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potassium rich feldspars (Orthoclase group) are important parts of rock forming minerals, granite, granite pegmatites, carbonatites, and hornfels. Calcium rich feldspars (plagioclase group) are also important rock forming minerals, gabbro, nepheline syenites, schists, and hornfels.

The generic term “feldspar” comes from the Germanic term “feldt spat”, meaning “mineral with prominent cleavage from the field”. It was a prime constituent of many of the rocks over turned by farmers while plowing their fields. The feldspars are made up of three fundamental members and a wide number of chemical mixtures. There is the potassium rich member KAlSi3O8 (Orthoclase), the sodium rich member NaAlSi3O8 (albite), and the calcium member CaAl2Si2O8 (anorthite). These three members make up the vertices of a solid solution phase diagram. Orthoclase takes its name from the Greek “orthos”, meaning “upright” and “klasis” meaning “fracture”. This is due to its perfect right angle cleavage. The “albite” member gets its name from the Latin “albus” which means “white” illuding to its color. The calcium end member (anorthite) gets its name from the Greek, “an-” (meaning a negative), and “orthos” meaning “upright”, ie. “not-upright” cleavage. The sodium-rich (albite) and the calcium-rich (anothite) form a continuous series of what is known as the “plagioclase” feldspars. The ratios are as follows: Albite (100) – Anorthite( 0) Albite Albite ( 90) – Anorthite( 10) oligoclase Albite ( 70) – Anorthite( 30) andesine< Albite ( 30) – Anorthite( 70) labradorite Albite ( 10) – Anorthite( 90) bytownite Albite ( 0) – Anorthite(100) Anorthite “oligoclase” comes from the Greek “oligos”, meaning “little” and “klasis” meaning “fracture”. “Andesine” is named for the after a locality in the Andes, “Labradorite” is named for Labrador, and “Bytownite” is named for the locality Bytown in Ottawa, Canada. “Orthoclase” composition has a variety of different crystal structures based on their temperature of formation. Adularia, sanadine, and microcline. The potassium-sodium series are known as the “potash feldspars”. The name “Andularia” comes from a locality in Switzerland known as the Adula Mts., “Sanadine” comes from the Greek “sanis” and “inos”, which mean “tablet” and “like”. “like a tablet” from its tabular habit. “Microcline” comes from the Greek “mikos” and “klinein” meaning “small” and “to incline”. The green variety of microcline is called “amozonite” and is often cut as a cabochon. The Adularia variety is called “moonstone” in the gem trade, and has what is described ad “adularescence” (a blue-white schiller effect.) There is also a variety of plagioclase that shows this same effect and is known as “moonstone” too. Labradorite may show what is called “labradorescence”, a bluish to yellow schiller effect. Labradorite is sometimes found in pale-yellow, transparent crystals as is orthoclase, and both are cut into faceted gemstones. When tiny hematite or goethite platelets are trapped in feldspar it is known as Aventurine feldspar or “sunstone”.

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth’s crust.

Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock.

Etymology

Feldspar is derived from the German Feld, “field”, and Spath, “a rock that does not contain ore”. “Feldspathic” refers to materials that contain feldspar. The alternative spelling, felspar, has now largely fallen out of use.

Compositions

This group of minerals consists of framework or tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:

Potassium-Feldspar (K-spar) endmember KAlSi3O8

Albite endmember NaAlSi3O8

Anorthite endmember CaAl2Si2O8

Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, or more properly plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth. Albite is considered both a plagioclase and alkali feldspar. In addition to albite, barium feldspars are also considered both alkali and plagioclase feldspars. Barium feldspars form as the result of the replacement of potassium feldspar.

Alkali Feldspars

The alkali feldspars are as follows:

orthoclase (monoclinic), — KAlSi3O8
sanidine (monoclinic) —(K,Na)AlSi3O8
microcline (triclinic) — KAlSi3O8
anorthoclase (triclinic) — (Na,K)AlSi3O8

Sanidine is stable at the highest temperatures, and microcline at the lowest.Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Plagioclase Feldspars

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):

albite (0 to 10) — NaAlSi3O8
oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8
andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8
labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8
bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8
anorthite (90 to 100) — CaAl2Si2O8

Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solution are complex compared to the gap in the alkali feldspars. The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.

Barium Feldspars

The barium feldspars are monoclinic and comprise the following:

celsian — BaAl2Si2O8
hyalophane — (K,Na,Ba)(Al,Si)4O8.

Feldspars can form clay minerals through chemical weathering.

Uses

Feldspar is a common raw material in the production of ceramics and geopolymers.
Feldspars are used for thermoluminescence dating and optical dating in earth sciences and archaeology
Feldspar is one of several abrasive ingredients in Bon Ami, a brand of household cleaner in the USA.

In 2005, Italy was the top producer of feldspar with almost one fifth of world share, followed by Turkey, China and Thailand—reports the International Monetary Fund.