Chemistry Reference
In-Depth Information
are linked by Ca ions (X sites), cleavage (001) of the melilites corresponds to
that sheet structure. The tetragonal symmetry is due to the square centered
pavement. The general formula of melilite is:
X
2
Y Z
2
O
7
X
=
Ca, Na, (K, Sr)
Y
=
Al, Mg, (Fe
2+
, Mn)
Z
=
Si, Al
In
åkermanite
Ca
2
MgSi
2
O, Y sites are occupied by Mg. The formula
of
gehlenite
Ca
2
Al
2
SiO
7
is deduced from it by the substitution Mg Si Al
IV
Al
VI
. The formula of
soda melilite
(Na-melilite, or melilite) NaCaAlSi
2
O
7
is
deduced by the substitution Ca Mg Na Al. There are certainly gaps of
solubility between these end members, but they remain poorly investigated.
Åkermanite and gehlenite are minerals under-saturated in silica. They
are only very rarely associated with plagioclase.
Minerals of the melilite group are high temperature minerals over 700°C
for åkermanite (this mineral is characteristic of the sanidinite facies), 600°C
for gehlenite.
3.4.1.3 Non aluminous calcium silicates
The only relatively common, calcium silicate is
wollastonite
CaSiO
3
.
Wollastonite is an inosilicate, the chains of SiO
4
4-
tetrahedra differ from
those of pyroxenes by the fact that the pattern is formed by two tetrahe-
dra pointing in one direction alternating with a tetrahedron pointing in the
opposite direction (Figure 1.8).
There are three polymorphs: triclinic pseudowollastonite of very high
temperature (above 1120°C), monoclinic wollastonite-2M and triclinic
wollastonite-Tc at lower temperatures. The usual form of wollastonite is
the triclinic form.
Wollastonite accepts small quantities of iron and manganese in substitu-
tion to calcium.
Wollastonite is stable at high temperatures. The reaction:
CO
2
occurs around 650-750°C, depending on the CO
2
pressure. Towards higher
temperatures wollastonite is transformed into spurrite in the presence of
calcite at about 1000°C.
There are many other calcium silicates, most of them are anhydrous but
they may contain anions such as CO
3
: for instance,
larnite
(Ca
2
SiO
4
),
ran-
kinite
(Ca
3
Si
2
0
7
),
spurrite
(2Ca
2
SiO
4
· CaCO
3
),
tilleyite
(Ca
3
Si
2
O
7
· CaCO
3
).
These are rare minerals, limited to high temperature skarns. Larnite and
spurrite are presented in the CD.
calcite
+
quartz
wollastonite
+
