As shown in the photo, haematite is a dark grey mineral.Explain why we can't assume that haematite has a dark grey streak.

The streak of a mineral, which is the color of its powder when it is rubbed on a porcelain plate, can differ from the color of the mineral itself. Here are a few reasons why we cannot assume that haematite has a dark grey streak simply based on its appearance:

1. Streak Color vs. Mineral Color: The color of a mineral can be influenced by various factors including impurities and the way light interacts with its structure. For instance, two minerals may look the same in bulk but have different streaks.

2. Chemical Composition: Haematite (Fe₂O₃) can contain impurities that might change the color of its streak. Depending on the presence of other elements or minerals, the actual streak color could vary.

3. Physical State: The physical conditions and the way a mineral breaks or weathers can affect the powder produced, leading to a different streak color than expected.

4. Testing the Streak: The only way to know the true streak color of haematite is to perform a streak test, which involves rubbing the mineral on a streak plate and observing the color of the powder left behind.

In summary, the appearance of a mineral does not definitively indicate its streak color; empirical testing is necessary to determine the streak of haematite accurately.