Spinel

oxide

pyrrhotite

Formula: MgAl2O4 multiple oxide
Specific gravity: 3.6
Hardness: 8
Streak: White
Colour: Red, violet, blue, yellow, colourless
Solubility: Insoluble in hydrochloric and nitric acid; slightly soluble in sulphuric acid
Environments:

Pegmatites (USGS bulletin 445 Pegmatites of Maine)
Placer deposits
Metamorphic environments

Spinel is a common high-temperature mineral occurring in contact metamorphosed limestone and metamorphic argillaceous (clay-rich) rocks poor in SiO2. It also occurs as an accessory mineral in many dark igneous rocks and it is frequently found frequently as rolled pebbles in stream sands.
In contact metamorphic rocks such as marble spinel is associated with phlogopite, pyrrhotite, chondrodite and graphite.
It is a mineral of the amphibolite and granulite facies.

Alteration

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopitephlogopite
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).

corundum and forsterite to spinel and enstatite
2Al2O3 + 2Mg2SiO4 ⇌ 2MgAl2O4 + Mg2Si2O6
At 10 kbar pressure the equilibrium temperature is about 570oC (amphibolite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

enstatite and corundum to cordierite and spinel
5Mg2Si2O6 + 10Al2O3 ⇌ 2Mg2Al4Si5O18 + 6MgAl2O4
At 6 kbar pressure the equilibrium temperature is about 715oC (amphibolite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC).

enstatite and kyanite to spinel and cordierite
5Mg2Si2O6 + 10Al2OSiO4 ⇌ 2MgAl2O4 + 4Mg2Al4Si5O18
Increasing temperature favours the forward reaction (SERC).

enstatite and spinel to forsterite and cordierite
5Mg2Si2O6 + 2MgAl2O4 ⇌ 5Mg2SiO4 + Mg2Al4Si5O18
At 4 kbar pressure the equilibrium temperature is about 715oC (amphibolite facies). The equilibrium moves to the right at higher temperatures and to the left at lower temperatures (SERC)

enstatite-ferrosilite and andalusite to Fe-rich cordierite and spinel-hercynite
5(Mg,Fe2+)SiO3 + 5 Al2SiO5 → 2(Mg,Fe2+)2Al4Si5O18 + (Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction (DHZ 2A p134).

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4
(DHZ 2A p258)

forsterite and anorthite to clinoenstatite, diopside and spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 + CaMgSi2O6 + MgAl2O4
The reaction can proceed in either direction, depending on the ambient conditions.

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

forsterite and kyanite to spinel and pyrope
5Mg2SiO4 + 4Al2OSiO4 ⇌ MgAl2O4 + 3Mg3Al2 (SiO4)3
Increasing temperature favours the forward reaction (SERC).

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)

hypersthene, augite and Fe and Cr-rich spinel to garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4
(DHZ 2A p258)

kyanite and forsterite to enstatite and spinel
2Al2OSiO4 + 4Mg2SiO4 ⇌ 3Mg2Si2O6 + 2MgAl2O4
Increasing temperature favours the forward reaction (SERC).

kyanite and pyrope to cordierite and spinel
2Al2OSiO4 + Mg3Al2(SiO4)3 ⇌ Mg2Al4Si5O18 + MgAl2O4
Increasing temperature favours the forward reaction (SERC).

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).

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)

spinel, enstatite and cordierite to pyrope
MgAl2O4 + Mg2Si2O6 + Mg2Al4Si5O18 ⇌ Mg3Al2(SiO4)3
Increasing pressure favours the forward reaction (SERC).

spinel-hercynite, sillimanite and quartz to sapphirine
7(Mg,Fe2+)Al2O4 + 2Al2SiO5 + SiO2 → 4(Mg,Fe)1.75Al4.5Si0.75O10
(DHZ 2A p633)

spinel and tremolite to forsterite and magnesio-hornblende MgAl2O4 + Ca2Mg5Si8O22(OH)2 ⇌ Mg2SiO4 + Ca2(Mg4Al)(Si7Al)O22(OH)2
This reaction occurs in some strongly metamorphosed serpentinite (DHZ 1A p261).

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).

Fe and Cr-rich spinel, diopside and enstatite to olivine, anorthite and chromite
MgFe2+Al2Cr2O8 + CaMgSi2O6 + 2MgSiO3 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4
In high temperature and high pressure environments olivine is produced according to the above reaction.

spinel, forsterite and cordierite to pyrope
MgAl2O4 + 5Mg2SiO4 + 2Mg2Al4Si5O18 ⇌ 5Mg3Al2(SiO4)3
Increasing pressure favours the forward reaction (SERC).

spinel, kyanite and enstatite to pyrope
2MgAl2O4 + 2Al2OSiO4 + 5Mg2Si2O6 ⇌ 4Mg3Al2(SiO4)3
Increasing temperature favours the forward reaction (SERC).

Common impurities: Ti,Fe,Zn,Mn,Ca

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