Alteration (geology)

Rocks transformed by hydrothermal alteration (chloritization) and discolored light turquoise green at the "Falla de Los Azulejos" in the El Teide National Park , Tenerife , Canary Islands .

As alteration (from latin alterare "change, change" for) is referred to in the geology , Lagerstättenkunde and mineralogy the mineralogical and chemical changes in an ore body and / or its surrounding rocks , wherein the minerals in the rock in secondary minerals are converted. The resulting transformation products can be formed in various ways, for example by changing the crystal structure of the minerals while the chemical composition remains the same . Alteration can also occur through a change in chemical composition, for example when hydrothermal solutions or volcanic gases react with rocks and new substances are added to or removed from the minerals they contain. The transformations can be syngenetic or epigenetic , ascending or descending. The underlying principle is the adaptation of the mineral to changed chemical environmental conditions to a stable state. Often the conversion products form pseudomorphoses according to the starting minerals. In contrast, changes caused by meteoric waters are usually referred to as weathering in German usage .

Classification according to temperature range

Alterations are caused by circulating (infiltrating) or diffusing solutions of hydrothermal , deuteric or pneumatolytic origin or by circulating, heated sea ​​water in areas of submarine volcanism .

Depending on the prevailing reaction temperatures, a distinction is made between

• the deuteric alteration (also autometasomatosis or pneumatolytic alteration ) at temperatures above 400 ° C
• of hydrothermal alteration at temperatures below 400 ° C

The phenomenon cannot be sharply delimited from a low-grade metamorphosis or a metasomatosis .

Deuteric or autometasomatic transformation processes involved reactions between primary igneous minerals and the water-rich solutions that separate from the same magma body at a later point in their cooling history. These processes include silicification , albitization , potassium feldspatization , tourmalinization and aging . They represent either all-pervasive, only selectively pervasive, cavity-filling and / or duct-shaped types of alteration.

Hydrothermal alteration is the chemical and mineralogical change in rock under the influence of hydrothermal solutions. In doing so, equilibrium is established between the hydrothermal solution and the rock and an alteration fringe or courtyard is formed. Hydrothermal alterations are an important indicator of the possible presence of hydrothermal mineralization. Alterations in the wake of hydrothermal ore deposits are known as wall-rock alteration. Examples of hydrothermal alteration are Sericitisierung , chloritization or Propylitisierung . Ore bodies are often surrounded by alteration zones in the host rock (e.g. porphyry copper deposits ), which play a major role in exploration geology.

Classification according to reaction products

Numerous alteration reactions can be named based on their distinctive reaction products, whereby many of these transformation reactions have their own name. These different alteration reactions are described below.

Most of the tabular compilation of the alteration reactions comes from the book by Hans Jürgen Rösler "Textbook of Mineralogy".

Starting mineral	Type of transformation (alteration)	minerals formed during alteration	Name of the alteration process
Olivine	hydrothermal or epizonal-metamorphic	serpentine	Serpentinization
Olivine	hydrothermal or epizonal-metamorphic	Hornblende, talk	Pilitization
garnet	hydrothermal	Chlorite	Chloritization
garnet	Aureole formation of pyrope in olivine (in pyroxene-amphibole rocks with the supply of thermal energy)	Hornblende + pyroxene + spinel	Kelyphitization
Cordierite	hydrothermal or epizonal-metamorphic	Sericite + Chlorite	Pinitization
Clinopyroxene	epizonal-metamorphic	Amphibole	Uralitization
Clinopyroxene	hydrothermal	Chlorite	Chloritization
Clinopyroxene	hydrothermal	serpentine	Serpentinization
Clinopyroxene	hydrothermal	Antigorite	Bastitization
Clinopyroxene	epizonal-metamorphic	Epidote	Epidotization
Orthopyroxene	hydrothermal	Serpentine (tangled)	Serpentinization
Orthopyroxene	hydrothermal	few large antigorite crystals	Bastitization
Common hornblende	hydrothermal	Chlorite	Chloritization
Common hornblende	hydrothermal	Serpentine (tangled)	Serpentinization
Common hornblende	Decay after effusion	Magnetite + hypersthene	Opacification
Biotite	hydrothermal	green biotite, chlorite, kaolinite, illite, vermiculite	Building treatment, bleaching
Biotite	High temperature disintegration after effusion	Magnetite + spinel + hematite + augite	Opacification
Muscovite	hydrothermal	Illite	Illitization
Muscovite	hydrothermal	Montmorillonite	Montmorillonization
Muscovite	hydrothermal	Beidellite, kaolinite, halloysite	Kaolinitization
Nepheline	hydrothermal	Analcime + zeolites	"Sunburn"
Nepheline	epizonal-metamorphic	finely flaky mica
Leucite	hydrothermal	Kaolinite	Kaolinitization
Leucite	hydrothermal	Zeolites	Zeolitization
Feldspar, aluminosilicates	pneumatolytic	Topaz, tourmali, mica, kaolinite	Senile education
Alkali feldspar	hydrothermal	Kaolinite + quartz	Kaolinitization
Alkali feldspar	hydrothermal or epizonal-metamorphic	Serious	Sericitation
Plagioclase	hydrothermal	Alunit	Alunitization
Plagioclase	hydrothermal	Clay minerals (nontronite, montmorillonite, illite) carbonates	Carbonation
Plagioclase	epizonal-metamorphic	Epidote + zoisite + sericite	Saussuritization
topaz	hydrothermal	Gilbertite (kaolinite + nakrite / muscovite)

