Potassium Feldspar

Over half the Earth’s crust is composed of a single group of minerals known as the feldspars. Feldspars are an important component of many building stones, but in economic terms these minerals are more important for what they become than what they are. As feldspars weather they produce clay minerals and aluminum ores, two resources used throughout our modern society.

Based on their chemical composition, feldspars can be subdivided into two mineral groups – plagioclase feldspars and potassium feldspars. Potassium feldspars are a group of polymorphs, polymorphs being minerals that have the same chemical composition but slightly different crystal structures. Despite their different crystal structures, the potassium feldspars are very similar to one another in appearance, so in the field they are often identified simply as ‘potassium feldspar’.

Although their color varies, potassium feldspars typically form pink, reddish or white, hard blocky crystal masses that exhibit cleavage in two directions. The resulting cleavage faces meet nearly at right angles. Potassium feldspars are particularly common in felsic igneous rocks, metamorphic rocks, pegmatites and low temperature hydrothermal veins.

Description and Identifying Characteristics

‘Feldspar’ is derived from the German word feld,which means ‘field’, and spar, which means ‘crystal’. This name reflects the minerals’ abundance, as most crystalline material found in the field is feldspar. Potassium feldspars are the feldspar minerals in which the silicate tetrahedral and aluminum tetrahedra are bound with potassium ions, rather than sodium or calcium ions as in the plagioclase feldspar subgroup. The potassium feldspar group is composed of three mineral polymorphs, each having the same chemical composition, but slightly different crystal structures. Technically these are distinct minerals, but their physical properties are so similar that they are usually only identified as ‘potassium feldspar’ in the field. Using other minerals in the rock to determine the host rock’s identity is often the most useful guide to their probable identity.

All of the feldspar minerals exhibit a blocky crystal habit, hardness greater than glass, and two directions of cleavage that meet at nearly right angles. Together with their non-metallic luster and typically opaque character, these properties will usually distinguish the feldspars from other common non-metallic minerals. Within the feldspar group, the potassium feldspars are only differentiated from the plagioclase minerals by the lack of fine parallel striations, which are usually present on some cleavage surfaces in the plagioclase minerals. In general, potassium feldspars commonly have pink to reddish hues, while the plagioclase feldspars tend to be white or gray, but both mineral groups may exhibit similar colors, so the presence or absence of striations is more diagnostic.

The crystal structure of the plagioclase and potassium feldspar groups are so similar that the two may develop as thin alternating bands within a single specimen. This mixed variety of feldspar is called perthite.

Potassium feldspars are an important component of most felsic igneous rocks, pegmatites, low temperature hydrothermal veins and many metamorphic rocks, especially those that formed from the alteration of shale or felsic igneous rocks. Their crystals are often particularly well developed in pegmatites, coarsely crystalline igneous rocks that form during late water-wet stages of a magma cooling. Within the potassium feldspar group, the occurrence of each potassium feldspar member varies slightly. Orthoclase is the most common potassium feldspar, forming the majority of potassium feldspar found in felsic plutonic igneous rocks, some high temperature veins, and higher-grade metamorphic rocks. Sanidine has a less ordered crystal structure than orthoclase and is primarily found in volcanic igneous rocks where magmas cooled more rapidly and crystals had less time to form. Microcline tends to form at lower temperatures and is the most common potassium feldspar found in pegmatites, low temperature hydrothermal veins, and some low-grade metamorphic rocks. It may also occur in significant amounts in granite.

Although not as resistant as quartz, feldspar minerals are resistant enough that they form a significant component of many sand deposits and sandstones. Ultimately though, these minerals will break down to form clay minerals. These clay minerals form the bulk of the fine-grained muds, mudstones and shales that cover much of the land surface and shallow seafloor.

The potassium feldspars usually occur with quartz, muscovite and the more sodium-rich plagioclase minerals. In metamorphic rocks, they may be accompanied by micas and garnets.

