Crystal chemistry, structure analyses and phase transition experiment on an omphacite from eclogitic metagabbro from Syros island, Greece

Pyroxene samples, from the Greek island of Syros, taken from a blueschist-eclogite facies Mg-rich metagabbro, were investigated by chemical and XRD analyses and Mössbauer spectroscopy. Single-crystal XRD and microprobe analysis showed that the natural sample is a typical omphacite of intermediate composition in the Ca-Na pyroxene solid solutions. The space group P2/n was confirmed and the cations Mg, Al and Ca, Na were found to be ordered in the M1 and M2 positions, respectively. Mössbauer spectroscopy showed that there is both ferrous and ferric iron in the structure, with the ratio 1.38:1. The M2 sites are fully occupied by Ca and Na, thus the iron (Fe2+ and Fe3+) can substitute only for Mg and Al in the M1 sites. Partial disorder was attained by annealing the sample at 850 °C, 20 kbar for 7 days, as confirmed by decrease of intensity of reflections affected by the C-type extinction. © Springer-Verlag 2007

A multi-analytical study of the crystal structure of unusual Ti-Zr-Cr-rich Andradite from the Maronia skarn, Rhodope massif, western Thrace, Greece

Unusual Ti-Cr-Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mössbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO2, 8 wt.% Cr2O3 and 4 wt.% ZrO2, representing a solid solution of andradite (Ca3Fe3+2 Si3O12 ≈46 mol%), uvarovite (Ca3 Cr2Si3O12 ≈23 mol%), grossular (Ca3Al2Si3O12 ≈10 mol%), schorlomite (Ca3Ti2[Si,(Fe3+, Al3+)2]O12 ≈15 mol%), and kimzeyite (Ca3Zr2 [Si,Al2]3 O12 ≈6 mol%). The Mössbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al3+, Fe3+ and Ti4+. Cr3+ and Zr4+ are found at the octahedral site along with Fe3+, Al3+ and Ti4+. The measured H2O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca2.99Mg0.03) ∑=3.02 (Fe3+0.67Cr0.54Al0.33 Ti0.29Zr0.15)∑=1.98 (Si2.42Ti0.24Fe0.18Al0.14) ∑=2.98O12OH0.11. Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets. © Springer-Verlag 2008

Quartz from Allchar as monitor for cosmogenic 26 Al: Geochemical and petrogenetic constraints

Results of a multidisciplinary study on quartz concentrates (mineralogically separated) and etched concentrates (stoichiometric quartz) from three locations at Allchar (Macedonia) are presented. The investigation of quality and composition of these quartz samples is of great interest because the same material has been previously used as monitor for 26Al Acceleration Mass-Spectrometry (AMS) erosion rate estimates. Two genetically different types of quartz are distinguished in the studied samples which petrologically can be described as hydrothermally altered dacites or quartz latites; i.e. volcanic (QV) and hydrothermal (QH) quartz with relative proportions of QH:QV around 3:2. QH is genetically related to the Allchar Sb–As–Tl–S mineralization having very high Sb (85–785 ppm), As (7.6–78 ppm) and (Tl 3.3–4.0 ppm) contents. This type of quartz is also characterized by very high Li (129–138 ppm), Al (2424–2520 ppm) and Ti (153–219 ppm) concentrations. QV appears to be much less enriched in trace elements having Al and K contents ranging from 0 to 280 ppm and from 50 to 85 ppm, respectively. 26Al AMS measurements were done on the samples containing two genetically different types of quartz but this had no effects on the interpretation and erosion rate determinations. However, the extremely high Al concentrations in the analyzed quartz have generally negative effects, mainly by decreasing 26Al/27Al ratios and thus causing an increase of the detection limit. The disagreement between the results of 26Al AMS analyses and quantitative geomorphologic data for one location is probably caused by different geographical position with respect to the direction of cosmic rays