaxy. Their characteristics also reflect the conditions encountered in interstellar space, in the solar nebula, and in the host meteorites.
Annealing of gem-quality diamonds at very high pressures (above 5 GPa) and temperatures (above ~1800°C) can produce significant changes in their color. Treatment under these high-pressure–high- temperature (HPHT) conditions affects certain optically active defects and their absorptions in the visible spectrum. In the jewelry industry, laboratory- treated diamonds are valued much less than those of natural color. Polished diamonds are carefully examined at gemological laboratories to determine the “origin of color” as part of an overall assessment of their quality. Currently, the recognition of HPHT-treated diamonds involves the determina- tion of various visual properties (such as color and features seen under magnification), as well as characterization by several spectroscopic tech- niques. HPHT-treated diamonds were introduced into the jewelry trade in the late 1990s, and despite progress in their recognition, their identification remains a challenge. While some detection methodologies have been estab- lished, the large number of diamonds requiring testing with sophisticated analytical instrumentation poses a logistical problem for gemological laboratories.
The synthesis of large single-crystal diamonds by chemical vapor deposi- tion (CVD) at high growth rate has opened a new era for applications of the material. Large and thick single crystals can now be produced at very high growth rates, and the mechanical properties, chemistry, and optical and electronic properties of the material can be tuned over a wide range. The single crystals can have extremely high fracture toughness and exceptionally high hardness following high-pressure/high-temperature annealing. CVD single-crystal diamonds will make possible a new generation of high-pressure–temperature experimentation to study Earth and planetary materials and should enable a variety of other new scientific and technological applications.
