Alterations

Magmatic rocks undergo secondary transformations postdating their solidification. As a consequence, they change chemical composition and new minerals may also originate. The changes are due to variations in temperature and pressure, influx of fluids and chemical components (metasomatism), and weathering.

The following fundamental transformations of magmatic rocks can be distinguished:

1. 1. Albitization is partial or total replacement of plagioclase and alkalic feldspar to albite caused by addition of sodium. This late-stage transformation of some acid rocks is often accompanied by silicification.
2. 2. Alunitization is decomposition of feldspars to opal, quartz and alunite in volcanic rocks, under influence of fluids rich in SiO₄^2-. The transformation is typical of acid and intermediate volcanic rocks (rhyolite, dacite, andesite). Gypsum is sometimes generated in Ca-rich rocks.
3. 3. Bastitization is transformation of mafic Fe, Mg-containing silicates (pyroxenes, amphiboles, phlogopite, but not olivine) to serpentine. Pyroxenes are the most sensitive minerals to this type of transformation. Bastite originates by the transformation of orthopyroxene; it represents a pseudomorph of serpentine-group minerals after enstatite.
4. 4. Baueritization is leaching-out of iron and magnesium from mafic micas, which lose their colour and pleochroism, and become similar to a white mica - muscovite.
5. 5. Epidotization (zoisitization) is transformation during which secondary epidote, zoisite and/or clinozoisite originate at the expense of plagioclase and amphibole. Epidotization of plagioclases is accompanied by crystallization of secondary albite, and Ca-rich anortite molecule is stored in epidote-zoisite. This type of transformation is often connected with chloritization.
6. 6. Fenitization is alkalic metasomatism caused by an influx of sodium. This type of transformation is genetically associated with intrusions of alkalic rocks and carbonatites. Specific mineral association (albite, alkalic pyroxene – aegirine, alkalic amphibole – arfvedsonite, rarely nepheline) creates fenite, which is named after the type locality of Fen in Norway.
7. 7. Chloritization is transformation accompanied by crystallization of secondary chlorite-group minerals at the expense of other, mainly mafic minerals (biotite, amphibole, pyroxene) or volcanic glass.
8. 8. Kaolinitization is transformation of aluminosilicates (mainly feldspars) to kaolin in acid (rhyolite, granite) or intermediate rocks (andesite).
9. 9. Kelyphitization is pseudomorphic recrystallization of garnets giving rise to a mixture of pyroxenes, spinel-group minerals, and sometimes amphiboles at decreasing pressure (decompression). As a consequence, ring-shaped kelyphitic rims are formed around the restitic garnet. Intensity of the kelyphitization depends on the duration of appropriate P-T conditions during decompression. This type of transformation is typical of pyrope from garnet peridotites of upper mantle origin transposed into shallower crustal levels.
10. 10. Pinitization is retrograde transformation of cordierite, nepheline, topaz, andalusite and feldspars to sheet silicates (biotite, muscovite, chlorite, pyrophylite), which often create psedomorphs after decomposed minerals, or rims around them.
11. 11. Propylitization is hydrothermal transformation of mafic minerals of volcanic and volcanoclastic rocks to sericite, zeolite, calcite, epidote, chlorite and kaolin. The pervasive rock transformation is often accompanied by silicification and impregnation by pyrite.
12. 12. Saussuritization is hydrothermal transformation of intermediate to basic plagioclases to a fine-grained mixture of zoisite, epidote, scapolite, sericite, calcite, quartz and albite. This type of transformation occurs due to a weak metamorphism of mafic magmatic rocks.
13. 13. Sericitization is a hydrothermal-metasomatic process, during which finely scaled white mica – sericite originates from feldspars and plagioclases. The internal metasomatism is caused by influx of potassium triggered by chloritization of biotite.
14. 14. Serpentinization is transformation of mainly olivine, but also other Fe-Mg silicates (pyroxene, amphibole) to serpentine-group minerals and antigorite, lizardite, chryzotile, brucite, magnetite and talc. This type of transformation occurs mainly in ultramafic and mafic rocks.
15. 15. Spilitization is complex metasomatic transformation of seafloor basalts, involving albitization of basic plagioclases accompanied by precipitation of albite and decomposition of mafic minerals to epidote and chlorite. Product of this transformation is called spilite or metabasalt.
16. 16. Steatization is transformation of mafic minerals to talc
17. 17. Uralitization is transformation of pyroxenes to fibrous tremolite. This type of transformation occurs in gabbros and some syenites.
18. 18. Zeolitization is a low-temperature hydrothermal transformation of feldspar-bearing rocks to zeolites. It is also one of rock transformations during zeolite-facies metamorphism. Mafic effusive rocks very frequently undergo this type of transformation. Low-temperature zeolitization of volcanic glass gives rise to natrolite, chabasite, heulandite, phillipsite and stilbite.