A Raman and infrared spectroscopic analysis of the phosphate mineral wardite NaAl3(PO4)2(OH)4⋅2(H2O) from Brazil

•A wardite mineral sample from Lavra Da Ilha, Minas Gerais, Brazil was analysed.•Using SEM with EDX and vibrational spectroscopy.•The calculated formula is (Na0.97Ca0.03)Σ1.00Al3(PO4)2(OH)4⋅2(H2O).•Observation of multiple bands supports the concept of non-equivalent phosphate units in the structure.

A wardite mineral sample from Lavra Da Ilha, Minas Gerais, Brazil has been examined by vibrational spectroscopy. The mineral is unusual in that it belongs to a unique symmetry class, namely the tetragonal-trapezohedral group. The structure of wardite contains layers of corner-linked –OH bridged MO6 octahedra stacked along the tetragonal C-axis in a four-layer sequence and linked by PO4 groups. Consequentially not all phosphate units are identical. Two intense Raman bands observed at 995 and 1051 cm−1 are assigned to the ν1PO43- symmetric stretching mode. Intense Raman bands are observed at 605 and 618 cm−1 with shoulders at 578 and 589 cm−1 are assigned to the ν4 out of plane bending modes of the PO43-. The observation of multiple bands supports the concept of non-equivalent phosphate units in the structure. Sharp infrared bands are observed at 3544 and 3611 cm−1 are attributed to the OH stretching vibrations of the hydroxyl units. Vibrational spectroscopy enables subtle details of the molecular structure of wardite to be determined.

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