Chemistry Reference
In-Depth Information
sanidine
nepheline; that explains its absence in evolved rocks. Sometimes
early crystallized leucite is destabilized in a very fine aggregate of sani-
dine
+
nepheline which retains its crystalline forms (pseudo-leucite). Leucite
is unstable under high pressure, which explains its absence in the plutonic
rocks.
Kalsilite is a very rare mineral of ultrabasic potassic volcanic rocks.
Cancrinite is essentially a secondary mineral formed on nepheline.
Nepheline, sodalite (and cancrinite) appear by metasomatism (fenitiza-
tion) of very varied rocks (gneiss, amphibolite, limestone) around alkaline
intrusions.
Analcime occurs in the matrix of basic to intermediate volcanic rocks.
It may be primary, but rather comes from devitrification of glass. It also
appears in the amygdala, bubbles, vesicles of these rocks; it is often associ-
ated with zeolites (and calcite, prehnite, axinite, apophyllite, etc.).
Analcime appears as an authigenic mineral in clastic sedimentary rocks,
shales (especially lacustrine argillites), silts and sometimes impure sand-
stones. Some of these rocks contain volcanic material (glass shards, ash,
tuffs), but analcime remains a relatively minor component in them. There
are rocks with no trace of such a volcanic contribution where analcime
appears in a significant proportion (analcimolites) over areas of significant
extension and thicknesses (up to 20 m). The fact that analcime is present in
such rocks, in the presence of quartz, indicates very little diagenetic evolu-
tion. The presence of dolomite and magnesian chlorites suggests a chemical
sedimentation.
Analcime
+
quartz association is characteristic of a very low grade of
metamorphism (early zeolite facies). At higher metamorphic grade analcime
is replaced by albite.
+
3.2 MAJOR FERRO-MAGNESIAN MINERALS:
MICAS, CHLORITES, AMPHIBOLES,
PYROXENES, OLIVINES, SERPENTINES
3.2.1
Micas and related minerals
3.2.1.1
Structure and chemical composition
Micas are phyllosilicates whose TOT-type sheets are made of two tetra-
hedral layers sandwiching an octahedral layer (Figure 3.11). The tetra-
hedral layers consist of a hexagonal pavement of tetrahedra (SiO
4
)
4−
in
which each tetrahedron shares three apexes with the neighboring tet-
rahedra: the chemical composition of such layers is (Si
4
O
10
)
4−
. In each
sheet, the tetrahedra of the upper tetrahedral layer point downwards,
and the ones of the lower tetrahedral sheet point upwards. This pattern
