The number of use cases for feldspars are as diverse as the material itself - from glassmaking to ceramics to fillers. African Pegmatite is a leading miller and supplier of the finest quality feldspars and other related materials and minerals to the most exacting customers globally; proudly boasting the broadest reach, the most experience and the widest selection of in house milling technologies to provide the right product at the right time.
Feldspar are a class of rock-forming tectosilicate minerals that contains calcium, potassium or sodium and makes up more than half of the earth’s core by weight. Its minerals are largely found in igneous, sedimentary and metamorphic rocks spread across different parts of the world. These minerals are such essential components of these rocks that the classification of a number of rocks is based on the feldspar mineral content.
For the most part, feldspars are naturally occurring in opaque shades of white to gray and pink to very dark grey and have other vitreous lusters. Since feldspars are made up of two split planes intersecting at 90degrees, parts of pure feldspar crystals have a huge tendency to form rectangular blocks with varying ends.
It is important to note that feldspars can be split into two primary groups which are potassium feldspars and plagioclase. Both forms of feldspars have two visible cleavages and a wealth of colors which overlap. Plagioclase, in particular, has small grooves on one of its cleavages which are commonly called striations.
The mineral composition of most feldspars can be expressed in the ternary form including albite, orthoclase, and anorthite. The minerals of which composition is based primarily on the solid solution range between albrite and anorthite is what is known as the plagioclase feldspars while those between albrite and orthoclase or mono line are what is collectively known as alkali feldspars. They are referred to as alkali feldspars due to the presence of potassium and sodium minerals.
These alkali feldspars, especially alkali feldspars of potassium origin are of particular to industries because of its wide range of applications. They play pivotal roles in the production of fillers in the paint, rubber, adhesive and plastic industries. They are also used as fluxing agents in glass and ceramic applications.
In general, most of the products we use on a daily basis are made with feldspar. This includes floor tiles and shower basins bathrooms, glass for drinking water and other fluids, fiberglass used for insulation and different types of tableware used for dishing meals.
In fact, a number of popular gemstones like sunstone, moonstone, labradorite, spectrolite, and amazonite are feldspar minerals.
Physical Properties of Feldspar Minerals
Feldspar minerals have a surprisingly consistent range of physical properties common to them. Most of the minerals present in feldspar have two directions of perfect cleavage which intersect at nearly right-angles. Not only do they have about the same angle of inclination for their cleavages, but they also have similar Mohs hardness and specific gravity.
Most feldspar minerals have a specific gravity within the 2.5-2.8 range while their Mohs hardness hovers about 6 to 6.5. In addition to all these similarities in physical properties, all feldspar minerals have a vitreous luster that is mostly pearly on cluster faces. This is a great way of identifying feldspar minerals.
These properties of feldspar minerals which are mostly consistent in all forms of these minerals are pretty useful even when the feldspar crystals to be identified are very small. It is extremely easy for people who are conversant with feldspar cleavages to identify feldspar in igneous rocks. No matter how small the feldspar crystals are, they can be easily identified as separate from other minerals when an igneous rock is examined using a hand lens, as its properties are easy to identify.
With minimal practice, one can determine the Mohs hardness of even tiny feldspar minerals using a set of mineral hardness picks and a hand lens.
As detailed later, many of feldspar’s applications stem from its unique behaviour at temperature. Feldspars tend to have a brittle to ductile transition in the 700 to 800 °C range - far lower than many of the materials with which they are mixed - and therefore it often behaves as a flux.
Uses and Applications of Alkali Feldspar
The most important properties of feldspar to downstream industries and many other industries are their alumina and alkali content. For this reason, alkali feldspars (potassium feldspars) have a wide range of applications in paint industries, glass, and ceramic making industries. Here are some of the applications of potassium feldspars.
Glass: Feldspar is a critical material in the production of glass. It is also a crucial raw material in its production as it tends to act as a fluxing catalyst. As a fluxing catalyst, it reduces the temperature of quartz while simultaneously helping to keep the viscosity of the produced glass at the right correct range. It is the alkali content of the feldspar that helps it function effectively as a flux in glass making.
