How do geologist determine the mineral compisition of granite?

Answer 1

answ2. Simple visual inspection is always the first method.

For a tiny sample, a few mg of pure material, x-ray diffraction XRD.

If a small sample, (<1gm) x-ray fluorescence XRF in an electron microscope.

A larger sample <100 gm) x-ray fluorescence using a powder sample.

The above methods assume easy access to the equipments.

Thin section microscopy is a simple technique, but takes some time in sample preparation.

The determination would start with the visual inspection of the hand sample.

You will know what the origin of the rock is, and that gives clues to the cohort of minerals expected.

This would be followed by grinding the rock to sort out the particular minerals you are interested in.