Feldspar: Properties, Facts and Photos
What is Feldspar?
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.
Types of Feldspar
All feldspar minerals fall into two main groups: potassium-rich feldspars (K-feldspars) and plagioclase feldspars, which contain varying amounts of sodium and calcium.
Potassium, which defines K-feldspars, can influence colour, crystal structure, and the types of rocks these minerals form. K-feldspars are also known as alkali feldspars.
Common examples include amazonite and moonstone. Sunstone can be either K-feldspar or plagioclase feldspar, depending on its composition, while labradorite is a plagioclase feldspar.
The chemical symbol K for potassium comes from the Latin word kalium, reflecting its classification as an alkali element.
Albite is a sodium-rich plagioclase feldspar and one of the most common members of the feldspar group. It's typically white or colourless but can exhibit grey, bluish, or pale green tones depending on impurities.
Albite is an important component of many igneous and metamorphic rocks and is present in minerals such as moonstone, peristerite, and some varieties of sunstone.
Common Feldspar Varieties
These feldspars are defined by their chemical composition and crystal structure.
- Orthoclase – potassium feldspar, common in granite
- Albite – sodium-rich plagioclase feldspar
These minerals sit within the broader K-feldspar and plagioclase groups. The following feldspars are named for their appearance or optical effects rather than chemistry.
- Labradorite – plagioclase feldspar known for its blue flash (labradorescence)
- Amazonite – blue to green variety of potassium feldspar
- Moonstone – feldspar with adularescence, usually rich in orthoclase
- Sunstone – feldspar with aventurescence caused by metallic inclusions
What is Orthoclase?
Orthoclase is a common potassium feldspar and an abundant rock-forming mineral found in most types of granite. It typically forms as white or pink crystals.
Orthoclase has been identified in igneous rocks brought back from the Moon and in rocks analysed 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: Amazonite: Courtesy of Igor Schwartzmann. Orthoclase (albite on orthoclase) and Granite: Courtesy of Stan Celestian.
Albite: Courtesy of Ron Wolf. Moonstone: Courtesy of Géry Parent.
The polished labradorite and amazonite are from our collection.
