DEFINITION OF HALIDE MINERALS

Minerals that have a halogen such as fluorine,

chlorine, iodine, or bromine as the anion. Two common

halides are fluorite (CaF2) and halite (NaCl). Both minerals are

cubic. Fluorite has perfect octahedral cleavage and vitreous luster

and commonly occurs as purple, violet, green, yellow, or

brightly colored crystals. It occurs as a late-crystallizing

hydrothermal mineral in some granites, syenites, and other plutonic

rocks and is most often found in pegmatites. It also occurs

in some hydrothermal veins along with barite, sphalerite, galena,

calcite, and quartz. Halite, including the form commonly

known as rock salt, is normally colorless but may be red, yellow,

or blue because of iron and other impurities. It can be distinguished

by its perfect cubic cleavage, saline taste, softness,

and solubility in water. Halite forms primarily in sedimentary

rocks where it has been formed by evaporation from seawater,

or saline lakes or sabkhas. Halite begins to precipitate when the

volume of seawater has been reduced to about 10 percent of its

original volume by evaporation. It may also form as a volcanic

sublimate and as a surface efflorescence in arid regions.

Halite is a very weak rock, and it often deforms into salt

domes when it gets buried and is subject to large overburden

pressures. It may also act as a decollement horizon in mountain

and extensional belts, accommodating large contractional

or extensional strains because it is typically one of the

weakest, most deformable horizons in a sedimentary/volcanic

sequence of rocks.

See also MINERALOGY; DIAPIR.

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