Feldspar Properties, Facts and Photos

What are Feldspars?

Feldspar is a general term for one of Earth's most abundant groups of minerals. The different varieties account for more than half of Earth’s crust.

Feldspars are important rock-forming minerals found worldwide. They're a major component of most igneous, sedimentary, and metamorphic rocks.

Feldspar minerals have also been found on the moon and in meteorites.

The name 'feldspar' (originally 'felspar' but no longer used) comes from the German word 'feld', meaning 'field'. This is because feldspars release nutrients into the soil during weathering and decomposition.

The variation in colour of different feldspar minerals depends on the impurities present.

Feldspar minerals are widely used in industry, mainly for producing glass, ceramics, and porcelain. Varieties such as labradorite, moonstone, sunstone, and amazonite are used for decorative purposes, as well as for their metaphysical healing properties.

What is K-Feldspar?

Some feldspar minerals are classified as K-feldspar which indicates they contain potassium. This distinguishes them from other feldspar minerals that contain sodium or calcium.

Potassium, which makes K-feldspars unique, can influence colour, crystal structure, and the types of rocks they form.

K-feldspars are also known as alkali feldspars.

All feldspar minerals share similar physical properties. 

In 1814, the letter K was chosen as a symbol for potassium. It comes from the Latin word 'kalium', which comes from the Arabic 'qali', meaning alkali.

The association with alkali reflects the role of potassium in forming alkaline compounds.

In chemistry, elements are often represented by symbols derived from their Latin names. 'K' was chosen to avoid confusion with other elements, such as phosphorus, which uses P.

The name 'potassium' comes from the English word 'potash,' which refers to the process of extracting potassium compounds by leaching wood ashes in pots.

Leaching wood ashes involves pouring water over them to dissolve the potassium salts. The resulting liquid, known as lye, evaporates, leaving behind potassium carbonate or potash.

Potash was produced for soap, glass-making, and fertilisers.

High-potash plant feeds benefit plants because potassium is essential for strong root growth, improving drought resistance, and enhancing overall health.

Potassium was first isolated by British chemist and inventor Humphry Davy in 1807 through the electrolysis of potash (the breaking down of potash using electric currents).

Amazonite and Moonstone are K-feldspars, Sunstone can be K-feldspar or plagioclase feldspar, depending on its specific composition. Labradorite is a plagioclase feldspar.

Feldspars are broadly categorized into two groups: K-feldspars, which include minerals rich in potassium, and plagioclase feldspars, which contain varying amounts of sodium and calcium.

What is Sunstone-Moonstone?

Sunstone-moonstone is a trade name for a variety of feldspar correctly known as Peristerite. It's not a combination of sunstone and moonstone.

Peristerite is a variety of the feldspar mineral albite. White peristerite is often sold as rainbow moonstone.

Rainbow Moonstone is a trade name for whitish-grey feldspar minerals that exhibit a blue flash or a silky, pearlescent lustre.

White peristerite can be called 'rainbow moonstone', however this name also includes other types of feldspar with similar effects.

Most moonstone is a combination of orthoclase and albite. The layered structure of this material produces the light-scattering effect known as adularescence. This gives moonstone the characteristic glow that appears to float just below the stone's surface.

The trade name sunstone-moonstone has recently appeared because some peristerite can exhibit the optical effects seen in both sunstone and moonstone. This has led some to believe it's a combination of the two minerals.

Trade names, which are created to enhance a mineral's appeal, are often misleading regarding its chemical composition. The name 'Jasper' is often used for materials that contain no actual jasper. Dalmatian Jasper is a perfect example. 

Peristerite is named after the Greek word 'peristera', meaning 'dove.' The name comes from a perception that its colour is thought to resemble a dove's feathers.

The blue flash is caused by the interference of light between thin layers of different feldspar minerals.

Orthoclase, which is a potassium feldspar, is an abundant rock-forming mineral found in most types of granite. It usually appears as white or pink crystals.

Orthoclase has been identified in igneous rocks brought back from the Moon and also in those identified on Mars.

Article Photos

The feldspar at the top of our article and the orthoclase in the photo above are both courtesy of Stan Celestian.

The moonstone is courtesy of James St. John.

Images are clickable and redirect to the original photo.

Pop-up photos: Sunstone - Courtesy of James St. John. Amazonite - Courtesy of Igor Schwartzmann. Moonstone (peristerite-oligoclase feldspar) - Courtesy of James St. John. Albite - Courtesy of Ron Wolf. 

The polished labradorite stone and amazonite tumbled stones are from our collection.