Fenitization

The Fenitisierung or the process of Fenitbildung is induced by magmatic fluids formation of nepheline , Aegirin , Na-amphibole and Kalifeldspäten the contact and the halos of carbonatite - and alkali rock intrusions. Fenite is a rock rich in feldspar that has formed through alkali metasomatosis at the contact of carbonatite or alkali rock intrusions from granites , gneisses , migmatites and other rocks of the surrounding basement . Its typical mineral components are alkali feldspars , aegirine and sodium amphiboles, more rarely quartz , albite-rich plagioclase , nepheline or biotite .

Albitization

The Albitisierung is the deuteric or autometasomatische conversion of calcium-rich plagioclase Kalifeldspäten or in the pure sodium -Plagioklas albite by hydrothermal sodium metasomatism according to the following equation:

${\displaystyle \mathrm {KAlSi_{3}O_{8}+Na^{+}\longrightarrow NaAlSi_{3}O_{8}+K^{+}} }$

Potassium feldspar reacts with sodium ions to form albite and potassium ions

Sodium is either added to the rock or relatively enriched by removing other cations from the rock. In the vicinity of hydrothermal veins, but also independently of them, various minerals of the parent rock are displaced, e.g. B. potassium feldspar, calcite , zoisite u. a. Albitization in regional metamorphic parent rocks can lead to almost monomineral vein-like or vein-like albitites. With regard to the mineral stock, similar “albitite veins” are also formed in granites by SiO ₂ removal and sodium metasomatosis. Typical mineral associations are albite, paragonite (high-sodium sericite ), chlorite and quartz, which are generally accompanied by orthoclase , anchorite or other carbonate minerals.

Potash feldspatization

Kalifeldspatisierung (also Kali (metasomatism um) or K-metasomatism) is a deuteric or autometasomatische alteration in kalkalkalischen rocks, triggered by the supply of potassium and the removal of calcium and sodium. Characteristic minerals of the transformed rock are potassium feldspars ( adulara , orthoclase, microcline ), biotite or chlorite, sericite and quartz, which can be accompanied by albite, anhydrite , Fe-Mg carbonates and / or apatite.

Senile education

The deuteric or autometasomatische Greis formation (also Vergreisenung or aging) takes place in the intermediate to acidic rocks and is the most pneumatolytic or high-thermal conversion of feldspars, micas and other aluminosilicates by solutions, typically to fluorine , boron and alkali metals (sodium, potassium Lithium ) are enriched. The resulting rocks are called greisen and represent a mixture of mainly topaz , minerals of the tourmaline group , minerals of the mica group (muscovite, zinnwaldite, lepidolite), quartz, fluorite , alkali feldspars and / or kaolinite .

${\displaystyle \mathrm {CaAl_{2}Si_{2}O_{8}+4\ F^{-}+4\ H^{+}\longrightarrow Al_{2}[F_{2}|SiO_{4}]+SiO_{2}+CaF_{2}+H_{2}O} }$

Anorthite reacts with fluoride ions and hydrogen ions to form topaz, quartz, fluorite and water

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  Wolframite-bearing old manor houses in Cligga Granite, Cornwall

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  Old town with cassiterite in granite from Geyer , Ore Mountains

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  fine-grained Greisen, "Huber" stock near Horní Slavkov , Czech Republic

Silicification

Silification (also silicification , silicification or quartzization) is the secondary filling of pore spaces or the supply of or metasomatic displacement of existing minerals by SiO ₂ , which either comes from hydrothermal solutions or is the product of mineral alteration in the host rock. The SiO ₂ can come from the hydrothermal solution or be a product of mineral aging in the adjacent rock. The result is fine-grained quartz, chalcedony or opal , which either fill the pores between the minerals of the educt or partially or completely displace them.

