Chemistry Reference
In-Depth Information
replaced by åkermanite (wollastonite
+
monticellite
åkermanite at 700°C),
spurrite, merwinite.
Humites group
includes four minerals which are very similar in struc-
ture, chemical composition, optical properties and occurrences:
Norbergite
Mg
2
SiO
4
· Mg(OH, F)
2
orthorhombic
Chondrodite
2Mg
2
SiO
4
· Mg(OH, F)
2
monoclinic
Humite
3Mg
2
SiO
4
· Mg(OH, F)
2
orthorhombic
Clinohumite
4Mg
2
SiO
4
· Mg(OH, F)
2
monoclinic
Fe, Mn, Zn may replace Mg in very small quantities. Titanium (Ti
4+
)
may enter as a substitution to magnesium (Mg
2+
) in Mg(OH)
2
groups, the
electrical neutrality being maintained by the substitution of OH
-
by O
2-
.
Humites are medium to high temperature minerals. The association
humite
forsterite/tremolite
or talc associations. The stability of these associations depends on the relative
pressures of CO
2
and H
2
O. Their appearance, of course, depends of fluorine.
Talc
Mg
6
(Si
8
O
20
)(OH)
4
is a phyllosilicate whose structure is similar to
that of the TOT sheets of trioctahedral micas: two layers of SiO
4
4-
tetrahedra
sandwiching an octahedral layer which sites are occupied by Mg. Unlike
mica there is no interlayer site. The connection between the layers is a very
weak bonding of the Van der Waals type. This explains the cleavage of talc,
its very low hardness (1 by definition in the Mohs scale) and greasy touch.
The substitutions Mg Fe and Mg Si Al
IV
Al
VI
are very limited.
Talc is a mineral of low to medium temperature. Its stability towards
higher temperatures, controlled by the water pressure is limited by the
appearance of anthophyllite and enstatite.
Periclase
MgO (cubic mineral, magnesium may be replaced by Fe in
very small quantities) is a mineral from high temperature metamorphism of
dolostones. It is formed at higher temperatures than wollastonite, but lower
than monticellite.
Periclase is generally destabilized into
brucite
Mg(OH)
2
(with very little
substitution of Mg by Fe), a fibrous mineral, trigonal, with a layered struc-
ture. Brucite is altered into hydromagnesite 3MgCO
3
Mg(OH)
2
3H
2
O.
+
calcite is formed at the expense of dolomite
+
3.4.1.5 Ferro-magnesian calcic silicates
These minerals share the same occurrences as other minerals presented in
this chapter; have been treated previously. The most important are amphi-
bole and pyroxene:
-
Calcic amphiboles
- non aluminous
tremolite-actinolite
-
ferroactinolite series
Ca
2
(Fe, Mg)
5
Si
8
O
22
(OH)
2
