Color and streak are among the most noticeable properties of minerals, providing valuable clues for identification. While color can be variable due to impurities, streak—the color of a mineral's powder—is often a more reliable diagnostic feature.
Mineral Color
How Minerals Get Their Color
The color of a mineral is primarily determined by its chemical composition and atomic structure. Specific elements or impurities within the crystal lattice can absorb certain wavelengths of light, resulting in the characteristic colors we observe.
Types of Color in Minerals
- Idiochromatic color: Inherent color determined by the mineral's essential chemical composition (e.g., sulfur is always yellow)
- Allochromatic color: Color caused by trace impurities or defects in the crystal structure (e.g., corundum is colorless in pure form but becomes ruby with chromium impurities)
- Pseudochromatic color: Color resulting from optical effects such as light interference or diffraction (e.g., the play of colors in opal)
Limitations of Color for Identification
While color is often the first property we notice, it can be misleading for several reasons:
- Many minerals occur in multiple colors (e.g., quartz can be clear, white, purple, pink, or brown)
- Weathering can change a mineral's surface color
- Impurities can significantly alter color even within the same mineral species
The Streak Test
What is Streak?
Streak is the color of a mineral when it is powdered by being scratched across an unglazed porcelain plate. This test reveals the true color of the mineral, unaffected by surface coatings or weathering.
How to Perform the Streak Test
- Obtain an unglazed porcelain streak plate
- Select a fresh, clean surface of the mineral sample
- Gently but firmly rub the mineral across the streak plate
- Observe the color of the powder left behind
Special Considerations
- Minerals harder than porcelain (Mohs hardness >6.5) will scratch the streak plate instead of leaving a powder
- For very hard minerals, you can crush a small sample to observe the powder color
- Some minerals may have a streak that is different from their visible color
Common Minerals and Their Streak Colors
| Mineral | Common Color(s) | Streak Color |
|---|---|---|
| Hematite | Silver, gray, red, brown | Reddish-brown |
| Magnetite | Black | Black |
| Pyrite | Brass yellow | Greenish-black |
| Galena | Lead gray | Gray to black |
| Chalcopyrite | Brass yellow to golden | Greenish-black |
| Calcite | Colorless, white, gray, yellow, green, blue | White |
| Quartz | Colorless, white, purple, pink, brown | White (usually no visible streak) |
| Gypsum | Colorless, white, gray | White |
Practical Applications
Color and streak analysis are essential tools for field geologists, mineral collectors, and professionals in mining and mineral exploration. These simple tests can quickly narrow down the identification of unknown specimens and provide insights into the mineral's composition and potential uses.
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