Feldspar Chemical Composition

Feldspar refers to a group of minerals that contain alumina and silica (SiO₂) in their chemical makeup, and are natural minerals. Feldspar can be described as a large group of rock-forming silicate minerals. Aluminum silicates of soda, potassium, or lime belong to this group of natural minerals. This is a common mineral group and makes up about 60% of the exposed rocks, as well as sand, clay, and other unconsolidated sediments, and are important parts of rock classification schemes. Included in this group are orthoclase, microcline, and plagioclase feldspars.

Chemical Formula of Feldspar

Potassium feldspar chemical  formula or (K-spar) formula- KAlSi₃O₈

Sodium feldspar chemical formula or albite formula- NaAlSi₃O₈

Calcium feldspar chemical formula- CaAl₂Si₂O₈.

Bowen's Reaction Series

Alkali feldspar is a solid solution between K-feldspar and albite and the Solid solutions that are found between albite and anorthite are called plagioclase, also known as plagioclase feldspar. A 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 used as plagioclase and also as alkali feldspar.

Calcium-rich Plagioclase is the first feldspar that crystallizes from cooling magma, but with the progress in crystallization, the plagioclase gradually becomes sodium-rich.

The structure of  Feldspar is based on Aluminosilicate tetrahedra, where each of the tetrahedra has an aluminum or silicon ion which is surrounded by four oxygen ions. It forms a three-dimensional network, each oxygen ion is shared by a nearby tetrahedron. Here, we can see long chains of aluminosilicate tetrahedra which are often called the crankshaft chains.

Properties of Feldspar

Feldspar has different chemical formulas each depending on its composition.  The specific chemical formula is KAlSi₃O₈ – NaAlSi₃O₈ – CaAl₂Si₂O₈.

The minerals are chemically alert and have a stable pH value.

In the Physical properties of Feldspar, we can say that the Feldspars have a monoclinic or a triclinic crystal system and the refractive index of this mineral is 1.518 – 1.526. 

The cleavage of feldspar is two and it consists of two planes that intersect each other at an angle of 90º. On the Mohs hardness scale, the hardness of this mineral is 6 – 6.5. The density of Feldspar is 2.56 g/cm3 and the double refraction is of the first order. Feldspar has a conchoidal or a brittle fracture that can be noticed along the cleavage planes. Feldspar consists of a vitreous, pearly luster with a white streak. Feldspars are not known to have any kind of optical phenomenon.

Feldspar is the name given to a group of minerals that have alumina and silica (SiO₂) in their chemical makeup. Aluminum silicates of soda, potassium, or lime belong to this group of natural minerals. This natural mineral is the most common mineral group on the planet. They make up about 60% of exposed rocks, as well as sands, clays, and other unconsolidated sediments, and are important parts of rock classification schemes. The orthoclase, microcline, and plagioclase feldspars are included in this group. The subject of feldspar has been thoroughly discussed in this article. We've covered everything you need to know about feldspar, including its chemical name, uses, structure, and history.

Feldspars form intrusive and extrusive igneous rocks from magma, and they're also found in a variety of metamorphic rocks. Anorthosite is a rock composed almost entirely of calcic plagioclase feldspar.

Feldspars can be present in a wide variety of sedimentary rocks. Feldspars are potassium and aluminium salts, sodium and aluminium salts, and calcium and aluminium salts, respectively. Potassium aluminium silicates (KAlSi₃O₈), sodium aluminium silicates (NaAlSi₃O₈), and calcium aluminium silicates are the chemical names for feldspar (CaAl₂Si₂O₈). The form of alkali metal or alkaline earth metal found in the molecule determines the chemical name of feldspar. These minerals are a class of rock-forming tectosilicate minerals that account for around 41% of the continental crust's weight.

Feldspar Composition

Tectosilicates, or silicate minerals, are a type of natural mineral. Silicon ions are connected by shared oxygen ions to form a three-dimensional network in this type of mineral. Three endmembers may be used to express the compositions of major elements in common feldspar. Let's look at the chemical formulas for various forms of feldspar.

