Will powdering the entire shell and XRD analysis give good results? or do we have to section the shell in some some specific way and then find? do the CaCO3 forms change with age?
Why not dry and weigh a known mass, dissolve in HCl, filter and weigh the residue and estimate the CaCO3 proportion this way? I assume close to 100%... Works unless there are other soluble carbonates (e.g. MgCO3) present. I assume you'll have to correct for seaweed and limpets on the shell etc
Your only route between differentiating between calcite and aragonite etc is via XRD, but I don't know if the differences are sufficient to get a handle on percentages. I'd be trawling the literature simply to find the form of calcium carbonate in mussel shells.. For example: http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392012000200006