Geoscience Reference
In-Depth Information
10
Mineral reactions
In this chapter we take a look at the chemical and mineralogical changes accompanying the
formation of sedimentary and metamorphic rocks. Marine mud is consolidated into sedi-
ment through early diagenetic reactions. These sediments and the other rocks that form
the bedrock may be affected by percolating hot water, mineralized to varying degrees,
and acids. This is hydrothermal metamorphism. When rock is dragged deep down by sub-
duction and thereby heated and dehydrated, it is transformed, producing a great variety
of different mineral assemblages. This process is termed “metamorphism.” Some of these
thermal processes concern the transformation of organic matter, whose ultimate products
are the fossil fuels such as natural gas, petroleum, and coal.
The principal geochemical issues raised by these processes are to identify the nature
of the rock before its transformations, the physical conditions (temperature and pressure)
of the transformations, and the nature and intensity of exchanges between the trans-
formed rocks and the interstitial solutions. Once again the essential analytical tool is
thermodynamics and readers may wish to refresh their knowledge of this by referring to
Transformations are often controlled by water pressure and temperature. Let us take the
example of the important reaction whereby muscovite (white mica) disappears from gneiss
and schist, and which characterizes the entry of metamorphic rocks into granulite facies:
KAl
3
Si
3
O
10
(OH)
2
+
⇔
+
+
SiO
2
KAlSi
3
O
8
Al
2
SiO
5
H
2
O
(muscovite)
(quartz)
(K-feldspar)
(sillimanite)
(10.1)
