Chemistry Reference
In-Depth Information
The end members of aluminous garnets (pyralspites) are:
Pyrope Mg
3
Al
2
Si
3
O
12
Almandin Fe
3
Al
2
Si
3
O
12
Spessartine Mn
3
Al
2
Si
3
O
12
Grossular Ca
3
Al
2
Si
3
O
12
Although the grossular is also part of the group of calcium garnets and
is usually treated with them, it is here associated to the group of pyralspites
because they always contain a larger or less proportion grossular molecule
(usually less than 10%, sometimes much more).
Aluminous garnets are common minerals in metamorphic rocks, par-
ticularly in rocks deriving from pelites, and in peraluminous igneous rocks.
They do not show other substitutions than the ones between the elements in
X site, mostly Fe
2+
, Mg, Mn and Ca.
Garnets from kimberlites and some peridotites may contain a significant
proportion of chromium, defining a series between the pyrope end member
and a knorringite Mg
3
Cr
2
Si
3
O
12
end member.
3.3.1.3 Staurolite
Staurolite is a nesosilicate formed of Al
2
SiO
5
slabs the structure of which
is identical to those of kyanite, alternating with iron-layers of approximate
composition AlFe
2
O
2
(OH)
2
. This analogy with the structure of kyanite
makes that these minerals occur quite often in epitaxic association. The alu-
minum sites are not fully occupied. Thus the precise formula of staurolite is
not exactly known:
(R
2+
)
2
(R
3+
)
about 9
Si
about 4
O
22
(OH)
about 4
<
where R
2+
=
Fe
2+
, Mg, Zn, (Co, Li)
R
3+
=
Al, (Fe
3+
, Ti, Cr)
Staurolite contains mostly iron as R
2+
, with moderate substitution of
iron by magnesium. It contains very commonly zinc in small quantities. Sub-
stitutions of Al are very small. So the simplified formula of staurolite is:
(Fe, Mg, Zn)
2
Al
9
Si
4
O
22
(OH)
2
Staurolite is typically a mineral of medium grade metamorphism: its
stability domain ranges from about (500)-550°C to 650-700°C. At higher
temperatures it is destabilized into garnet (almandine)
+
aluminum sili-
cate
water. Towards lower temperatures, the reaction of formation of
staurolite varies with initial parageneses (an example will be developed
+
