igneous rocks

Igneous rocks result when molten rock cools (± crystallization) from plutonic (intruded) magma or from volcanic (extruded) lava.

Left - click to enlarge image - texture and mineral composition of common igneous rocks:

Increasing to the left: SiO2 content, viscosity; Increasing to the right: darkness, mafic : felsic composition, (Fe, Mg, Ca) : (K, Na) ratio, temperature of melting:

◘ a/p - aphanitic or porphyritic texture, derived from extruded magma (lava),
◘ 1. rhyolite ◘ 3. dacite ◘ 5. andesite ◘ 7. basalt
phaneritic texture, emplaced as magma (plutonic)
◘ 2. granite ◘ 4. granodiorite ◘ 6. diorite ◘ 8. gabbro to peridotite
0-100: percentage mineral content:
◊ a. quartz ◊ b. K-feldspar ◊ c. Na-feldspar ◊ pl. plagioclase to Ca-rich plagioclase ◊ d. muscovite
◊ e. biotite ◊ f. amphiboles ◊ g. pyroxenes ◊ h. olivine

Igneous rocks, predominantly plutonic, comprise about 95% of the Earth's crust, though their burial by sedimentary rocks and association with metamorphic rocks disguises their true extent.
The crystalline basement rock (shield) at the core of most continents is ancient, having arisen predominantly during a period from 3.0 to 2.5 billion years ago, which was the period of maximum continent formation. Most of the earliest rocks have been greatly altered through regional metamorphic processes, but later rocks (3.2-2.5 Ga) are mostly pillow-basalts that formed beneath the vast oceans. Archean sedimentary rocks are mostly coarse and poorly sorted sandstones and conglomerates.

| 0 Guide-Glossary

crystallization

Crystallization is the process by which minerals form an ordered state (crystal). In chemistry, crystallization occurs from solution, and in geology from molten rock. Crystals have a regularly repeating internal structure in which constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions, and hence display external plane faces.

Recrystallization involves an internal reorganization through partial melting (metamorphism), interaction with percolating chemical solutions (metasomatism), or through a diagenetic process by which unstable minerals in buried sediment are transformed into more stable minerals. Thus, recrystallization can involve a reorganization of elements of the original minerals in a rock in response to changes in temperature, and/or pressure, and/or the activity of pore fluids. Recrystallization can result in an increase in the degree of crystallinity, or crystal perfection or both.

Diagenetic alterations take place in deposited sediments, during and after lithification, and occur at relatively low temperatures and pressures. There exists a gradational rather than distinct boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure. Diagenesis alters a rock's original mineralogy and texture, and can also contribute to fossilization of buried skeletal material by replacing the collagen of bones with minerals.

The crystal structure of most rock minerals depends upon the variety of arrangements available to tetravalent silicon, which can bond to various anions or be substituted by small cations, such as aluminum. Silicon can adopt triangular, tetrahedral, cubic, octahedral, and dodecahedral configurations upon bonding to other atoms. In the silicates, silica tetrahedra and octahedra can arrange/rearrange as:
● simple tetrahedra – nesosilicates [SiO₄]^4- – olivine
● double tetrahedra – sorosilicates [Si₂O₇]^6- – epidote
● rings – cyclosilicates [Si_nO_3n]^2n- – tourmaline
● chains – inosilicates [Si_nO_3n]^2n- – pyroxene
● double chains – inosilicates [Si_4nO_11n]^6n- – amphibole
● sheets – phyllosilicates [Si_2nO_5n]^2n- – micas and clays
● 3D frameworks (lattices) – tectosilicates [Al_xSi_yO_2(x+y)]^x− – quartz, feldspars, zeolites
● complex intermediates between the above configurations

[Jmol kaolinite, porphyrillite]