Determining the properties and composition of minerals is an important part of geology and materials science. One simple but useful test that geologists and mineralogists use is the streak test, which reveals the color of a mineral’s powdered form. This can provide valuable information about a mineral’s chemical composition and structure. In this article, we’ll examine how the streak test works and what it tells us about minerals.
What is Streak?
The streak of a mineral refers to the color of its powder. This is different from a mineral’s color in solid form, as the atoms are arranged differently when finely ground. To do a streak test, a mineral is rubbed across a piece of unglazed porcelain known as a “streak plate.” This abrades off a powdered sample that exposes the mineral’s underlying color.
Geologists runs streak tests in the field to quickly identify minerals based on their streak color. However, the streak test is more accurate when performed in controlled lab conditions. There are standard streak testing procedures to ensure consistent results.
Why is Streak Important?
A mineral’s streak reveals details about its chemical composition and crystal structure. Different elements and arrangements of atoms in the mineral lattice produce distinctive colors. For example, very pure forms of the mineral hematite have a red to reddish-brown streak. This tells geologists that iron is a major constituent of the mineral’s makeup.
Streak also differentiates minerals that appear similar. The minerals galena and graphite are both dark gray to black in color. But galena has a light gray streak, while graphite’s streak is black. This indicates differences in their elemental makeup and atomic bonding. Therefore, streak is a key diagnostic feature used in mineral identification and characterization.
How Does the Streak Test Work?
The streak test is simple to perform, but there are some key steps to ensure accurate results:
Materials Needed:
– Mineral sample to be tested
– Streak testing plate – Undecorated white porcelain tile
– Metal file or sandpaper
– Soft brush
Procedure:
1. File a small corner or edge of the mineral across the streak plate, applying moderate pressure. This creates a powdered sample on the plate.
2. Brush the powdered streak sample with quick strokes of the brush. This spreads it into a thin, uniform coating.
3. Closely observe the color of the powdered streak and compare to known mineral streak colors. Streak color may differ from the solid mineral color.
4. Repeat test in different areas to confirm consistency. Variations in color may indicate impurities.
5. To reset between tests, rub the streak plate clean with sandpaper. This removes any residue from prior samples.
6. Optionally, streak test can also be performed by rubbing minerals directly against white, unglazed ceramic. The color of the abraded powder is then observed.
Standard Streak Color Charts
Geologists reference standard color charts to identify minerals based on their streak test results. Here are some of the most common streak colors and associated minerals:
| Streak Color | Associated Minerals |
|---|---|
| Red to reddish brown | Hematite, cuprite, cinnabar |
| Yellow to brownish yellow | Sulfur, molybdenite, limonite |
| Green to black | Chrysocolla, malachite, azurite, hornblende |
| White | Halite, gypsum, talc |
| Colorless to white | Quartz, diamond, zircon |
| Light blue | Vivianite, azurite |
| Dark blue to black | Hematite, galena, obsidian |
| Gray to black | Graphite, sphalerite, pyrite, galena |
There can be variance in streaks, so it is best to test known mineral samples alongside unknowns. Subtle color differences help distinguish minerals with similar streak hues. Recording precise descriptions like “greenish-gray” or “reddish-brown” provides the most diagnostic information.
Limitations of the Streak Test
While a useful basic technique, the streak test has some limitations:
– Streak color alone is not definitive proof of mineral identity. Other diagnostic tests are needed to confirm composition.
– Subtle color variations may be hard to discern. Comparisons against known standards are important.
– Some minerals do not powder well, so no streak is produced. Alternative abrasion techniques may be needed.
– Streaks can be biased by impurities and inclusions in the mineral sample. Testing multiple areas helps improve accuracy.
– The test is destructive, as it grinds off part of the mineral being identified. Small, expendable samples should be used.
– Porcelain streak plates retain residue that can bias future tests. Proper cleaning between samples is essential.
Despite these drawbacks, the simplicity and speed of streak testing make it a widely used first step in mineral identification. The color produced gives geologists crucial clues about elemental composition that guide further testing. It remains one of the field’s most fundamental techniques.
Examples of Streak Testing in Action
Here are two examples illustrating how the streak test is applied to identify unknown mineral samples:
Example 1 – Distinguishing Galena from Graphite
A dull gray mineral sample is found, suspected to be either galena or graphite. Both are grayish black in color. A streak test is performed by rubbing the sample across a porcelain streak plate. This produces a light silvery gray streak, indicating the mineral is galena. Graphite would have produced a blackish streak.
Example 2 – Identifying Chalcopyrite
A brass-yellow mineral is discovered. A possible identity is chalcopyrite. The mineral is streak tested, yielding a greenish black streak color. This confirms the mineral is chalcopyrite, since that streak is distinctive for the compound. The test ruled out other brass/gold-hued minerals like pyrite or molybdenite that would not produce the same streak.
These examples demonstrate how streak tests deliver rapid preliminary identification of unknown mineral samples in the field. Geologists can then pursue more rigorous lab analysis to determine additional characteristics.
Advanced Streak Testing Methods
While the traditional streak test only distinguishes color, there are more advanced methods that reveal additional mineral traits:
Streak Luster –
The visual reflective quality of the streak can indicate details about a mineral’s structural lattice. Metallic lusters signify certain elemental makeups.
Streak Hardness –
Rubbing a mineral across materials of known hardness, like glass or steel, can help characterize its scratch resistance. Harder minerals abrade softer plates.
Streak Texture –
Particulate properties of the streak powder provide clues about the crystalline structure of the parent mineral. Fibrous or flaky texture points to certain atomic lattices.
Magnetic Streak –
Testing the streak powder’s reaction to magnets reveals if the original mineral contained magnetic elements like iron. This can help identify substances like magnetite.
When combined with the basic streak color test, these advanced methods provide much more rigorous data for identifying unknown mineral samples in the field. However, they require additional equipment and training to perform accurately.
The Role of Streak in Mineral Identification
While sophisticated modern analyses are available, the humble streak test remains a crucial first step in mineral identification. The simplicity and speed of grinding off a small powder sample to determine color provides rapid preliminary data that guides further testing. It quickly steers geologists towards the elements and structures that may be present before more rigorous lab work is undertaken.
Advanced methods like spectroscopy and crystallography have not made the streak test obsolete. It still delivers fast and useful information about mineral composition. Streak color screening will likely continue playing a key role in field mineralogy for the foreseeable future. The technique joins other foundational identification methods like hardness, cleavage, and magnetism tests. While limited in scope individually, together these simple methods build a picture of an unknown mineral’s properties to inform targeted study. Thanks to centuries of established knowledge and standards, the streak test occupies a key place in the geologist’s toolkit.
Conclusion
The streak test is a simple but powerful technique for identifying minerals based on the color of their powdered forms. Rubbing a small mineral sample across a porcelain plate reveals elemental properties not visible in its intact state. Standardized streak color charts allow geologists to rapidly screen for known minerals in the field before more detailed lab analysis. Though limited in scope, the ease and speed of streak testing make it a critical first step in mineral characterization alongside other basic identification methods. After centuries of use, geologists continue to rely on this fundamental test to uncover the chemical makeup of diverse mineral types.