Tourmalinization

The deuteric or autometasomatic tourmalinization is a pneumatolytic formation at the end of the solidification process of acidic melts or a contact pneumatolytic effect on horn rock and slate . It is absent in volcanic rocks. The contact pneumatolytic change in the adjacent rock takes place on both sides of the contact through the action of boron-containing, highly volatile components with the consumption of biotite, with tourmaline being formed.

Schematic cross-section through a porphyry copper deposit with the sequence of hydrothermal alteration zones around the intrusion

Other important forms of host rock alteration are Argillitisierung , Sericitisierung and Propylitisierung . These forms of alteration occur especially in porphyry copper deposits (Porphyry Copper Ores) in andesitic and dioritic rocks, but they are also present in the wake of hydrothermal vein mineralization. A zonal arrangement of propylitization , argillitization and sericitation (from outside to inside) can usually be observed around porphyry copper deposits , with potassium alternation (potassium metasomatosis) with secondary potassium feldspar and / or biotite in the central area (see the adjacent drawing). The composition of the alteration products depends on the one hand on the composition of the hydrothermal solution and on the other hand on the chemistry of the parent rock. Acid rocks tend to show sericitation , argilitization , silicification and pyritization , while in basic rocks chloritization , carbonation and propylitization occur; Serpentinization is found in ultra-basic rocks.

Propylitization

The Propylitisierung is a low temperature-hydrothermal Alterationsprozess to many ore body in particular in porphyry copper deposits, wherein the host rock by newly formed minerals such as chlorite, epidote , Zoisit, Klinozoisit is displaced Carbonatminerale (Mg-Fe-Ca-carbonate) and alkali feldspar (albite) partially . Sericite, pyrite , magnetite , montmorillonite and zeolites, occasionally also hematite , jarosite and goethite, can form subordinately . If one of the alteration products occurs more intensely, it is also referred to as chloritization , albitization or carbonation .

Argillization

During argillization, dickite , kaolinite, pyrophyllite and quartz are formed. Furthermore, alunite occur, topaz and tourmaline.

Sericitation

During sericitation, a very frequent alteration in the wake of hydrothermal mineralization and at low pH values , feldspars and mica are converted into sericite, a fine-grained, dense, steatite-like muscovite. Depending on the precursor mineral - in particular the feldspar - sericitation takes place with consumption (equation 1) or release (equation 2) of potassium and the formation of quartz (equations 1-3) and albite (equation 3). The destruction of plagioclase can lead to the formation of paragonite. Primary Fe contents of the starting minerals are often fixed as pyrite in the alteration product.

${\displaystyle \mathrm {3\ NaAlSi_{3}O_{8}+K_{(aq)}^{+}+2\ H_{(aq)}^{+}\longrightarrow KAl_{3}Si_{3}O_{10}(OH)_{2}+3\ Na_{(aq)}^{+}+6\ SiO_{2}} }$

Albite reacts with potassium ions and hydrogen ions to form muscovite, sodium ions and quartz (equation 1)

${\displaystyle \mathrm {3\ KAlSi_{3}O_{8}+2\ H_{(aq)}^{+}\longrightarrow KAl_{3}Si_{3}O_{10}(OH)_{2}+2\ K_{(aq)}^{+}+6\ SiO_{2}} }$

Potassium feldspar reacts with hydrogen ions to form muscovite, potassium ions and quartz (equation 2)

${\displaystyle \mathrm {4\ (Na,K)AlSi_{3}O_{8}+2\ H_{(aq)}^{+}\longrightarrow NaAlSi_{3}O_{8}+KAl_{3}Si_{3}O_{10}(OH)_{2}+6\ SiO_{2}+2\ Na_{(aq)}^{+}} }$

Alkali feldspar reacts with hydrogen ions to form albite, muscovite, quartz and sodium ions (equation 3)

Chloritization

Chloritized intra-caldera tuffs. "Fuente de los Azulejos", Gran Canaria , Canary Islands, Spain.

