HALITE

The geological name for rock salt. It is a member of the halide group which is usually white or colourless but may be tinged with red, purple or blue. Halides are composed of a metallic element combined with a halogen e.g. chlorine or bromine. Halite has a greasy feel and is soluble in cold water. It is very soft (2 on the mohs scale).

Halite is used as a source of sodium and chlorine. The salt used for cooking and in food is extracted from halite found on land. Sodium and chlorine is extracted from halite found in sea water. Chlorine is produced as a side effect when sodium is extracted from sea water by electroloysis. Sodium was first isolated by Sir Humphrey Davy in early 1800's.

The name halite is derived from the word hals which means sea or salt. Sodium is used as a coolant in nuclear reactors. Halite is also a source of sodium hydroxide which is a constituent of soaps and paper.

It is found in evaporite deposits, sometimes in association with gypsum. Evaporites form when enclosed areas of salt water evaporate. Halite can be found in Saltzburg (Austria), Ontario (Canada), India, Italy and Siberia. It may also be found in Cheshire (UK).

HEMIMORPHITE

A common silicate mineral which can appear in many colours but is usually white or blue.

It is a secondary mineral which forms when zinc bearing ores are oxidised by air and water. It is fairy soft (4.5-5 on the mohs scale) and gets it's name from the shape of it's crystals which are blunt at one end and sharp at the other. Crystals which have different forms at each end are known as hemimorphic crystals. Tourmaline is an example of another hemimorphic mineral.

When it occurs in large deposits it may be mined for zinc. Zinc forms a corrosion resistant layer when in contact with moist air and is therefore used as protective coating for metals such as iron to prevent erosion. It is also used for alloys such as brass and for galvanisation.

Hemimorphite is usually found in great masses known as consecrations in areas where there are zinc and lead sulphide minerals. It is deposited when zinc salts react with water that is rich in silica and may sometimes be mixed with smithsonite, anglesite and cerussite. It may also be found in limestones. It may be found in locations such as Italy, Mexico, Nevada and California (US) and Cumbria, and Derbyshire (UK).