The acquisition of Visible and Near-infrared reflectance (VNIR) spectra is a tested methodology in order to infer compositional information about the surface of Solar System bodies. Nonetheless, because this methodology is sensitive to Crystal Field absorptions and Vibrational Overtone, different types of uncertainties on the interpretation of the mineralogy can occur using VNIR spectra. In this regard it would be desirable to be able to count on spectroscopic database of minerals (and rocks) to be compared with the acquired spectra of Solar System bodies. The database can be assembled taking into consideration both natural (terrestrial or extra-terrestrial) and synthetic materials. Aim of this study is to synthesize Fe2+- bearing plagioclases and characterize them through the acquisition of VNIR spectra. In fact, this mineral represents a main phase on the surface of many Solar System bodies, like Vesta, as it was found in Howardite - Eucrite - Diogenite (HED) meteorites, and Moon, as it was found in lunar samples and meteorites, as Anorthitic (An) rich phase containing Fe2+ as a minor element. Plagioclase is a common phase on the Earth too, but Fe can be present as 3+ and 2+ oxidation state. Thus, a treatment process in a controlled atmosphere was necessary in order to synthesize An90 plagioclases with variable amounts of FeO (up to 1 wt%). Synthesis were performed at 0.1 MPa using a Deltech DT-31 vertical gas mixing quench furnace in which oxygen fugacity is controlled by means of a mixture of CO and CO2 flowing through the furnace and measured by a SIRO2 oxygen sensor. Starting materials were usually contained on a Pt wire loops and suspended in the hot spot by means of a thin Pt wires. Different synthesis procedures were followed in order to get the best homogeneous composition of products. Indeed the varied parameters were the following: bulk of starting material (Fe2+ or Fe3+ in oxides and carbonates mixtures), vitrification cycles (0-2), T and duration of vitrification (1550-1580 °C, 15-90 minutes), duration of sintering at 1400 °C and logfO2= -9 (18-28 hours). Products were analyzed by XRD and EPMA before VNIR characterization. In general, synthesized plagioclases have a good homogeneity as regards An contents but not for their FeO contents when nominal FeO is > 0.5 wt%. VNIR spectra on both glasses and plagioclases, besides showing the typical Fe absorption bands, proved to be a sensitive approach to test the compositional homogeneity of synthetic products.
Synthesis of Fe2+- bearing plagioclases: a valuable tool for interpreting reflectance spectra from Solar System bodies
Orlando A;
2014
Abstract
The acquisition of Visible and Near-infrared reflectance (VNIR) spectra is a tested methodology in order to infer compositional information about the surface of Solar System bodies. Nonetheless, because this methodology is sensitive to Crystal Field absorptions and Vibrational Overtone, different types of uncertainties on the interpretation of the mineralogy can occur using VNIR spectra. In this regard it would be desirable to be able to count on spectroscopic database of minerals (and rocks) to be compared with the acquired spectra of Solar System bodies. The database can be assembled taking into consideration both natural (terrestrial or extra-terrestrial) and synthetic materials. Aim of this study is to synthesize Fe2+- bearing plagioclases and characterize them through the acquisition of VNIR spectra. In fact, this mineral represents a main phase on the surface of many Solar System bodies, like Vesta, as it was found in Howardite - Eucrite - Diogenite (HED) meteorites, and Moon, as it was found in lunar samples and meteorites, as Anorthitic (An) rich phase containing Fe2+ as a minor element. Plagioclase is a common phase on the Earth too, but Fe can be present as 3+ and 2+ oxidation state. Thus, a treatment process in a controlled atmosphere was necessary in order to synthesize An90 plagioclases with variable amounts of FeO (up to 1 wt%). Synthesis were performed at 0.1 MPa using a Deltech DT-31 vertical gas mixing quench furnace in which oxygen fugacity is controlled by means of a mixture of CO and CO2 flowing through the furnace and measured by a SIRO2 oxygen sensor. Starting materials were usually contained on a Pt wire loops and suspended in the hot spot by means of a thin Pt wires. Different synthesis procedures were followed in order to get the best homogeneous composition of products. Indeed the varied parameters were the following: bulk of starting material (Fe2+ or Fe3+ in oxides and carbonates mixtures), vitrification cycles (0-2), T and duration of vitrification (1550-1580 °C, 15-90 minutes), duration of sintering at 1400 °C and logfO2= -9 (18-28 hours). Products were analyzed by XRD and EPMA before VNIR characterization. In general, synthesized plagioclases have a good homogeneity as regards An contents but not for their FeO contents when nominal FeO is > 0.5 wt%. VNIR spectra on both glasses and plagioclases, besides showing the typical Fe absorption bands, proved to be a sensitive approach to test the compositional homogeneity of synthetic products.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