Under chloritization is a series of very different metasomatic processes. What they all have in common is the conversion of existing minerals into minerals of the chlorite group or chlorite sensu lato (hereinafter referred to as "chlorite"). In the chloritization to chlorite is hydrothermally or diaphthoretisch (in retrograde metamorphosis) from garnet , biotite, green hornblende and / or cordierite . Sericite, quartz and pyrite can arise as a subordinate. The chloritization of magmatites takes place postmagmatically through acidic solutions at low temperatures> 500 K. The following reaction equation expresses the conversion of pyroxene ( enstatite ) and plagioclases to chlorite, albite and calcite.

${\displaystyle \mathrm {5\ Mg_{2}[Si_{2}O_{6}]+4\ Ca[Al_{2}Si_{2}O_{8}]+4\ Na^{+}+4\ HCO_{3}^{-}+6\ H_{2}O\longrightarrow 2\ Mg_{5}Al[AlSi_{3}O_{10}(OH)_{8}]+4\ Na[AlSi_{3}O_{8}]+4\ Ca[CO_{3}]} }$

Pyroxene (enstatite) reacts with plagioclase (anorthite), sodium ions, hydrogen carbonate ions and water to form chlorite, albite and calcite

Carbonation

Carbonates are formed during carbonation , which corresponds to a CO ₂ gain. In order to remain in an existing rock, the carbon must be bound to Ca, Mg and / or Fe, which were either brought into the system by a fluid or originate from the constituents of the fresh rock. In the latter case, the maximum carbonate content depends on the composition of the precursor. When the carbon combines with the calcium from the plagioclase, paragonite and quartz are also formed as by-products, while the destabilization of other minerals - alkali feldspar and clinopyroxene - creates muscovite and chlorite. It is therefore not easy to quantify the intensity of the carbonation and to distinguish the phyllosilicate by-products from those associated with sericitization and chlorination processes. The characterization of the carbonate phases is also important, since Ca and Fe-Mg carbonates are observed in both weakly and strongly modified rocks.

${\displaystyle \mathrm {3\ (Mg,Fe)_{5}Al_{2}Si_{3}O_{10}(OH)_{8}+15\ CaCO_{3}+K^{+}+15\ CO_{2}\longrightarrow KAl_{3}Si_{3}O_{10}(OH)_{2}+15\ Ca(Mg,Fe)(CO_{3})_{2}+3\ SiO_{2}+11\ H_{2}O+5\ O_{2}} }$

Chlorite, calcite and fluid react to form white mica minerals, anchorite dolomite, quartz and fluid

Serpentinization

The serpentinization is the hydrothermally running conversion of forsterite or magnesium rich members of the olivine - mixed crystal series in dunites , peridotite , Harzburgiten and / or other ultramafic rocks in minerals of the serpentine group according to the reaction

${\displaystyle \mathrm {5\ (Mg,Fe)_{2}[SiO_{4}]+8\ H^{+}\longrightarrow 2\ Mg_{3}[(OH)_{4}|Si_{2}O_{5}]+4\ (Mg,Fe)^{\,2+}+SiO_{2}} }$

Olivine reacts with hydrogen ions to form serpentine , magnesium and iron ions and quartz

Fe ²⁺ , Mg ²⁺ and Ca ²⁺ ions have to be released, and silica (quartz) is also released at the same time . First, fibrous chrysotile and lizardite are formed , both of which are detached from flaky antigorite by metamorphic reactions at higher temperatures (> 250 ºC) . In addition to olivines, other magnesium-rich silicates such as clino- and orthopyroxenes as well as amphiboles can also be serpentinized:

${\displaystyle \mathrm {2\ Mg_{2}[SiO_{4}]+Mg_{2}[Si_{2}O_{6}]+4\ H_{2}O\longrightarrow 2\ Mg_{3}[(OH)_{4}|Si_{2}O_{5}]} }$

Forsterite reacts with enstatite and water to form serpentine

Together with serpentine and / or chrysotile, magnetite, carbonates such as magnesite , anchorite and siderite , brucite , talc and quartz carbonate rocks ( listwänite ) are often formed . Since the starting minerals have a much denser lattice than serpentine, a significant amount of silica has to be removed in order to keep the volume constant. The serpentinization therefore only takes place when pressure is relieved along fractures or in passages. The olivine-containing rocks are often completely converted into serpentine, whereby the olivine is first converted from the edge and cracks into the finest serpentine fibers, always perpendicular to the edge or crack. With further transformation towards the core, the “mesh structure” is ultimately formed. The serpentinization of orthopyroxenes usually results in tangled serpentine.

