Chemistry Reference
In-Depth Information
stishovite
P kbars
tetragonal
80
70
40
Coesite
monocliniic
30
20
10
α−
quartz
β−
quartz
trigonal
hexagonal
0
0
573
867
1470
1713
T°C
tridymite
hexagonal
(low T orthorthombic)
Cristobalite
cubic
(low T tetragonal)
Figure 3.1
Polymorphs of silica (from Tuttle and Bowen, 1958, modified).
(diatoms, radiolarians, siliceous sponges) and in the sediments derived from
them. Cristobalite is unstable at room temperature, opal tends to transform
into fibrous quartz (chalcedony and lutecite); this transformation is very
slow.
The solubility of cristobalite in the surface waters is 120 ppm; the one
of quartz is 7 ppm, it is therefore the main source of silica in solution. The
lutecite seems crystallized directly from water at high pH, particularly in an
evaporitic environment.
In
hydrothermal rocks
, opal precipitates from waters at temperatures
lower than 100-150°C. At higher temperatures it is chalcedony or quartz
(
-quartz) that precipitates. In the veins and particularly ore veins, quartz
often occurs in elongated prismatic crystals that tend to grow perpendicular
to the walls (comb structure).
In most
metamorphic rocks
the stable form of silica is quartz:
α
α
-quartz
or
-quartz during the fallout
of temperature. In the ultra-high pressure facies (white schist), the stable
form is coesite; it largely tends towards retrograde quartz at the pressure
drop.
Coesite and stishovite are found in meteorite impacts, and, for coesite,
atomic bombs impacts. Coesite also occurs in the enclaves uplifted by mag-
mas of very ddep origin, especially in kimberlites.
The conditions of formation of
plutonic rocks
are the same as that
of metamorphic rocks (possibly with higher temperatures: 650-1100°).
Quartz crystallizes when the silica content of the rock is high enough, this
limit depends on the composition of the rock: it is about 50 wt% in meso-
β
-quartz. The latter is transformed into
α
