ALBERT

Description: Structural reconstitution upon dry thermal annealing of mildly to strongly radiation-damaged, gem-quality zircon from Sri Lanka has been studied by single-crystal X-ray diffraction and Raman spectroscopy. Results of structure refinement of a strongly radiation-damaged zircon (sample GZ5, calculated alpha dose ~4 × 1018 g−1) indicate the existence of an interstitial oxygen site that is sparsely occupied (about 4% of all O atoms). Annealing of this sample at Ta (annealing temperature) = 700 °C has resulted in nearly complete recrystallization of its amorphous volume fraction and significant decrease in the occupation of O-interstitial sites. For all samples studied, annealing up to Ta ≤ 650–700 °C is characterised by preferred recovery of Raman shifts (compared to Raman FWHMs; full width at half band maximum) and extensive contraction of the unit-cell volume, in particular along unit-cell dimension a. This low-T annealing is dominated by epitaxial growth of the crystalline volume fraction at the expense of the amorphous volume fraction, and general recovery of low-energy defects. During annealing at Ta = 700–1400 °C there is preferred recovery of Raman FWHMs (compared to Raman shifts) and only mild unit-cell contraction. High-T annealing is dominated by the recovery of high-energy defects such as recombination of cation Frenkel pairs. Here, unit-cell parameter a shows a remarkable behaviour (namely, mild re-increase at Ta = 700–1150 °C and mild final shrinking at Ta = 1000–1400 °C), which is attributed to enhanced contortion of ZrO8 polyhedrons due to cation repulsion. The combined data set of Raman band and unit-cell parameter presented herein will help analysts to assign Raman spectra of annealed unknowns to certain recovery stages.

Description: A multi-methodological study was conducted in order to provide further insight into the structural and compositional complexity of rare earth element (REE) fluorcarbonates, with particular attention to their correct assignment to a mineral species. Polycrystals from La Pita Mine, Municipality de Maripí, Boyacá Department, Colombia, show syntaxic intergrowth of parisite–(Ce) with röntgenite–(Ce) and a phase which is assigned to B3S4 (i.e., bastnäsite-3–synchisite-4; still unnamed) fluorcarbonate. Transmission electron microscope (TEM) images reveal well-ordered stacking patterns of two monoclinic polytypes of parisite–(Ce) as well as heavily disordered layer sequences with varying lattice fringe spacings. The crystal structure refinement from single crystal X-ray diffraction data – impeded by twinning, complex stacking patterns, sequential and compositional faults – indicates that the dominant parisite–(Ce) polytype M1 has space group Cc. Parisite–(Ce), the B3S4 phase and röntgenite–(Ce) show different BSE intensities from high to low. Raman spectroscopic analyses of parisite–(Ce), the B3S4 phase and röntgenite–(Ce) reveal different intensity ratios of the three symmetric CO3 stretching bands at around 1100 cm−1. We propose to non-destructively differentiate parisite–(Ce) and röntgenite–(Ce) by their 1092 cm−1 / 1081 cm−1 ν1(CO3) band height ratio.

Description: A strongly dichroic (vivid blue to near-colourless) tourmaline specimen from the Elahera gem field, central Sri Lanka, was identified as dravite. The sample shows strong Fe2+-related optical absorption (E⊥c) in the red range and comparably weak red emission due to Cr3+. The discovery of virtually Cu-free blue dravite (with a Li content of only about 3 ppm) shows that blue colour in gem tourmaline is not restricted to Cu- and/or Fe-bearing elbaite and other lithium tourma-lines. Also, this study reconfirms the usefulness of Raman spectroscopy for non-destructive mineral identification. The sensitivity of Raman shifts of O–H stretching bands to their nearest-neighbour cations allowed us to exclude the presence of significant amounts of Li ions, even before obtaining results of light-element analysis.