Anorthite

minerals

tectosilicate

plagioclase

labradorite

Formula: Ca(Al2Si2O8)
Tectosilicate (framework silicate)
Anorthite is a plagioclase feldspar.
Bytownite is a variety of anorthite
Labradorite is a variety of anorthite
Specific gravity: 2.74 to 2.76, bytownite 2.61 to 2.77
Hardness: 6 to 6½
Streak: White
Colour: Anorthite is colourless, reddish grey or white. Bytownite is colourless, white, greenish, reddish or grey.
Solubility: Anorthite is slightly soluble in hydrochloric acid
Melting point: About 1,550oC at atmospheric pressure (JVW p275)
In the continuous branch of the Bowen reaction series anorthite is the first major mineral to crystallise.
Environments (anorthite):

Plutonic igneous environments (labradorite)
Pegmatites (bytownite)
Metamorphic environments

Anorthite is a plagioclase feldspar found in rocks rich in dark minerals, in druses of ejected volcanic blocks and in granular limestone of contact metamorphic deposits. Anorthite also may be found in serpentinite, and hornfels.
Bytownite is found in granite, with hornblende and augite in gabbro, anorthosite, gneiss, granulite, and pegmatites, and lining Alpine cavities and fissures in ore veins.
Labradorite is found with hornblende in basalt, with hornblende and augite in gabbro.
Anorthite is a mineral of the greenschist, amphibolite and granulite facies.

Alteration

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4
(DHZ 2A p651)

Ca-Fe amphibole and anorthite to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2
(JVW p363)

calcium-iron amphibole and anorthite to garnet (grossular and almandine), clinozoisite and quartz
Ca2Fe3Si8O22(OH)2 + 6Ca(Al2Si2O8) ⇌ 4/3Ca3Al2(SiO4)3 + 5/3Fe3Al2(SiO4)3 + 2Ca2Al3[SiO7][SiO4]O(OH) + 5SiO2
(MM 48.206)

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction (DHZ 2A p272).

anorthite to calcite and kaolinite in the early Earth's atmosphere
CO2 + H2O + anorthite → calcite + kaolinite
CO2 + 2H2O + CaAl2Si2O8 → CaCO3 + Al2Si2O5(OH)4
(JVW p634)

anorthite to grossular, kyanite and quartz
3CaAl2 Si2O8 → Ca3Al2(SiO4)3 + 2Al2OSiO4 + SiO2
At 20 kbar pressure the equilibrium temperature is about 1,000oC and at 30 kbar it is about 1,400oC

anorthite and CO2 to meionite (scapolite series), corundum and quartz
4Ca(Al2Si2O8) + CO2 ⇌ Ca4Al6O24(CO3) + Al2O3 + 2SiO2
(DHZ 4 p334)

anorthite, H2O and CO2 to kaolinite and calcite
2CaAl2 Si2O8 + 4H2O + 2CO2 ⇌ Al4Si4O10(OH)8 + 2CaCO3
calcite is found as a low-temperature, late-stage alteraation product according to the above reaction. (DHZ 5B p128).

anorthite, H2SO4 and H2O to gypsum and kaolinite
CaAl2 Si2O8 + H2SO4 + 3H2O → CaSO4.2H2O + Al2Si2O5(OH)4
(DHZ 5B p65)

anorthite, albite and H2O to jadeite, lawsonite and quartz
CaAl2 Si2O8 + NaAlSi3O8 + 2H2O → NaAlSi2O6 + CaAl2(Si2O7)(OH)2.H2 + SiO2
(DHZ 2A p475)

anorthite and calcite to meionite (scapolite series)
3Ca(Al2Si2O8) + CaCO3 ⇌ Ca4Al6O24(CO3)
This reaction occurs in the presence of a high CO2 pressure in an environment deficient in (Al+Na+K) (DHZ 4 p331).

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 10MgSiO3 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs in the granulite to amphibolite facies (DHZ 2A p631).

calcium amphibole, calcite and quartz to diopside- hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction (DHZ 2A p272).

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction (DHZ 2A p272).

anorthite variety labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure (DHZ 2A p449).

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2
This reaction occurs at high temperature and pressure (DHZ 2A p449).

augite and andalusite to enstatite- ferrosilite and anorthite
Ca(Fe,Mg)Si2O6 + Al2SiO5 → (Mg,Fe2+)SiO3 + Ca(Al2Si2O8)
(DHZ 2A p126)

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).