Feldspar Structure

Aluminosilicate tetrahedra make up the backbone of feldspar crystals. An aluminium or silicon ion is surrounded by four oxygen ions in each tetrahedron. A three-dimensional network is formed when each oxygen ion is shared by a neighbouring tetrahedron. Long chains of aluminosilicate tetrahedra can be seen, which are often referred to as crankshaft chains due to their kinked form. Each crankshaft chain forms a three-dimensional network of fused four-member rings by connecting to neighbouring crankshaft chains. The structure is open enough for cations (usually sodium, potassium, or calcium) to fit inside and balance the charge.

Types of Feldspar

Alkali feldspars are grouped into two types: 

1. Containing potassium in combination with sodium, aluminium, or silicon.

2. Containing barium (barium replaced potassium). 

The first of these include:

• Orthoclase (monoclinic) KAlSi₃O₈

• Sanidine (monoclinic) (K,Na)AlSi₃O₈

• Microcline (triclinic) KAlSi₃O₈

• Anorthoclase (triclinic) (Na,K)AlSi₃O₈

Potassium and Sodium Feldspars 

Since these aren't perfectly miscible in the melt at low temperatures, intermediate alkali feldspar compositions can only be found in higher-temperature settings. Sanidine is stable at high temperatures and microcline is stable at low temperatures. Perthite is a common texture in alkali feldspar, resulting from the exsolution of different alkali feldspar compositions during the cooling of intermediate composition. Many granites have perthitic textures in their alkali feldspars that can be seen with the naked eye. With a light microscope, one can see micro perthitic textures in crystals, but only an electron microscope can see crypto perthitic textures.

The QAPF classification of igneous rock is based on the ratio of alkali feldspar to plagioclase feldspar, as well as the proportion of quartz. Plagioclase, which is calcium-rich, is the first feldspar to crystallize from a cooling magma, but as crystallization progresses, the plagioclase gradually becomes sodium-rich. The continuous Bowen's reaction series is described by this. The last feldspar to crystallize from the magma is K-feldspar.

Barium Feldspars

Alkali feldspars are also known as barium feldspars. Barium feldspars are formed when barium is substituted for potassium in the mineral structure. The monoclinic barium feldspars contain the following:

• celsian BaAl₂Si₂O₈

• hyalophane (K,Ba)(Al,Si)₄O₈

The triclinic plagioclase feldspars are a type of plagioclase feldspar. The following is the plagioclase series (with percent anorthite in parentheses):

1. albite (0 to 10) NaAlSi3O8

2. oligoclase (10 to 30) (Na,Ca)(Al,Si)AlSi₂O₈

3. andesine (30 to 50) NaAlSi₃O₈–CaAl₂Si₂O₈

4. labradorite (50 to 70) (Ca,Na)Al(Al,Si)Si₂O₈

5. bytownite (70 to 90) (NaSi,CaAl)AlSi₂O₈

6. anorthite (90 to 100) CaAl₂Si₂O₈

Plagioclase feldspar with intermediate compositions can also exsolve to two feldspars with contrasting compositions during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths are usually too fine-grained to be visible with optical microscopes. In comparison to the difference in alkali feldspars, the immiscibility differences in plagioclase solid solutions are more complicated. The colour play apparent in some feldspar of labradorite composition is due to Boggild intergrowth, which is a very fine-grained exsolution lamellae. From albite (2.62) to anorthite (2.72–2.75), the specific gravity of the plagioclase sequence increases.

Uses of Feldspar

• Dinnerware, bathroom and construction tiles are all made of feldspar.

• In the manufacture of ceramics and glass.

• As a flux, feldspar is used. A flux is a substance that decreases the melting temperature of another substance, such as glass in this case.