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  Serpentinized peridotite, Atlantis massif . Size: 20 cm.

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  Yellow olivine in serpentinized peridotite. Field of view: 2 mm.

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  Chrysotile asbestos aisle in serpentine komatiite . Ontario , Canada .

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  Magnetite-rich serpentinite. Montana , USA . Size: 5.9 cm.

Iddingsitization

The iddingsitization of olivines takes place in the highly hydrothermal range at temperatures <400 ° C, with the oxidation of Fe ²⁺ and the addition of water causing an intense yellow-brown to brown-red discoloration of the olivines in their marginal areas and along the cracks. This creates submicroscopic mixtures of goethite , clay minerals , chlorite, etc. Such pseudomorphoses, known as iddingsites , are only found in volcanic and sub- volcanic rocks , but never in plutonites .

Pilitization

In the hydrothermal or metamorphic-epizonally running pilitization , olivine forms a mixture of hornblende and talc.

Kelyphitization

Kelyphitized Garnet Crystals. Västerby, Askersund , Örebro County ( Närke ), Sweden . Step size: 15cm × 10.6cm × 9.3cm.

When thermal energy is supplied, amphiboles, pyroxenes and spinels are formed from garnet. Ultimately, aureoles (corona-like formations) develop around pyrope and other representatives of the garnet group in mostly mafites and ultramafites such as B. olivine (pyroxene amphibole) rocks. These mixtures of newly formed and intergrown minerals are called Kelyphites . Kelyphitization typically occurs in serpentinites , gneisses and rocks of the green schist facies . Reaction zones between olivine and calcium-rich plagioclases in gabbros are called kelyphite zones. They also consist of mixtures of radially fibrous green hornblende, pyroxene, garnet and spinels.

Pinitization

Pinitized cordierite crystals. Västerby, Askersund , Örebro County ( Närke ), Sweden . Step size: 15cm × 10.6cm × 9.3cm.

The hydrothermal or metamorphic-epizonal transformation of cordierite into a finely flaked to dense mixture of sericite and / or chlorite and biotite is called pinitization . This always takes place with the absorption of water. The name comes from the type locality of the pinite , a pseudomorphosis of muscovite (sericite) after cordierite from the "Pini-Stolln" near Aue in the Ore Mountains .

Gilbertitization

The pneumatolytic to hydrothermal decomposition of topaz produces pseudomorphoses from muscovite to topaz, which are known as gilbertite .

Uralitization

Clinopyroxenes uralitized in actinolite amphiboles. "Calumet Mine", Colorado , USA . Field of view: 5.6 cm × 5.0 cm × 3.3 cm.

In the contact area of ​​intruding plutons and during regional metamorphosis , under pneumatolytic to hot-hydrothermal conditions, a secondary conversion of magmatically formed clinopyroxenes into actinolite amphiboles, starting from edges or cracks, often takes place, with the conversion mostly polycrystalline, fibrous aggregates with a coaxial orientation to the displaced clinopyroxene arise. This process is known as Uralitization . Uralites form both late magmatic-hydrothermal and during epizonal metamorphosis. The conversion from Augit to Hornblende describes the following reaction equation:

${\displaystyle \mathrm {8\ CaMgAl_{0,125}[Al_{0,125}Si_{1,875}O_{6}]+3\ Mg^{\,2+}+2\ Na^{+}+2\ H_{2}O\longrightarrow 2\ NaCa_{2}Mg_{5}[AlSi_{7}O_{22}(OH)_{2}]+4\ Ca^{\,2+}+SiO_{2}} }$

Augit reacts with magnesium and sodium ions to form hornblende, calcium ions and quartz

Bastitization

"Bastitization" of pyroxenes to minerals of the serpentine group. Kohlebornkehre, Harz.

During bastitization , orthopyroxenes and z. T. also clinopyroxene, mostly starting from cracks, hydrothermally into a few large antigorite crystals or other minerals of the serpentine group. Due to its golden yellow or bronze colors on the cleavage surfaces, it can be easily recognized in decomposed rocks containing enstatite. The term "Bastitisierung" was derived from the type locality, converted bronzite rocks from the valley of the Baste near Bad Harzburg in the Harz Mountains . Willy Bruhns describes the bastitization of pyroxenes in detail as a result of his microscopic examinations of the “hornblende porphyrites” by Potschappel and the “mica porphyrites” by Wilsdruff: “The pyroxene transforms into a fibrous substance of e.g. Th. Yellow, z. Th. Greyish-greener, z. Th. Grass green color. The grain is visible in longitudinal and cross-sections ... In some occurrences the pyroxene is completely converted into bastite and this mineral gains such a development here that it emerges macroscopically in the form of very shiny, lively shimmering leaves. "

