An unknown mineral scratches apatite and is scratched by corundum. what can you conclude about this mineral’s hardness?

If we guide ourselves with the Moh´s scale:

Material scratches apatite → material is harder than 5.

Material is scratched by corundum → material is softer than 9.

Conclusion:

The hardness of the unknown material stands between 5 and 9.

Note:  

Both apatite and corundum are defining materials (reference materials in the Moh´s scale). So, you can also give an answer using defining materials:

"It can either be an orthoclase feldspar, a quartz or a topaz".

Hope it helped,

BiologiaMagister

This answer is dedicated to JaySL

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.