Talc carbonate
Encyclopedia
Talc carbonate is a geologic term for a suite of rock and mineral compositions found in metamorphic
ultramafic rocks.
The term refers to the two most common end-member minerals found within ultramafic rocks which have undergone talc-carbonation or carbonation reactions, talc
and the carbonate mineral magnesite
.
Talc carbonate mineral assemblages are controlled by temperature and pressure of metamorphism and the partial pressure of carbon dioxide within metamorphic fluids, as well as by the composition of the rock.
.
Thus, the MgO content of a metamorphosed ultramafic rock can be estimated roughly by understanding the mineral assemblage of the rock. Magnesium content determines the proportion of talc and/or magnesite and aluminium-calcium-sodium content determines the proportion of amphibole and/or chlorite.
At amphibolite facies, the diopside-in isograd
is reached (dependent on carbon dioxide partial pressure) and metamorphic assemblages trend toward talc-pyroxene and eventually toward metamorphic olivine.
Forsterite - aqueous silica → Serpentine
Carbonation of serpentine to form talc-magnesite
Talc carbonate is present in many of the ultramafic bodies of the Archaean Yilgarn Craton
, Western Australia. Notably, the Widgiemooltha Komatiite
shows pure talc-carbonation on the eastern flank of the Widgeimooltha Dome, and almost pure serpentinite metamorphism on the western flank.
Felsic and mafic rocks tend to be less affected by carbon dioxide due to their higher aluminium content. Ultramafic rocks lack aluminium, which allows carbonate to react with magnesium silicates to form talc. In rocks with extremely low aluminium contents, this reaction can progress to create magnesite.
Advanced carbonation of felsic and mafic rocks, very rarely, creates fenite, a metasomatic
alteration caused particularly by carbonatite
intrusions. Fenite alteration is known, but very restricted in distribution, around high-temperature metamorphic talc-carbonates, generally in he form of a sort of aureole around ultramafics. Such examples include biotite-rich zones, amphibolite-calcite-scapolite alteration and other unusual skarn
assemblages.
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
ultramafic rocks.
The term refers to the two most common end-member minerals found within ultramafic rocks which have undergone talc-carbonation or carbonation reactions, talc
Talc
Talc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
and the carbonate mineral magnesite
Magnesite
Magnesite is magnesium carbonate, MgCO3. Iron substitutes for magnesium with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts...
.
Talc carbonate mineral assemblages are controlled by temperature and pressure of metamorphism and the partial pressure of carbon dioxide within metamorphic fluids, as well as by the composition of the rock.
Compositional controls
In a general sense, talc carbonate metamorphic assemblages are diagnostic of the magnesium content of the ultramafic protolithProtolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
.
- Lower-magnesian ultramafic rocks (12-18% MgO as a rule of thumb) tend to favor talc-chlorite assemblages
- Medium-MgO rocks (15-25% MgO) tend to produce talc-amphibole assemblages.
- High-MgO rocks with in excess of 25% MgO tend to form true talc-magnesite metamorphic assemblages.
Thus, the MgO content of a metamorphosed ultramafic rock can be estimated roughly by understanding the mineral assemblage of the rock. Magnesium content determines the proportion of talc and/or magnesite and aluminium-calcium-sodium content determines the proportion of amphibole and/or chlorite.
Talc carbonate minerals
Several minerals are diagnostic of talc carbonated ultramafic rocks;- TalcTalcTalc is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg34 or Mg3Si4O102. In loose form, it is the widely-used substance known as talcum powder. It occurs as foliated to fibrous masses, its crystals being so rare as to be almost unknown...
- ChloriteChlorite groupThe chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
, generally magnesian bluish-green - TremoliteTremoliteTremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades...
-CummingtoniteCummingtoniteCummingtonite is a metamorphic amphibole with the chemical composition 7Si8O222, magnesium iron silicate hydroxide.Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being...
-GruneriteGruneriteGrunerite is a mineral of the amphibole group of minerals with formula Fe7Si8O222. It is the iron endmember of the grunerite-cummingtonite series. It forms as fibrous, columnar or massive aggregates of crystals. The crystals are monoclinic prismatic. The luster is glassy to pearly with colors...
amphiboles in greenschist facies rocks - AnthophylliteAnthophylliteAnthophyllite is an amphibole mineral: 7Si8O222, magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthophyllite are lamellar or fibrous and are used as asbestos...