Saussuritization

The Saussuritisierung is a process, in which converted by hydrothermal or epizonal-metamorphic processes plagioclase in "Saussurit". “Saussurite” is a dense, white to light green mixture of zoisite, scapolite , epidote, sericite and the like. a. Understand minerals. "Saussurite" is created by thermal or hydrothermal effects in deep rocks such as gabbros and the like. a. It is typical that within the plagioclase only the anorthite component of the nuclei is converted, while the albite component of the outer zones remains stable. The result is what are known as “filled feldspars”.

Epidotization

Epidotization is the conversion of clinopyroxenes such as z. B. Augit or plagioclase in epidote. The plagioclase is albitized, the released anorthite component is available for the formation of epidote. The process is often accompanied by chloritization . The metasomatic, autopneumatolytic to autohydrothermal formation of epidotes from pyroxenes, amphiboles and the anorthite component of plagioclases on crevices and in cavities of magmatites is also called epidotization .

Zoisitization

Similar to the Epidotisierung takes place at the running under Metamorph epizonal conditions Zoisitisierung a Albitisierung instead of plagioclase, the released anorthite component is in the course of the formation of epidote available. This process is also often accompanied by chloritization .

Opacification

In the case of hornblende crystals and biotite in volcanic rocks, after the effusion as a result of pressure relief, the lattice disintegrates with the formation of a new dense mixture of magnetite, hematite, low-iron clinopyroxenes such as hypersthene and augite as well as spinels. This opacitization usually starts from the edge of the crystals (" weeping edge"), but also covers the entire crystal, of which only the outlines are preserved ("hornblende or biotite corpses").

Civilization

Baueritization is the hydrothermal conversion of biotite into hydrobiotite, montmorillonite, vermiculite , e.g. Sometimes also in green biotite, chlorite, kaolinite and illite . This transformation is often associated with bleaching of the rock. Because the biotites change their color to light brown-yellow and take on a metallic sheen, they are called "fool's gold".

Illitization

Under Illitisierung means the hydrothermal-metasomatic transformation of the country rock u. a. of fluorspar deposits, whereby hydromuscovite (illite) and sericite are formed by the conversion of muscovite and potassium feldspars.

Montmorillonization

During montmorillonitization , muscovite is transformed into montmorillonite as a result of hydrothermal processes.

Kaolinization

Under kaolinisation or Kaolinitisierung the hydrothermal or auto hydrothermal formation of kaolinite from feldspars and micas alkalifeldspatführender acidic igneous rocks such as granites, is arkose u. a understood. A high H ⁺ / K ⁺ ratio in the hydrothermal solutions is required for the kaolinization of the starting minerals. Potassium must be removed, otherwise illite will form instead of kaolinite. Besides kaolinite, beidellite and halloysite can arise.

${\displaystyle \mathrm {2\ K[AlSi_{3}O_{8}]+11\ H_{(aq)}^{+}\longrightarrow Al_{2}[(OH)_{4}|Si_{2}O_{5}]+2\ K^{+}+4\ Si^{\,4+}+7\ OH^{-}} }$

Potassium feldspar reacts with hydrogen ions to form kaolinite, potassium ions, silicon ions and hydroxide ions

${\displaystyle \mathrm {4\ K[AlSi_{3}O_{8}]_{(s)}+6\ H_{2}O_{(aq)}\longrightarrow Al_{2}[(OH)_{4}|Si_{2}O_{5}]_{(s)}+8\ {SiO_{2}}_{(s)}+4\ KOH_{(aq)}} }$

Feldspar reacts with water to kaolinite, quartz and potassium hydroxide to

Spilitization

Spilit

In the Spilitisierung it also is a Albitisierung (see there) or a sodium metasomatism, wherein the in submarin formed basalts are converted plagioclase and pyroxene contained (z. B. enstatite) in albite and chlorite. The sodium can come directly from the sea ​​water . Spilite (from the Greek σπιλάς [ spilás ] for "rock") are therefore basic magmatites that arise metasomatically from oceanic basalt.

