Feldspars correspond to a group of minerals that form a series of solid solutions such as plagioclase feldspars and alkaline feldspars, whose general formula is X (Al, Si)4 O8. These minerals are igneous, sedimentary and metamorphic rock-formers and more than 50% of feldspars are present in the earth’s crust.
Physical properties and characteristics of feldspars
| Feldspar | Physical properties and characteristics |
|---|---|
| class, group | Silicate, Tectosilicate |
| Brightness | Vitreous, in some faces of pearly exfoliation. |
| Color | Usually white, pink, brown, or gray. They are also colorless, orange, yellow, red, blue, green, and black. |
| Stripe | White |
| Chemical composition | X(Al,Si)4O8, X represents potassium, sodium, calcium and in some cases barium, strontium or rubidium. |
| Mohs hardness | 6 to 6.5 |
| Specific weight or density | 2.5 to 2.8 g/cm3 |
| Cleavage or exfoliation | Perfect in two directions, the cleavage planes typically intersect at approximately a 90 degree angle. |
| Fracture | Conchoidal to Unequal |
| crystalline system | triclinic, monoclinic |
| Way of introducing yourself (habit) | Prismatic, tabular, massive, broad-leaved. |
| Origin or geological environment | During the crystallization of igneous rocks, hydrothermal action on pegmatite veins and drusen in the rocks. Regional metamorphism of sedimentary rocks. |
| associated rocks | Granite, granodiorite, diorite, gabbro, rhyolite, andesite, basalt, trachyte, syenite, gneiss, arkosa, schist. |
| Associated minerals | Quartz, biotite, muscovite, hornblende. |
This group of minerals has similar chemical compositions, structures, and physical properties.
The continental earth’s crust is made up of more igneous rocks such as granite, granodiorite, diorite, which are rocks that have a high content of feldspars, as well as these minerals are present in the oceanic crust in rocks such as gabbro and basalt.
The following diagram shows the range of common igneous rock-forming minerals.
As mentioned before, granite, diorite and gabbro are rocks that make up the majority of the earth’s crust, in this image it can be seen that feldspars are the main minerals for the formation of these rocks.
Classification and chemistry of feldspars
The minerals that belong to this group have a generalized chemical formula X (Al, Si)4 O8, X represents seven ions, which are replaced among them, to form the minerals, the most common ions are sodium, potassium and calcium and among the rarest are barium, strontium, rubidium, and iron feldspars.
The following diagram shows the two solid solution series encompassed by the feldspar group.
Depending on the chemical composition, the feldspar minerals have been classified as observed in the ternary diagram. Plagioclase feldspars are made up of a series of solid solutions between Albite and Anorthite and on the left side of the diagram is the series of solid solutions of alkali feldspars.
Types of mineral feldspars
Plagioclase feldspars (plagioclase)
It corresponds to an isomorphic series of chalcosodic feldspars, the two extreme minerals of the series are albite (Si3AlO8Na) and anorthite (Si2Al2O8Ca).
These two minerals replace each other and depending on the relative proportion in the replacement, the series is classified as follows:
| plagioclase | % albite | % anorthite |
|---|---|---|
| Albite | 100 – 90 | 0 – 10 |
| Oligoclase | 90 – 70 | 10 – 30 |
| Andesine | 70 – 50 | 30 – 50 |
| Labradorite | 50 – 30 | 50 – 70 |
| Bitownite | 30 – 10 | 70 – 90 |
| Anorthite | 10 – 0 | 90 – 100 |
We have a whole article on plagioclase, you can click here.
These minerals are formed from the crystallization of a magma, in many cases these magmas have abundant sodium or calcium content.
They occur through a fusion with sodium and calcium ions, which throughout the solution are dissolved and suspended in various positions to form the complete series of solid solutions between pure sodium plagioclase and pure calcium plagioclase.
Alkali feldspars
They are minerals with chemical compositions that vary between NaAlSi3O8 and KAlSi3O8, which are:
| Mineral | Chemical composition |
| Albite | NaAlSi3O8 |
| anorthoclase | (Na,K)AlSi3O8 |
| sanidine | (K,Na)AlSi3O8 |
| orthoclase | KAlSi3O8 |
| microcline | KAlSi3O8 |
Sanidine, orthoclase and microcline can have the same chemical composition KAlSi3O8, being polymorphic minerals (same chemical composition but different crystal structure).
Temperature is the factor for these minerals to form with the same chemical composition. Myclocline forms at low temperature, orthoclase at medium temperature, and sanidine forms at high temperature.
Origin, formation and geological environment.
They are formed by the crystallization of igneous rocks, by hydrothermal action in pegmatite veins and drusen in the rocks, from the crystallization of aqueous solutions at low temperatures in hydrothermal veins.
Plagioclase feldspars are also formed by regional metamorphism of a sedimentary rock or inherited from an igneous protolith, being a major component of some metamorphic rocks.
Optical properties of feldspars
It explains how feldspars are observed in thin sheets:
| Color: | They are usually colorless and transparent. |
| Form: | It is euhedral with rectangular or square shapes |
| Relief: | Absent |
| Interference Colors: | First order, grey. |
| Others: | Maclado Carlsbad, divides the crystal into two twins that are parallel to the elongation |
Learn about plagioclase and how to identify it in a thin section in our unique article, click here.
We have a whole article about orthoclase and how you can identify it in thin film samples, click here.
Sedimentary rocks and sediments containing feldspars
These minerals are found in sedimentary deposits resulting from the weathering of igneous rocks and metamorphic rocks rich in feldspars, and they are found in greater quantities near the source or area of origin.
At a greater distance from the source, they are found in less quantity because they are affected by mechanical weathering because their two directions of perfect cleavage make them vulnerable, decreasing their size and exposure to chemical weathering, altering them to clayey minerals.
Arkosa is a sedimentary rock that is formed by the weathering of rocks rich in feldspar, contains at least 25% feldspars and are easy to identify grains. This rock is found on slopes and near the source where the feldspar grains were weathered.