Spectroscopic and structural investigations of blue afwillite from Ma'ale Adummim locality, Palestinian Autonomy

Until now, only the colourless crystals of mineral afwillite, Ca₃(HSiO₄)₂·2H₂O, were known from several localities around the world. Present work focuses on blue afwillite counterparts from the Ma'ale Adummim locality in Palestine. Using the wide spectrum of analytical methods we attempted to identify the causes of this unusual colour. Structural investigation confirms the presence of two tetrahedral SiO₃OH units connected by hydrogen bonds. The Raman spectrum of afwillite, obtained for the first time, shows the increased number of bands in the range of 785-970 cm^-1, whose assignation was correlated with the presence of two different kinds of structural units: (SiO₃OH)^3- and its deprotonated counterpart (SiO₄)^4-. The heating process at 250 °C, in addition to the colour changes from blue to pastel green, shows the intensity reduction and disappearing of some Raman bands attributed mainly to SiO₃OH units. The IR investigation confirms also the presence of that unit and provides information that the position and designation of infrared bands above ∼2300 cm^-1 is related to the strength of hydrogen bonds within the structure. The stretching and bending OH vibrations of afwillite sample show the partial shift to the lower spectral frequencies after the H/D isotopic exchange in OH or H₂O groups. Based on the results of the electron absorption and luminescence analyses it has been proposed that the blue colour of afwillite is caused by hole oxygen defect, most probably SiO₃^-.