Mean content of aluminium at 50 cm depth in mineral soil under forest, (expressed as mass percentage of aluminium oxide)

Factfile

Metallic element belonging to the non-metals, and subgroup 3A in the periodic system. Aluminium is a silvery-white metal that is readily covered by a passive layer consisting of an oxide film. It is therefore relatively stable in air and water. Soluble in hydrochloric acid and sodium hydroxide (lye) but not in nitric acid. Discovered in 1825 by Hans Christian Örsted in Copenhagen. Aluminium is the third commonest element in the Earth's crust (counting oxygen as the commonest). In nature aluminium never occurs in free form but in compounds, mainly with silicon and oxygen and in various constellations of alkali metals and alkaline earth metals. The mean concentration by mass of aluminium in rocks in the Earth's crust is 8.1% Al, in the oceans (0.1-9.7)*10^-4 % Al. To facilitate review of the principal element in total geochemical analysis, in MarkInfo aluminium is expressed as its oxide.

In Swedish rocks, regoliths and soils, aluminium occurs mainly as rock-forming minerals and to a certain degree as secondary minerals formed by weathering processes. The rock-forming minerals are dominated by various types of feldspar, including alkali feldspars in the form of microcline, orthoclase and albite, as well as plagioclases, which form a continuous series with differing chemical composition between the end members. In the Earth's crust, the proportion of silicate minerals is estimated at 95%, of which the feldspars comprise 60% and quartz 12%. Aluminium is also contained to a greater or lesser extent in other silicate minerals. The most important source for production of aluminium is the ore bauxite, which contains minerals such as diaspore and boehmite AlO(OH) and gibbsite Al(OH)₃.

It is worth noting that the minerals in Swedish clays consist to a major extent of various alumino-silicates. Up to 20% by mass of the clay fraction (material < 0.002 mm particle diameter) can be aluminium expressed as Al₂O₃.

With a composition corresponding to the Swedish average bedrock (mafic rocks making up 7-8% of the crystalline basement), the content by mass is 15.1% Al2O3. The arithmetic mean of forest soils lies around 12.6% Al₂O₃, showing that the aluminium content of the regolith is somewhat underrepresented in relation to the composition of the underlying bedrock. A possible explanation for this is the relative effect that an increased quartz content in the regolith has on other elements in the calculation. A median value for 24000 analyses of regolith (mat <0.063 mm) is 13.6% Al₂O₃ (SGU).