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Feldspar in granite.

Feldspar is the name of a group of rock-forming minerals that make up as much as 60% of the Earth's crust.

Feldspar forms crystals from magma in both intrusive and extrusive rocks, and they can also happen as compact minerals, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar is known as anorthosite. Feldspar is also found in many types of sedimentary rock.

Feldspar is typically reddish or pale pink in color, and has a hardness of 6 on the Mohs scale, making it roughly as hard to scratch as glass.

There are two groups of feldspar, both made out of silica and aluminum:

  1. Orthoclase or "alkali" feldspars: a group of minerals that all have the same formula (KAlSi3O8 with sodium sometimes replacing the potassium in some pieces), but are formed in different ways because of how different the times and places where they formed were.
  2. Plagioclase feldspars: A series of minerals that are made up of a series of two chemicals (Albite, NaAlSi3O8, and anorthite CaAl2Si2O8), with a given piece having its place on the series determined by the relative amounts of the two chemicals.

Production and uses

Feldspar output in 2005.

About 20 million tonnes of feldspar were produced in 2010, mostly by three countries: Italy (4.7 Mt), Turkey (4.5 Mt), and China (2 Mt).

Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber. In glassmaking, alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. Ceramics (including electrical insulators, sanitaryware, pottery, tableware, and tile) and other uses, such as fillers, accounted for the remainder.

In earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, and luminescence dating.

In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content.

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