Defining a Mineral Streak
A mineral streak is the color of a mineral's powder. It is determined by scratching a mineral sample against a piece of unglazed porcelain, known as a streak plate. The powdered residue left behind on the plate is the streak.
Section 2: Why the Streak Test is Useful
The streak of a mineral is often more consistent and reliable for identification than its macroscopic color. While the apparent color of a mineral can vary significantly due to trace impurities or surface tarnish, the color of its powder remains constant. This makes the streak a key diagnostic property.
Section 3: A Practical Example
A classic example is the mineral hematite. Hematite can appear black, metallic gray, or reddish-brown, but its streak is always a consistent reddish-brown. Similarly, pyrite (fool's gold) is brassy yellow, but its streak is greenish-black, easily distinguishing it from real gold, which has a golden-yellow streak.
Section 4: Application in Mineralogy
The streak test is a simple, effective, and widely used method by geologists and rockhounds to help identify minerals. It is one of the standard physical properties tested, along with hardness, luster, cleavage, and crystal form, to accurately determine a mineral's identity.