Chemistry Reference
In-Depth Information
greater or lesser alterability, ability to endure a more or less long trans-
portation, which is a character linked to their hardness and their brit-
tleness (presence of cleavage) and of course of their frequency and
abundance in potential source areas. Sedimentary sorting separates the
fine fraction, mostly clay (mudstone, shale) from the coarser clastic
fraction (mainly quartz but also feldspar, mica). An extreme example
of sedimentary sorting is given by the beach or fluvial “black sands”
that may be of economic interest for their deposits and that concentrate
“heavy minerals”, like garnet, kyanite, zircon, monazite, magnetite,
chromite, ilmenite, cassiterite, gold, platinum, diamond, ruby, sapphire.
Some clastic rocks contain a more or less important proportion of car-
bonates, either as clasts, cement or matrix.
Chemical and biochemical rocks have much more particular compo-
sitions. Chemical rocks form from direct precipitation (from seawater,
brackish or freshwater): some carbonate rocks (dolostone), evaporites
(gypsum, rock salt, potash), some siliceous rocks (chert, flint), (siliceous
and carbonated) oolitic ironstones and manganese ores. Biochemical
rocks are the result of biochemical activity of living organisms (cor-
als, molluscs, foraminifera, stromatolites): many carbonate rocks (lime-
stone), some siliceous rocks (radiolarite, diatomite) and carbonaceous
rocks (coal, oil).
The size of the grain of sedimentary rocks is often too small for
their mineralogy to be studied under the petrographic microscope. Their
study then requires X-ray diffraction.
3
Magmatic or igneous rocks
are the result of the cooling and solidification
(most times crystallization) of a magma (most often a silicate magma).
They are classified according to their level of emplacement in the Earth's
crust: extrusive or
volcanic rocks
,
hypabyssal rocks
and intrusive or
plu-
tonic rocks
. A second classification is done by the relative proportion of
dark- (ferromagnesian minerals, oxides) to light colored (quartz, feld-
spar, feldspathoids) minerals, expressed as color index. Ultramafic rocks
are rocks containing more than 90% of colored minerals.
Chemical terms used to designate the igneous rocks are:
-
acidic rocks
with a SiO
2
content higher than 66 wt%); such rocks
are also riche in alkalis Na, K, and often in iron (sometimes ferric
iron) and hygromagmaphiles elements (F, B, Zr, Nb, Sn, W); acid
rocks are often felsic rocks, which is a different notion referring to
their mineralogy and not to their chemistry;
- intermediate rocks,
with a SiO
2
content comprised between 52 and
66 wt%;
- basic rocks
(SiO
2
between 45 and 52 wt%);
- ultrabasic rocks
(SiO
2
<
45%).
The notions of ultramafic and ultrabasic rocks are distinct: an ultrama-
fic rock is not necessarily ultrabasic and vice versa.
