Streak minerals are those whose powdered color, known as their streak, provides a consistent and valuable diagnostic tool for identification. Key examples include hematite, which leaves a reddish-brown streak; pyrite, known for its black streak; and real gold, which produces a distinct gold streak.
Understanding Mineral Streak
The streak of a mineral refers to the color of the fine powder produced when the mineral is scraped across an unglazed porcelain plate, known as a streak plate. While a mineral's apparent body color can vary greatly due to impurities or weathering, its streak color is remarkably consistent, making it a reliable property for identification. This consistency is why the color of the streak can help to identify an unknown mineral.
For instance, differentiating between iron pyrite (fool's gold) and real gold is made easy by their streak: pyrite leaves a distinct black streak, whereas real gold produces a bright gold streak. This simple test can prevent misidentification between visually similar minerals.
Common Examples of Streak Minerals
Many minerals exhibit characteristic streak colors that differ from their external appearance. Here's a table showcasing some notable examples:
| Mineral | Common Appearance | Characteristic Streak Color |
|---|---|---|
| Hematite | Silvery-gray, metallic, or dull reddish-brown | Reddish-brown |
| Goethite | Brown, yellowish-brown | Yellowish-brown |
| Pyrite | Metallic brass-yellow ("fool's gold") | Black |
| Gold | Metallic yellow | Gold |
| Galena | Metallic lead-gray, cubic crystals | Lead-gray |
| Chalcopyrite | Metallic brass-yellow, often tarnished | Greenish-black |
| Magnetite | Black, metallic, magnetic | Black |
| Sphalerite | Resinous yellow, brown, black | Yellowish-brown to white |
| Cinnabar | Bright red | Scarlet red |
| Chromite | Black, metallic | Brownish-black |
| Bornite | Metallic reddish-brown, often iridescent | Grayish-black |
Note: Minerals harder than the streak plate (e.g., quartz, feldspar, calcite) will not leave a powder and typically have a white or no streak on a standard plate, as they scratch the plate instead.
Why Streak is a Key Identification Property
- Consistency: Unlike a mineral's external color, which can be influenced by trace impurities, weathering, or crystal structure, the streak color represents the true color of the powdered mineral, making it a more consistent and reliable indicator.
- Differentiation: Streak is particularly useful for distinguishing between minerals that look similar. For example, several black minerals exist, but their streaks can range from black (magnetite) to reddish-brown (hematite).
- Simple Test: The streak test is a quick and non-destructive method (for minerals softer than the streak plate) that requires minimal equipment.
For more information on mineral identification and properties, you can consult resources from the U.S. Geological Survey (USGS) or geological educational sites like Mindat.org.
