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Formula: Ca3Al2(SiO4)3
Nesosilicate (insular SiO4 groups), garnet group, forms
series with
andradite, with uvarovite and
with pyrope
Varieties
Hessonite is a variety of grossular coloured orange due to a small iron content
Hibschite is a variety of grossular with part of the orthosilicate ions SiO4 replaced by hydroxide (OH) ions
Hydrogrossular is a general term for OH-bearing garnet compositions from the grossular-dominant portion of the
grossular-katoite series
Tsavorite is a green chrome and vanadium rich variety of grossular
Properties
Crystal System: Isometric
br
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
Common impurities: Fe,Cr
Environments:
Volcanic igneous 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.
Associated minerals include calcite, dolomite,
epidote, clinozoisite,
wollastonite, scapolite,
vesuvianite, diopside,
tremolite and quartz
(HOM).
Localities
At Kantiwa, Nuristan, Afghanistan, grossular variety hessonite has been found associated with
vesuvianite on massive white calcite
(Lauf p123).
Grossular from Kantiwa - Image
The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia,
are skarn deposits at the contact between
granodiorite and volcanic rocks.
Grossular is a primary silicate that occurs as
crystals up to 4 mm and as aggregates. Associated minerals include hornblende,
diopside and actinolite
(AJM 22.1.35).
At the Jeffrey mine, Asbestos, Les Sources RCM, Estrie, Quebec, Canada, grossular occurs with
diopside
( R&M 94.5.437, 439), and on albite
(R&M 94.5.440).
Grossular from the Jeffrey Mine - Image
At the Orford Nickel mine, St-Denis-de-Brompton, Le Val-Saint-François RCM, Estrie, Québec, Canada, grossular has been found on
diopside
(Lauf p121).
Grossular from Orford - Image
At Xingtai, Hebei, China, well formed, yellow, vitreous, transparent crystals of grossular have been found
(AESS).
Grossular from Xingtai - Image
At Mianning County, Liangshan Yi, Sichuan, China, grossular has been found in a metamorphic
skarn as microscopic brown isometric crystals, together with acicular brown vitreous
diopside crystals up to 5 mm long (Identified by Jolyon Rolph, but Myanmar University
told Denis Ho that the acicular mineral is zircon)
(AESS).
Grossular from Mianning County - Image
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).
Grossular from Alpe delle Frasse - Image
At the Scorpion mine, Tsavo National Park, Voi, Taita-Taveta county, Kenya, grossular variety tsavorite occurs in
gneiss interbedded with schist and
marble layers
(R&M 95.3.252-259).
Tsavorite from the Scorpion Mine - Image
At the Tsavorite mining area, Gogogogo, Ampanihy, Atsimo-Andrefana, Madagascar, grossular variety tsavorite occurs in
gneiss mixed with marble and associated with
diopside, titanite,
quartz and plagioclase
(R&M 95.3.252-259).
At Itrafo, Ampanivana area, Andrembesoa, Betafo, Vakinankaratra, Madagascar, grossular variety tsavorite occurs in
amphibolite - marble country
rock transected by a graphite - rich vein in turn cut by
calcite veinlets that contain the grossular. vanadium
is the main cause of the green colour,
although traces of chromium are also present
(R&M 95.3.252-259).
At Sierra da Cruces, Mexico, grossular has been found in crystalline limestone
associated with vesuvianite
(Lauf p120).
Near Mingora, Swat District, Khyber Pakhtunkhwa Province, Pakistan, grossular variety tsavorite occurs in
quartz veins in
schist. The green colour is due to vanadium, and the
crystals have inclusions of
graphite, mica,
wollastonite and zircon
(R&M 95.3.252-259).
At the Lindi Region, Tanzania, grossular variety tsavorite has been found in a
quartz/graphite matrix
(Lauf p127).
At the Merelani Hills, Manyara Region, Tanzania, grossular variety tsavorite has been isotopically dated to 620 to 580 ma,
presumably when the metamorphism in the region occurred. The tsavorite is coloured green by vanadium,
chromium and manganese, and at
this locality the manganese content is always greater than the chromium. Associated minerals include
axinite-(Mg), chromium-rich diopside,
fluorapatite, graphite and
pyrite
(R&M 95.3.252-259).
Grossular from the Merelani Hills - Image
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 Nightingale district, Pershing county, Nevada, USA, grossular occurs with calcite
in skarn
(Lauf p127).
Grossular from the Nightingale District - Image
At Eden Mills, Vermont, USA, and Warren, New Hampshire, USA, grossular variety 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).
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