Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 387

Box 9.2.1 Minerals

Silicate minerals are broadly classifi ed as either tectosilicates (also known as frame-
work silicates) where each SiO 4 (or AlO 4 ) tetrahedron is linked to four neighboring
tetrahedra in a three-dimensional network, or phyllosilicates (also known as sheet sili-
cates, most of which are clay minerals) where each SiO 4 tetrahedron is linked to three
neighboring tetrahedra to form a two-dimensional sheet.
The tectosilicates include quartz, composed only of SiO 4 tetrahedra, and feld-
spars, composed of both SiO 4 and AlO 4 tetrahedra. In feldspars, the charge defi cit
caused by the presence of Al instead of Si is balanced by structural K + , Na + , or Ca 2 +
ions. Feldspars that are fully balanced by these ions are known as K-feldspar, albite,
and anorthite, respectively. Most feldspars are either solid solutions of K-feldspar and
albite (or alkali feldspar ) or solid solutions of Na-feldspars and Ca-feldspars (or
plagioclase ).
The phyllosilicates are based on a structural motif in which a sheet comprised of
AlO 6 , FeO 6 or MgO 6 octahedra is bound to a sheet of SiO 4 tetrahedra (the so-called
1:1 structure found particularly in kaolinite) or sandwiched between two sheets of SiO 4
tetrahedra (the so-called 2:1 structure of mica, illite, smectite, and most other phyllo-
silicates). Most 2:1 structure phyllosilicates have a negative structural charge (caused
by isomorphic substitutions of tetrahedral Si or octahedral Al, Fe, or Mg by cations of
lower valence) that is balanced by loosely bound interlayer cations (Na + , K + , Ca 2 + , or
Mg 2 + ). Because of their lamellar structure, phyllosilicates tend to form stacks where
the interlayer space is either inaccessible to water (as in the case of kaolinite, illite, and
mica) or can be propped open by the solvation of interlayer cations (as in the case of
smectite, a swelling clay ).
Several observations relevant to carbon sequestration can be made regarding the
silicate minerals in Figure 9.2.3. First, the weathering rates of silicate minerals tend to
decrease with increasing Si:O ratio (hence among the tectosilicates weathering rates
increase in the order quartz < alkali feldspar < plagioclase) and increasing connectivity
of the SiO 2 network (hence tectosilicates tend to weather more slowly than phyllosili-
cates). Second, most phyllosilicates (except mica) are clay minerals and they have
very small particle sizes. For example, smectite crystals are fl ake-shaped nanoparti-
cles with a thickness of 0.94 nm and a diameter on the order of 200 nm (specifi c sur-
face area a s 800 m 2 /g), whereas quartz grains are roughly spherical with a diameter ≈

0.02 m 2 /g). Because of their small crystal size,
clay minerals can strongly infl uence the properties of porous media, for example, by
forming surface coatings on larger minerals.

of ∼ 10 µ m (specifi c surface area a s ≈

As tectonic forces continue to act on the rock formations over geo-
logical time, folding, faulting, and fracturing of the sediments may occur
on large and small scales, giving the once-sub-horizontal sediments
upward and downward-arching structures (anticlines and synclines,