Minerals are classified as metallic or nonmetallic. This distinction is fundamental to geology and dictates how minerals are used. Determining a mineral’s category does not require complex chemical analysis or specialized equipment, but relies instead on a careful examination of its observable physical characteristics. These observable properties provide a reliable and quick method for preliminary identification.
Luster: The Primary Visual Distinction
Luster describes how a mineral reflects light, serving as the fastest initial way to categorize it. A mineral displaying metallic luster appears similar to a polished metal surface, such as chrome or silver. This appearance is due to the mineral’s electron structure, which allows light to be reflected uniformly and strongly. Minerals like galena and pyrite exhibit this mirror-like reflectivity.
Nonmetallic minerals exhibit a wider range of reflective qualities that do not resemble metal. They lack the strong, unified reflection of metallic minerals and are often subdivided based on the specific type of light reflection observed. Common nonmetallic lusters include:
- Vitreous, which is the bright, reflective quality of glass (quartz).
- Pearly, resembling the soft, iridescent sheen found on the inside of a seashell.
- Silky, observed in fibrous minerals like gypsum.
- Resinous, appearing similar to solidified tree sap or plastic.
- Dull or earthy, where the mineral reflects little to no light.
The term submetallic is sometimes used for highly reflective but slightly duller samples, which are generally grouped with nonmetallic minerals for field work.
The Streak Test Confirmation
The color of a mineral’s powder, known as its streak, offers a reliable confirmation. A mineral’s surface color can be highly variable due to impurities or weathering, but the streak color tends to remain consistent.
The streak test is performed by dragging the mineral across unglazed porcelain, which is harder than most common minerals. This action grinds the sample into a fine powder, revealing its true color and providing a definitive characteristic for identification.
Minerals with metallic luster typically leave a distinct, dark, or heavily colored streak. For instance, hematite, which can appear silvery-gray or black, consistently leaves a reddish-brown streak. Similarly, the metallic mineral galena often produces a dark gray or black streak.
In contrast, nonmetallic minerals generally produce a colorless, white, or very light-colored streak. Many common nonmetallic silicate minerals, such as feldspar and quartz, are harder than the porcelain plate, meaning they leave no powder streak at all. If they are softer, the resulting powder is usually a pure white, indicating a lack of the dense, colored metal ions typical of metallic minerals.
Comparing Other Physical Properties
After observing luster and streak, other physical properties provide supporting evidence to finalize the classification. Specific gravity, the ratio of a mineral’s density to the density of water, is a particularly useful property.
Specific Gravity
Metallic minerals generally possess a significantly higher specific gravity than nonmetallic minerals. This difference arises because metallic minerals often contain elements with high atomic masses, such as iron, lead, or gold, packed tightly into their crystal structures. A sample of a metallic mineral, therefore, feels noticeably heavier for its size compared to a nonmetallic sample of the same volume. For example, galena has a specific gravity around 7.5, while quartz has a specific gravity of approximately 2.65.
Tenacity
Tenacity describes a mineral’s resistance to breaking, crushing, bending, or tearing, and it differs reliably between the two groups. The strong metallic bonds found in metallic minerals often lend them properties like malleability (hammered into thin sheets) and ductility (drawn into thin wire). Native metals, such as copper or silver, exhibit these properties, allowing them to deform plastically under stress rather than fracturing. This structural feature is a direct result of their characteristic electron sharing and lattice structure.
Nonmetallic minerals are typically characterized by brittleness. These minerals, held together by weaker ionic or covalent bonds, shatter easily. Rather than deforming, they commonly exhibit cleavage, breaking along flat, parallel planes that reflect weaknesses in their crystal structure, such as the perfect cleavage seen in mica or calcite.