Introduction

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The Rock Cycle

The molten magma in the Earth's mantle may cool and crystallise underground, forming plutonic igneous rocks, or it may be forced out of erupting volcanoes, then crystallise to form volcanic igneous rocks.
Either way, the igneous rocks may be eroded to form sediments which are subsequently compacted to form sedimentary rocks, or they may be heated and deformed by the pressure of overlying material to form metamorphic rocks, or they may be pushed back under the Earth's surface at converging plate boundaries, where they will be subjected to such high temperatures that they will become molten magma again.
The sedimentary rocks, formed by erosion and sedimentation, may themselves be eroded to sediment which is compacted to form new sedimentary rocks. Alternatively they may become buried so deeply that they undergo diagenesis or metamorphism, forming metamorphic rocks.
Metamorphic rocks may be formed regionally from igneous or sedimentary rocks; they also may be formed locally where intruding magma comes into contact with pre-existing rock. These are contact metamorphic rocks. All types may be eroded to form sedimentary rocks, or they may be subjected to such high temperatures that they melt into magma again, completing the cycle.

Mineral Formation

Minerals may form directly by crystallisation from molten magma, precipitation from magmatic fluids or sublimation from gases. They may also form from alteration of rocks, by weathering, metamorphic or hydrothermal processes.
When molten magma cools, minerals crystallise out in order of their melting points, highest first, as given by Bowen's Reaction Series, which has two branches.
If the magma is rich in iron and magnesium, but poor in silicon, then the order is olivine, then pyroxene, amphibole and biotite; this is the discontinuous branch. If the magma is rich in silicon but poor in iron and magnesium then there is a continuous series from calcium-rich plagioclase feldspar, anorthite, to sodium-rich plagioclase feldspar, albite. Thereafter the two branches combine, and orthoclase crystallises, followed by muscovite and finally quartz. These are the major rock- forming minerals, although other minerals may be present as minor constituents.
Excess silica SiO2 separates out as quartz, and excess alumina Al2O3 crystallises as corundum.
During crystallisation the more mafic parts of the magma are enriched in chromium, nickel, platinum and sometimes phosphorus. More silicic parts of the magma are enriched in tin, zirconium and thorium. Titanium and iron are found throughout the range of composition and are found in all types of igneous rocks.