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Chlorite refers to the chlorite group, 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)
Specific gravity: 2.6 to 3.3
Hardness: 2
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
Environments:
Carbonatites
Sedimentary environments
Metamorphic environments (typical)
Hydrothermal environments
Basaltic cavities
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.
(R&M 85.4.320-322).
At Palabora, South Africa, it occurs in mineralised cavities in
carbonatite
(R&M 92.5.438).
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.
Localities
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 Kwun Yum Shan, Yuen Long District, New Territories, Hong Kong, China, the deposit is a hydrothermal deposit which lies
along a fault zone within altered acid volcanic rocks, consisting mainly of
chlorite, biotite,
sericite and actinolite with
scattered quartz
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
At the Lin Ma Hang mine, North District, New Territories, Hong Kong, China, the
lead-zinc deposit is a hydrothermal
deposit which lies along a fault zone within altered acid volcanic rocks, consisting mainly of
chlorite, biotite,
sericite and actinolite, with
scattered quartz
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
At Lettermuckoo Quarry, Kinvarra, Connemara, Galway County, Connacht, Ireland, the minerals are hosted by a
megacrystic pink to grey monzogranite, with occasional
pegmatitic segregations.
Patches of a powdery green chlorite-group mineral are present on a few specimens. One analysis by
energy dispersive X-ray spectrometry revealed that iron was present in much
greater amounts than magnesium, and the species is therefore chamosite.
Another analysis of a fluorite-contaminated sample revealed that molar
iron and magnesium were about equal, corresponding to a composition near the
boundary between chamosite and
clinochlore
(JRS).
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).
Alteration
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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