Mineral Alterations
Adinolization
Adinolization is a type of albitization, and the rock “adinole” is a type of albitite.
Adinolization is probably a sodic metasomatic process related to the intrusion of mafic igneous rocks into pelitic rocks such as shales, for example, converting them into an adinole, basically consisting of albite.
Adinole is a very fine-grained, compact metasomatic rock, forming splinters, usually banded in gray, green, or reddish. Formed by albite and some quartz, it contains um to 10% Na.
Albitization
Partial or total replacement of potassium feldspars or plagioclase by virtually pure albite. The process can occur due to final crystallization fluids of granitic bodies or by the reaction of ocean floor basalts with seawater.
Alteration of opaques
It consists of the development of bright red iron hydrates (blood-red, may have orange tones), which spread, forming spots, around the opaque grains (usually iron oxides or hydroxides) in thin sections. It can occur with magnetites in volcanic rocks, for example.
Alunitization
Formation of alunite (KAl3(SO4)2(OH)6) by the action of hydrothermal solutions containing sulfuric acid on volcanic rocks rich in potassium feldspars, such as trachytes and rhyolites. The process is accompanied by kaolinitization and silicification and generates masses, sometimes enormous, of alunite, an important ore (source of alum, K+Al, etc.).
Argilization
Also known as kaolinization, it is the formation of clay minerals (usually kaolinite and clay minerals of the Montmorillonite Group) in minerals, generally through weathering processes.
The altered minerals, originally clear and colorless, become cloudy and yellowish in color under ND (no-determined spectroscopy). The process is common in feldspars in general. It may be present only in the core, only on the edges, or disseminated throughout the mineral.
Looking at the thin section with the naked eye, one usually perceives colors in shades of pink, which are the intensely altered feldspars.
Bastitization
A process that involves the replacement of orthopyroxenes (e.g., enstatite) by minerals from the Serpentine Group.
The replacement begins along the fractures, with the parallel fibers of bastite arranged perpendicularly to the fracture walls.
If the replacement is complete, pseudomorphs of bastite on orthopyroxene are generated.
Bowlingite
Bowlingite is a mixture of smectite, quartz, chlorite, serpentine, and talc, with a greenish appearance under no-dose light, formed by the alteration of olivine.
This alteration begins with the fractures that olivine grains usually exhibit and then extends throughout the mineral.
Olivine completely altered to bowlingite is a pseudomorph.
Olivine can alter to bowlingite or iddingsite, depending on the fugacity of O2.
Calcitization
Also called carbonatization, it consists of the development of calcite in minerals.
Calcite can even completely replace the grain, forming pseudomorphs.
This can occur with plagioclase.
Celadonitization
Formation of celadonite replacing clinopyroxenes (augite) in magmatic rocks.
Pseudomorphs of celadonite form on augite.
Cymatolite
“Cymatolite” is a designation given to an alteration of spodumene, which is replaced by a mixture of muscovite and albite. It has a white color, a silky, pearly or dull luster, and a somewhat lamellar or parallel fibrous structure.
The material is brittle, shows irregular or splintery or fibrous or micaceous fracture, and breaks along curved planes rich in muscovite.
Cymatolite can form complete pseudomorphs on spodumene and occurs associated with quartz (rock crystal or smoky), albite (clevelandite variety), spodumene (including the kunzite variety), and eucryptite (another mineral resulting from the alteration of spodumene).
Records of cymatolite are very rare, with one occurrence in Australia, another in China, one in Zimbabwe, and a dozen in the USA. It is probably much more common, not having been identified as such in the lithium-bearing pegmatites where it occurs.
Damouritization
It consists of the development of damourite in the host mineral.
Damourite is an extremely fine-grained, compact muscovite with an unctuous feel, resembling serpentine. Under a microscope, it appears as extremely fine grains.
Devitrification
Typical alteration of volcanic glass (obsidian), where the glass is replaced by finely crystalline aggregates composed of the minerals that would have formed if the magma had cooled slowly. Many textures are possible; see the “Volcanic Glass” fact sheet.
Dolomitization
This refers to the various processes discussed in the literature, through which a limestone formed by sedimentary processes and consisting mainly of calcium carbonate (calcite) transforms into dolomite, formed by calcium and magnesium carbonate (dolomite).
To read more, search for “The dolomite question” and “Fairbridge 1957”.
Epidotization
A metamorphic, hydrothermal process, during which epidote forms in mainly granitic rocks due to the albitization of the plagioclase, that releases the anorthite molecule. Epidote and zoisite are formed, often accompanied by chloritization.
Fenitization
It is a metasomatic process that transforms the surrounding rocks into alkaline rocks (ijolites), carbonatite complexes, nepheline syenites, peralkaline granites, gneisses, and migmatites.
They can form monomineralic rocks of alkali feldspars.
Glimmeritization
Post-magmatic, metasomatic process that produces an igneous rock composed primarily of dark micas (biotite and phlogopite).
