Grossular

nesosilicate

metamorphic

Formula: Ca3Al2(SiO4)3 nesosilicate (insular SiO4 groups), garnet group
Specific gravity: 3.594
Hardness: 6½ to 7
Streak: White to pale brownish white
Colour: Brown, orange, red, yellow, green, white, colourless. colourless when pure (rare), commonly red orange to brown.
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Pegmatites
Metamorphic environments

Grossular is typically found in contact and regionally metamorphosed limestone and skarn, but may also be found in serpentinite and rodingite (Lauf p120); it is a mineral of the hornblende-hornfels, pyroxene-hornfels, amphibolite and granulite facies.

At Kantiwa, Nuristan, Afghanistan, grossular variety hessonite has been found associated with vesuvianite on massive white calcite (Lauf p123).

At the Orford nickel mine, Quebec, Canada, grossular has been found on diopside (Lauf p121).

At Passo Del Faiallo, Liguria, Italy, grossular variety hibschite has been found associated with clinochlore (Lauf p128).

At Val d'Ala, Piemonte, Italy, grossular variety hessonite is associated with diopside (Lauf p121).

At Alpe delle Frasse, Val di Susa, Torino, Italy, grossular variety hessonite is associated with clinochlore (Lauf p122).

At Sierra da Cruces, Mexico, grossular has been found in crystalline limestone associated with vesuvianite (Lauf p120).

At the Lindi Province, Tanzania, grossular variety tsavorite has been found in a quartz/graphite matrix (Lauf p127).

At the Commercial quarry, Crestmore, Riverside county, California, USA, grossular has been found in calcite (Lauf p126).

At Garnet Hill, Valaveras county, California, USA, grossular occurs in skarn associated with calcite (Lauf p126).

At the Nightingal district, Pershing county, Nevada, USA, grossular occurs with calcite in skarn (Lauf p127).

At Eden Mills, Vermont, USA, and Warren, New Hampshire, USA, grossular variey hessonite has been found with diopside (Lauf p125).

Alteration

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, 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 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

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 (http://www.tulane.edu/~sanelson/eens212/metaminerals.htm).

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 (granulite facies), with the equilibrium to the right at higher temperatures, and to the left at lower temperatures) (SERC).

grossular, diopside, monticellite, calcite and H2O to vesuvianite, quartz and CO2
10Ca3Al2(SiO4)3 + 3CaMgSi2O6 + 3CaMg(SiO4) + 2CaCO3 + 8H2O ⇌ 2Ca19Al10Mg3(SiO4)10 (Si2O2)4O2(OH)8 + 3SiO2 + 2CO2
A common association in calc-silicate metamorphism can be represented by the above equation. Vesuvianite stability will tend to increase with increasing water and decrease as the activity of CO2 rises (DHZ 1A p714).

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).

grossular and lawsonite to zoisite, prehnite and H2O
Ca3Al2(SiO43 + 3CaAl2(Si2O7)(OH)2.H2O ⇌ 2Ca2Al3[Si2O7][SiO4]O(OH) + Ca2Al(Si3Al)O10(OH)2 +10H2O
Increasing temperature faours the forward reaction (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, 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).

lawsonite and grossular to zoisite, quartz and H2O
5CaAl2(Si2O7)(OH)2.H2O + Ca3Al2(SiO43 ⇌ 4Ca2Al3[Si2O7][SiO4]O(OH) + SiO2 + 18H2O
Increasing temperature favours the forward reaction (SERC)

lawsonite, grossular and quartz to prehnite
CaAl2(Si2O7)(OH)2.H2O + Ca3Al2(SiO43 + SiO2 ⇌ 2Ca2Al(Si3Al)O10(OH)2
Increasing temperature favours the forward reaction (SERC)

meionite (scapolite series), calcite and quartz to grossular and CO2
Ca4Al6O24(CO3) + 5CaCO3 + 3SiO2 ⇌ 3Ca3Al2(SiO4)3 + 6CO2
(DHZ 4 p334)

prehnite to zoisite, grossular, quartz and H2O
5Ca2Al(Si3Al)O10(OH)2 ⇌ 2Ca2Al3[Si2O7][SiO4]O(OH) + 2Ca3Al2(SiO43 + 3SiO2 + 4H2O
Increasing temperature favours the forward reaction (SERC)

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 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).

Common impurities: Fe,Cr

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