Molten Rock Material From Which Minerals Can Crystallize Is Called Magma.
Igneous Rocks
Of the three principal rock types (igneous, sedimentary, and metamorphic), igneous rocks tin be thought of as "chief" rocks considering they crystallize from a liquid. Sedimentary and metamorphic rocks, which we volition study later, may be thought of as derivative rocks.
Igneous rocks are rocks formed from the crystallization of a liquid (molten rock). Igneous rocks may be divided into two categories. Intrusive or plutonic rocks crystallize from magma below the earth's surface. Extrusive or volcanic rocks crystallize from lava at the earth'due south surface.
The texture of an igneous rock (fine-grained vs fibroid-grained) is dependent on the rate of cooling of the cook: wearisome cooling allows big crystals to class, fast cooling yields pocket-sized crystals. Magmas and their resultant plutonic rock bodies cool and crystallize slowly and are characterized by coarse-grained texture, in which the mineral crystals are visible to the unaided middle. On the other hand, lavas absurd speedily at the earth's surface and are characterized past fine-grained texture, in which the crystals are besides minor to exist seen past the unaided middle.
Very quickly cooled lavas, typically those quenched in h2o, will have a glassy texture. They cool too apace to course crystals. Spectacles do not have an orderly arrangement of atoms and in that location are therefore no minerals, in the strict sense, in them. Volcanic glass is called obsidian.
In add-on to texture, igneous rocks may are classified according to their chemical composition. The nigh general nomenclature is based on the relative abundance in a rock of felsic (feldspar and silica-quartz) minerals vs mafic (magnesium and ferrum or iron) minerals. Felsic minerals (quartz, 1000 feldspar, etc) are light colored while mafic minerals (hornblende, pyroxenes) are usually dark colored. Felsic minerals have the lowest melting points (600 to 750 °C) and mafic minerals have higher melting points (1000 to 1200 °C).
(5x magnified view)
Bowen'south Reaction Series tin can exist thought of as an idealized gild of crystallization of a cooling magma. However, not all of these minerals will be crystallized together in the same stone. A mafic magma will begin crystallizing olivine and continue with pyroxenes and calcium rich plagioclase feldspar. Some amphiboles may also crystallize earlier the melt is used up. Mafic melts don't accept plenty silica to crystallize potassium feldspar, quartz, etc. Felsic melts don't have enough iron, magnesium, and calcium to class olivine, pyroxene, or calcium plagioclase. The starting time-formed minerals in a felsic melt are amphiboles (hornblende) or biotite mica, forth with some intermediate or sodium plagioclase. Eventually, as the melt continues to absurd and becomes richer in silica (as the metal cations are used upward preferentially in the double chain and sail silicates) potassium feldspar and quartz crystallize.
Igneous rocks may be simply classified according to their chemic/mineral composition equally felsic, intermediate, mafic, and ultramafic, and by texture or grain size: intrusive rocks are class grained (all crystals are visible to the naked eye) while extrusive rocks may be fine-grained (microscopic crystals) or drinking glass (no crystalline structure; i.e., no minerals). Volcanic rocks, peculiarly felsic and intermediate, often have a porphyritic texture characterized past visible crystals floating in a fine-grained groundmass.
Shown below is a simple classification scheme. Alternative, circuitous classification schemes take into account finer gradations in limerick and especially the varying amounts of the elements potassium, aluminum, sodium, and calcium.
Note that felsic rocks are low-cal in colour; intermediate rocks range through grays, and mafic rocks are black in colour. Ultramafic rocks (peridotite) may range black to olive green (dunite) from the mineral olivine.
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Source: http://www.columbia.edu/~vjd1/igneous.htm
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