Potassium：Uses, mineral groups and crystal structures

Uses of Potassium

K, the third most important fertilizer element, is the eighth most abundant element in the rocks of the Earth’s crust. K occurs in nearly all rocks and soils, although its quantity varies widely. It occurs in igneous and metamorphic rocks primarily as potassium feldspar (microcline and orthoclase, KAlSi3O8); weathering releases K, which is then incorporated in clay minerals such as illite (KAl2(Al,Si)4O10(OH)2). The K+ ion of clay minerals is exchanged with plants by substitution of a H+. Unlike N and P, K does not form an integral part of the plant components, but it is vital as a catalytic agent in numerous plant functions such as N metabolism, synthesis of proteins, activation of enzymes, and maintenance of water content.

Although K occurs in most rocks, the only occurrences that can be economically extracted and processed into fertilizers are those in evaporite sequences. Nearly complete evaporation results in the deposition of large amounts of halite (NaCl) and smaller amounts of a variety of K and K-Mg salts, among which the most important are sylvite (KCl) (Fig.1), langbeinite (K2Mg2(SO4)3), kainite (KMg(SO4)Cl· 3H2O), and carnallite (KMgCl3· 6H2O). Because the K salts are very soluble, they are only preserved in very arid regions, or in salt beds that are buried below zones of groundwater flow.

Mineral groups and Crystal structures

Potassium is a common element in many minerals. So far, 553 minerals have been recognized as containing, or may contain through substitution, K in their structure. By far, the largest group of K-containing minerals, however, is found under the silicates (more than 250 minerals). These include many very well-known minerals, for example, apophyllite ((Na,K)Ca₄(Si₈O₂₀)(F,OH)· 8H2O) (Fig. 2), astrophyllite (K₂NaFe²⁺ ₇ Ti₂Si₈O₂₈ (OH)₄F), fluorapophyllite-(K) (KCa₄(Si₈O₂₀)(F,OH)· 8H2O), leucite (K(AlSi₂O₆)) (Fig. 3), microcline (Fig. 4) and orthoclase (K(AlSi₃O₈)), muscovite (KAl₂(AlSi₃O₁₀)(OH)₂) (Fig. 5), phlogopite (KMg₃(AlSi₃O₁₀)(OH)₂), and sanidine (K(AlSi₃O₈)) (Fig. 6). Finally, there are two organic compounds with K: antipinite (KNa₃Cu₂(C₂O₄)₄) and minguzzite (K₃Fe³⁺(C₂O₄)₃· 3H2O).

Translucent green apophyllite, (Na,K)Ca4(Si8O20)(F,OH)· 8H2O), crystal clusters to 1.5 cm with stilbite.

Leucite, K(AlSi2O6). The crystal in this specimen is 4 mm. Leucite crystals, like these, are trapezohedral, but mostly rounded.

Microcline, K(AlSi3O8). The crystals are 5~10 mm with white glossy albite.

A 2.5-cm group of star-like (stellate), crystallized muscovite, KAl2(AlSi3O10)(OH)2.

A single somewhat translucent, sharp sanidine crystal, K(AlSi3O8), 3 3 2.5 3 1.5 cm. Sanidine is a high-temperature polymorph of the potassium feldspars. It is found in volcanic rocks as phenocrysts and easily weathered out. This is a common Carlsbad twin.