By XRD the strong peak is d=2.94 and it appears a structure like kutnohorite CaMn(CO3). But by Rietveld analysis MgCalcite also fits. In fact I don´t have enough Mn to have Kuntnohorite, but it is not a normal MgCalcite... What does it means in terms of structure?
It is definitely expected to have the same structure for kutnohorite and CaMg(CO3)2, also known as dolomite. The cell parameters are close. Considering the likelihood of solid solutions (Mg/Mn/Fe and intermixing of these ions with Ca in the Ca planes) and of variations (order/disorder) due to formation conditions, you may not be able to identify your sample just by cell parameters.
See the Wikipedia article about kutnohorite; also see for example
American Mineralogist, Vol. 69, pp. 520-527, 1984 - Variation of lattice parameters in some sedimentary dolomites, R. J. Reeder and C. E. Sheppard
The diffraction patterns of pure calcite, Mg-calcite, dolomite, kutnohorite and isomorphous mixed phases such as Mn or Fe-rich dolomite will differ slightly in the intensities of the superstructure diffraction lines. It concerns the so-called "b reflections", i.e. with h-h0l, l = odd. Disordering in the structure significantly reduces the intensities of these "b-lines" (the problem of the so-called "proto-dolomite"). There is a lot of literature on this.
My opinion is that from powder diffraction alone it is often not possible to reliably and accurately identify real carbonates of the calcite dolomite group, with the exception of the end members. I know of many cases where macroscopically homogeneous samples gave diffraction patterns of mixtures of very close carbonates (slightly split and asymmetric lines).
d= approx. 2.94A can also be found in strongly calcareous dolomite, i.e. dolomite with an excess of Ca in the structure.