Chlorite refers to a group of minerals, the most common members being
clinochlore: Mg5Al(AlSi3O10)(OH)8 and
chamosite: (Fe2+,Mg,Al,Fe3+)6(Si,Al)4 O10(OH,O)8
phyllosilicates (sheet silicates) chlorite group
Specific gravity: 2.6 to 3.3
Streak: Green, more rarely brown
Colour: Dark green to brown
Solubility: Slightly soluble in sulphuric acid
Common impurities: clinochlore: Cr,Ca; chamosite: Mn,Ca,Na,K
Metamorphic environments (typical)
Chlorite is a common mineral in metamorphic rocks, such as chlorite
schist; it is also formed by hydrothermal alteration of igneous rocks often
associated with quartz and
siderite; it is found in
cavities in igneous volcanic rocks and mixed with
clay minerals in argillaceous sediments.
At Palabora, South Africa, it occurs in mineralised cavities in
Chlorite is an essential constituent of phyllite.
It also may be found in hornfels.
Commonly associated minerals are actinolite and epidote, and an assemblage of quartz, albite, muscovite variety sericite and garnet.
Chlorite is characteristic of the greenschist facies; it is also a mineral of the zeolite, albite-epidote-hornfels, prehnite-pumpellyite, amphibolite and blueschist facies.
At the Mount Lyell Mines, Queenstown district, West Coast municipality, Tasmania, Australia, chlorite is common and generally the earliest phase to crystallise, although it is occasionally found overgrowing and including quartz and, very rarely, siderite. Green quartz crystals have chlorite inclusions. Other associated minerals include chalcopyrite, apatite and hematite. Clinochlore is more common in the outer parts of the ore bodies, and chamosite in the main ore zones (AJM 21.2.23).
At Barrasford Quarry, Chollerton, Northumberland, England, UK, very small spots of chlorite have been found within radiating crystalline masses of pectolite in vesicles (JRS 21.8).
Chlorite forms as an alteration product of Mg-Fe silicates such as pyroxene, amphibole, biotite and garnet.
albite, chlorite and calcite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz, CO2 and H2O
8Na(AlSi3O8) + (Mg4.0Fe2.0)(AlSi3O10)(OH)8 + CaCO3 → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 2SiO2 + CO2 + 2H2O
In low to intermediate metamorphism jadeite-glaucophane assemblages may arise from reactions such as the one above (DHZ 2A p475).
Ca-Fe amphibole, anorthite and H2O to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2 (JVW p363)
amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above (DHZ 2A p453).
amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction (DHZ 2A p453).
chlorite (clinichlore), actinolite and albite to glaucophane, iron-poor epidote, SiO2 and H2O
9Mg5Al(AlSi3O10)(OH)8 + 6☐Ca2Mg5Si8O22(OH)2 + 50Na(AlSi3O8) → 25☐Na2(Mg3Al2)Si8O22(OH)2 + 6Ca2Al3[Si2O7][SiO4]O(OH) + 7SiO2 + 14H2O
This is a metamorphic reaction (DHZ 3 p156).
chlorite (clinochlore), iron-poor epidote and SiO2 to amphibole (tschermakite), anorthite and H2O
3Mg5Al(AlSi3O10)(OH)8 + 6Ca2(Al2Fe3+)[Si2O7][SiO4]O(OH) + 7SiO2 → 5☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2 + 2Ca(Al2Si2O8) + 10H2O
This reaction occurs at a fairly high metamorphic grade (DHZ 3 p154).
chlorite, muscovite and quartz to biotite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al(AlSi3O10)(OH)8 + KAl2(AlSi3O10)(OH)2 + 2SiO2 → K(Mg,Fe2+)3(AlSi3O10)(OH)2 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
This reaction ocurs when the metamorphic grade increases (http://www.tulane.edu/~sanelson/eens212/metaminerals.htm).
chlorite and quartz to enstatite- ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)4Al4Si2O10(OH)8 + 5SiO2 → 2(Mg,Fe2+)SiO3 + (Mg,Fe2+2)2Al4Si5O18 + 4H2O
enstatite-ferrosilite also occurs in medium grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content according to the above equation (DHZ 2A p134).
epidote and chlorite to hornblende and anorthite
6Ca2Al3(SiO4)3(OH) + Mg5Al2Si3O18(OH)8 → Ca2Mg5Si8O22(OH)2 + 10CaAl2Si2O8
This reaction represents changes when the metamorphic grade increases from the greenschist facies to the amphibolite facies (KB p429 diagram p430).
kaolinite, dolomite, quartz and H2O to chlorite, calcite and CO2
Al2Si2O5(OH)4 + 5CaMg(CO3)2 + SiO2 + 2H2O ⇌ Mg5Al(AlSi3O10)(OH)8 + 5CaCO3 + 5CO2
Chlorite often forms in this way from reactions between clay minerals such as kaolinite and carbonates such as dolomite (KB p377).
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