Chemistry Reference
In-Depth Information
Pectolite
NaCa
2
Si
3
O
8
(OH)
2
(triclinic) is an inosilicate with a structure simi-
lar to that of wollastonite. Mn may partially replace calcium, Mg, Fe as sub-
stitution to calcium, Al as substitution to Si may also be present. It has been
synthetized at a temperature of 180°. So it is mostly a hydrothermal mineral (but
may also occur as a primary mineral in skarns and rare alkaline igneous rocks).
Charoite
K
5
Ca
8
(Si
6
O
15
)
2
(Si
2
O
7
)Si
4
O
9
(OH) · 3H
2
O (=K(Ca, Na)
2
Si
4
O
10
(OH, F) · H
2
O) (monoclinic) is a very rare inosilicate, known only in the
Murunskii massif (Yakutia, Russia). It has a quite unique structure of a
3-periodic single and multiple chains. The only justification to show it in the
CD is its strange beauty.
3.4.1.4 Magnesian, non-aluminous silicates (oxides
and hydroxides) (anhydrous and hydrated;
saturated and under-saturated in silica)
These are minerals of metamorphism of siliceous dolomitic rocks and of
skarns developed on dolostones. Monticellite and humites are also of pri-
mary minerals of alkaline ultramafic rocks. Talc and brucite are also prod-
ucts of alteration of olivine and serpentine.
This group includes:
peridots and minerals close to them: forsterite Mg
2
SiO
4
, monticellite
CaMgSiO
4
, humites group(1-4) Mg
2
SiO
4
· Mg(OH,F)
2;
diopside CaMgSi
2
O
6;
merwinite Ca
3
MgSi
2
O
6
, very rare mineral of high temperature (not dis-
cussed here);
talc, hydrated phyllosilicate Mg
6
(Si
8
O
20
)(OH)
4;
periclase MgO and brucite Mg(OH)
2.
Clintonite and xantophyllite Ca
2
(Mg, Al)
6
Si
1,5
Al
5,5
O
20
(OH)
4
, are mag-
nesian (and aluminous) brittle micas (§3.2.1.7) which have the same occur-
rences as the previous minerals.
Neither enstatite MgSiO
3
nor anthophyllite Mg
5
Si
8
O
22
(OH)
2
occur
in this type of deposits because they are replaced in presence of calcite by
forsterite
+
diopside.
3MgSiO
3
+
CaCO
3
CaMgSi
2
O
6
+
Mg
2
SiO
4
+
CO
2
Forsterite and diopside have been previously discussed with olivines and
pyroxenes.
The composition of
monticellite
CaMgSiO
4
does not vary very much as
the substitution of Mg by Fe or Mn is very limited. Monticellite is a mineral
of high temperature, formed at temperatures higher than diopside, forsterite
and wollastonite (650-850°C depending f
CO2
). At higher temperatures, it is
