Feldspar

Feldspar (pronounced FEL(D)-spar, and sometimes spelled felspar) refers to a group of rock-forming minerals made of aluminium tectosilicates, which also include other positively charged ions like sodium, calcium, potassium, or barium. The most widespread types are the plagioclase feldspars (containing sodium and calcium) and the alkali feldspars (containing potassium and sodium). By weight, feldspars account for roughly 60% of the entire Earth's crust and 41% of its continental crust. These minerals form from cooling magma in both intrusive and extrusive igneous rocks, and they also appear in many metamorphic rocks. A rock composed almost entirely of calcium-rich plagioclase feldspar is called anorthosite. Additionally, feldspars are common in various sedimentary rocks.

The name comes from the German *Feldspat*, a combination of *Feld* ("field") and *Spat* ("flake"). *Spat* had long been used for rocks that split easily into flakes; *Feldspat* was introduced in the 18th century, possibly because the mineral was often found in rocks in fields (Urban Brückmann, 1783) or because it appeared as "fields" within granite and other minerals (René-Just Haüy, 1804). The spelling shift from *Spat* to *-spar* was influenced by the English word *spar*, which describes a non-opaque mineral with good cleavage. *Feldspathic* describes materials that contain feldspar. The alternative spelling *felspar* is now obsolete. The term *felsic*, meaning light-colored minerals like quartz and feldspars, is an acronym derived from *feldspar* and *silica*—unrelated to the old spelling *felspar*.

In terms of composition, feldspars are tectosilicates: silicate minerals where silicon ions connect through shared oxygen ions to form a three-dimensional network. The major elements in common feldspars can be expressed using three endmembers: orthoclase (KAlSi₃O₈), albite (NaAlSi₃O₈), and anorthite (CaAl₂Si₂O₈). Solid solutions between orthoclase and albite are called alkali feldspar, while those between albite and anorthite are called plagioclase (or plagioclase feldspar). There is only limited solid solution between K-feldspar and anorthite, and in the other two solid solutions, immiscibility occurs at temperatures typical of the Earth's crust. Albite is considered both a plagioclase and an alkali feldspar. The ratio of alkali feldspar to plagioclase feldspar, along with the amount of quartz, forms the basis for the QAPF classification of igneous rocks. During magma cooling, calcium-rich plagioclase crystallizes first, with the plagioclase becoming increasingly sodium-rich as crystallization continues—this defines the continuous Bowen's reaction series. K-feldspar is the last feldspar to crystallize from magma.

Alkali feldspars are divided into two types: those containing potassium with sodium, aluminium, or silicon; and those where potassium is replaced by barium. The first group includes orthoclase (monoclinic, KAlSi₃O₈), sanidine (monoclinic, (K,Na)AlSi₃O₈), microcline (triclinic, KAlSi₃O₈), and anorthoclase (triclinic, (Na,K)AlSi₃O₈). Potassium and sodium feldspars do not mix perfectly in the melt at low temperatures, so intermediate compositions of alkali feldspars are only found in higher-temperature environments. Sanidine is stable at the highest temperatures, while microcline is stable at the lowest. Perthite is a common texture in alkali feldspar, caused by the exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. In many granites, perthitic textures are visible to the naked eye; microperthitic textures require a light microscope, and cryptoperthitic textures can only be seen with an electron microscope.

Buddingtonite is an ammonium feldspar with the formula NH₄AlSi₃O₈, associated with hydrothermal alteration of primary feldspar minerals.

Barium feldspars form when barium replaces potassium in the mineral structure. They are sometimes classified as a separate group or as a sub-group of alkali feldspars. These feldspars are monoclinic and include celsian (BaAl₂Si₂O₈) and hyalophane ((K,Ba)(Al,Si)₄O₈).

Plagioclase feldspars are triclinic. The plagioclase series, with the percentage of anorthite in parentheses, includes: albite (0–10%, NaAlSi₃O₈), oligoclase (10–30%, (Na,Ca)(Al,Si)AlSi₂O₈), andesine (30–50%, NaAlSi₃O₈–CaAl₂Si₂O₈), labradorite (50–70%, (Ca,Na)Al(Al,Si)Si₂O₈), bytownite (70–90%, (NaSi,CaAl)AlSi₂O₈), and anorthite (90–100%, CaAl₂Si₂O₈). During cooling, intermediate compositions exsolve into two feldspars of contrasting composition, but diffusion is much slower than in alkali feldspar, so the resulting intergrowths are usually too fine to see with an optical microscope. The immiscibility gaps in plagioclase solid solutions are more complex than those in alkali feldspars. The play of colors seen in some labradorite feldspar is due to very fine exsolution lamellae called Bøggild intergrowth. Specific gravity in the plagioclase series increases from albite (2.62) to anorthite (2.72–2.75).

The structure of a feldspar crystal is based on aluminosilicate tetrahedra. Each tetrahedron consists of an aluminium or silicon ion surrounded by four oxygen ions. Every oxygen ion is shared with a neighboring tetrahedron, forming a three-dimensional network. This structure can be visualized as long chains of aluminosilicate tetrahedra, sometimes described as crankshaft chains because of their kinked shape. Each crankshaft chain connects to neighboring chains to create a three-dimensional network of fused four-member rings. The structure is open enough to accommodate the various cations.