Home test for magnesium carbonate
If all you want to know is the relative amounts of calcium and magnesium carnbonates disolve it in vinegar (it will fizz). Now add lye until the pH is about neutral (use pH test strips) in order to neutralize any excess acid. Obtain a test kit for determining water hardness that will give both the magnesium and calcium hardness values for a sample. Dilute with DI (or nearly DI i.e. RO) water to be within range of the kit Be sure you understand what units the kit is reporting in. Usually it is units related to the equivalence of the calcium and magnesium so that if calcium harness turns out to be 1/5 magnesium hardness that tells you the original mix has 1/5 of a mole of CaCO3 for each mole of MgCO3.