Potassium feldspars are a significant component of many felsic igneous rocks used as decorative building stones. By volume, however, the most significant use of potassium feldspar is as aggregate in concrete and asphalt. Large volumes are also used in the manufacture of glass, enamel and scouring powder. Microcline is particularly important in the manufacture of ceramics and ceramic glazes. Ground up microcline is mixed with clay and quartz, and then fired to form a wide range of ceramic and porcelain products, from fine pottery and dinnerware, to sinks, bathtubs, electrical insulators, and floor tiles.

Indirectly, the feldspar group provides some crucial resources that form as feldspars weather and break down. Water will react with feldspars to produce a host of clay minerals upon which all ceramics and pottery, from the crudest mud-covered reed basket to the finest porcelain dinnerware and figurines. Without ceramics, fired clay, most human cultures would have had dramatically different histories. More recently, another weathering product of feldspar has had nearly as great an impact on our industrial society. As feldspars breakdown under the intense weathering conditions of tropical climates, almost all of the feldspar breaks down and is dissolved away except for some hydrous aluminum oxide minerals that are left behind. These concentrated deposits of aluminum ores are collectively called bauxite and are our major source of aluminum.

In addition, some varieties of potassium feldspar have significant value as gemstones. Some glassy sanidine crystals exhibit a distinctive bluish iridescence and are commonly known as moonstones. Amazonstone is a variety of microcline that has a distinctive bright green color and is used as jewelry or as ornamental stone. Perthite is another gemstone feldspar variety that consists of thin alternating bands of potassium feldspar and plagioclase that gives the mineral sample a distinctive striped pattern.

Potassium feldspars are important components of felsic igneous rocks throughout the Upper Midwest. Together with the plagioclase minerals, potassium feldspars form the bulk of many igneous and metamorphic decorative building stones found in the region, including the St. Cloud Granite, Morton Gneiss and Wausau Granite. Feldspars are also significant components of the Precambrian sandstones that filled the Mid-Continent Rift valleys, such as the Hinckley and Fond du Lac Sandstones.

A combination of hardness, non-metallic luster and two good cleavage directions at right angles to one another serves to distinguish the feldspar minerals from most of the common rock-forming minerals. It is far more difficult to discriminate between the feldspar group's internal divisions to distinguish plagioclase minerals from potassium feldspars. The presence of fine parallel striations on some plagioclase cleavage surfaces is the best way to distinguish the two.

Plagioclase:

The various plagioclase minerals share a similar composition, crystal structure and origin with many of the potassium feldspars, so the two groups are difficult to distinguish from one another. Typically, potassium feldspars tend to have more pink and reddish hues, while plagioclase minerals usually range from white to gray. However, the hues of both mineral groups overlap, so color is seldom definitive. Fine parallel lines or grooves, called striations, occur on some cleavage surfaces in the plagioclase minerals and are a more definitive means to distinguish the two groups.

Perthite:

Because the crystal structures of both feldspar groups are so similar, the two may actually occur as thin alternating bands within a single specimen. This mixed variety of feldspar is called perthite. The thin alternating bands may be mistaken as striations, but the alternating bands usually have slightly different colors, and as you follow individual bands across the cleavage surfaces, they tend to be more irregular than striations, with their boundaries not being as strictly parallel.

Quartz:

Quartz is a common mineral in many metamorphic and felsic igneous rocks that, like the potassium feldspars, also has a non-metallic luster and hardness greater than glass. Unlike the potassium feldspars, however, quartz does not exhibit cleavage, which easily distinguishes it from feldspar minerals. Quartz also tends to be more transparent and typically occurs as clear glassy masses.

Calcite:

Although calcite and potassium feldspar may have similar colors and both exhibit well-developed cleavage, the two are easily distinguished as calcite is softer than glass or metal, reacts readily with dilute acid, and its rhombic cleavage surfaces do not meet at right angles.

Dolomite:

Although dolomite and potassium feldspar may have similar colors and both exhibit well-developed cleavage, the two are easily distinguished as dolomite is softer than glass or metal, and its rhombic cleavage surfaces do not meet at right angles.