Geoscience Reference
In-Depth Information
1.1 Subsurface Solid Phase
The solid phase of the subsurface is a porous medium composed of a mixture of
inorganic and organic natural materials in various stages of development. The
surface area and the surface (chemical) properties of the solid phase are major
factors that control the behavior of chemicals.
Based on their origin, minerals are grouped into two broad classes: primary
minerals and secondary minerals. Primary minerals have not been altered chemi-
cally since the time of their crystallization from molten lava and their subsequent
deposition. This group includes quartz (SiO
2
), feldspars (e.g., (Na,K)AlSi
3
O
8
; i.e.,
aluminosilicates containing varying amounts of calcium, potassium, or sodium),
micas (e.g., muscovite: KAl
2
(AlSi
3
O
10
)(OH)
2
and/or chlorites: (Mg,Fe)
3
(Si,Al)
4
O
10
(OH)
2
-(Mg,Fe)
3
(OH)
6
, which belong to the phyllosilicate mineral group),
amphibole (e.g., hornblende: (Ca,Na)
2,3
(Mg,Fe,Al)
5
Si
6
(Si,Al)
2
O
22
(OH)
2
, i.e.,
magnesium-iron silicates, often with traces of calcium, aluminum, sodium,
titanium, and other elements), pyroxenes (e.g., augite: (Ca,Na)(Mg,Fe,Al)
(Si,Al)
2
O
6
, i.e., silicate minerals rich in calcium, iron, and magnesium and com-
monly found in basalt), olivine (magnesium-iron silicate (Mg,Fe)
2
SiO
4
) and rutile
(TiO
2
). Secondary minerals originate from the decomposition of primary minerals
and subsequent reprecipitation into new, chemically distinct minerals. This group
contains many minerals, including kaolinite, smectite, vermiculite, chlorite, imog-
olite, gibbsite, goethite, hematite, birnessite, calcite, and gypsum. Layer alumino-
silicates are the dominant minerals formed in most temperate region soils. These
layer silicates are composed of various arrangements of silicon/oxygen sheets in
tetrahedral coordination and aluminum/oxygen sheets in octahedral coordination.
Primary minerals with low surface area (e.g., silica minerals) and low reactivity
mainly affect the physical transport of water, dissolved chemicals, colloids,
immiscible (in water) liquids, and vapors. Secondary minerals generally have high
surface area (e.g., clay minerals) and high reactivity that affect the transport of
chemicals, their retention and release onto and from the solid phase, and their
surface-induced transformations. The solid phase also can indirectly induce the
degradation of chemical compounds, through its effects on the water-air ratio in
the system and, thus, on microbiological activity.
1.1.1 Silica Minerals
Silica minerals are a primary mineral classified as tectosilicates, characterized by
repeating SiO
4
units in a frame-like structure. Quartz, one of the most abundant
minerals on earth, often comprises up to 95 % of all sand and silt fractions. It
therefore is representative of the structure and properties of silica minerals.
Wilding et al. (
1977
) showed that the silica tetrahedron in quartz is almost
symmetrical and has a Si-O distance of 0.16 nm. They noted that the structure of
