Micro and FT-Raman characterisation of destinezite

Destinezite, a crystalline form of hydrated iron(III) hydroxylsulphatophosphate was identified in Silurian graptolitic black shales in the Bardzkie Mountains (West Sudetes, Poland), where the mineral occurs in forms of nodules, colloform crusts and cements of tectonic microbreccia. They are dull, cream, cream-beige to pale orange, macroscopically amorphous and pulverulent. But at very high magnification (SEM) well-formed columnar, tabular and blocky microcrystals up to 10 μm in size are observed. Powder X-ray diffraction studies indicated that the mineral of Bardzkie Mts locality exhibits crystalline character (destinezite) of various content of diadochite phases. The lattice parameters of four samples of destinezites, comparable with literature data are very similar and equal to: a=9.5798(9) Å, b=9.7463(8) Å, c=7.3346(10) Å, α=98.821(10)°, β=107.992(9)°, γ=63.881(9)° (data given for the most ordered phase Dc1). FT-Raman and FTIR (powdered samples) and microRaman (polished samples) spectra have been collected for cream and pale orange mineral forms. PO4 and SO4 symmetric and antisymmetric stretching and bonding frequencies are generally similar, irrespective of the amorphous phase content.