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

¹ Chemistry Department, Universidade Federal de Ouro Preto, 35400, Ouro Preto, Minas Gerais, Brazil
² Geology Department, Universidade Federal de Ouro Preto, 35400, Ouro Preto, Minas Gerais, Brazil

Correspondence: ^* E-mail: magela{at}iceb.ufop.br

A second occurrence of tinsleyite, KAl₂(PO₄)₂(OH)·2H₂O, is reported. The mineral exists as a thin layer in a quartzite wall partially covered by rock paintings, and was characterized by X-ray diffraction, thermal and chemical analysis, and by Mössbauer spectroscopy. The calculated cell parameters are a = 9.58(6), b = 9.53(4), c = 9.54(6) Å, ß = 103.2(4)°. Chemical analysis showed the presence of 3.9% Fe which probably replaces Al in the octahedral site. The Mössbauer spectra from room temperature down to 85 K show the existence of two Fe³⁺ doublets with E_Q1 ~ 0.57 mm/s and E_Q2 ~ 1.0 mm/s. At 77 K the spectrum drastically changes, suggesting that a structural transition might have occurred. The formation of tinsleyite might be due to the reaction of phosphate-rich water which runs along the fractures of the wall. The existence of tinsleyite in such a relatively large abundance indicates that this mineral might not be rare as previously thought.

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