Fe-rich cordierite and diopside- hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2
(DHZ 2A p126)

Al-rich enstatite and Al-rich diopside to forsterite, enstatite, diopside and anorthite
Mg9Al2Si9O30 + Ca5Mg4Al2Si9O30 ⇌ 2Mg2SiO4 + 3Mg2Si2O6 + 3CaMgSi2O6 + 2Ca(Al2Si2O8)
This reaction occurs at fairly low temperature and pressure (DHZ 1A p233).

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2
This reaction represents metamorphic reactions between the granulite and amphibolite facies (DHZ 2A p139).

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).

epidote and quartz to anorthite, grossular and H2O
4Ca2Al3(SiO4)3(OH) + SiO2 → 5CaAl2Si2O8 + Ca3Al2(SiO4)3 + 2H2O
This reaction occurs as the degree of metamorphism increases.

forsterite and anorthite to clinoenstatite, diopside and spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 + CaMgSi2O6 + MgAl2O4

forsterite and anorthite to enstatite, diopside and spinel
2Mg2SiO4 + Ca(Al2Si2O8) = Mg2Si2O6 + CaMgSi2O6 + MgAl2O4
(DHZ 1A p242)

grossular to anorthite, gehlenite and wollastonite
2Ca3Al2(SiO4)3 ⇌ CaAl2Si2O8 + Ca2Al2SiO7 + 3CaSiO3
The equilibrium temperature for this reaction at 5 kbar pressure is 1,110oC (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

grossular and corundum to anorthite and gehlenite
2Ca3Al2(SiO4)3 + Al2O3 ⇌ CaAl2Si2O8 + Ca2Al2SiO7
The equilibrium temperature for this reaction at 5 kbar pressure is about 950oC At 4.3 kbar pressure the equilibrium temperature is about 890oC (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

grossular and kyanite to anorthite and corundum
Ca3Al2(SiO4)3 + 3Al2OSiO4 ⇌ 3CaAl2Si2O8 + Al2O3
The equilibrium temperature for this reaction at 10 kbar pressure is about 540oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

grossular, kyanite and quartz to anorthite
Ca3Al2(SiO4)3 + 2Al2OSiO4 + SiO2 ⇌ 3CaAl2Si2O8
The equilibrium temperature for this reaction at 10 kbar pressure is about 510o (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

kyanite to anorthite and corundum
Ca3Al2(SiO4)3 + 3Al2OSiO4 ⇌ 3CaAl2Si2O8 + Al2O3
The equilibrium temperature for this reaction at 10 kbar pressure is about 540oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

grossular, kyanite and quartz to anorthite
Ca3Al2(SiO4)3 + 2Al2OSiO4 + SiO2 ⇌ 3CaAl2Si2O8
The equilibrium temperature for this reaction at 10 kbar pressure is about 510o (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

grossular and quartz to anorthite and wollastonite
Ca3Al2(SiO4)3 + SiO2 ⇌ CaAl2Si2O8 + 2CaSiO3
The equilibrium temperature for this reaction at 5 kbar pressure is 730oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

hornblende, calcite and quartz to Fe-rich diopside, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical (DHZ 2A p272).

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical (DHZ 2A p272).

Al-rich hornblende, spinel, quartz, K2O and H2O to anorthite, Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 4 MgAl2O4 + 6SiO2 + K2O + H2O → 2.5Ca(Al2Si2O8) + Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
(DHZ 2A 631)

kyanite and zoisite to anorthite, corundum and H2O
2Al2O(SiO4) + 2Ca2Al3[Si2O7][SiO4]O(OH) ⇌ 4CaAl2Si2O8 + Al2O3 + H2O
The equilibrium temperature for this reaction at 5 kbar pressure is 480oC (greenschist facies), and at 10 kbar it is about 720oC (amphibolite facies). The equilibrium is to the right at higher temperatures, and to the left at lower temperatures (SERC, AM61.699-709).

kyanite and zoisite to margarite and anorthite
2Al2O(SiO4) + 2Ca2Al3[Si2O7][SiO4]O(OH) ⇌ CaAl2(Al2Si2O10)(OH)2 + Ca(Al2Si2O8)
The equilibrium temperature for this reaction at 6 kbar pressure is about 520oC (amphibolite facies), and at 9 kbar it is about 675oC (amphibolite facies). At any pressure the euqilibrium is displaced to the right at higher temperatures, and to the left at lower temperatures (SERC, AM61.699-709).

