Split crystallization

See also: Crystallization

The split crystallization is the progressive crystallization of a magma due to changes of physicochemical conditions. These changes of physicochemical conditions are in general a decrease in pressure and of temperature due to the rise of the magma through the Earth's crust, or an increase in pressure due to the confrontation of several plates tectonics S.

Progressively of split crystallization, the mineral formed form a deposit at the bottom of the magmatic pocket. To schematize, the ferro-magnesian minerals crystallize and form a deposit the first, to leave a differentiated magma, the residual magma enriched out of silica. the various rocks resulting from these more or less differentiated magmas constitute a differentiated Série.

Is the fact that the crystallization of silicates in a magma is done in a well defined order, according to the reactional continuation of Bowen, product of the different mineralogical assemblies: ultramafic, mafic, intermediate and felsic. These four assemblies define four great types of igneous rocks.

Let us take as example the crystallization of a magma which cools in a magmatic room. The crystals will not be formed all at the same time as the series of Bowen expresses it. The first minerals to be crystallized will be obviously the minerals of high temperature, Olivine initially, Pyroxène S and Amphibole S then. These crystals will be formed in the magma and will form a deposit towards the base of the magmatic room to form a rock rich in olivine, pyroxene and amphibole, a mafic igneous rock, a Gabbro for example. The residual liquid will thus be impoverished in these minerals; there will be thus a magma of composition different from his initial composition. This magma will have a composition say intermediate.

If this magma is introduced into a secondary room and that it continues its cooling, the first minerals to be crystallized will be the amphiboles, biotites, the quartz and some Feldspath S plagioclases, which will produce an intermediate rock igneous, a diorite for example. If this magma makes its way to surface, there will be andesitic lava. Thus, starting from a magma of composition given, one can obtain more than one type of igneous rock.

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