The classification of minerals within igneous rocks relies fundamentally on their chemical composition. Geologists use the terms “felsic” and “mafic” to categorize these minerals, providing a shorthand for understanding Earth’s crustal composition. This distinction is based on the relative abundance of light versus heavy elements, which determines a mineral’s color, density, and crystallization temperature.
Defining the Chemical Distinction
The term “felsic” is derived from “feldspar” and “silica,” referencing the elements that dominate these minerals. Felsic minerals are highly concentrated in Silicon (Si) and Oxygen (O), which form the basic building block of all silicate minerals: the silica tetrahedron. They also contain lighter metal ions such as Aluminum (Al), Potassium (K), and Sodium (Na) within their structure. The high proportion of silica content, typically greater than 65 percent by weight, is the defining chemical characteristic of felsic minerals.
In contrast, the term “mafic” is derived from “Magnesium” (Ma) and “Ferrum” (Fe), the Latin word for iron. Mafic minerals are characterized by a significant concentration of these two heavier elements, sometimes including Calcium (Ca). They contain a lower overall silica content, generally ranging between 45 and 55 percent by weight. The scarcity of silica means their crystal structures have less complex linkages of the silica tetrahedra compared to the network structures found in felsic minerals.
Observable Physical Properties
The disparity in chemical composition directly translates into differences in physical properties, making the minerals easily distinguishable. Mafic minerals generally exhibit a dark color, often appearing black, dark green, or brown. This deep coloration, known as melanocratic, is caused by the high concentration of iron and magnesium, which efficiently absorb light.
Conversely, felsic minerals are light-colored, or leucocratic, typically appearing white, pink, light gray, or colorless. This is because lighter elements like silicon, aluminum, and potassium do not absorb visible light strongly. The color serves as an immediate visual clue to the mineral’s chemical nature.
The density of the minerals is significantly affected by their elemental composition. Mafic minerals are notably denser than felsic minerals because iron and magnesium atoms are substantially heavier than silicon, aluminum, and alkali metal atoms. Mafic minerals generally have a higher specific gravity, often greater than 3.0. Felsic minerals, due to their lighter atomic constituents, are less dense, with a specific gravity typically below 3.0.
The crystallization temperature of the minerals follows a predictable pattern based on their chemistry. Mafic minerals, which are silica-poor, generally crystallize at much higher temperatures, often between 1000°C and 1200°C. Felsic minerals, being silica-rich, form at lower temperatures, typically in the range of 600°C to 750°C. This difference illustrates that minerals rich in iron and magnesium solidify earlier from a cooling magma body, a key principle of Bowen’s Reaction Series.
Common Minerals and Geological Setting
Felsic and mafic minerals dominate different parts of Earth’s crust due to their chemical and physical distinctions. Felsic minerals like Quartz, Potassium Feldspar (Orthoclase), and Muscovite Mica are common examples of this light-colored, silica-rich group. These minerals are the primary components of the continental crust, forming rocks such as granite.
Mafic minerals include common rock-forming silicates such as Olivine, Pyroxene, Amphibole, and Biotite Mica. These dark, dense minerals are the building blocks of the oceanic crust and the upper mantle. Basalt and gabbro are rocks composed predominantly of these iron- and magnesium-rich minerals.
Plagioclase Feldspar is one mineral that can span both categories, exhibiting a continuous range of composition. The calcium-rich end of the plagioclase series is considered mafic, while the sodium-rich end is categorized as felsic. This variation highlights the compositional gradation that exists in nature. The prevalence of less dense felsic minerals in continental landmasses explains why continents “float” higher on the mantle compared to the high density of mafic minerals in the oceanic crust.