ratio is therefore 4 : 11, giving a general formula Si 4 O 11. The amphibole
group of minerals has double-chain structure—for example, tremolite
(Ca 2 Mg 5 Si 8 O 22 (OH) 2 ).Sheet silicates
The next step in polymerization is to cross-link chains into a continuous, semi-
covalently bonded sheet, such that every tetrahedron shares three oxygens with
neighbouring tetrahedra (Fig. 4.4e). This structure has one non-bridging oxygen
and the overall Si : O ratio is 4 : 10, giving a general formula Si 4 O 10. The hexag-
onal rings formed by the cross-linkage of chains are able to accommodate addi-
tional anions, usually hydroxide (OH-). This structure is the basic framework for
the mica group—for example, muscovite (Mg 3 (Si 4 O 10 )(OH) 4 )—and all of the clay
minerals. These minerals are thus stacks of sheets, giving rise to their ‘platy’
appearance.Framework silicates
In this class of silicates every tetrahedral oxygen is shared between two tetrahe-
dra, forming a three-dimensional semicovalent network. There are no non-bridg-
ing oxygens, the overall Si : O ratio being 1 : 2, as in the simplest mineral formula
of the class, quartz (SiO 2 ). Substitution of aluminium into some of the tetrahe-
dral sites (the ionic radius of aluminium is just small enough to fit) gives rise to
a huge variety of aluminosilicate minerals, including the feldspar group, the most
abundant mineral group in the crust. Substituting tetravalent silicon for trivalent
aluminium causes a charge imbalance in the structure, which is neutralized by
the incorporation of other divalent or monovalent cations. For example, in the
feldspar orthoclase (KAlSi 3 O 8 ), one in four tetrahedral sites is occupied by alu-
minium in place of silicon. The charge is balanced by the incorporation of one
K+for each tetrahedral aluminium.4.3 Weathering processes
The surface of the continental crust is exposed to the atmosphere, making it vul-
nerable to physical, biological and chemical processes. Physical weathering is a
mechanical process which fragments rock into smaller particles without substan-
tial change in chemical composition. When the confining pressure of the crust is
removed by uplift and erosion, internal stresses within the underlying rocks are
removed, allowing expansion cracks to open. These cracks may then be prised
apart by thermal expansion (caused by diurnal fluctuations in temperature), by
the expansion of water upon freezing and by the action of plant roots. Other
physical processes, for example glacial activity, landslides and sandblasting,
further weaken and break up solid rock. These processes are important because
they vastly increase the surface area of rock material exposed to the agents of
chemical weathering, i.e. air and water.76 Chapter Four