To observe a mineral's streak, you scrape a specimen of the mineral across a piece of unglazed porcelain, known as a streak plate. This action produces a small amount of powdered mineral on the plate's surface, and the color of this powder is the mineral's characteristic "streak."
How Do You Observe Streaks?
Observing a mineral's streak is a fundamental and highly effective method for identifying minerals, often revealing their true, underlying color, which can differ significantly from the external appearance.
What is a Streak Test?
The streak test is a simple yet powerful diagnostic tool in mineralogy. It involves creating a fine powder of the mineral by abrading it against a surface harder than the mineral itself but softer than many common minerals. This specially designed surface is called a streak plate, typically made of unglazed porcelain, which has a Mohs hardness of about 7. The color of the resulting powder, or "streak," is a consistent property for many minerals, regardless of variations in their external color due to impurities or weathering.
The Process: Observing a Mineral's Streak
Performing a streak test is straightforward and requires minimal equipment.
Materials You'll Need
- Mineral Specimen: The rock or mineral you wish to identify.
- Streak Plate: A small piece of unglazed porcelain. These are commonly available in geological supply kits or can sometimes be found as the unglazed back of a ceramic tile.
- Magnifying Glass (Optional): Helpful for observing faint or very fine streaks.
- Water (Optional): For cleaning the streak plate between tests.
Step-by-Step Guide
- Prepare the Streak Plate: Ensure your streak plate is clean and free of any debris or previous mineral powder. A clean surface ensures an accurate streak color.
- Scrape the Mineral: Hold the mineral specimen firmly and scrape it across the unglazed surface of the streak plate. Apply steady, firm pressure, similar to drawing with a crayon. You should aim to leave a visible mark.
- Observe the Powder Color: A small amount of powdered mineral will be produced on the surface of the plate. The powder color of that mineral is known as its "streak." Carefully note the color of this powder. Is it white, black, red, yellow, or another distinct hue?
- Compare and Identify: Compare the observed streak color to a reliable mineral identification guide or a known reference sample. This color, along with other physical properties like hardness and luster, helps in narrowing down or confirming the mineral's identity.
Practical Tips for Accurate Observation
- Cleanliness is Key: Always start with a clean streak plate. Residue from previous tests can contaminate your observation.
- Apply Firm Pressure: Some minerals require more pressure than others to produce a visible streak.
- Beware of Hard Minerals: If a mineral is harder than the streak plate (Mohs hardness greater than 7), it will not leave a streak but instead scratch the plate. In such cases, the streak test is not applicable, and other identification methods must be used.
- Observe in Good Lighting: Natural light is ideal for accurately discerning colors. Avoid shadows or colored lights that could distort your perception.
- Distinguish from Surface Tarnish: Ensure you are observing the actual streak (powdered mineral) and not just a rub-off of surface tarnish or weathering. The streak reveals the mineral's internal color.
Why is Streak Important?
The streak test is invaluable because:
- Consistent Property: Unlike external color, which can be highly variable due to impurities, tarnish, or crystal structure, the streak color is a more consistent and diagnostic property for many minerals.
- Reveals True Color: It often reveals the true color of the mineral's finely powdered form, which is less affected by light absorption and reflection from the crystal faces.
- Distinguishes Look-Alikes: It's particularly useful for differentiating minerals that appear similar but have different streak colors. A classic example is distinguishing gold from pyrite ("fool's gold").
Common Streak Colors and Examples
Here are a few examples illustrating how streak color can aid in identification:
| Mineral | Apparent Color | Streak Color | Insight |
|---|---|---|---|
| Hematite | Black, silver-gray, red-brown | Cherry red to brown | Despite its varied appearance, hematite always produces a reddish streak. |
| Pyrite | Brass-yellow, metallic | Greenish-black | Differentiates it from gold, which has a yellow streak. |
| Gold | Golden yellow, metallic | Golden yellow | Its streak matches its color, but it's much softer than pyrite. |
| Sphalerite | Yellow, brown, black | Yellowish-brown | Consistent streak helps distinguish it from other dark minerals. |
| Galena | Silver-gray, metallic | Lead-gray | Its streak confirms its identity. |
| Malachite | Bright green | Light green | The green streak reinforces its characteristic body color. |
Limitations of the Streak Test
While powerful, the streak test has limitations:
- Hardness: Minerals with a Mohs hardness greater than 7 (e.g., quartz, topaz, diamond) will not leave a streak on a standard unglazed porcelain plate; instead, they will scratch the plate.
- No Streak/White Streak: Many minerals produce a white streak, which can be less diagnostic as numerous minerals share this property. Some minerals may produce no discernible streak.
Further Exploration and Resources
To deepen your understanding of mineral identification and the streak test, consider exploring resources from reputable geological organizations:
- Learn more about general Mineral Identification Techniques.
- Understand the Mohs Hardness Scale to better utilize the streak test and other hardness tests.
- Consult detailed Mineral Databases for specific mineral properties.