Substitutes and Alternative Sources of Feldspar

Other minerals and mineral mixtures with identical physical properties may be substituted for feldspar. Pyrophyllite, clays, talc, and feldspar-silica (quartz) mixtures are some of the minerals that could be used to replace feldspar. For the near future, these substitutions would be unnecessary due to the abundance of feldspar.

Mining of Feldspar

North Carolina, Virginia, California, Oklahoma, Idaho, Georgia, and South Dakota, in order of estimated tonnage, are the top states producing feldspar. Mica and silica sand were recovered as co-products by feldspar processors.

Feldspar is mined from large granite bodies (called plutons by geologists), pegmatites (formed when a crystallizing granite's last fluid stages become concentrated in small liquid and vapor-rich pockets that allow the growth of extremely large crystals), and feldspar-rich sands.

Since feldspar is such a large component of the Earth's crust, it is believed that feldspar supply would be sufficient to satisfy the demand for a very long time. The demand for raw feldspar is currently being met by existing mines around the world.

Hard-rock feldspar mining is achieved by open-pit methods by the mine owner or by contractors. Secondary breakage is done with a traditional drop ball after the feldspar ore has been drilled and blasted. The ore is then loaded onto trucks using a hydraulic shovel and hauled to the crushing facility, which is located near the flotation plant.

Weathering of Feldspar

Hydrolysis causes chemical weathering of feldspars, resulting in clay minerals such as illite, smectite, and kaolinite. The first step in the hydrolysis of feldspars is for the feldspar to dissolve in water, which occurs better in acidic or basic solutions and less so in neutral solutions. The rate at which feldspars are dissolved determines how easily they weather. Clays are formed when dissolved feldspar reacts with H+ or OH ions. The reaction also produces new ions in solution, with the form of feldspar reacting controlling the variety of ions produced.

Clays are plentiful weathering materials due to the presence of feldspars in the Earth's crust. Clays make up about 40% of the minerals in sedimentary rocks, and clays are the primary minerals in mudrocks, the most common sedimentary rocks. They're also an integral part of the soil. Clay-replaced feldspar appears chalky in comparison to unweathered grains of feldspar, which is more crystalline and glassy.

Due to their high formation temperature, feldspars, especially plagioclase feldspars, are not very stable at the earth's surface. Feldspars are quickly weathered to clays due to their lack of stability. Feldspars are rarely found in sedimentary rocks due to their proclivity for weathering. Significant concentrations of feldspar in sedimentary rocks suggest that the sediment was not subjected to much chemical weathering before being buried. This indicates that it was most likely transported a short distance in cold or dry conditions that slowed weathering and were easily buried by other sediments. Arkoses are sandstones that contain a lot of feldspar.

Feldspar as a Mineral

Feldspars are the most common and widespread rock-forming minerals, making up the majority of major rock types including granites and gabbros. They are aluminosilicates, which undergo complex reactions when cooled from melts or metamorphosed at high temperatures due to the ordering of Si and Al.

Below 600 °C, there is a limited solid solution in the alkali feldspars between the albite and sanidine end members. A solid solution is also common between the end members albite and anorthite in plagioclase feldspars, with more unmixing occurring below 400 °C.

Even when the feldspar crystals are very thin, these consistent properties of feldspar are extremely useful. People who are familiar with feldspar cleavage can pick up an igneous rock with a few millimeter-sized crystals, analyze it with a hand lens, and easily distinguish the feldspars from the other minerals in the rock. They may also use a set of mineral hardness picks and a hand lens to assess the Mohs hardness of such tiny grains with a little practice.

Did You Know

• Feldspar is a mineral that accounts for up to 60% of the Earth's crust.

• Feldspar is divided into two types, both of which are composed of silica and aluminium.

• In 2010, about 20 million tons of feldspar were produced, mainly by three countries: Italy (4.7 million tons), Turkey (4.5 million tons), and China (4.5 million tons) (2 Mt).

• Feldspars are used for K-Ar dating in earth sciences and archeology.