A streak plate is a simple but effective tool used to help identify minerals, specifically by testing their hardness relative to the plate itself. By rubbing a mineral across the surface of an unglazed porcelain tile, you can observe whether the mineral leaves a "streak" or scratches the plate.
The key to how the streak plate helps determine hardness lies in the hardness of the plate itself. The streak plate (porcelain tile) only has a hardness (Mohs scale) of 7. This specific hardness provides a benchmark against which the mineral's hardness is compared.
Here's how the process works:
- The Test: You take the mineral sample and rub it firmly across the surface of the unglazed porcelain streak plate.
- Observation 1: A Streak is Left: If the mineral is softer than the streak plate, particles of the mineral will be scraped off as you rub it across the plate. These powdered particles leave behind a line of color, known as the mineral's streak. According to the reference, only minerals with a hardness less than 7 will leave a streak.
- Observation 2: The Plate is Scratched: If the mineral is harder than the streak plate, it will not leave a powder of the mineral's color. Instead, the mineral will scratch the surface of the porcelain plate itself. This is because anything with a hardness greater than 7 is harder than the streak plate and will “scratch” the streak plate rather than leave a “streak”.
In summary, the streak plate test tells you if a mineral's hardness is greater than or less than 7.
- Mineral Hardness < 7: Leaves a colored streak on the plate.
- Mineral Hardness > 7: Scratches the plate, leaves no mineral streak.
- Mineral Hardness ≈ 7: May leave a faint streak or scratch, results can be less definitive.
This test provides a quick way to place a mineral's hardness within a range relative to the Mohs hardness scale at the crucial value of 7.