${\displaystyle \mathrm {6\ CaAl_{2}Si_{2}O_{8}+4\ Mg_{2}Si_{2}O_{6}+4\ Na^{+}+4\ HCO_{3}^{-}+2\ CO_{2}+6\ H_{2}O\longrightarrow 4\ NaAlSi_{3}O_{8}+2\ Mg_{4}Al_{2}[Al_{2}Si_{2}O_{10}(OH)_{8}]+6\ CaCO_{3}+4\ SiO_{2}} }$

Anorthite reacts with enstatite, sodium ions, hydrogen carbonate ions, carbon dioxide and water to form albite, chlorite, calcite and quartz

sunburn

In certain rocks (" sunburner basalts "), nepheline is formed into mixtures of analcime and zeolites through intense solar radiation and thus heating of the rock ("sunburn") . The process is associated with a volume increase of approx. 5% by volume.

Zeolitization

Near-surface hydrothermal conversion processes in basaltic and other lavas with the addition of calcium or sodium often lead to the formation of zeolites in cavities. The temperatures vary between 100 ° C and 350 ° C. This so-called zeolitization is the introduction of, conversion to or replacement by a mineral or mineral associations that contain zeolites as characteristic, if not necessarily abundant components. Zeolitization results from the action of almost neutral hydrothermal solutions with relatively low temperatures, which cause a recombination of sodium, calcium and / or potassium in the adjacent rock. Zeolites occur most often as alteration products of volcanic glass and calcium-rich plagioclases and are accompanied by other alteration products such as adulara, prehnite , pumpellyite and minerals of the propylitic facies, especially epidote, albite and carbonates. The most common zeolites formed during zeolitization are clinoptilolite , mordenite , analcime , heulandite , laumontite and wairakite .

Alunitization

In the Alunitisierung is a hydrothermal conversion of alkali feldspars (plagioclase) or Foiden by an extreme hydrolytic leaching of the surrounding rocks (acidic to intermediate volcanic rocks such as rhyolite to andesite ) under oxidizing conditions, primarily in the range of solfataras under the action of H ₂ SO ₄ , which lead to the formation of alunite and clay minerals (mainly montmorillonite and nontronite ). In addition to alunite, the most common newly formed minerals in alunitization are quartz, chalcedony, cristobalite , tridymite , opal and kaolinite, sericite, diaspore , barite , jarosite , rutile , Zunyite , pyrite and hematite.

Appearances in the terrain

The alteration fringes or courtyards resulting from the alterations have strongly varying thicknesses. Along dikes, an alteration fringe is often only a few centimeters thick, while the alteration of some deposit types (e.g. in porphyry copper deposits) can encompass the entire intrusive complex. Depending on the degree of transformation, the alteration of a rock can be completely inconspicuous to the naked eye (if, for example, only individual secondary parts are affected, such as the serpentinization of individual olivines in basalt), or it can cause discoloration of individual components or the entire rock ( greening of plagioclase crystals ; Red coloration of rhyolites) or completely change the character of the rock (e.g. when ultramafic rocks are completely transformed into serpentinite ).

literature

• John Leslie Jambor : Wall rock alteration . In: The Canadian Mineralogist . tape 11 , no. 1 , 1971, p. 272–304 (English, rruff.info [PDF; 4.5 MB ; accessed on March 22, 2020]).
• Charles Meyer, Julian Hemley: Hydrothermal alteration in some granodiorites . In: Clays and Clay Minerals . tape 6 , no. 1 , 1957, pp. 89–100 (English, clays.org [PDF; 4.5 MB ; accessed on March 22, 2020]).
• Anne JB Thompson, John FH Thompson: Atlas of Alterations: a field and petrographic guide to hydrothermal alteration minerals . 1st edition. Geological Association of Canada Mineral Deposits Division, St. John's 1996, ISBN 0-919216-59-5 , pp. 1–119 (English, geokniga.org [PDF; 49.0 MB ; accessed on March 22, 2020]).
• Hans Pichler, Cornelia Schmitt-Riegraf: Rock -forming minerals in thin sections . 1st edition. Enke, Stuttgart 1987, ISBN 3-432-95521-9 , pp. 1-230 .
• Friedrich Klockmann : Klockmann's textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke , Stuttgart 1978, ISBN 3-432-82986-8 , pp. 577 (first edition: 1891).
• Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p. 716 .
• Pat WC Shanks III: Hydrothermal Alteration . In: Pat WC Shanks III, Roland Thurston (Ed.): Volcanogenic massive sulfide occurrence model . US Geological Survey Scientific Investigations Report 2010-5070-C. 1st edition. tape 1 , no. 11 . US Geological Survey, Denver 2012, p. 169–180 (English, pubs.usgs.gov [PDF; 895 kB ; accessed on March 22, 2020]).