-CummingtoniteCummingtoniteCummingtonite is a metamorphic amphibole with the chemical composition 7Si8O222, magnesium iron silicate hydroxide.Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being...
amphibole in weakly carbonated serpentinite at greenschist facies or very rarely, uncarbonated amphibolite facies serpentinites - MagnesiteMagnesiteMagnesite is magnesium carbonate, MgCO3. Iron substitutes for magnesium with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts...
, and rarely dolomiteDolomiteDolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
in association with amphibolitic compositions
At amphibolite facies, the diopside-in isograd
Isograd
In geology, an isograd is a plane of constant metamorphic grade in the field; it separates metamorphic zones of different metamorphic index minerals. On geologic maps focusing on metamorphic terranes , the boundaries between rocks of different metamorphic grade are commonly demarcated by isograd...
is reached (dependent on carbon dioxide partial pressure) and metamorphic assemblages trend toward talc-pyroxene and eventually toward metamorphic olivine.
Mineral reactions
Serpentinisation of olivineForsterite - aqueous silica → Serpentine
- 3Mg2SiO4 + SiO2 + 2H2O → 2Mg3Si2O5(OH)4
Carbonation of serpentine to form talc-magnesite
- 2Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3MgCO3 + H2O
Occurrence
Because carbon dioxide is such a common component of metamorphic fluids, talc-carbonated ultramafics are relatively common. However, the degree of talc-carbonation is usually somewhere between the two end-member compositions of pure serpentinite and pure talc-carbonate. It is common to see serpentinites which contain talc, amphibole and chloritic minerals in small proportions which infer the presence of carbon dioxide in the metamorphic fluid.Talc carbonate is present in many of the ultramafic bodies of the Archaean Yilgarn Craton
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
, Western Australia. Notably, the Widgiemooltha Komatiite
Widgiemooltha Komatiite
The Widgiemooltha Komatiite is a formation of komatiite in the Yilgarn Craton of Western Australia.- Stratigraphy :The stratigraphy of the Widgiemooltha Komatiite is well known to be part of the regional komatiite magmatic event also seen at the Kambalda Dome, to the north. There are comparisons...
shows pure talc-carbonation on the eastern flank of the Widgeimooltha Dome, and almost pure serpentinite metamorphism on the western flank.
Carbonation of other rocks
Carbon dioxide has less severe impacts on mafic, felsic and rocks of other composition, such as carbonate rocks, chemical sediments, etcetera. The exception to this rule is the calc-silicate family of metamorphic rocks, which are also subjected to wide variations in mineral speciation due to the mobility of carbonate during metamorphism.Felsic and mafic rocks tend to be less affected by carbon dioxide due to their higher aluminium content. Ultramafic rocks lack aluminium, which allows carbonate to react with magnesium silicates to form talc. In rocks with extremely low aluminium contents, this reaction can progress to create magnesite.
Advanced carbonation of felsic and mafic rocks, very rarely, creates fenite, a metasomatic
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
alteration caused particularly by carbonatite
Carbonatite
Carbonatites are intrusive or extrusive igneous rocks defined by mineralogic composition consisting of greater than 50 percent carbonate minerals. Carbonatites may be confused with marble, and may require geochemical verification....
intrusions. Fenite alteration is known, but very restricted in distribution, around high-temperature metamorphic talc-carbonates, generally in he form of a sort of aureole around ultramafics. Such examples include biotite-rich zones, amphibolite-calcite-scapolite alteration and other unusual skarn
Skarn
Skarn is an old Swedish mining term originally used to describe a type of silicate gangue, or waste rock, associated with iron-ore bearing sulfide deposits apparently replacing Archean age limestones in Sweden's Persberg mining district. In modern usage the term "skarn" has been expanded to refer...
assemblages.
See also
- Ultramafic rocks
- SerpentiniteSerpentiniteSerpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
- Metamorphic rocks
- List of rock types
- KomatiiteKomatiiteKomatiite is a type of ultramafic mantle-derived volcanic rock. Komatiites have low silicon, potassium and aluminium, and high to extremely high magnesium content...
- GreenschistGreenschistGreenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
, AmphiboliteAmphiboliteAmphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
, GranuliteGranuliteGranulites are medium to coarse–grained metamorphic rocks that have experienced high temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure... - SoapstoneSoapstoneSoapstone is a metamorphic rock, a talc-schist. It is largely composed of the mineral talc and is thus rich in magnesium. It is produced by dynamothermal metamorphism and metasomatism, which occurs in the areas where tectonic plates are subducted, changing rocks by heat and pressure, with influx...