What Minerals Are Potassium Bearing?

November 18, 2025 •

3 min read

Potassium is the seventh most abundant element in the Earth's continental crust. A wide variety of minerals contain potassium, with the most common being potassium feldspars and micas. These minerals are vital sources of potassium for both industrial uses and for enriching the soil to support plant growth.

Quick Summary

This guide details the major categories of minerals containing potassium, including common rock-forming silicates like feldspars and micas, as well as evaporite deposits. It explains the different types within these groups, their formation, and their importance as natural sources of potassium in geology and agriculture.

Key Points

Feldspars are Abundant: Potassium feldspars, including orthoclase and microcline, are the most common potassium-bearing minerals, forming a large portion of the Earth's crust in igneous and metamorphic rocks.
Micas are Layered Silicates: Key mica minerals like muscovite and biotite contain potassium within their sheet-like crystalline structures and are important natural sources of the element.
Evaporites are Soluble Salts: Minerals like sylvite (KCl) and carnallite (KMgCl₃·6H₂O) are highly soluble evaporite salts, making them the primary commercial sources for potash fertilizers.
Clays Hold Reserve Potassium: Secondary clay minerals such as illite and vermiculite form from the weathering of primary minerals and play a crucial role in holding and slowly releasing potassium into soils.
Weathering Releases Potassium: The availability of potassium from silicate minerals like feldspars and micas depends on weathering, with some minerals releasing potassium more readily than others.

Potassium-Bearing Feldspar Minerals

Feldspars constitute a significant portion of the Earth's crust, and potassium-rich varieties are key components of various igneous, metamorphic, and sedimentary rocks. Potassium feldspar (K-feldspar) has the basic chemical formula KAlSi$_3$O$_8$, with different types forming at varying temperatures.

Common Varieties of K-Feldspar

Orthoclase: A monoclinic K-feldspar frequently found in granite and other felsic plutonic rocks.
Microcline: A triclinic K-feldspar that crystallizes at lower temperatures; amazonite is a green variety of microcline.
Sanidine: The high-temperature K-feldspar, typically occurring in volcanic rocks.
Amazonite: A green microcline colored by a lead impurity.

Potassium-Bearing Mica Minerals

Micas are phyllosilicate minerals known for their perfect basal cleavage, allowing them to cleave into thin sheets. Several micas contain potassium.

Key Potassium-Containing Micas

Muscovite: Also called white mica, it's a hydrated phyllosilicate of aluminum and potassium. A common rock-forming mineral in granites and metamorphic rocks, muscovite is resistant to weathering.
Biotite: An iron-magnesium-potassium mica found in igneous and metamorphic rocks. Biotite weathers more easily than muscovite, providing a faster release of potassium to soil.
Illite: A non-expanding, potassium-bearing clay mineral and a secondary source of potassium in soils derived from weathered micas.

Evaporite Potassium Minerals

Evaporite minerals are water-soluble salts formed by the evaporation of saline waters. These deposits are significant commercial sources of potassium, known as potash.

Notable Evaporite Minerals

Sylvite (KCl): A simple potassium chloride salt, it's a primary source for potash fertilizer. It is anhydrous.
Carnallite (KMgCl₃·6H₂O): A hydrated double salt of potassium and magnesium chlorides, highly deliquescent. It is a major potash source.
Langbeinite (K₂SO₄·2MgSO₄): A potassium and magnesium sulfate mineral mined from evaporites for use as a potassium and magnesium source.

Clay Minerals and Soil Potassium

Clay minerals are fine-grained silicates resulting from the weathering of other minerals like feldspars and micas. Many clays, particularly those with a 2:1 layer structure, can hold potassium ions, acting as a slow-release reservoir in soils. Illite is a notable example. Other clays like smectite and vermiculite can also contain potassium, with availability influenced by moisture and clay type.

Comparison Table: Major Potassium-Bearing Minerals


Mineral Group	Common Examples	Primary Composition	Typical Occurrence	Rate of K Release (Weathering)
Feldspars	Orthoclase, Microcline	KAlSi₃O₈	Igneous and metamorphic rocks	Slow
Micas	Muscovite, Biotite	KAl₂(AlSi₃O₁₀)(OH)₂	Igneous and metamorphic rocks	Varies (Biotite > Muscovite)
Evaporites	Sylvite, Carnallite	KCl, KMgCl₃·6H₂O	Sedimentary deposits (saline lakes, oceans)	Rapid (soluble salts)
Clays	Illite, Vermiculite	(K,H₃O)Al₂(Si,Al)₄O₁₀((OH)₂,H₂O)	Soils, sedimentary rocks (weathered)	Slow to moderate

Conclusion

Potassium-bearing minerals are diverse, ranging from feldspars and micas in rocks to soluble evaporite salts. While feldspars and micas contain the bulk of crustal potassium, they release it slowly through weathering. Evaporite minerals like sylvite offer highly soluble, commercially important sources of potassium for fertilizer. Weathering of primary minerals contributes to soil clay mineralogy, with clays such as illite holding potassium in reserve. This cycle is essential for the availability of this vital nutrient.

More Mineralogical Resources

For deeper insights into mineral classification and properties, consider exploring the Mindat database, a leading resource for mineralogy and geology. Mindat.org: The world's largest open database of minerals, rocks, meteorites and the localities they come from.

Frequently Asked Questions

The most common potassium-bearing minerals are the potassium feldspars, which include orthoclase and microcline. These are essential components of many igneous and metamorphic rocks, such as granite.

Evaporite minerals like sylvite and carnallite are formed from the evaporation of ancient seas or saline lakes. As the water evaporates, it concentrates dissolved salts until they crystallize and precipitate, forming large layered deposits.

Sylvite (potassium chloride) is the principal commercial source of potash fertilizer. Carnallite is also a significant source.

Mica minerals, particularly biotite and muscovite, are important sources of native potassium in soil. As they weather, they slowly release potassium that can be absorbed by plants. Biotite releases potassium faster than the more resistant muscovite.

Certain clay minerals, such as illite and vermiculite, have a layered structure with spaces between the layers that can trap or 'fix' potassium ions. This fixed potassium is released slowly over time and serves as a reservoir for plant nutrition.

No, most potassium in soil is in an unavailable, mineral form, primarily within the crystal lattices of primary minerals like feldspars and micas. A much smaller fraction is available to plants, existing in the soil solution or held in an exchangeable form on clay and organic matter surfaces.

The main difference is their composition and properties: Sylvite (KCl) is a simple, anhydrous salt, whereas carnallite (KMgCl₃·6H₂O) is a hydrated double salt containing both potassium and magnesium, and it is highly deliquescent.