Manganese occurs in a wide range of minerals. But what are the most common element-mineral links of Mn?
Manganese (symbol Mn; atomic number 25; relative atomic mass 54.938044) [Link to webelements.com]
Mn has five oxidation states, +2, +3, +4, +6, and +7.
Although Mn commonly occurs in concentration of less than 1 wt.% in rocks, it is referred to as a major element. It is commonly reported in its oxide form as MnO.
Common element-mineral links of Mn
The most common element-mineral links of Mn: main detrital minerals containing Mn are ferromagnesian silicates, ilmenite and magnetite. Mn is more common in secondary Mn-oxides in form of coatings on other minerals or as concretions. In carbonates Mn may substitute for Mg, e.g. in dolomite.
Mn in siliciclastic rocks
Mn2+ has a similar ionic radius like Fe2+ and Mg2+ (67 pm vs. 61 pm and 72 pm, respectively) and thus readily substitutes for these elements in ferromagnesian silicates, such as pyroxenes, and Fe-Ti oxides. Through substitution Mn for Fe and Mg, it can also be present in clay minerals (e.g., chlorite) and mica (e.g., biotite). Further, igneous (particularly mafic/ultramafic) lithic fragments can elevate the Mn concentrations in clastic sediments. Mn-bearing carbonate (see below) cements may contribute to the overall Mn contents in siliclastic rocks, too.
Without detailed mineralogical analysis (e.g., QXRD) it is difficult to judge from geochemical analysis alone which mineral(s) may host Mn in a rock sample. However, some element associations may be useful to make a general assumption:
- igneous (mafic, ultramafic) minerals or provenance: Fe, Mg, Ti, Cr, Nb, and Ni
- clay minerals and mica (biotite): Al, Fe, Mg, and possibly Ti
- carbonate cements: Ca, Mg, Fe, and Sr, as well as reduced sum of oxides (see, due to CO3; see article about data quality).
- reducing conditions (organic-rich shales): Co, Cu, Mo, Ni, U, V, and Zn
Mn in carbonate rocks
The principal Mn carbonate mineral is Rhodochrosite (MnCO3), which however, is an authigenic carbonate in low- to moderate-temperature hydrothermal rains, metamorphic rocks, as well as in carbonatites, and carbonate rocks rocks. Traces can be found in calcite (CaCO3), where it substitutes for Sr, dolomite (Ca,Mg(CO3)2), siderite (FeCO3), magnesite (MgCO3), and of course ankerite (Ca(Fe,Mg,Mn)(CO3)2).
In limestones, Mn may be an indicator for diagenetic processes, where it substitutes for Sr in calcite. Ratios of Ca/Mn (and Ca/Sr) may thus indicative for these alteration processes.
Mn in igneous rocks
Like outlined above, the similar ionic radii of Mn2+ (67 pm), Fe2+ (67 pm) and Mg2+ (72 pm) enables Mn2+ to substitute for these elements in the crystal latices of ferromagnesian silicates (e.g., pyroxene, amphibole, biotite). Mn is therefore relatively enriched in ultramafic and mafic rocks relative to felsic rocks.