A mineral streak is the color of the powdered form of a mineral, a fundamental physical property used in mineral identification that often differs significantly from the mineral's apparent external color.
The Essence of Mineral Streaks
The streak of a mineral is observed by rubbing it across an unglazed porcelain plate, known as a streak plate. This process grinds a small amount of the mineral into a fine powder, revealing its true intrinsic color, which can be a more reliable diagnostic tool than the mineral's bulk color, especially for metallic minerals.
Key Physical Properties of Mineral Streaks
Several key physical properties define mineral streaks, making them invaluable for identification:
Streak Color
The color of a mineral's streak is its most defining characteristic. It's often consistent even when the mineral's external color varies due to impurities or weathering.
- Metallic Minerals: Streak is particularly vital for metallic minerals. These minerals consistently produce a very dark-colored streak, irrespective of their surface appearance. For example, hematite can be red, silver, or black, but its streak is always reddish-brown. Similarly, galena, a silvery-gray mineral, leaves a gray to black streak.
- Nonmetallic Minerals: For minerals with a nonmetallic luster, streak is generally not a useful property. Many nonmetallic minerals, especially those that are lighter in color, will leave a white or very light-colored streak, which provides little diagnostic information. Some colored nonmetallic minerals might have a streak that matches their body color, but this isn't always reliable.
Consistency
Unlike the varying external color of many minerals, which can be influenced by trace elements or surface oxidation, the streak color tends to be remarkably consistent for a specific mineral. This makes it a much more reliable diagnostic property. For instance, fluorite can be purple, green, yellow, or clear, but it always has a white streak.
Luster Correlation
The effectiveness of the streak test is directly related to a mineral's luster.
- Minerals with a metallic or submetallic luster often yield a distinctively colored streak.
- Minerals with a nonmetallic luster (e.g., vitreous, pearly, earthy) typically produce a white or very light streak, or no streak at all if they are harder than the streak plate, rendering the test less informative for identification.
Form and Texture
A streak is essentially the powdered form of a mineral. It appears as a fine, often opaque line on the streak plate. The texture of this powdered material is uniform and fine-grained.
Hardness Requirement
For a mineral to leave a streak, it must be softer than the streak plate, which typically has a Mohs hardness of about 7. Minerals harder than 7 (e.g., quartz, topaz, corundum) will scratch the streak plate without leaving a powdered trace, thus producing no streak.
Practical Examples of Mineral Streaks
The difference between a mineral's apparent color and its streak color can be striking, as shown in these examples:
| Mineral Name | Apparent Mineral Color (Common) | Streak Color | Luster |
|---|---|---|---|
| Hematite | Red, black, silver | Reddish-brown | Metallic |
| Pyrite | Brassy yellow (gold-like) | Greenish-black | Metallic |
| Galena | Silvery-gray | Gray-black | Metallic |
| Magnetite | Black | Black | Metallic |
| Limonite | Yellowish-brown, brown | Yellowish-brown | Earthy, dull |
| Sphalerite | Yellow, brown, black | Yellowish-brown | Resinous |
| Malachite | Green | Light green | Silky, dull |
| Fluorite | Purple, green, clear | White | Vitreous |
| Calcite | White, clear, various colors | White | Vitreous |
Why Streak Matters in Mineral Identification
The streak test is a quick and straightforward method for preliminary mineral identification because:
- Reliability: It reveals the mineral's true color, minimizing confusion caused by superficial variations.
- Consistency: The streak of a specific mineral is almost always the same, making it a dependable diagnostic property.
- Differentiation: It helps distinguish between minerals that look similar externally. For instance, gold (yellow streak) can be differentiated from pyrite (greenish-black streak), often called "fool's gold."
Understanding the physical properties of mineral streaks is crucial for accurately identifying minerals, especially those with metallic lusters where this test provides highly reliable information. For further reading on mineral properties, refer to Mindat.org or similar geological resources.
