I usually face the problem to distinguish between the dolomite and ankerite during the evaluation on powder XRD. Both of these minerals are found within the carbonate sedimentary rocks and belong to rhombohedral division of the hexagonal crystal system. So far having same crystal system, the XRD patterns overlap to each other and it is very difficult to distinguish that which one of these or both of these are present in a sample. Though, at some extent the chemical analysis helps due to the presence of iron in the crystal structure of ankerite Ca(FeMg)(CO3)2 and it is absent in dolomite CaMg(CO3)2. But again it is not the solution because it is also a common phenomenon that the ferrous variety of dolomite exists and there is possibility that the iron that is found in the chemical analysis could be of that variety of dolomite instead of ankerite.
Though, on the basis of petrography, the etching and staining tests are helpful but my concern is the PXRD.
Therefore, I want to know that how do we can differentiate between these two minerals on the basis of PXRD?
Hello.
Reliable identification of rhombohedral carbonates from the dolomite-ankerite-rhodochrosite series using PXRD is practically unrealistic. There are continuous transitions between them. The definition of ankerite has also changed relativelly recently. Now ankerite must have Fe > Mg and Fe > Mn (apfu) (application of the rule of the predominance cation with predominance valence in the each position). Most so-called ankerites are iron-rich dolomites and true ankerite is evidently wery rare mineral. Vegard's rule for isomorphic series does not work because it is not a simple series. The only option is to use of the electron microanalysis.
Thanks alot dear Dalibor Matýsek for your support and feed back. So far,it is very much clear from your answer that PXRD is not a powerful technique to distinguish the above series of carbonates and the only possible way of identification of such series is the chemical analysis either EPMA or EDS.
Yes, as Dalibor wrote, the identification of rhombohedral carbonates from the dolomite-ankerite series by XRPD is really very difficult. There is a continuous transition between dolomite-ankerite depending on Fe/Mg ratio (apfu). The recent redefinition of the name means the majority of old specimens labelled "ankerite" are now considered to be ferroan dolomite, and true ankerite is quite rare.
I suggest chemical analysis by EMPA.
My best
Michele
Dear Michele Mattioli,
Thanks alot for your input and suggestion.
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