Minerals are naturally occurring solids, each possessing a specific chemical composition and characteristic atomic structure. These underlying chemical and structural differences manifest as distinct physical properties that allow for identification and categorization. The most basic classification divides all minerals into two major groups: metallic and nonmetallic. Determining which of these two categories a mineral belongs to is often the first step in identification.
What Defines Metallic and Nonmetallic Minerals
Metallic minerals typically contain metal elements (native forms like gold or copper, or compounds like sulfides and oxides). This composition allows for delocalized electrons, facilitating high electrical and thermal conductivity. Structurally, these minerals tend to be opaque, meaning light cannot pass through them.
Nonmetallic minerals are composed primarily of nonmetal elements, often forming compounds like silicates, carbonates, or halides (e.g., quartz and calcite). Lacking free-moving electrons, nonmetallic minerals are poor conductors of heat and electricity, acting as insulators. Their internal structure often allows light to pass through, making them transparent or translucent.
Luster: The Key Visual Property
Luster is the single most effective property for visually distinguishing between these two groups, describing how a mineral’s surface interacts with and reflects light. A quick assessment of luster provides the most reliable clue to a mineral’s classification, stemming from the internal arrangement of atoms and bonding.
Minerals with metallic luster appear like polished metal, reflecting light intensely and evenly (e.g., pyrite or galena). This high reflectivity is a direct consequence of the mineral’s opaque nature. A mineral is classified as metallic if it displays this characteristic shine, even if the sample is tarnished or dull.
Nonmetallic luster encompasses a wide array of appearances that do not resemble metal. These lusters are described using terms related to common materials, such as vitreous (glassy, typical of quartz), pearly (like talc), or earthy (dull and non-reflective, often found in clay minerals).
Secondary Physical Tests for Confirmation
Other physical properties are used to confirm the distinction, especially when a mineral’s surface is weathered or ambiguous. Streak involves rubbing the mineral across an unglazed porcelain plate to observe the color of its powdered form. Metallic minerals typically leave a distinctive, dark, or colored streak (e.g., the greenish-black streak of chalcopyrite).
Nonmetallic minerals most often produce a colorless or white streak. Density, or specific gravity, offers another point of differentiation: metallic minerals are generally much denser than nonmetallic minerals due to the high atomic weight of the metal elements they contain.
Hardness, the mineral’s resistance to scratching, is a final test. While hardness varies widely, metallic minerals are often softer and more malleable compared to many common nonmetallic minerals like quartz. However, hardness is more useful for identifying a specific mineral within a category than for the initial metallic versus nonmetallic classification.