Feldspar

Feldspar is a group of minerals that are very common in the world. It is made mostly of aluminium and other elements like sodium, calcium, potassium, or barium. The two main types of feldspar are called plagioclase and alkali feldspars.

Feldspars make up a big part of the Earth's crust—they are about 60% of it by weight. They are found in many rocks, including those formed from cooled magma, changed rocks, and rocks that have settled over time.

These minerals form when magma cools and become solid. You can find feldspar in many places, from deep inside the Earth to the surface in different kinds of rocks.

Etymology

The word feldspar comes from the German word Feldspat. This German word is made from two parts: Feld, meaning "field", and Spat, meaning "flake". Long ago, people used the word Spat to describe rocks that could be split easily into thin pieces. Later, in the 1700s, the word Feldspat was created to describe a special kind of rock often found in fields or looking like little fields inside other rocks. When this word came into English, it changed a little to feldspar, because of another English word, spar, which describes minerals that are clear and break easily. Today, we use the word feldspathic to talk about things that have feldspar in them. There was once another way to spell it, felspar, but we don’t use that anymore. The word "felsic", which describes light-colored minerals like quartz and feldspar, comes from the words feldspar and silica, not from the old spelling "felspar".

Compositions

The feldspar group of minerals is made of special kinds of rocks called tectosilicates. In these rocks, silicon and oxygen atoms connect in a pattern that looks like a net. Feldspars are made mostly of silicon, oxygen, and metals like potassium, sodium, or calcium.

Feldspars come in different types based on which metals they contain. The three main types are:

• Orthoclase: Contains potassium, aluminium, silicon, and oxygen.
• Albite: Contains sodium, aluminium, silicon, and oxygen.
• Anorthite: Contains calcium, aluminium, silicon, and oxygen.

Some feldspars mix together. For example, mixtures of orthoclase and albite are called alkali feldspar, while mixtures of albite and anorthite are called plagioclase feldspar. These mixtures happen when the minerals cool and change shape.

Structure

Feldspar crystals are made from tiny shapes called aluminosilicate tetrahedra. Each of these shapes has an aluminium or silicon atom in the middle, surrounded by four oxygen atoms. These oxygen atoms are shared with nearby tetrahedra, creating a three-dimensional network.

You can imagine these networks as long chains that look like crankshafts because they are kinked. These crankshaft chains connect to each other, forming a three-dimensional web of fused four-member rings. This structure has space for atoms like sodium, potassium, or calcium to fit inside and keep the crystal balanced.

Weathering

When feldspars are exposed to water, they break down through a process called hydrolysis. This creates clay minerals such as illite, smectite, and kaolinite. Feldspars dissolve more quickly in water that is either acidic or basic, and slower in neutral water. As they dissolve, they form new ions in the water.

Because feldspars are so common in the Earth's crust, clays are also very common. About 40% of minerals in sedimentary rocks are clays, and clays are the main minerals in mudrocks. They are also important in soils. When feldspar changes into clay, it looks chalky instead of clear and glassy.

Feldspars, especially plagioclase feldspars, are not very stable at the Earth's surface because they form at high temperatures. This makes them break down easily into clays. Because of this, feldspars are usually not found in many sedimentary rocks. When a sedimentary rock has a lot of feldspar, it means the material was moved only a short distance and buried quickly, without much chance to break down. Sandstones with lots of feldspar are called arkoses.

Applications

Feldspar is a common material used to make glass, ceramics, and as a helper in paints, plastics, and rubber. In the US, most feldspar is used to make glass, like bottles and fibers. The rest is used for ceramics, including things like dishes, tiles, and insulators, as well as fillers in other products.

Feldspar helps make glass by providing important ingredients and keeping the cost low. In ceramics, it helps create a smooth finish and is used in glazes. It can also help tell the age of very old objects in science and archaeology.

Some household cleaners, like Bar Keepers Friend and Bon Ami, use feldspar to gently scrub surfaces.

Glass Ceramics Earth sciences Potassium–argon dating Argon–argon dating Luminescence dating Bar Keepers Friend Bon Ami

Production

The USGS estimated that in 2020, the world produced about 26 million tonnes of feldspar. The top four countries for making feldspar were China, India, Italy, and Turkey.

Commercial grades

Typical mineralogical and chemical analyses of three commercial grades used in ceramics are:

Extraterrestrial

In October 2012, the Curiosity rover discovered a rock on Mars that contained a lot of feldspar. This finding helped scientists learn more about the composition of rocks on other planets.