Question

A halide with octahedral cleavage can scratch calcite and can be scratched by quartz. Identify the halide.

• A. Sylvite
• B. Fluorite
• C. Apatite
• D. Halite

Hint:

Fluorite is known for its octahedral cleavage and a hardness of 4, which allows it to scratch calcite but be scratched by quartz.

Answer 1

The Correct Option is B

Step 1: Read the two clues.
We want a halide that has octahedral cleavage. It must also scratch calcite but be scratched by quartz.

Step 2: Turn the scratch test into hardness.
Calcite is hardness 3 and quartz is hardness 7. To scratch calcite the mineral is above 3, and to be scratched by quartz it is below 7. So its hardness sits between 3 and 7.

Step 3: Use the cleavage clue.
Halite and sylvite both break in cubes, not octahedra, and both are very soft near hardness 2. Fluorite is the halide famous for octahedral cleavage.

Step 4: Check fluorite hardness.
Fluorite has hardness 4. That is above calcite and below quartz, so it scratches calcite and is scratched by quartz, just as asked.

Step 5: Final choice.
The halide that fits every clue is fluorite.
\[ \boxed{\text{Fluorite}} \]