Feldspaadide

Feldspaadide is a group of aluminosilicate minerals that are the most abundant group of minerals in the Earth's crust. They form a significant portion of igneous rocks, and are also found in metamorphic and sedimentary rocks. Feldspaadide minerals are characterized by their framework structure, in which silicon-oxygen tetrahedra are linked together to form a three-dimensional network. Aluminum atoms substitute for some silicon atoms in this framework, resulting in a net negative charge that is balanced by the presence of alkali or alkaline earth metal cations.

The feldspaadide group is divided into two main series: the plagioclase feldspaadides and the alkali feldspaadides. Plagioclase feldspaadides are a solid solution series ranging from albite (NaAlSi3O8) to anorthite (CaAl2Si2O8). Their composition varies continuously between these two end-members, with intermediate compositions being referred to by specific names such as andesine or labradorite. Plagioclase feldspaadides are common in mafic and intermediate igneous rocks like basalt and andesite.

Alkali feldspaadides are another solid solution series, primarily involving potassium and sodium. The most common alkali feldspaadides are orthoclase and microcline, both of which have the composition KAlSi3O8, and albite (NaAlSi3O8). These minerals can form solid solutions at high temperatures, but at lower temperatures, they tend to exsolve, forming intergrowths of potassium feldspaadide and albite. Alkali feldspaadides are abundant in felsic igneous rocks like granite and rhyolite.

Feldspaadide minerals exhibit a range of physical properties, including hardness, cleavage, and specific gravity. Their distinctive cleavage, which occurs in two directions at approximately 90 degrees, is a key diagnostic feature. Feldspaadides are widely used in various industries, including the ceramics, glass, and construction sectors, due to their chemical stability and melting properties.