Ferraioloite from the Sítio do Castelo mine, Folgosinho (Guarda, Portugal), description and Raman spectroscopy

Abstract

A zinc phosphate corresponding to ferraioloite with low Mg and high Na cation contents in interlayer space was identified in samples from the Sítio do Castelo mine, Folgosinho (Guarda, Portugal). It occurs as sky-blue, pearly lustrous, radial or irregular aggregates up to 1 mm in size composed of very thin elongated flaky crystals. The mineral is monoclinic, space group I2/m, with unit-cell parameters refined from X-ray powder diffraction data: a = 25.417(7), b = 6.342(4), c =15.186(5) Å, β = 90.02(4)°, V = 2448.0(17) Å3. The chemical composition generally agrees with published data for ferraioloite, but differs significantly in elements present at sites assumed to be in the interlayer of the crystal structure. The corresponding empirical formula based on 8 (PO)4 and 4 (OH) groups pfu is (Mg0.16Na0.15Ca0.09K0.01)Σ0.41Mn4.22(Fe1.64 Fe1.15Al1.47)Σ4.26Zn3.87(PO4)8(OH)4(H2O)20. The Raman spectra and tentative assignment of observed bands are given. The most prominent bands are attributed to stretching and bending vibrations of phosphate tetrahedra and complex metal-centered polyhedra. Bands of O–H stretching vibrations are weak, and bending vibrations of water molecules were not observed. The origin of the mineral is related to in-situ supergene weathering of zwieselite–triplite and isokite–fluorapatite assemblages with admixtures of sphalerite.