Igneous Rocks In the Mantle and Crust

A mineral is a naturally occuring solid, with a particular crystal structure and either a particular chemical composition or a restricted range of chemical composition that can be described as a solid solution. (Thus the two dominant crystal forms of CaCO₃, calcite and aragonite, are different minerals.) Rocks are solids made up of a collection of minerals. Rocks can be of three types: igneous, sedimentary, and metamorphic. Igneous rocks are produced by cooling of molten lava, and so are closely associated with heat transfer processes.

Some important minerals found in igneous rocks are summarized in this table:

Common Minerals in Oceanic Lithosphere

olivine	(Fe,Mg)SiO	Mg-rich: forsterite
		Fe-rich: fayalite
pyroxene	(M)(SiO)	Mg: enstatite	(orthopyroxene)
		(Mg,Fe): hypersthene
		Ca(Mg,Fe): diopside	(clinopyroxene)
plagioclase	CaAlSiO	anorthite
	NaAlSiO	albite
garnet	(Fe,Mg)Al(SiO)	Fe-rich almandine; Mg-rich pyrope
quartz	SiO
spinel	MgAl2O4

Classification of Igneous Rocks

From (3).

Important aspects of this classification of igneous rocks are:

differentiation, as measured by silica content. The silica content reflects fractionation between liquid and solid composition as material melts. The general character of melts is the reverse of the observed order of crystallization (known as Bowen's series), from first to crystallize (high temperature) to last (low temperature):

Upon melting (from bottom to top of the table) a high silica content reflects high degree of fractionation, low temperature of melting, liquid melt least like solid, i.e., most differentiated, from which derived. A low silica content reflects lower degree of fractionation, higher temperature of melting, liquid melt most like solid from which derived, i.e. less differentiated.

rate of cooling, controlling crystallinity. Volcanic rocks extruded at surface are subject to rapid cooling and develop either glassy or very finely crystalline texture; plutonic rocks intruded into crust are insulated and cool slowly developing a coarsely crystalline texture.

The rock names assigned depend on chemistry/mineralogy (more or less differentiated) and texture (more or less rapidly cooled).