DEFINITION OF CRUSTACEAN

Arthropods characterized by joints and segmented

bodies with an exoskeleton, with two pairs of antennae

on their heads. Most crustaceans live or lived in the

marine environment, although some varieties also live in

freshwater and subaerial settings. They are second in abundance

of individuals only to insects. They range from Early

Cambrian to the present, with modern examples including

shrimp, lobster, barnacles, and ostracods.

crystal Homogeneous solid structures composed of chemical

elements or compounds, having a regularly repeating

arrangement of atoms. All minerals are solids, and in all minerals

the atoms are arranged in a very regular geometric form

that is unique to that mineral. Every mineral of that species

has an identical crystalline structure. It is this regular structure

that gives each mineral its characteristic color, chemistry,

hardness, and crystal form.

Many minerals may not have a well-developed external

crystal form, but they still must have a regularly repeating

crystal lattice composed of the constituent atoms. Crystal

forms and the internal atomic lattice exhibit symmetry,

which may be of several different varieties. There are four

main types of crystal symmetry. Mirror plane symmetry is

the simplest, in which the crystal can be divided by an imaginary

mirror plane that would divide the crystal into two

halves that are mirror-images of one another. Crystals may

have symmetry about an axis that runs through the center

of the crystal, in which the crystal lattice would be rotated

into an identical configuration two, three, four, or, six times

in a 360° circuit. These symmetry systems are known as

diads, triads, tetrads, and hexads. A more complex form of

symmetry is known as roto inversion, characterized by a

rotational operation followed by inversion of the lattice

across its center point. Finally, there is a simple inversion

across the center of the crystal leading to a crystal face diametrically

opposite to every other crystal face. Using various

combinations of these symmetry operations, all crystals are

found to belong to one of seven crystal systems. These

include cubic, tetragonal, orthorhombic, monoclinic, triclinic,

hexagonal, and trigonal.

Crystals may appear to be close to perfect but always

have millions of atomic defects. These include vacancies in

the crystal lattice, various types of defects in the arrangement

of the atoms and lattice, and replacement of one type of atom

or ion by another with a similar charge and size.

See also MINERALOGY.

cyclone See HURRICANE.

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