Pyrite, or iron disulfide, is a common mineral recognized for its metallic luster and pale brass-yellow color. This golden appearance has earned it the nickname “Fool’s Gold,” as it has often been mistaken for true gold. Pyrite forms in a wide variety of geological environments, from sedimentary rocks to hydrothermal veins. While its outward color is visually striking, reliable identification requires diagnostic tests beyond simple visual inspection. Mineralogists rely on the streak test because a mineral’s surface color can be misleading due to trace impurities or weathering.
Defining Mineral Streak
The mineral streak is defined as the color of a mineral’s powder, which is revealed when a specimen is scraped across an unglazed porcelain plate. Unlike the apparent color of the mineral, which can vary significantly, the streak color tends to be a far more consistent and reliable property for identification. This consistency occurs because the powder is composed of tiny, randomly oriented crystals whose color is less affected by the larger crystal structure or minor surface contaminants. The powdered form allows light to interact with the mineral’s fundamental composition, providing a truer color signature. The streak test is particularly valuable for minerals that are opaque and have a metallic or sub-metallic luster.
The True Color of Pyrite’s Streak
The streak of pyrite is definitively dark, ranging from greenish-black to brownish-black. This dark powder provides a stark contrast to the mineral’s bright, brass-yellow appearance, making the streak test one of the easiest ways to distinguish pyrite from other yellow minerals. Pyrite’s chemical composition of iron and sulfur is responsible for this dense, dark powder.
The dark streak is the most effective way to separate “Fool’s Gold” from genuine gold. Gold is soft (Mohs hardness 2.5 to 3) and leaves a distinct yellow or golden-yellow streak. Pyrite (Mohs hardness 6 to 6.5) is much harder and leaves the unmistakable dark streak that confirms its identity as iron disulfide.
Performing the Streak Test
To properly perform the streak test, a piece of unglazed porcelain tile, known as a streak plate, is necessary. The streak plate must be harder than the mineral being tested to successfully crush the mineral into a fine powder. Standard streak plates typically have a Mohs hardness around 7, which is sufficient for pyrite, as its hardness is slightly lower. For dark-colored minerals like pyrite, a white streak plate is preferable to make the resulting dark powder more visible.
The procedure involves holding the streak plate firmly and drawing a sharp edge of the mineral specimen across the unglazed surface. Only a moderate amount of pressure is needed to leave a small line of powder. The color of this powdered residue is the mineral’s streak.