In the vicinity of the city of Rio Claro (S. Paulo, Brazil) occurs a type of calcareous rock which was studied chemically, spectrographically and roentgenographically. The roentgenograph investigations showed that the rock belong to the hydromagnocalcite, CaCO3+Mg(OH)2, and not the dolomitic type as it was considered up to the present. The first four roentnogramms show the crystalline structure of the mineral and the fifth the crystalline structure of the synthetic mixture of CaCO3+Mg(OH)2 in the proportions indicated by the chemical analysis of table I. The spectrographical analysis revealed the presence of: Mn, Fe, Al, Sn, Cu, Na, Sr, Zn and Pb, the percentages of which reached a value less than 0,1%.