copper pyrites and other sulphide

Pyrrhotite, copper pyrites and other sulphide ores may separate from the magma at an early stage to form irregular bodies which may also give rise to substantial ore deposits. In magmas very rich in iron and phosphorus, the iron may separate out early and sink to the bottom of the magma chamber to form large masses of magnetite with apatite. The mineral assemblages formed during early crystallisation are invariably characterised by an abun¬dance of iron and magnesium ores and a scarcity of silica—feldspar often being completely absent. It is possible to get a number of more or less distinct mineral assemblages if, during the early stages of crystallisation, the liquid magma is squeezed out of the mesh of crystals and moves elsewhere to crystallise into a separate igneous body.

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The igneous and closely related metamorphic rocks

The igneous and closely related metamorphic rocks contain only a relatively small proportion of the total number of known minerals, but they form, nevertheless, by far the greater part of the earth’s crust. It is estimated that they make up about 95 per cent of the volume of the upper 10 miles of the crust.

The crystallisation of magma into plutonic rocks at depth takes place in a number of fairly well defined stages. The earliest minerals to crystallise are those with the highest melting point. These include minerals containing the elements titanium, phosphorus, chromium, vanadium, platinum and other elements related to platinum. The most important minerals in this class are magnetite, titaniferous magnetite, ilmenite, rutile, chromite, spinel and native platinum. Titanium ores are

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Igneous Minerals

The term magma is used to describe the molten rock material within the earth’s crust which eventually consolidates to become igneous rock. If the magma crystallises in the deeper parts of the crust, the resulting rock is termed plutonic. If the magma pushes its way up to the surface through cracks or along volcanic vents, it forms lava flows which consolidate into volcanic or extrusive rocks. The crystallisation of minerals in lava takes place at temperatures ranging from 1100° C down to 700° C.

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