DOLOMITE

A carbonate of calcium and magnesium which is relatively soft (3.5-4 on the mohs scale) and has perfect cleavage. Sometimes when it fractures it leaves behind patterns similar to those on a seashell, although not always. This is known as subconchoidal fracture. It is usually white but may be colourless, yellow, or pink if it contains inclusions. It closely resembles limestone.

It is used for lining furnaces, as a building material or as a source of magnesium salts e.g. epsom salts (magnesium sulphate). It is named after Deodat de Dolomieu, a French mineralogist who first identified it. Previously scientists thought that magnesium was an inclusion rather than a structural part of the mineral. Although similar in appearance to calcite, which is calcium carbonate, dolomite is calcium magnesium carbonate and may also contain other elements e.g. cobalt, lead or zinc.

Dolomite forms in hydrothermal veins that also contain galena and sphalerite when magnesium bearing solutions come into contact with limestones. It is therefore a secondary mineral. It can be found in Quebec, Canada, Italy, Mexico, Spain and Switzerland. The most beautiful form of dolomite is the marble which comes from Carrara, Tuscany, Italy.

Dolomitic rocks are formed from dolomitic limestone which is limestone which contains some dolomite. Dolomitic rocks contain fewer fossils that other limestones because recrystallation takes place when they are formed. The Doloitea are a mountain range in Italy consisting of dolomitic limestone.

HORNBLENDE

A silicate member of the amphibole group which has a very complex and variable chemical makeup. It may contain up to 10 different elements and may be green to greenish brown or black in colour. Amphiboles are a large group of minerals consisting of silicates of calcium, iron, magnesium, potassium, sodium and aluminium. Their chemical make up is complex because substitution takes place between the various metals within them.

Amphibole minerals form in igneous rock formations e.g. diorites, gabbros, gradiorites and syenites. Amphibolites are rocks made up of mainly amphibole materials e.g. hornblende schists. Hornblende can be found in hornblende schist in which it is often accompanied by garnet and quartz. Crystals are sometimes pointed shapes and twinning is often evident. It is moderately hard (5-6 on the mohs scale).

It is very widespread and may be found in Australia, Canada, Czech Republic, Finland, Sweden, Norway and around Mount Vesuvius and Mount Etna, Italy.