Minerals are identified by physical characteristics like luster, hardness, and color. Since many properties can be misleading due to trace impurities or weathering, the streak of a mineral is one of the most consistent and reliable identification methods. It offers a standardized point of comparison, helping geologists accurately determine a mineral’s identity when its outward appearance is deceptive.
Defining Mineral Streak
Mineral streak is the color of a mineral when reduced to a fine powder. This powdered color is more fundamental than the mineral’s apparent mass color, which is highly variable. For instance, quartz can appear in almost every color, from clear to purple, due to minute chemical impurities.
The difference in color occurs because grinding the mineral breaks down its macroscopic crystal structure into microscopic, randomly oriented particles. In mass form, light absorption is influenced by the crystal structure and trace impurities. When powdered, these microscopic particles absorb and scatter light uniformly, revealing the true, inherent color of the mineral’s chemical composition with less interference.
The Practical Test: Determining Streak
The standard procedure for determining a mineral’s streak is the streak test. This involves firmly scraping the mineral specimen across a piece of unglazed porcelain, known as a streak plate. The friction grinds a small amount of the sample into a powder, leaving a line of residue that represents the streak color.
The unglazed porcelain plate is chosen for its consistent hardness, typically 6.5 to 7 on the Mohs scale. This physical constraint dictates the test’s practicality: any mineral harder than the plate will scratch the porcelain instead of being ground into powder. Minerals like quartz or topaz will therefore either leave no streak or simply scratch the plate, which is itself a diagnostic observation.
Why Streak is a Reliable Diagnostic Tool
Streak is a far more reliable diagnostic tool than the surface color of a mineral because the powdered color is chemically consistent. While the bulk color of a mineral can change drastically due to surface weathering or trace elements, the streak color remains constant for a specific mineral species. This consistency makes the streak test invaluable for distinguishing between minerals that look similar.
The iron-oxide mineral hematite is a classic example. Hematite exhibits a variable appearance, ranging from silvery-black metallic luster to a dull red or brown earthy form. Despite these varied appearances, all forms consistently produce a distinctive reddish-brown streak, immediately ruling out many other black or silvery-looking minerals.
Streak is also particularly useful in differentiating between “fool’s gold” and genuine gold. Pyrite, or fool’s gold, has a bright brassy-yellow color that closely resembles real gold. However, when tested, pyrite leaves a greenish-black to brownish-black streak, a clear indication that it is an iron sulfide mineral. In contrast, genuine gold, which is much softer and chemically distinct, leaves a true golden-yellow streak, confirming its elemental composition.