Chemistry Reference
In-Depth Information
temperature and andalusite of low temperature and low pressure. The com-
position of natural minerals does not differ very much from the ideal for-
mula. The only notable substitutions are those of aluminum by ferric iron in
andalusite (up to 2 wt% of Fe
2
O
3
), and to a lesser extent in the kyanite, and
manganese in andalusite. The iron-rich andalusites have a pink color under
the microscope, those rich in manganese and iron (viridine) a green color.
Alumina silicates consist of chains of octahedra joined by an edge, con-
taining aluminum. These chains are linked by isolated silica tetrahedra (these
minerals are nesosilicates) alternating with sites containing the other half of
aluminum the coordination of which varies in these various polymorphs:
the pressure tends to favor the phases where aluminum has a 6-fold
coordination. The structural formula of kyanite is Al
VI
Al
VI
SiO
5
the sym-
metry of this mineral is triclinic but the network is close to a face-cen-
tered cubic lattice;
high temperatures favor the phases where aluminum has a 4-fold coor-
dination: sillimanite, high-temperature polymorph of orthorhombic
symmetry, has the formula Al
IV
Al
IV
SiO
5
;
half of the aluminum of andalusite have a 5-fold coordination: Al
VI
A
lV
SiO
5
;
this mineral has orthorhombic, almost tetragonal, symmetry.
Only the curve of equilibrium between sillimanite and kyanite is pre-
cisely known. The other curves andalusite-sillimanite and andalusite-
kyanite, are poorly known so that different values for the position of
the triple point have been proposed: 350 to 600°C, 3-8 kb. Recent data
suggest 500
±
50°C and 4
±
0.5 kb (Figure 1-9).
3.3.1.2 Aluminous garnets
Garnets are made of a three-dimensional network of tetrahedra (site Z) and
octahedra (Y site). Octahedra and tetrahedra alternate and share an oxygen
to each apex. This structure makes room for distorted cube-shaped sites
(X sites) of 8-fold coordination. Thus the formula of garnet is:
X
3
Y
2
Z
3
O
12
X
=
Fe
2+
, Mg, Mn Ca, (Y)
Y
=
Al, Fe
3+
, Cr, (V, Mn
3+
, Ti, Zr, Fe
2+
)
Z
=
Si
In most garnet silicon is the only element entering in tetrahedral site.
In titanium or zirconium garnets aluminum can enter this site. Tin enters in
this site in some rare calcium garnets.
Garnets are traditionally divided into aluminous garnets and calcic gar-
nets. These two groups differ not only by their chemical composition but
also by their occurrences.
