The physical property that denotes the color of a powdered mineral is called its streak. Understanding streak is essential for mineral identification, as it provides a consistent characteristic often more reliable than the mineral’s outward appearance.
Understanding Streak in Mineral Identification
Streak is the color of a mineral in powdered form. It’s determined by scraping the mineral across an unglazed porcelain plate, known as a streak plate. This process reveals the mineral’s true color, regardless of any surface tarnish or weathering that might affect the apparent color of the solid specimen. Why is streak important? Because a mineral’s outward color can be quite variable due to impurities or structural variations, while its streak tends to be much more consistent, making it a valuable diagnostic tool.
Why is Streak More Reliable Than Apparent Color?
The color you see when you look at a mineral specimen is due to the way the mineral absorbs and reflects light. This can be influenced by trace elements, surface coatings, and even the size of the mineral crystals. Streak, on the other hand, gets to the heart of the matter, revealing the fundamental color of the finely powdered mineral. Think of it like this: a piece of quartz can appear purple (amethyst), yellow (citrine), or even pink (rose quartz), but its streak will always be white. This consistency makes streak a powerful tool for identification.
How to Perform a Streak Test
Performing a streak test is straightforward. You simply take your mineral sample and firmly drag it across the unglazed porcelain streak plate. The pressure applied should be enough to leave a powdered residue on the plate. This residue’s color is the mineral’s streak. It’s important to use a streak plate with a hardness of around 6.5-7 on the Mohs hardness scale to avoid scratching the plate instead of the mineral.
Common Mineral Streaks and Their Significance
Some minerals have characteristic streak colors that are diagnostic. For example, hematite, despite often appearing metallic gray or black, has a distinctive reddish-brown streak. Similarly, pyrite, known as “fool’s gold,” has a greenish-black streak that helps differentiate it from real gold, which has a golden-yellow streak. Knowing these common streak colors can be immensely helpful in the field.
What if the Mineral is Harder than the Streak Plate?
If the mineral is harder than the streak plate (like corundum or diamond), it will scratch the plate and won’t leave a streak. In these cases, the mineral’s streak is described as “colorless” or “none.” This is still a useful piece of information for identification.
Streak: A Crucial Tool in the Geologist’s Toolkit
Streak, the color of a powdered mineral, is a fundamental physical property used in mineral identification. It provides a reliable and consistent characteristic that is often more informative than the mineral’s external color. By understanding how to perform a streak test and knowing the common streak colors of different minerals, you can unlock a powerful tool for exploring the world of geology.
Conclusion
Understanding What Physical Property Denotes The Color Of A Powdered Mineral—its streak—is critical for proper mineral identification. By using this simple technique, you can move beyond a mineral’s deceptive outward appearance and uncover its true color identity. Remember, streak is a consistent characteristic that can help you distinguish between minerals that might otherwise look similar.
FAQ
- What is the purpose of a streak test? To determine the true color of a mineral in its powdered form.
- How do I perform a streak test? Scrape the mineral across an unglazed porcelain streak plate.
- Why is streak more reliable than a mineral’s apparent color? Because apparent color can be affected by impurities, while streak is more consistent.
- What if a mineral doesn’t leave a streak? This indicates that the mineral is harder than the streak plate, and its streak is described as “colorless” or “none.”
- What is the streak of hematite? Reddish-brown.
- What is the streak of pyrite? Greenish-black.
- What is the hardness of a streak plate? Around 6.5-7 on the Mohs hardness scale.
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