How could mineral hardness be used to tell quartzite from marble?
I don’t know that hardness would be a good test since both of these rocks have undergone metamorphism. In theory, quartzite should be harder because it is made of Quartz which has a hardness of 7 on the Mohs scale and Marble is made of calcite which is has a hardness of 3 on Mohs scale. The best test is the “fizz test.” If you drop a dilute acid on the rocks, the marble will bubble or fizz. You can use vinegar since it is a very weak acid and see if you can observe the fizz. Anything made of calcite should fizz but quartz will not.
