Silicon is the second most abundant element (after oxygen) on earth, and makes 25.7% of the earth’s crust, mainly in the form of SiO2 (=silica) or SiO4 (=silicate). But, which are the element-mineral links of Si?
Silicon (symbol Si; atomic number 14, relative atomic mass 28.085) [Link to webelements.com]
Si has three main oxidation states, -4, +2, and +4, of which +4 is the most common.
Common element-mineral links of Si
The common mineral associated with Si is quartz, but it is also part of any other silicate mineral.
Si in siliciclastic rocks
Si is also the most abundant element in siliciclastic sedimentary rocks, mainly in quartz (SiO2), but also as a principal element in silicates such as of the feldspar, mica, and clay mineral groups, as well as in accessory minerals such as zircon, tourmaline, garnets, and spinel.
Quartz is one of the most resistant minerals against chemical alteration during weathering and diagenetic processes, and thus one of the residual minerals in weathered rocks and soil profiles.
Geochemical analyses, such as ICP, XRF, and LIBS cannot differentiate between quartz (detrital), amorphous silicates such as chert, opal, agate, chalcedony, which form during diagenetic processes, authigenic quartz cement, and biogenetic silica, e.g., spiculae, diatoms, or radiolaria.
SiO2 as a lithology indicator in siliciclastic rocks
SiO2 concentrations can to some degree be used for a lithology estimation of siliciclastic sediments, though concentrations can be diluted by non-silicate minerals such as calcite, dolomite, anhydrite, etc. (e.g., in form of cement), or enriched by biogenetic or authigenic silica. In general (rule of thumb):
| SiO2 [%] | Lithology |
|---|
| ~35 to ~55 | claystone and ‘mudrocks’ |
| ~55 to ~70 | siltstone and argillaceous sandstone |
| ~70 to ~90 | sandstone |
| ~90 to 100 | quartz sandstone (quartz arenite) |
Table 1: SiO2 concentrations in siliciclastic rocks.Very high SiO2 concentrations in sandstones i.e., > 95 %, indicate a high mineralogical maturity, as quartz is one of the most stable minerals during sedimentary processes and thus gets relatively enriched compared to less stable minerals. High mineralogical maturities may be used to estimate depositional environments, such as well sorted tidal bars or glaciogenic sands, as well as sandstones that have been reworked and recycled (often multiple times).
Silica combined with aluminum, expressed in the SiO2/Al2O3 ratio, can give further insights into the likely lithology of siliciclastic sediments. As a rule of thumb:
| SiO2/Al2O3 | Lithology |
|---|
| < 3 | claystone |
| < 5 | claystone and ‘mudrocks’ |
| 5-7 | siltstone and argillaceous sandstone |
| > 7 | sandstone |
| > 10 | quartz sandstone (quartz arenite) |
Table 2: SiO2/Al2O3 ratios in siliciclastic rocks.Si in carbonate minerals
Si concentrations in carbonate and evaporite sediments are commonly low. Together with other elements, such as Al and Ti, it is however a good indicator for terrigenous input, and thus of value for chemostratigraphic interpretations. Si can, however, get enriched in stylolites (together with Al, K, and Fe; Hassan, 2007).
Si in igneous rocks
In igneous rocks, Si is associated with silicates, such as mainly quartz and feldspars, and olivine, pyroxene, amphibole, and biotite.
Based on the SiO2content, igneous rocks are geochemically differentiated into:
| SiO2 | Classification |
|---|
| > 63% SiO2 | acidic |
| 55-63% SiO2 | intermediate |
| 45-55% SiO2 | basic |
| < 45% SiO2 | ultrabasic |
Table 3: Geochemical differentiation of igneous rocks based on silica content.