The occurrence of tinsleyite in the archaeological site of Santana do Riacho, Brazil

A second occurrence of tinsleyite, KAl2(PO4)(2)(OH). 2H(2)O, is reported. T he mineral exists as a thin layer in a quartzite wall partially covered by rock paintings, and was characterized by X-ray diffraction, thermal and che mical analysis, and by Mossbauer spectroscopy. The calculated cell paramete rs are a = 9.58(6), b = 9.53(4), c = 9.54(6) Angstrom, beta = 103.2(4)degre es. Chemical analysis showed the presence of 3.9% Fe which probably replace s Al in the octahedral site. The Mossbauer spectra from room temperature do wn to 85 K show the existence of two Fe3+ doublets with DeltaE(Q1) similar to 0.57 mm/s and DeltaE(Q2) similar to 1.0 mm/s. At 77 K the spectrum drast ically changes, suggesting that a structural transition might have occurred . The formation of tinsleyite might be due to the reaction of phosphate-ric h water which runs along the fractures of the wall. The existence of tinsle yite in such a relatively large abundance indicates that this mineral might not be rare as previously thought.