Magnesium (Mg) – element-mineral links of Mg

Magnesium is the eighth most abundant elements in the Earth’s crust. So what are the element-mineral links of Mg?

Magnesium (symbol Mg; atomic number 12; relative atomic mass 24.305) [Link to webelements.com]

Mg has one main oxidation stage, +2.

Mg has three naturally occurring isotopes (²⁴Mg, ²⁵Mg, and ²⁶Mg), with ²⁴Mg the most abundant one.

Most common element – mineral links: Sedimentary rocks can have a diverse range of Mg-bearing minerals including silicates, carbonates, sulphates, phosphates, and borates.

Mg in siliciclastic rocks

In siliciclastic sediments, the mineral assemblage can comprise detrital and authigenic minerals. Mg is commonly associated with clay minerals, such as those of the chlorite and smectite groups, mica e.g., biotite and glauconite (the latter an indicator for marine origin), serpentine and talk, as well as olivine (e.g., forsterite), amphibole, pyroxene, spinel, and garnet (e.g., pyrope). Authigenic minerals may include carbonate minerals, such as dolomite, ankerite, and magnesite.

Due to its strong affinity with phyllosilicates (clay and mica minerals) and the instability of most of the igneous minerals (olivine, amphibole, pyroxene), Mg concentrations are commonly higher in ‘shales’ than in sandstones.

High Mg concentrations may indicate a mafic or ultramafic provenance (see below), particularly when in association with trace metals such as Cr, Ni, and V). However, when associated with higher values of Ba, Ca, and Sr, high Mg concentrations indicate calcareous rocks.

Mg in carbonate rocks

Mg-bearing carbonate minerals are mainly dolomite (CaMg(CO₃)₂), ankerite (Ca(Fe,Mg,Mn)(CO₃)₂), and magnesite (MgCO₃). However, calcite (CaCO₃) can incorporate Mg into its crystal lattice to a certain amount leading to the differentiation of low-Mg calcite (< 5 mol% MgCO₃) and high-Mg calcite (> 5 mol% MgCO₃) (which is reflected in the ‘calcite – aragonite sea’ cycles, e.g., Sandberg, 1983, Stanley et al., 2010).

Mg in igneous rocks

In magmatic rocks, Mg has crystallized relatively early as high-temperature minerals, e.g. olivine (e.g., forsterite Mg₂SiO₄) and (ortho-)pyroxene, or under ‘wet’ conditions (high water and oxygen fugacity) as amphibole and mica. Mg is thus relatively abundant in mafic/basic igneous rocks, but also in granite and granodiorite in form of biotite.