Page 8 - Curriculum Visions Dynamic Book

Halide minerals
Most halides – compounds containing chlorine, fluorine, iodine or bromine – readily dissolve
in water. They are only precipitated when
water is evaporated, such as happens when desert lakes dry up or in coastal lagoons.
The precipitates form white crystals, most commonly seen as the white “ghost lakes” or playas in deserts.
White deposits also form on the surface
of some soils, especially those that have
been irrigated with water containing high concentrations of dissolved halides. Soils
that exhibit white surface encrustations are contaminated with halides to a level where they are toxic to most plants. Such soils
are examples of desertification.
 Fluorine is found as fluorite (calcium fluoride), a blue, yellow or green crystalline mineral. Often known as Blue John, this mineral makes beautiful cubic (box-shaped) crystals.
 Each time a wave breaks, tiny droplets
of water spray are thrown into the air. Many
of these are carried aloft by the wind, where the sodium chloride they contain attracts even more water. These tiny particles are called condensation nuclei by meteorologists because they cause water to condense on them, gradually growing bigger and bigger until they reach a size at which they can fall out of the clouds as rain. This is why many freshwater raindrops contain tiny amounts of salt.
Chloride ion
 The glassy character of rock salt (halite) can clearly be seen in this specimen.
 The structure of halite (sodium chloride) is made by sodium and chloride ions occupying alternate corners of a cubic lattice.
Sodium ion
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