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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.
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
(P&M.
Paragenesis is the association of minerals in rocks considered in relation to their origin. The paragenetic sequence is the sequential
order of mineral deposition, usually in an ore deposit, but the term can be applied to any mineral assemblage, including assemblages
in igneous, sedimentary and metamorphic rocks, as well as in ore deposits.