The name comes from “Glimmer,” which is the German term for micas in general.
Iddingsite
It is a deep red to brown color change that occurs in olivine. It consists of the formation of an aggregate of fibrous serpentine, chlorite, iron oxides (hematite, goethite), and others.
The change begins at the intergranular boundaries and along the fractures.
Kelyphitization
It consists of the formation of minerals around the grain in question, forming bands. These bands are called cheliphytic texture, cheliphytic crowns, or reaction borders.
Many minerals can exhibit them, such as garnet.
Leucoxene
Leucoxene is a submicroscopic association of iron and titanium oxides, with rutile, anatase, titanite, perovskite, titaniferous magnetite, and especially ilmenite.
The macroscopic color of leucoxene is white to pale yellow.
It is a characteristic alteration of ilmenite and titanite.
Limonitization
Alteration to oxides and hydroxides of iron in the form of dendritic aggregates of goethite (limonite) with “ice flower” shapes (radial aggregates).
It occurs in siderite and is an important diagnostic feature for this mineral.
It is also characteristic of pyrite, generating limonite pseudomorphs on pyrite.
Metamictization
The term refers to the process that leads to the gradual and ultimately complete destruction of a mineral’s structure due to the radioactivity of some of its components.
A metamictized mineral is amorphous and therefore isotropic.
Allanite undergoes this process.
Opacitization
It consists of the development of magnetite + hematite + iron-poor clinopyroxene and is caused by an imbalance in the magma.
It affects brown hornblende and biotite, and can start at the crystal margins and then occupy the entire crystal.
Opalization
Opal formation from the alteration of amorphous silica, often from organic remains of microorganisms in siliceous shells within lithified sedimentary rocks.
Phengitization
It consists of the development of phengite as an alteration product of the mineral.
Phengites are white micas that form a series between muscovite and celadonite.
Pinitization
It consists of the development of a dense aggregate of sericite and/or chlorite, and also biotite, from intergranular boundaries and fractures.
It occurs in cordierite; the pseudomorphs on cordierite are known as pinite.
Propylitization
A type of hydrothermal alteration that primarily affects volcanic rocks, forming a greenish aggregate of chlorite, epidote, albite, and carbonate in the minerals.
It is a complex, low-intensity alteration; sericitine, magnetite, pyrite, quartz, and zeolites may be present.
Pyritization
A process that involves the replacement of pre-existing minerals with sulfur (S) and iron (Fe), which combine to form the mineral pyrite – FeS2.
Saussuritization
Designation of an alteration process that consists of the development of a very fine-grained, greenish aggregate composed of clinozoisite, zoisite, albite, actinolite, and sericite.
It is common in plagioclases.
Scapolitization
This process occurs in mafic igneous rocks such as gabbros and diabases, and consists of the replacement of plagioclase by scapolite.
It is a metasomatic process that introduces chlorine into the rock in the form of vapor or solutions. All stages can be observed, from portions of the rock that are still the original gabbro to others where the plagioclase has been completely replaced by scapolite. The remaining plagioclase has an original appearance and does not seem altered. Apparently, the process is not superficial, but occurs under pressure at a greater depth.
The process generates a rock with white spots where rounded portions of scapolite are surrounded by granular aggregates of green hornblende, with the original structure having disappeared.
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Schilleritization
Also known as the “Schiller effect” or “Schiller structure,” it consists of small, oriented inclusions of a second mineral within a pre-existing mineral, through exsolution or alteration.
In a hand sample, it can generate an iridescence effect that is best known for labradorite (“labradorescence”) and adularia (“adulariescence”).
It also occurs in orthopyroxenes and green hornblende.
Sericitization
A very common alteration that consists of the formation of small flakes of white mica in the mineral.
The white mica is usually sericite (alteration muscovite), and the NC appears as small luminous spots.
It occurs in feldspars in general, often accompanied by argillization.
Serpentinization
It consists of a change in mineralogy, usually of mafic and ultramafic rocks such as peridotites and dunites, through the oxidation and hydrolysis of minerals (such as olivine), forming serpentine, brucite, and magnetite.
Silicification
Through the introduction of silica, partial or total replacement of the host rock or mineral occurs.
Fossilized tree trunks are a classic example of silicification.
Spilitization
Alteration process that consists of replacing anorthite-rich plagioclase with albite + calcite.
Steatitization
Formation of fine-grained talc, called “steatite” (“soapstone”), through hydrothermal alteration of mafic and ultramafic rocks under greenschist facies metamorphic conditions.
Uralitization
The term refers to the process by which a pyroxene (such as augite) is transformed into an amphibole, such as hornblende, tremolite, or actinolite.
Zeolitization
This alteration occurs under hydrothermal conditions, transforming the mineral into zeolites such as natrolite, thomsonite, scolecite, and heulandite.
Melilite undergoes this process.