Chemistry Reference
In-Depth Information
An example of the latter group is given by some alkaline granites (for
instance in Corsica), where the under-saturation in aluminum is marked
by the presence of wollastonite instead of anorthite. Some phonolites and
ijolites (igneous rocks with nepheline
+
augite) containing wollastonite are
other examples.
Melilite, monticellite, titaniferous (and zirconian) garnet, titaniferous
humites, perovskite (CaTiO
3
, discussed with accessory minerals, §3.5.4) and
carbonates are more or less important components of rocks strongly under-
saturated in silica: alkaline volcanic and hypabyssal rocks, and their plutonic
equivalents (when they exist). These alkaline rocks are classified into:
Sodic rocks: nephelinites;
Potassic rocks: leucitites, mafurites (rocks with kalsilite KAlSiO
4
);
Potassic rocks rich in biotite: kimberlites, alnöites (lamprophyres with
melilite), lamproites;
Calcic rocks: mélilitites (lavas containing more than 10% of melilite)
and carbonatites.
Carbonatites are plutonic and volcanic rocks made mainly of carbonates:
calcite (coarse-grained sövite and fine-grained alvikite), dolomite (beforsite),
iron-bearing carbonates. Lavas made of sodium carbonate (natrocarbonatite)
are known but, as this mineral is soluble in water, these rocks are not preserved.
Garnets of the andradite-melanite (Ti-andradite) series also occur in
nepheline syenite and some phonolites. Vesuvianite, pectolite and calcite
are also constituents of nepheline syenite. Pectolite also occurs in some rare
mica peridotite.
Calcite, aragonite associated with zeolites, pectolite (associated with
datolite, presented in §3.5.2.3) may crystallize in vesicules of basalts, andes-
ites and their hypabyssal equivalents.
3.4.2.4 Hydrothermal veins
Carbonates (calcite, dolomite, ankerite, siderite, rhodocrosite) frequently
appear as gangue minerals in ore veins (or massive orebodies). Some, such
as siderite, may be mined.
Epidote, prehnite (with quartz, albite, carbonate, etc.) form of low tem-
perature hydrothermal veins in crystalline terranes (a famous example are
the alpine clefts).
The aragonite may be formed in the hot springs.
3.4.2.5 Alteration minerals
Saussurite is an alteration of calcic plagioclase into epidote (
+
albite)
(§3.1.2.3).
