Potassium (K) is the seventh most abundant element on earth and makes 2.59% of the earth’s crust. But, which are the element-mineral links of K?
Potassium (symbol K; atomic number 19; relative atomic mass 39.0983) [Link to webelements.com]
K has one oxidation state, +1. It is the seventh-most abundant element in the Earth’s crust.
Potassium is present in three naturally occurring isotopes (39K, 40K, and 41K), of which 39K forms 93.3% of the total mass.
Common element substitutions for K are: Li, Na, Rb, and Cs
In geochemical analysis, potassium is commonly reported in its oxide form as K2O in weight percent [wt.%].
Common element-mineral links of K
Most common element-mineral links of K: K-feldspar, K-bearing clay-minerals, particularly illite, and micas (muscovite and glauconite (di-octahedral), and e.g. phlogopite and biotite (tri-octahedral).
K in siliciclastic rocks
In siliciclastic sediments, K is mainly found in alkali-feldspars, mica (such as muscovite and glauconite (di-octahedral), and phlogopite and biotite (tri-octahedral)), and clay minerals, particularly illite. K concentrations are in general higher in fine-grained siliciclastics than in coarser-grained sands, due to its abundance in clay and mica minerals.
| Mineral | Idealized chemical formula |
|---|
| alkali-feldspar (e.g. sanidine) | (K,Na)AlSi3O8 |
| muscovite | KAl2(Si3Al)O10(OH,F)2 |
| glauconite | (K,Na)(Fe,Al,Mg)2(Si,Al)4O10(OH)2 |
| biotite | K(Mg,Fe)3[AlSi3O10(OH,F)2 |
| phlogopite | KMg3AlSi3O10F(OH) |
| illite | K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)] |
Table 1: Common K-rich minerals in siliciclastic sediments.Potassium shows affinities with aluminum in (alumo-)silicates.
K2O/Al2O3 ratios may give some indication of clay mineralogy in mudstone and ‘shales’ when carefully interpreted. As a rule of thumb:
| K2O/Al2O3 | Clay Mineralogy |
|---|
| < 0.1 | kaolinite |
| 0.2-0.3 | illite, smectite, chlorite, mica |
| > 0.4 | K-feldspar |
Table 2: K2O/Al2O3 ratios and clay mineralogy.Potassium, when released during feldspar dissolution (weathering, diagenesis) is very soluble. Its mobility, however, is limited, because it is quickly incorporated into clay mineral lattices (due to its large size), and it is more strongly adsorbed onto clay mineral surfaces and organic matter than for instance sodium (Na) (in addition, it is also readily taken up by plants roots).
Rb (rubidium) substitutes for K in mica, e.g., muscovite, clay minerals (e.g., illite and montmorillonite), and to a lesser extend in K-feldspar. The K/Rb (or K2O/Rb) ratio may give an indication of the dominance of K-feldspar (higher K/Rb) over clay minerals (lower K/Rb) and vice-versa, although there are no universal values available.
Another source for K, worth mentioning, in siliciclastic sediments, can be lithic fragments from igneous or metamorphic parent rocks.
K in carbonate minerals
In carbonates, K concentrations are commonly very low and hosted in the non-carbonate fraction, and can get enriched in stylolites (together with Al, Si, and Fe; e.g. Hassan, 2007).
K in evaporite minerals
In evaporite sections, K is a common component of many phosphates, halide, and sulfate minerals. It forms a range of minerals in its own right, such as sylvite KCl and carnallite KMgCl3·6H2O, which are common in evaporite deposits.
NOTE:
K contamination in cuttings
KCl is a common drilling mud additive, and thus can affect/contaminate the chemical analysis, i.e. elevated K concentrations, if the sample is not cleaned effectively.
K in igneous rocks
In igneous rocks, K is mainly associated with alkali feldspar (e.g. (Na,K)AlSi3O8), leucite (KAlSi2O6), biotite (K(Mg,Fe)3[AlSi3O10(OH,F)2), muscovite (KAl2(Si3Al)O10(OH,F)2), phlogopite (KMg3AlSi3O10F(OH)), and some amphiboles. K-feldspars are more common in acidic igneous rocks, such as granite and rhyolite, and decrease through the series of intermediate (diorite and andesite) to basic/ultrabasic (gabbro and basalt) rocks.
good to know …
K radioactivity
Potassium, i.e. its radioactive isotope 40K, is a contributor to the natural radioactivity of rocks. Thus, K-feldspar or mica-rich sandstones and igneous rocks show higher radioactivity. K is part of the gamma-ray (GR) and spectral gamma-ray (SGR) measurements of borehole tools. One percent K contributes 16 API units to the GR curve. High mica content can thus result in ‘hot’ sandstones, i.e. their gamma-ray signatures may be higher than those from clay mineral-rich shales.