Introduction
The streak test is an important tool used in mineral identification. It involves taking a mineral specimen and rubbing it across a porcelain streak plate, leaving behind a powdery residue known as the mineral’s streak. This test helps determine key properties of the mineral, including color, hardness, and chemical composition. By analyzing the color and characteristics of the streak, mineralogists can identify unknown mineral samples and classify them into groups.
What is streak?
The streak of a mineral refers to the color of the powdered residue left behind when it is dragged across a rough, unglazed porcelain plate known as a streak plate. The plate is harder than many common minerals, so as the mineral is rubbed across its surface, tiny particles break off and deposit on the plate. The collected powder displays the mineral’s intrinsic color, which may be the same as or different from the outward color of the intact mineral sample.
Streak is a diagnostic physical property used to distinguish between minerals that may otherwise appear similar. It represents the true color of the powdered mineral, which can vary from the color of the mineral in bulk form due to impurities or external weathering. The color of the streak depends on the mineral’s chemical composition and light-reflecting properties.
Why is streak color important?
Analyzing a mineral’s streak color is important for several reasons:
- Reveals the mineral’s true color: The streak often differs from the color of the intact mineral specimen. Weathering and impurities can alter the outward appearance of a mineral. But the streak shows the pure, unaltered color.
- Aids mineral identification: Streak color is a key identifying feature used to classify minerals. Minerals with the same chemical composition typically share similar streak colors.
- Determines color consistency: Some minerals show variations or streaks of color on their surfaces. The streak test determines if these are consistent properties of the mineral or just surface coatings.
- Distinguishes look-alikes: Minerals that appear identical may produce different streak colors, revealing them to be separate mineral species.
- Points to light-absorbing elements: Dark or colored streaks indicate the presence of transition metal elements or crystal lattice defects.
By revealing the true color unaffected by external variables, the streak test provides an essential clue for identifying an unknown mineral specimen.
How to perform the streak test
The streak test only requires a porcelain streak plate and the mineral sample to examine. Here are the steps involved:
- Obtain a streak plate. These are unglazed porcelain tiles available from mineral testing suppliers. A shard of white porcelain, such as the underside of a ceramic tile, can also work in a pinch.
- Select a sharp corner or edge of the mineral specimen to rub across the plate. This should expose fresh unweathered surfaces.
- Hold the streak plate steady and firmly rub the mineral back and forth across the surface with moderate pressure. Apply about the same pressure as you would using a pencil.
- Continue abrading the mineral on the plate to produce a powdery streak at least 2-3 cm long.
- Closely observe the color of the powdered streak left behind on the plate. Compare it to the outward color of the intact mineral sample. Note any differences in shade or hue.
- Blow or brush away any leftover powder on the streak plate after making your observations.
- Repeat the process using a different surface of the mineral to confirm streak color consistency.
Ideally, perform the streak test on at least three different surfaces or facets of the mineral specimen and compare results. The streak color should be similar each time if it represents the true intrinsic color of that mineral.
Factors that affect streak color
Several key factors can influence the color produced by the streak test:
Chemical composition
The elements present in a mineral largely determine the streak color. For example, most iron-containing minerals like hematite and magnetite display reddish to black streaks. Sulfide minerals like sphalerite and pyrite often show yellowish to brownish streaks. Elemental metals exhibit metallic gray to black streak colors.
Crystal lattice defects
Defects in the mineral’s crystal lattice structure can produce color centers that absorb certain wavelengths of light. This results in colored streaks, even for minerals that appear white in bulk form. Diamond is a famous example, producing a yellow streak due to nitrogen defects.
Crystal structure
The arrangement of atoms in the mineral’s crystal lattice also affects how light is scattered and absorbed. The same mineral with different crystal structures can display different streak colors. For example, tridymite (orthorhombic) has a white streak while cristobalite (cubic) streaks grayish white.
Particle size
The smaller the particle size of the powdered mineral, the more uniform the streak color. Very fine-grained minerals like clays typically produce smoother, more consistent streaks. Larger grain sizes scatter light inconsistently, resulting in streaks that may show variations.
Direction of abrasion
Some minerals have different colors along different crystallographic directions. When rubbed perpendicular to these directions, the streak may show color variations that aren’t present when rubbed parallel. Pleochroism can cause this effect.
Typical streak colors of common minerals
Here are the usual streak colors for some of the most widely encountered minerals:
Non-metallic minerals
| Mineral | Streak Color |
| Calcite | White |
| Quartz | White |
| Fluorite | White |
| Apatite | White |
| Feldspar | White |
| Corundum | White to pale brown |
| Diamond | Yellow |
| Graphite | Black to dark gray |
| Hematite | Red to reddish brown |
| Limonite | Yellowish brown |
| Sphalerite | Pale yellow to brown |
| Chalcopyrite | Greenish black |
| Pyrite | Greenish to brownish black |
| Cassiterite | White to reddish brown |
Metallic minerals
| Mineral | Streak Color |
| Gold | Golden yellow |
| Copper | Reddish brown |
| Silver | Silver white |
| Chromite | Brownish black |
| Galena | Grayish black |
| Molybdenite | Grayish black |
| Hematite | Reddish gray |
| Magnetite | Black |
These are common target streak colors to look for during mineral identification. However, many metallic minerals exhibit dark grayish to blackish streaks, so other tests are needed to distinguish between them.
Limitations of the streak test
While an invaluable tool for mineralogists, the streak test has some limitations:
- Not diagnostic for all minerals: Some minerals still produce similar streaks, requiring additional tests for positive ID.
- Alters the specimen: It grinds away part of the mineral sample.
- Specimen needs hardness: Very soft or powdery minerals don’t produce streaks.
- Non-diagnostic for some groups: Silicates show mostly white streaks and require other tests.
- Can’t distinguish variations: Doesn’t reveal all pleochroism, banding, iridescence, or schiller effects.
- Subject to human error: Color perception varies between observers.
While a useful first test, streak color alone is not definitive for positive mineral identification without other supporting techniques. It works best when combined with observation of a mineral’s other physical properties.
Conclusion
The streak test is a simple but valuable technique in a mineralogist’s toolkit. By rubbing a mineral across a rough porcelain plate, tiny particles break off and deposit as a powdery residue. Analysis of the streak color provides quick clues about a mineral’s true color, hardness, composition, and identity. While subject to some limitations, streak remains one of the most easily performed and diagnostic tests for identifying minerals in the field or lab. Combined with other methods, this century-old procedure still represents a cornerstone of mineral studies.