kyanite, zoisite and quartz to anorthite and H2O
Al2OSiO4 + 2Ca2Al3[Si2O7][SiO4]O(OH) + SiO2 ⇌ 4Al2OSiO4 + H2O
Increasing temperature favours the forward reaction (SERC)

margarite to corundum, anorthite and H2O
CaAl2(Al2Si2O10)(OH)2 ⇌ Al2O3 + Ca(Al2Si2O8) + H2O
The equilibrium temperature for this reaction at 6 kbar pressure is about 610oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

margarite and quartz to anorthite, andalusite and H2O
CaAl2(Al2Si2O10)(OH)2 + SiO2 ⇌ Ca(Al2Si2O8) + Al2OSiO4 + H2O
The equilibrium temperature for this reaction at 2 kbar pressure is about 440oC (greenschist facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC, AM61.699-709).

margarite and quartz to anorthite, kyanite and H2O
CaAl2(Al2Si2O10)(OH)2 + SiO2 ⇌ Ca(Al2Si2O8) + Al2OSiO4 + H2O
The equilibrium temperature for this reaction at 5 kbar pressure is about 520oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC, AM61.699-709).

margarite, zoisite and quartz to anorthite and H2O
CaAl2(Al2Si2O10)(OH)2 + 2Ca2Al3[Si2O7][SiO4]O(OH) + 2SiO2 ⇌ 5Al2OSiO4 + 2H2O
Increasing temperature favours the forward reaction (SERC)

Fe and Cr-rich spinel , diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4
This reaction occurs at fairly low temperature and pressure (DHZ 1A p233).

tremolite, anorthite and clinochlore to tschermakite and H2O
3☐Ca2MgSi8O22(OH)2 + 8Ca(Al2Si2O8) + 2Mg5Al(AlSi3O10)(OH)8 ⇌ 5☐Ca2(Mg3Al2)(Si6Al2)O22(OH)2 + 4H2O
(AM 76.998)

wairakite to anorthite and quartz and H2O
Ca(Si4Al2)O10.2H2O ⇌ Ca(Al2Si2O8) + 2SiO2 + 2H2O
Increasing temperature favours the forward reaction (AM61.699-709).

wairakite, zoisite and margarite to anorthite and H2O
Ca(Si4Al2)O10.2H2O + 2Ca2Al3[Si2O7][SiO4]O(OH) + CaAl2Si2Al2O10(OH)2 ⇌ Ca(Al2Si2O8) + H2O
Increasing temperature favours the forward reaction (AM61.699-709).

zoisite to anorthite, grossular, corundum and H2O
6Ca2Al3[Si2O7][SiO4]O(OH) ⇌ 6CaAl2Si2O8 + 2Ca3Al2Si3O12 + Al2O3 + 3H2O
The equilibrium temperature for this reaction at 6 kbar pressure is about 760oC, and at 10 kbar it is about 950oC (granulite facies). For any given pressure, the reaction goes to the right at higher temperatures, and to the left at lower temperatures (SERC).

zoisite and kyanite to margarite and anorthite
2Ca2Al3[Si2O7][SiO4]O(OH) + 2Al2OSiO4 ⇌ CaAl2(Al2Si2O10)(OH)2 + 3Al2OSiO4
Increasing temperature favours the forward reaction (SERC)

zoisite, kyanite and quartz to anorthite and H2O
2Ca2Al3[Si2O7][SiO4]O(OH) + Al2OSiO4 + SiO2 ⇌ 4Ca(Al2Si2O8) + H2O
The equilibrium temperature for this reaction at 10 kbar pressure is about 690oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC, AM61.699-709).

zoisite and quartz to grossular, anorthite and H2O
4Ca2Al3[Si2O7][SiO4]O(OH) + SiO2 ⇌ Ca3Al2Si3O12 + 5CaAl2Si2O8 + 2H2O
The equilibrium temperature for this reaction at 5 kbar pressure is 650oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

zoisite, margarite and quartz to anorthite and H2O
2Ca2Al3[Si2O7][SiO4]O(OH) + CaAl2(Al2Si2O10)(OH)2 + 2SiO2 ⇌ 5Ca(Al2Si2O8) + 2H2O
The equilibrium temperature for this reaction at 6 kbar pressure is about 540oC (amphibolite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures (SERC).

Common impurities in anorthite: Ti,Fe,Na,K

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