Among other things, yes, mineral composition is a common part of geology.
A. A geologist would study the composition of minerals.
geology= study of rocks,minerals and earth. the answer to your question is A. the composition of a mineral.
Rocks
Which would usually be studied by a geologist?
I assume that would fall under something a geologist would study
That's a hard question to answer, since a Geologist is someone who researches rocks, minerals, and the composition of the earth, what would you consider the opposite of a geologist. Possibly someone who studies the stars such as an astronomer.
It would depend on their field of endeavor. See the related question.
Minerals are basically chemicals in a solid form.A2. Part of the definition of a mineral is that it has a known chemical composition. If its composition were variable, it would not qualify under the definition.
Composition stoichiometry
The correct term would be a mineral.
Felsic igneous rock with exact same composition of granite, just tiny crystals. granite= orhtoclase, quartz, biotite.
Geology is the general term given to the study of the planet earth. Geology has many branches and Petrology is the branch of geology that studies the origin, composition, distribution and structure of rocks, a Geologist specialising in that area would call themselves a "Petrologist".