A mineral streak is obtained by rubbing the mineral on a hard, white surface, typically a piece of unglazed porcelain.
Understanding Mineral Streak
The streak of a mineral is essentially the color of the mineral when it is in its powdered form. Unlike the apparent color of a mineral specimen, which can be affected by impurities, surface oxidation, or structural variations, the streak color is often a more reliable diagnostic property.
The Process of Obtaining a Streak
To obtain a mineral streak, a specific method is followed using a specialized tool:
- The Tool: A streak plate is commonly used. This is usually a small, unglazed porcelain tile. Porcelain is chosen because it is hard enough to scratch many minerals (Mohs hardness around 7), and its white color provides a neutral background to clearly observe the color of the resulting powder.
- The Method: The mineral specimen is gently scraped or rubbed firmly across the surface of the unglazed porcelain tile. This action grinds a small amount of the mineral into a fine powder.
- The Result: As the mineral is rubbed, it leaves a line or 'streak' of this fine powder on the white surface. The color of this powdered line is the mineral's streak color.
For example, the mineral hematite can appear metallic silver or dull red-brown in hand specimen, but its streak is consistently reddish-brown. Pyrite, often called "fool's gold," has a metallic yellow color, but its streak is black or greenish-black.
Key Aspects
Here are some key points about obtaining and interpreting mineral streaks:
- Not all minerals produce a streak on a porcelain plate. Minerals harder than the streak plate (Mohs hardness > 7) will scratch the plate instead of leaving a powder streak. In such cases, the mineral is said to have no streak, or its streak is white or colorless (as observed on a harder material like sandpaper).
- The streak color can be the same as the mineral's hand specimen color, different, or even variable depending on impurities (though streak is generally more consistent).
- Always use the unglazed side of a porcelain tile. Glazed surfaces are too hard and smooth to effectively powder the mineral.
Table: Mineral Streak Examples
| Mineral | Typical Hand Specimen Color(s) | Streak Color |
|---|---|---|
| Hematite | Red, red-brown, black | Reddish-brown |
| Pyrite | Metallic yellow | Black or greenish-black |
| Galena | Metallic gray | Lead-gray |
| Sulfur | Yellow | Yellow |
| Fluorite | Various (purple, green, blue) | White |
Practical Application
Identifying a mineral's streak is a fundamental step in mineral identification. It is often performed alongside other tests like examining luster, cleavage, fracture, and hardness to accurately classify a mineral specimen. It is a quick, simple, and effective test that provides valuable information about a mineral's composition, particularly regarding metallic elements which often leave distinctive streak colors.
