Geology Reference
In-Depth Information
9.2.3 Stage III: mineralisation and rock formation
The third stage is subdivided into two processes: mineralisation and rock forma-
tion. The former is a chemical process in which the molecules combine to form the
mineral. The energy involved in such processes is quite variable but in general lower
than the formation enthalpies of their corresponding oxides.
A typical reaction for this stage would be:
CaO(s) + SiO 2 (s) ! CaSiO 3 , H 0 = 88:8 kJ/mol
This stage also includes the processes of solid phase transitions, where the en-
thalpies of phase change are within the range of 0.5 J/mol and the associated Gibbs
free energies are zero.
A rock is seldom formed as a conglomerate of pure minerals. There often occurs
crystalline defects and composition variations, especially in silicates. The final solid
solution becomes an impure crystal with ions similar in size and electronegativity,
replacing and randomly occupying positions in the structure. For example OH
substitutes F in amphiboles, micas and gypsum.This also occurs with Mg 2+ re-
placing Ca 2+ in carbonates. The energy involved in the solid formation process of
a mineral containing dopant irregular ions is considerably less than if it were 100%
pure. Obviously, at the same time its entropy is higher, as discussed below. How-
ever, this entropic generation is much smaller than that for the thermal exchanges
associated with the formation of the compound, increases in temperature generally
or phase changes.
9.2.4 Stage IV: formation of a mineral deposit
Mineral deposits are characterised by concentrated mineral ores mixed with other
rocks found in the Earth's crust. From a thermodynamic point of view the cohesion
energy between the ores and the rocks is equal to that needed to separate them, i.e.
comminution exergy (see Sec. 9.5.2.3).
The Earth's crust is the result of solidification of the lighter components of
magma. Its basement rock is made up of mixtures of very old granite, gneiss,
schist, sedimentary and volcanic rocks between 10 to 70 km thick (Pidwirny, 2011).
The majority of solid planetary materials can be classified as ceramics with silicates
present in most rocks. Silicates are brittle with large lattice resistances (Spray,
2010), however tectonic compression and meteorisation naturally comminute them
into greater ductile mixtures and generate cohesion forces. Such forces among solids
highly depend on the mineral deposit and can range from hydrogen, hydration,
ionic and covalent bonds. The energy required to break any of these is several
orders of magnitude lower than the mineral formation enthalpy as explained below
in Sec. 9.5.2.3.
 
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