Chemistry Reference
In-Depth Information
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scapolite group, that, due to their occurrences, will be treated in this
topic with the calcic minerals;
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zeolites group (not treated here).
3.1.1 Silica group
There are six natural mineral with SiO
2
formula, three of them have high
and low temperature polymorphs:
Quartz
: trigonal
α
-quartz is stable up to 573°C; hexagonal
β
-quartz is
stable from 573°C to 870°C.
Chalcedony is a fibrous habit of low temperature quartz. The fibers are
elongated in the c direction.
Quartzine
has a positive elongation and
chal-
cedony
a negative one.
Lutecite
is a fibrous habit of low temperature silica
in which the fibers are elongated obliquely on the c axis.
Tridymite
: orthorhombic
α
-tridymite is an unstable polymorph that can
exist up to 117°C; hexagonal
-tridymite is unstable from 117°C-870°C
and stable from 870°C to 1470°C.
Cristobalite
: quadratic
β
-cristobalite, there may exist up to about
200-275°C. It is unstable at these temperatures. Cubic
α
-cristobalite exists
above 200-275°C. It is stable only at temperatures above 1470°C, its melt-
ing point is 1713°C.
Coesite
is a monoclinic polymorph, stable at pressures exceeding 28 to
30 kb.
Stishovite
is a quadratic polymorph of ultra-high pressure.
Moganite
(Florke et al., 1976, 1984, approved in 1999) is a mono-
clinic pseudohexagonal polymorph of silica of low-temperature. Moganite
is metastable and recrystallizes into quartz.
Quartz is, after the feldspars, the most common mineral in the Earth's
crust forming 10-15% of it. It occurs in all families of rocks and its stability
field covers almost the entire field of the geological phenomena
In
sedimentary rocks
:
Because of its mechanical and chemical resistance, quartz is a abun-
dant clastic mineral. Sedimentary sorting, especially concentrates quartz, in
sands and sandstone of various types. In these rocks, quartz can be cor-
roded or otherwise preexisting grains can be nourished by secondary quartz.
The quartz is also a common cement in sedimentary rocks. The newly
formed quartz is common, either as euhedral crystals (authigenic quartz)
or as fibrous aggregates (chalcedony). Some siliceous rocks such as chert,
buhrstone (= “meulière”), etc. consist of aggregates of chalcedony that leave
besides abundant micropores.
Opal is a form of silica that is either micro-crypto-crystalline, cristobal-
lite (possibly with tridymite) (C opal) or aggregates of colloidal silica (opal
A), that contains up to 20 wt% of water and the bulk composition SiO
2
, n
H
2
O. Opal is the form of silica that occurs in the skeleton of living creatures
β
