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CRYSTAL
STRUCTURE
A neutral carbon atom has 6 protons and 6 electrons surrounding its
nucleus. Four of the electrons in a carbon atom are valence electrons,
which are electrons that are available to form bonds with other atoms. In
graphite, each carbon atom bonds only 3 of its 4 valence electrons with
neighboring carbons. The resulting structure of these bonds is a flat
sheet of connected carbon atoms. Though individually strong, these layers
are only weakly connected to one another, and the ease with which they are
separated is what makes graphite so slippery.
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 This image of trigons was
created with Nomarski differential interference contrast microscopy
and is 0.29 mm across.
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This model shows how each carbon atom (ball) is connected to 4 other
carbon atoms by strong chemical bonds (rods), creating diamond's rigid
crystal structure. In diamond however, every carbon shares all 4 of its
available electrons with adjacent carbon atoms, forming a tetrahedral
unit. This shared electron-pair bonding forms the strongest known chemical
linkage, the covalent bond, which is responsible for many of diamond's
superlative properties. The repeating structural unit of diamond consists
of 8 atoms which are fundamentally arranged in a cube.
Using this cubic form and its highly symmetrical arrangement of atoms,
diamond crystals can develop in a variety of different shapes known as
"crystal habits." The octahedron, or eight-sided shape that we associate
with diamonds is its most common crystal habit. But diamond crystals can
also form cubes, dodecahedra, and even combinations of these shapes. All
of these shapes are manifestations of the cubic crystal system to which
the mineral diamond belongs. Two exceptions are the flat form called a
macle, which is actually a composite crystal, and etched crystals, which
have rounded surfaces and, sometimes, elongated shapes
HARDNESS
Diamond is renowned for its hardness. Hardness is the measure of a
substance's resistance to being scratched, and only a diamond can scratch
another diamond. Diamond is the hardest substance known.
The Mohs scale--a hardness scale developed in 1822 by Austrian Friedreich
Mohs as a criterion for mineral identification--can help us appreciate the
hardness of diamond. The scale ranks 10 minerals; harder minerals, with a
higher number, can scratch those with a lower number.
When the mineral hardness numbers from the Mohs scale are plotted against
those on the more quantitative Knoop scale (based on the force needed to
make indentations using a diamond), we can see how it doesn't adequately
express the extreme hardness of diamond. The Mohs scale is relatively
stable until it reaches the eighth mineral topaz, but it jumps
exponentially from corundum (colorless sapphire) to diamond. It is in fact
difficult to measure the hardness of diamond, because diamond must be used
to measure its own hardness.
FLOUROSCENCE AND
PHOSPORESCENCE
An interesting property of some
diamonds is that they can glow in the dark. When illuminated by
ultraviolet light, certain diamonds can absorb the high-energy radiation
and re-emit it as visible light. These diamonds are called fluorescent.
Some can even continue glowing after the ultraviolet source is turned off.
These diamonds are phosphorescent. This fluorescence is usually blue, but
it can also be white, violet and sometimes yellow, green or orange.
This 2.15-carat, cushion-cut diamond fluoresces in daylight but not in
incandescent light, such as that given off by a light bulb. It is a
color-change diamond, going from intense, greenish yellow in fluorescent
light or daylight to yellowish brown in incandescent light. This Brazilian
diamond has a historic pedigree, having been given by Pedro II de
Alcantara (1825-91), emperor of Brazil, to his niece. Dom Pedro II, a
descendant of the Hapsburg family and the Braganza royal family that ruled
Portugal, ascended to the throne of Brazil in 1831 and ruled until 1889.
He was a scientist in his own right and was greatly interested in
diamonds, mineral specimens, and geology
Diamond is a polymorph of the element carbon.
Graphite is another polymorph. The two share the same chemistry, carbon,
but have very different structures and properties. Diamond is hard,
Graphite is soft (the "lead" of a pencil). Diamond is an excellent
electrical insulator; Graphite is a good conductor of electricity. Diamond
is the ultimate abrasive; Graphite is a very good lubricant. Diamond is
transparent, Graphite is opaque. Diamond crystallizes in the Isometric
system and graphite crystallizes in the hexagonal system. Somewhat of a
surprise is that at surface temperatures and pressures, Graphite is the
stable form of carbon. In fact, all diamonds at or near the surface of the
Earth are currently undergoing a transformation into Graphite. This
reaction, fortunately, is extremely slow.
COMPOSITION
Diamond is carbon in its most concentrated form. Except for trace
impurities like boron and nitrogen, diamond is composed solely of carbon,
the chemical element that is fundamental to all life.
But diamond is distinctly different from its
close cousins the common mineral graphite and lonsdaleite, both of which
are also composed of carbon. Why is diamond the hardest surface known
while graphite is exceedingly soft? Why is diamond transparent while
graphite is opaque and metallic black? What is it that makes diamond so
unique?
The key to these questions lie in diamond's particular arrangement of
carbon atoms or its crystal structure--the feature that defines any
mineral's fundamental properties. A crystal is a solid body formed from
the bonding of atomic elements or compounds in a repeating arrangement.
Often, crystals possess smooth external faces. Due to their symmetrical
and finite nature, the building blocks of crystals are limited to
relatively small numbers of atoms, and their chemical compositions to
simple numerical combinations of elements.
THE MINERAL
DIAMOND
Chemistry: C, Elemental Carbon
Class: Native Elements & Subclass: Non-metallics
Group: Carbon
Uses: as a gemstone and abrasive.
Diamond is the ultimate gemstone, having few weaknesses and much strength.
It is well known that Diamond is the hardest substance found in nature,
but few people realize that Diamond is four times harder than the next
hardest natural mineral, corundum (sapphire and ruby). But even as hard as
it is, it is not impervious. Diamond has four directions of cleavage,
meaning that if it receives a sharp blow in one of these directions it
will cleave, or split.
A skilled diamond setter and/or jeweler will prevent any of these
directions from being in a position to be struck
while mounted in a jewelry piece.
As a gemstone, Diamond's single flaw (perfect cleavage) is far
outdistanced by the sum of its positive qualities.
It has a broad color range, high refraction, high dispersion or fire, very
low reactivity to chemicals, rarity,
and of course, extreme hardness and durability. Diamond is the April
Birthstone.
In terms of its physical properties, diamond is the ultimate mineral in
several ways:
Hardness: Diamond is a perfect "10", defining the top of the hardness
scale.
Clarity: Diamond is transparent over a larger range of wavelengths (from
the ultraviolet into the far infrared) than is any other solid or liquid
substance - nothing else even comes close.
Thermal Conductivity: Diamond conducts heat better than anything - five
times better than the second best element, Silver!
Melting Point: Diamond has the highest melting point (3820 degrees
Kelvin)!
Lattice Density: The atoms of Diamond are packed closer together than are
the atoms of any other substance!
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