Minerals are the fundamental, naturally occurring components that form the rocks of Earth’s crust and mantle. Understanding their characteristics is central to geology, revealing information about the planet’s formation and history. Minerals also hold immense significance for industry and technology, providing raw materials for construction and electronics. Studying these characteristics allows scientists to classify, identify, and utilize the vast array of solid materials found globally.
The Five Defining Requirements of a Mineral
A substance must satisfy five specific criteria to be scientifically categorized as a true mineral. First, a mineral must be naturally occurring, formed by geological processes without human intervention. Materials synthesized in a laboratory, such as artificial diamonds, do not qualify. Second, a mineral must be inorganic, excluding materials derived from living organisms like wood or coal.
The third requirement is that a mineral must exist as a solid under normal conditions, excluding liquids or gases. Ice meets this criterion and is classified as a mineral. Fourth, a mineral must possess a definite chemical composition, expressed by a chemical formula, such as quartz (SiO2). This composition can sometimes vary within a range due to the substitution of similar elements, known as solid solution.
The fifth and most fundamental criterion is that a mineral must have an ordered internal structure, meaning it is crystalline. The atoms are arranged in a precise, three-dimensional, repeating pattern. This internal order distinguishes minerals from non-crystalline solids like volcanic glass, which are called mineraloids. The crystalline structure dictates nearly every other physical and chemical property the mineral exhibits.
Physical Properties Used for Identification
Geologists rely on several observable physical properties to identify minerals quickly. Hardness is a mineral’s resistance to scratching or abrasion. It is quantified using the Mohs Hardness Scale, which ranks minerals from 1 (talc) to 10 (diamond). A mineral with a higher number can scratch any mineral with a lower number, aiding identification.
Luster describes how a mineral’s surface reflects light, categorized as either metallic or non-metallic. Metallic luster appears like polished metal, while non-metallic lusters include glassy, pearly, or earthy descriptions. Color is often noticed first, but it is frequently unreliable because small chemical impurities can drastically change a mineral’s color, as seen in quartz.
A more consistent property than color is streak, which is the color of the mineral when ground into a fine powder. Geologists determine the streak by rubbing the mineral across an unglazed porcelain plate. The resulting powder color is diagnostic even when the hand sample’s color varies; for example, hematite’s streak is consistently reddish-brown regardless of its external appearance.
The way a mineral breaks provides information regarding cleavage and fracture. Cleavage is the tendency of a mineral to break smoothly along flat, parallel planes of weakness within its crystal structure, where atomic bonds are weaker. The quality of cleavage is described as perfect, good, or poor, resulting in forms like sheets (mica) or cubes (halite). Conversely, fracture describes an irregular and rough break, occurring in minerals where atomic bonds are of similar strength in all directions, often resulting in smooth, curved surfaces (conchoidal fracture).
Chemical Composition and Structural Classification
The precise chemical composition and internal atomic arrangement are used to classify the world’s approximately 6,000 known mineral species. The classification system groups minerals based on their dominant anion or anionic group, as minerals with the same chemical group share similar properties. This system organizes minerals into major classes, such as the Silicates, which are the largest group, making up over 90% of Earth’s crust.
Silicate minerals are defined by the silicon-oxygen tetrahedron (SiO4) as their fundamental building block. Other major classes include the Carbonates (CO3), common in sedimentary rocks, and Oxides (metal atoms bonded with oxygen). Sulfides contain sulfur bonded with metal, often forming economically important ores.
The crystalline structure, the ordered internal arrangement of atoms, forms the second layer of classification. All minerals are assigned to one of seven crystal systems, such as cubic or hexagonal, based on the symmetry of their unit cells. The Silicate class is further subdivided based on how the silicon-oxygen tetrahedra link together—forming chains, sheets, or frameworks—which dictates the mineral’s cleavage and stability.