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Formula: Mg3Al2(SiO4)3
Nesosilicate (insular SiO4 groups),
garnet group
Crystal System: Isometric
Specific gravity: 3.582 measured, 3.563 calculated
Hardness: 7 to 7½
Streak: White
Colour: Dark red, blood red
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Common impurities: Fe,Mn,Ca
Environments:
Plutonic igneous environments
Placer deposits
Metamorphic environments
Pyrope is a high-pressure mineral, typically formed in mafic and ultramafic rocks
such as mica-peridotite,
kimberlite, eclogite,
serpentinite and anorthosite,
and in sand and gravel derived from
these rocks (Lauf p132). Garnet is one of the
commonest minor minerals in kimberlite.
Pyrope is an essential constituent of
eclogite, and a common constituent of
kimberlite.
It is a characteristic mineral of the eclogite facies and also
occurs in the granulite facies.
Alteration
almandine and phlogopite to pyrope
and annite
Fe2+3Al2(SiO4)3 +
KMg3AlSi3O12(OH)2 ⇌
Mg3Al2Si3O12 +
KFe3AlSi3O10(OH)2
(JVW p 179).
This assemblage is commonly formed during amphibolite facies metamorphism of pelitic rocks
(KB p129).
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).
enstatite and corundum to pyrope
3Mg2Si2O6 + 2Al2O3 ⇌
2Mg3Al2(SiO4)3
At 14 kbar pressure the equilibrium temperature is about 810oC
(eclogite facies). The equilibrium moves to the right at
higher temperatures and to the left at lower temperatures
(SERC).
forsterite and cordierite to pyrope
and quartz
2Mg2SiO4 + Mg2Al4Si5O18 ⇌
2Mg3Al2(SiO4)3 + SiO2
Increasing pressure favours the forward reaction
(SERC).
forsterite and kyanite to
spinel and pyrope
5Mg2SiO4 + 4Al2OSiO4 ⇌ MgAl2O4 + 3Mg3Al2
(SiO4)3
Increasing temperature favours the forward reaction
(SERC).
kyanite and enstatite to
quartz and pyrope
2Al2O(SiO4) + 3Mg2Si2O6 ⇌ 2SiO2 +
2Mg3Al2(SiO4)3
The equilibrium temperature for this reaction at 14 kbar pressure is about 950oC
(granulite facies), with equilibrium to
the right at higher temperatures, and to the left at lower temperatures
(SERC).
phlogopite, muscovite and
SiO2 to orthoclase,
pyrope and H2O
KMg3(AlSi3O10)(OH)2 +
KAl2(AlSi3O10)(OH)2 + 3SiO2
⇌ 2K(AlSi3O8) + Mg3Al2(SiO4)3 +
2H2O
(DHZ 3 p72)
orthopyroxene, Fe-rich diopside and Fe
and Cr-rich
spinel to Fe, Ca and Cr-rich pyrope and
olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 +
(Mg,Fe)(Al,Cr)2O4 ⇌
(Mg,Fe)2Ca(Al,Cr)2Si3O12 +
(Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing
peridotites
are considered to have originated in a high-pressure environment according to the reaction
(DHZ 2A p123).
quartz, pyrope and kyanite to
cordierite
5SiO2 + 2Mg3Al2(SiO4)3 + 4Al2OSiO4 ⇌
3Mg2Al4Si5O18
Increasing temperature favours the forward reaction
(SERC).
spinel, forsterite and
cordierite to pyrope
MgAl2O4 + 5Mg2SiO4 + 2Mg2Al4Si5O18 ⇌
5Mg3Al2(SiO4)3
Increasing pressure favours the forward reaction
(SERC).
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