MA. Allotropes
Allotropes are more of a challenge than the isotopes dealt with under ME. Chemical Elements. Atoms of elements can bond in different ways. Carbon atoms can bond to form diamonds, for example. Chemists usually treat the most stable form (graphite in the case of carbon) as the standard allotrope. Nevertheless the notation for carbon MEC refers to carbon in general rather than any allotrope.
Allotropes in general can be designated as MA.
Particular allotropes can be designated using MA instead of ME plus a cutter number:
MAC(Cutter for diamond) is Diamond
MAC(Cutter for graphite) is Graphite
This seems preferable to classifying all allotropes in some way.
The MA notation for particular elements should be linked to the ME notation.