ABSTRACT
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 785-970â¯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 â¼2300â¯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-.
ABSTRACT
A number of the baryte, BaSO4, - hashemite, BaCrO4, solid solution compounds were synthesized previously. In this study, Raman spectra of naturally occurring phases belonging to the baryte-hashemite series from the pyrometamorphic rocks of the Hatrurim Complex were investigated. The Raman spectrum of natural hashemite, obtained for the first time, shows the position of the fundamental bands for the chromate anion vibrations. The bands related to the stretching vibrations (ν1, ν3) occur at 864â¯cm-1 and in 871-909â¯cm-1 regions, whereas the bending vibrations (ν2, ν4) are visible in the 346-360â¯cm-1 and 400-422â¯cm-1 range, respectively. Received results allowed to observe a gradual shift of bands in baryte-hashemite solid solution as a consequence of the substitution by different cations. The position of bands depends on the Cr/S ratio in analysed samples, and it is determined by differences in atomic mass, and ionic radii between Cr6+ and S6+, which affect changes in the strength and length of bonds. The occupancy of the same atomic position by two different cations enables to notice variations of polyhedra geometry, and unit cell parameters despite that baryte and hashemite are isostructural and crystallize in the same Pnma space group. We also confirm that the immobilization of the toxic (CrO4)2- ion in the baryte structure may occur directly without oxygen state reduction, we propose to using a baryte-hashemite solid solution as a reservoir for the incorporation of Cr as an environmental pollutant.