Fluxes reduce the overall melting point of a given mixture - this has the benefit of reducing the overall energy requirement needed to achieve a particular melt. Fluxes are widely used throughout the glass and ceramic industries for their ability to promote complete liquefaction. Feldspar melts at an early stage in the overall firing process, efficiently and rapidly producing a glassy matrix that is responsible for holding the other materials in the melt together. Additionally, feldspar can give the glass enhanced desirable properties such as increased hardness, durability and resistance to chemical attack. Alumina and calcium ions are responsible for these effects.
It is the alkalis in the feldspar - chiefly potassium and sodium - that lower the melting temperature within the mixture. They promote the melting of and bonding of the other materials going into the glass batch.
One of the advantages to using feldspar as an additive in glassmaking as a flux is that its addition does not change the batch redox number at all. This is beneficial as any change in redox number will require the commensurate addition of another material to negate the change. Feldspar can therefore be thought of as a more environmentally friendly addition - both due to its fluxing capacity meaning less energy is needed, but also the fact that its addition does not force the addition of other materials.
Glassmaking accounts for around two thirds of feldspar used in the United States in any given year, with the majority of the balance being made up of use as fillers and in ceramics. For glassmaking purposes, feldspars are ground to 0.85 mm (20 mesh).
Fillers: Feldspars are functionally used as extenders and fillers in paints, rubbers, and plastics. It is an effective filler because of a number of factors. These factors include its stable pH, high chemical inertness, good dispersibility, impressive refractive index, resistance to frosting, high resistance to abrasion and low viscosity at high filler loading. In general, the products used for such purposes are fine-milled grades.
As a filler in plastics, feldspars find little use in thermoplastics due to their relatively high Mohs hardness, however in plastic films feldspars come into their own as leading components for antiblocking. Blocking is where two (or more) materials interact with one another at their surfaces - antiblocking materials aim to prevent this and are therefore commonly referred to as ‘anti-slip’ agents. It is especially useful in low density polyethylene (LDPE), polypropylene and polyvinyl chloride films. Additionally, feldspars can be used to make LDPE films more suitable to heat management, for example in greenhouses and polytunnels.
In paints, coatings and lacquers, ultra-finely milled feldspars are used as fillers. The non-toxic addition of feldspar is appealing as it reduces overall material cost, but does not impact on the colour density owing to its relatively low refractive indices in the region of 1.0 to 1.5.
Ceramics: Putting things in perspective, feldspar is considered the other most critical ingredient – besides clay – in the production of ceramics. It doesn’t have a definite melting temperature, as it melts slowly throughout various temperatures. This is one of its advantages as it accelerates the melting of clays and quartz, while simultaneously allowing modulation of this crucial stage in the production process. They are generally used as fluxing catalysts to produce a glassy coating at low temperatures. They are also used as a source of alumina and alkalis in glazes.
Most often than not, feldspars improve the toughness, durability, and strength of the ceramic body. Also, they melt, soften and wet other components of the batch mix. Ceramic grade feldspars are typically ground to no more than 75 μm (200 mesh).
Just like in glassmaking, the basic components of the feldspars (sodium, potassium and calcium) behave as fluxes - they work with soda ash (like in soda glass) to reduce the melting temperature and increase the fusibility of all components. Additionally, feldspar controls the degree of vitrification of the ceramic body during firing.
Porcelian: Feldspar, along with kaolin and quartz (as a form of silica) are the primary components in modern porcelain ceramics. These are ‘hard paste porcelain’ types, which compared to ‘soft paste’ are fired at temperatures around 1,400 °C, whereas ‘soft’ types are fired only at 1,200 °C. These types of porcelain are regarded as stronger and much less likely to crack than ‘soft paste’ variants. Depending on firing type, it can resemble conventional stoneware or earthenware. As with all ceramics, it is now understood that one of the major reasons for feldspar’s inclusion is for its superior fluxing capabilities.
Enamel glazes: In enamel glazes, feldspar helps to enhance the aesthetics of the final product by ensuring that there are no errors in the final product. It is useful in the manufacturing of sanitary ware, ceramic tile glazes, tableware, giftware, electrical, and enamel frits. In sanitary wares, feldspars are used to facilitate the optimization process.
Feldspar also has a host of other end-uses which include urethane, mild abrasives, paints, welding of red coating, production of steel and latex foam.
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