9: Oxide Minerals

a general term referring to silicon dioxide ($\text{SiO}_2$) in its various forms

quartz
opals

the most abundant and ubiquitous silica mineral
primary: polymorphs crystallized from cooling magmas
secondary: coarsly crystalline or microcrystalline, crystallize from supersaturated $\text{H}_4 \text{SiO}_4$ solutions

hydrated $\text{SiO}_2$ of variable crystallinity, often found in soil environments
form in aqueous environments such as soil solution, lakes, etc

almost no heterovalent isomorphic substitution; low amounts of silanol groups => small contribution

gibbsite is almost the only aluminum mineral appearing in an important extent in soils
thermodynamically should weather to diaspore, but kinetics are slow since it involves loss of water and change from monoclinic to orthorhombic structure

the thermodynamic reaction coefficient for solution reactions, e.g., $K_{SP} \text{(gibbsite)} = K_{s0} = a_{\text{Al}^{3+}} \left( a_{\text{OH}^-} \right)^3$

$SI = \log{I_{SP}} - \log{K_{SP}} = \log{a^0_{\text{Al(OH)}_3 \text{(aq)}}} - \log{K_{s0}}$, used to evaluate the thermodynamic tendency to precipitate of any solid phase

$I_{SP}=a^0_{\text{Al}^{3+}} \left( a^0_{\text{OH}^-} \right)^3$ <- e.g., for gibbsite in equilibrium with free ions

high surface concentration of aluminol groups =>
- acid-base reactivity => dependence of surface charge on pH
- can specifically bind both anions and cations

the minerals widely are common and widely distributed in soils, having properties with an important impact on soil, such as their redox behavior, active surface chemistry, pigmenting power, etc.

goethite ($\alpha \text{-FeOOH}$): most common & most thermodynamically stable, octahedral Fe(III) with O-OH hydrogen bonding => open structure
hematite ($\alpha \text{-Fe}_2 \text{O}_3$): denser than ^, octahedra are arranged similarly to phyllosilicate dioctahedral sheets

- isomorphic substitution is compensated internally (e.g., by change in number of vacancies) => no permanent charge
- surface contain OH groups yielding acid-base behavior, thus strong pH dependence of surface charge

- inner-sphere complex formation, preferentially edge or corner binding as opposed to basal plane binding, eventually forming small polynuclear complexes or surface polymers but more rarely surface precipitates
- ^true for transition metal and p-block representative cations; alkaline metals and alkali earth metals normally bind electrostatically