Chemistry Reference
In-Depth Information
3.3.2.4 Sedimentary rocks and alterites
Andalusite, kyanite, staurolite, garnet, topaz and corundum are rather inal-
terable and of high density minerals.They are concentrated in the clastic
alluvial and beach (intertidal) sediments (black sands). Placers are also the
main source of rubies and sapphires.
Diaspore, boehmite and gibbsite are the main constituents of bauxites.
3.4 CALCIC, MAGNESIAN AND CALC-
MAGNESIAN MINERALS
These calcic, magnesian and calc-magnesian minerals are the major miner-
als of calcic and magnesian carbonate rocks, metamorphic rocks (impure
marble, calc-silicate-gneiss) and metasomatic rocks (skarns). These miner-
als also occur in calcic and magnesian basic metamorphic rocks of igneous
origin. Some are rare primary minerals of alkaline igneous rocks. The tra-
ditional mineralogical classification ranges the various minerals discussed
here in various classes. The similarities in chemical composition and the fact
that these minerals are found associated in their occurrences, justify their
regrouping here.
3.4.1 Chemical composition and stability
These minerals are here classified according to their chemical composition
(Table 3.2 and Figure 3.35):
carbonates,
calcic aluminous silicates,
calcic non-aluminous silicates,
non-aluminous magnesian and calcic-magnesian silicates,
calcic ferro-magnesians silicates,
minerals of calcium and titanium: titanite and perovskite (in part 3-5).
This classification is somewhat arbitrary. Indeed, some minerals or
mineral groups, such as calcic garnets or melilite, can be classified in sev-
eral of these groups. Nevertheless, this classification allows us to link the
composition of these minerals with the one their host rock and thus their
occurrences.
3.4.1.1 Carbonates
Common carbonates belong to two groups: the trigonal carbonates and
orthorhombic carbonates.
