Aluminium (aluminum) is the third most abundant element on earth and makes 8.1% of the earth’s crust. It is a major constituent of many rock-forming minerals. So, what are the element-mineral links of Al?
Aluminium (symbol Al; atomic number 13; relative atomic mass 26.9815385) [Link to webelements.com]
It has one oxidation state, +3.
Aluminium vs. aluminum
British vs. American English
Just to avoid confusion, it is ‘aluminium’ in British English, but ‘aluminum’ (i.e. without ‘i’ before ‘um’) in American English.
Geochemical analyses often report aluminium (Al) in its oxide form as Al2O3.
Common element-mineral links of Al
The most common element-mineral links: mainly clay-minerals, particularly kaolinite and other aluminosilicates, such as micas and feldspars. In addition, it is a major constituent in pyroxenes, amphiboles, and garnets.
On the other hand, aluminum forms several minerals on its own, such as corundum (Al2O3) and sillimanite (Al2SiO5).
Aluminium and gallium (Ga), both in Group 13 in the Periodic System of Elements, have similar geochemical behaviors, and Ga can substitute for Al (and/or Fe) in feldspar, mica, clay minerals, and amphibole.
Al in siliciclastic rocks
Like silicon, aluminium is a very common constituent of minerals in sedimentary rocks (other than carbonates and evaporites). It is common in silicates such as feldspars, micas, and clay minerals, but also in accessory minerals such as the heavy minerals tourmaline, garnets, and epidote.
Al2O3 concentrations (like SiO2) can be used to some degree for a lithology estimation of siliciclastic sediments (Table 1), though concentrations can be diluted by non-silicate minerals such as calcite, dolomite, anhydrite, etc. (e.g., in form of cement). In general (rule of thumb):
| Al2O3 | Lithology |
|---|
| ca. 5-12% | sandstones |
| ca. 12-35% | claystones and ‘mudrocks’ |
| ca. 35-55% | bauxite and laterites |
Table 1: Al2O3 concentrations in siliciclastic rocks.Under extreme weathering conditions, aluminosilicate minerals weather to bauxite and/or laterite.
- Bauxite is a mixture consisting mainly of gibbsite (Al(OH)3), boehmite, and diaspore (γ-AlO(OH)) and α-AlO(OH), respectively). Also common are the iron oxides goethite (FeO(OH)) and hematite (Fe2O3), as well as the clay-mineral kaolinite (Al2Si2O5(OH)4). Small amounts of anatase (TiO2) and ilmenite (FeTiO3) may be present.
- Laterite develops under hot and wet (tropical) weathering conditions of igneous rocks. It consists of the above mentioned aluminum oxides, biotite (K(Mg,Fe3[AlSi3O10(OH,F)2), hematite (Fe2O3), hornblende, iron oxides like goethite (FeO(OH)), manganese oxides, micas, muscovite or illite, plagioclase, and pyroxene.
Al in carbonate and evaporite rocks
Al concentrations in carbonate and evaporite sediments are commonly low.
Together with other elements, such as Si, K, and Ti, it is however a good indicator for terrigenous input, and thus of value for chemostratigraphic interpretations.
In carbonates, Al (often together with Si, K, Fe, and Ti) can be enriched in stylolites (e.g. Hassan, 2007), which may need to be kept in mind when interpreting geochemical data from carbonates. It is recommended to avoid sampling rocks with stylolites (e.g. outcrop or dill core samples) when the rock itself is part of characterization.
Al in igneous rocks
Aluminium concentrations in igneous rocks generally decrease with increasing iron (Fe) and manganese (Mn) concentrations. Higher Al2O3 concentrations are often linked to feldspar-rich varieties, such as anorthosite. Aluminium content is higher in acidic igneous rocks (e.g., granite and rhyolites), decreases through the intermediate (e.g., diorite and andesite), and are lowest in basic plutonic and volcanic rocks (e.g., gabbro, diorite, and basalts).
References
Hassan, M.H., 2007. Stylolite effect on geochemistry, porosity and permeability: Comparison between a limestone and dolomite sample from Khuff-B reservoir in eastern Audi Arabia. The Arabian Journal for Science and Engineering, Volume 32, Number 2A, 139-148. [Link]