The amounts of carbonates calculated from total inorganic carbon are either higher or lower than the carbonates determined by XRD.
What are your samples and what is the extent of the difference?
Firstly, there will always be some experimental error, so it would be more surprising if you got identical results.
Secondly, in my field (concrete durability) we're interested in the early stage carbonation of cement paste. In these instances we often find that the carbonate content determined from thermal analysis is greater than that determined by XRD. The most likely explanation for this is the presence of non-crystalline carbonates within our system. (This is normally confirmed by observing a much lower decomposition temperature than would be expected for calcite or other crystalline CaCO3 polymorphs).
What are your samples and what is the extent of the difference?
Firstly, there will always be some experimental error, so it would be more surprising if you got identical results.
Secondly, in my field (concrete durability) we're interested in the early stage carbonation of cement paste. In these instances we often find that the carbonate content determined from thermal analysis is greater than that determined by XRD. The most likely explanation for this is the presence of non-crystalline carbonates within our system. (This is normally confirmed by observing a much lower decomposition temperature than would be expected for calcite or other crystalline CaCO3 polymorphs).
XRD onl present you the crystalline materialy portions of. So if you have some amorphus phase present, TIC will give you a higher value of carbonate
TIC based on XRD may not give an accurate value because the peak height of carbonate primarily depends on its crystallinity and not on its amount.
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