The Earth’s crust is almost entirely built from minerals, which are naturally occurring solids with a definite chemical composition and a regular atomic structure. Among the many mineral groups, the silicates are overwhelmingly the most important, constituting over 90% of the crust’s mass. This dominance stems from the chemical abundance of their primary components and their versatile atomic architecture. Silicate minerals form the fundamental matrix of nearly all rocks, and the most common individual mineral group within this class is responsible for the bulk structure of the planet’s surface layer.
The Defining Feature of Silicates
The defining characteristic that unites all silicate minerals is the basic structural unit known as the silicon-oxygen tetrahedron. This unit consists of a single silicon atom positioned in the center of four larger oxygen atoms, forming a four-sided pyramid. This structure results in an overall negative charge of \(4^-\) for the complex (\(SiO_4\))\(^{4-}\). To achieve electrical neutrality, this negatively charged tetrahedron must bond with positively charged metal ions, such as iron, magnesium, or calcium.
The immense variety in silicates arises from the diverse ways these tetrahedra link together. They can remain isolated (olivine) or share oxygen atoms at their corners to form more complex structures. Sharing oxygen creates distinct sub-groups, including chains (pyroxenes and amphiboles) or extensive flat sheets (micas). The most complex linkage occurs when all four oxygen atoms are shared, creating a three-dimensional framework structure.
Geological Reason for Silicate Dominance
The reason silicates form the vast majority of the Earth’s crust is directly tied to the abundance of their constituent elements. Oxygen and silicon are the two most common elements in the crust. Oxygen makes up approximately 46.1% of the crust’s mass, while silicon accounts for about 28.2%, providing the necessary components for the silicon-oxygen tetrahedron.
The formation of silicate minerals is governed by the cooling of molten rock, or magma, a process summarized by Bowen’s Reaction Series. This series outlines the sequence in which different silicate minerals crystallize at progressively lower temperatures. Minerals formed at high temperatures, such as olivine, have simple structures, while those that crystallize later, like quartz and feldspar, have more complex, silica-rich structures.
This systematic crystallization ensures that the entire solid crust is essentially a massive, interlocking silicate framework. The minerals that form are determined by the temperature and the specific mix of other available metal ions, such as potassium, sodium, calcium, iron, and magnesium.
The Most Abundant Specific Mineral
The single most common mineral group in the Earth’s crust, making up roughly 60% of its volume, is feldspar. Feldspars are aluminosilicate minerals, meaning their framework structure uses silicon-oxygen tetrahedra where some silicon atoms have been replaced by aluminum. This substitution introduces an electrical charge imbalance, which is corrected by the inclusion of large alkali or alkaline earth metal ions, typically potassium, sodium, or calcium.
The general chemical formula for feldspar is \(\text{XAl}(\text{Al},\text{Si})_3\text{O}_8\), where \(\text{X}\) is one of these balancing metal ions. As framework silicates, all four corners of the tetrahedra are shared to create a strong, three-dimensional network. This structure gives feldspar minerals a moderate hardness, generally ranging from 6.0 to 6.5 on the Mohs scale.
Feldspars exhibit two prominent cleavage planes that intersect at or near a 90-degree angle, a key physical property used for identification. Their color is highly variable, ranging from colorless and white to pink, gray, or green. These minerals crystallize across a wide range of temperatures and compositions, which explains their overwhelming prevalence in all types of igneous, metamorphic, and sedimentary rocks.
Practical Applications and Common Varieties
Feldspar is a group of minerals with two main series that form a continuous range of compositions: plagioclase feldspar and alkali feldspar. Plagioclase is a solid solution series that varies continuously between sodium-rich albite (\(\text{NaAlSi}_3\text{O}_8\)) and calcium-rich anorthite (\(\text{CaAl}_2\text{Si}_2\text{O}_8\)). The composition of plagioclase changes predictably as magma cools, evolving from calcium-rich to sodium-rich phases, which is the basis of the continuous branch of Bowen’s Reaction Series.
Alkali feldspar is a solid solution primarily between potassium feldspar (\(\text{KAlSi}_3\text{O}_8\)) and sodium-rich albite. Potassium varieties, such as orthoclase and microcline, are commonly found in granite and other silica-rich rocks. These two main groups are often identified by distinct physical traits, such as the characteristic fine parallel grooves, or striations, often visible on the cleavage surfaces of plagioclase.
The abundance and chemical composition of feldspar make it valuable for various industrial applications. It is a primary raw material in the manufacturing of ceramics, where it acts as a flux to lower the melting temperature during firing. In the glass industry, feldspar provides alumina, which improves the product’s hardness and durability. It is also used as a component in abrasive materials and as a filler in paints and plastics.