Web links

• Mineral Occurence. empr.gov.bc.ca, accessed March 19, 2013 . 
• pseudomorphic. britannica.com, accessed March 19, 2013 . 
• Lexicon of Geosciences. Spektrum.de, accessed on March 22, 2020 . 

Individual evidence

1. ↑ Christiane Martin, Manfred Eiblmaier (ed.): Encyclopedia of Earth Sciences . 1 (A-Edi). Spektrum Akademischer Verlag, Heidelberg 2000, ISBN 3-8274-0299-9 , p. 172 .
2. ↑ ^{a b c d e f g h i j k l m} Christiane Martin, Manfred Eiblmaier (Ed.): Lexicon of Geosciences . 2 (Edu-Insti). Spektrum Akademischer Verlag, Heidelberg 2000, ISBN 3-8274-0421-5 , p. 465-466 .
3. ↑ ^{a b} Roland Vinx: Rock determination in the field . 3. Edition. Spektrum Akademischer Verlag, Heidelberg 2011, ISBN 978-3-8274-2748-9 , pp. 157, 160 .
4. ↑ ^{a b c d e f g h i} Mineral Occurence. empr.gov.bc.ca, accessed March 19, 2013 .  
5. ↑ ^{a b c d e f g h i j k l m} Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-432-82986-8 , p.  775-776 .
6. ↑ ^{a b c d e f g h i j} Lexicon of Geosciences. Spektrum.de, accessed on March 22, 2020 .  
7. ↑ Viorica Morogan: Mass transfer and REE mobility during fenitization at Alnö, Sweden . In: Contributions to Mineralogy and Petrology . tape 103 , 1989, pp. 25-34 , doi : 10.1007 / BF00371362 (English).
8. ^ Hans Murawski, Wilhelm Meyer: Geological dictionary . 12th edition. Springer Spectrum, Heidelberg 2017, ISBN 978-3-662-54050-3 , p. 1-221 .
9. ↑ ^{a b c d e f g h i} Hans Pichler, Cornelia Schmitt-Riegraf: Rock -forming minerals in thin sections . 1st edition. Enke, Stuttgart 1987, ISBN 3-432-95521-9 , pp. 1-230 .
10. ^ Richard H. Sillitoe: Porphyry Copper Systems . In: Economic Geology . tape 105 , no. 1 , 2010, p. 3–41 , doi : 10.2113 / gsecongeo.105.1.3 (English).
11. ^ ^{A b} Lucie Mathieu: Quantifying Hydrothermal Alteration: A Review of Methods . In: Geosciences . tape 8 , no. 7 , 2018, p. 245 , doi : 10.3390 / geosciences8070245 (English, mdpi.com [PDF; 1.9 MB ; accessed on March 22, 2020]).
12. ↑ ^{a b c d e f} Peter Möller: Inorganic Geochemistry (Heidelberger Taschenbücher Vol. 240) . 1st edition. Springer, Berlin 1986, ISBN 978-3-642-70845-9 , pp. 124–161 , doi : 10.1007 / 978-3-642-70845-9 ( available online in Inorganische Geochemie , p. 146 ff. In the Google book search).
13. ↑ Pilitization. In: Spektrum.de. Retrieved April 2, 2020 . 
14. ^ Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke , Stuttgart 1978, ISBN 3-432-82986-8 , pp.  747 (first edition: 1891).
15. ↑ ^{a b} Willy Bruhns: The Porphyry Train by Wilsdruff-Potschappel (inaugural dissertation to obtain the philosophical doctorate at the University of Leipzig) . 1st edition. JF Starcke, Berlin 1886, p. 1-28 .
16. ↑ Felix Machatschki : Special mineralogy on a geochemical basis . 1st edition. Springer, Vienna 1953, ISBN 978-3-7091-8006-8 , p.  54 , doi : 10.1007 / 978-3-7091-8006-8 ( available online in Special Mineralogy on a Geochemical Basis , p. 54 in the Google book search).
17. ^ Pentti Eskola : The Origin of Rocks: A Textbook of Petrogenesis . Ed .: Carl W. Correns. 1st edition. Julius Springer, Berlin 1939, ISBN 978-3-642-86244-1 , p.  382 , doi : 10.1007 / 978-3-642-86244-1 ( available online in The Origin of Rocks: A Textbook of Petrogenesis , p. 382 in the Google book search).