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Formula: MgAl2O4
Multiple oxide, spinel subgroup,
oxyspinel group,
spinel supergroup
Crystal System: Isometric
Specific gravity: 3.6 to 4.1 measured, 3.578 calculated
Hardness: 8
Streak: White
Colour: Red, violet, blue, yellow, colourless
Solubility: Insoluble in hydrochloric and nitric acid; slightly soluble in sulphuric acid
Common impurities: Ti,Fe,Zn,Mn,Ca
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.
Localities
At lots 10 and 11 of concession 1, Bathurst Township, Lanark County, Ontario, Canada (DeWitts corner), the deposit is
located in the Grenville Geological Province, which consists mostly of
marble, gneiss, and
quartzite.
Syenite-migmatite was
also reported in the area where the vein-dikes are located. Characteristic features of the vein-dikes include the
fact that perfectly formed euhedral crystals of different minerals can often be found floating in
calcite with no points of contact with the walls. Sometimes these crystals
have inclusions of calcite, irregular or rounded in shape. It has been argued
that at least some of the vein-dikes were formed as a result of melting of Grenville
marble.
Spinel is abundant in the vein-dikes. It forms black octahedral crystals to 6 cm in a
calcite matrix. Octahedra are frequently severely distorted and numerous
inclusions of rounded calcite can be found in them. The spinel
crystals show a fractal-like nature. They are composed of an ascending hierarchy of octahedra in parallel growth,
beginning at about 1 mm and then grouping to form composite octahedra to 5 mm. These suboctahedra are then stacked
together to form larger composite crystals. The spaces between the suboctahedra of the composite may or may not be
filled, partially or completely, by 1 to 2 mm octahedra in parallel growth. Octahedra with smooth uninterrupted octahedral
faces are rare. Isolated octahedra to 1 mm are sometimes linked into chains. Some spinel crystals are partially
or fully replaced by corundum, while associated
magnesiohögbomite remains unaltered
(R&M 97.6.556-564).
Amity, Town of Warwick, Orange county, New York, USA, is an area of
granite intrusions into
marble and associated
gneiss. The marble is
mostly composed of white crystalline calcite that often has small flakes
or spheres of graphite and
phlogopite. This locality is the source of some of the world’s
largest and best-formed spinel crystals, being large, black and greenish-black octahedra found lying loose
in the soil at a site 2 kilometers south of Amity. The crystals recently recovered from Amity are octahedral, but
many have etched faces or growth layers. The most common colours are grey to brown to black, with pink or reddish
crystals uncommon.
There are two distinct environments where spinel occurs. One is in white
marble associated with
chondrodite/clinohumite.
The other is in serpentine veins associated with
fluorite
(R&M 96.5.439-440).
The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in
marble. The development of veins of large crystals probably occurred as
a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during
this primary event, with additional species resulting from the
subsequent alteration of scapolite. There seems to be little, if any,
secondary, late-stage mineralisation present.
Spinel rarely occurs as millimetre-sized transparent pink octahedral crystals in
calcite associated with phlogopite
(R&M 96.6.552).
Alteration
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).
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
cordierite and spinel
5Mg2SiO4 + 10Al2OSO4 ⇌ 3Mg2Al4Si5O18 +
4MgAl2O4
At 400oC the equilibrium pressure for this reaction is about 4 kbars; increasing temperature 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).
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)
enstatite-ferrosilite,
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).
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