Chemistry Reference
In-Depth Information
R
3
+
=
Al
−
spinels group
There is a continuous solid solution between MgAl
2
O
4
spinel
and FeAl
2
O
4
hercynite
; the intermediate Mg-rich terms are called
pleonast
.
There is a continuous series between MgAl
2
O
4
spinel and the zincian
end member,
gahnite
ZnAl
2
O
4
and between spinel and the manganiferous
end member
galaxite
MnAl
2
O
4
. In this last series, ferric iron replaces alumi-
num. The latter two minerals are very rare.
Above 858°C, there is also a continuous series, between hercynite FeAl
2
O
4
and magnetite Fe
2+
Fe
3+
2
O
4
. At lower temperatures there is a solvus.
The intermediate terms Fe
2+
(Fe
3+
,Cr
3+
)
2
O
4
of the series between hercynite
and chromite Fe
2+
Cr
3+
2
O
4
are called
picotites
.
R
3
+
=
Fe3
+
−
magnetite group
Magnetite Fe
2+
Fe
3+
2
O
4
or Fe
3
O
4
is a major constituent of the rocks. In mag-
netites, Ni, Co, Zn, Mg, Mn, Ca can substitute to Fe
2+
(MgFe
3+
2
O
4
is
mag-
nesioferrite
) and Cr, V, Mn
3+
to ferric iron.
There is a continuous series between magnetite and
ulvospinelle
Fe
2+
2
TiO
4
. Most magnetites of the igneous rocks contain titanium. Titanifer-
ous magnetite and ilmenite FeTiO
3
(which forms a continuous series with
hematite Fe
2
O
3
) are frequently associated, and the partition of titanium and
ferrous/ferric iron between the two minerals is used to estimate temperature
and oxygen fugacity.
Some natural magnetites contain an excess of ferric iron, indicating a
solid solution between magnetite and
maghemite
Fe
2
O
3
, which is a low
temperature polymorph of hematite. Maghemite is the produce of supergene
alteration of magnetite. This mineral is unstable and transforms into hema-
tite at increasing temperatures.
Other members of this group are rather unimportant: franklinite
ZnFe
3+
2
O
4
(with Zn
γ
Mn
2+
substitution), jacobsite Mn
2+
Fe
3+
2
O
4
(with Mn
2+
Fe
2+
, Mg and Fe
3+
Mn
3+
substitutions), trevorite NiFe
3+
2
O
4
(with Ni
Fe
2+
, Mg substitution).
R
3
+
=
Cr
−
chromite group.
There is a continuous series between chromite Fe
2+
Cr
2
O
4
and magnesiochr-
omite MgCr
2
O
4
. Chromium can also be replaced by aluminum and ferric
iron. Zinc may substitute to ferrous iron in small quantities. The general
formula of chromite is:
(Fe
2+
, Mg, Zn)(Cr, Al, Fe
3+
)
2
O
4
Compositions of chromites depend on the type of deposit. Aluminous
chromites are used in the refractory industry (and to a lesser extend in met-
allurgy), chromites rich in chromium, in metallurgy.
