The structural role of light elements in magmas: P, Cl and S

This study is focused on the geochemical and structural behaviour of light elements in magmas. In particular we are interested in investigating the role of phosphorus, chlorine and sulfur in silicate glasses in order to get important information to apply in a natural model. The geochemical and structural role of P, S and Cl in silicate glasses of composition relevant for the Earth Sciences has been investigated by X-ray Absorption Spectroscopy (XAS). The geochemical behavior of these elements in silicate melts and glasses is of major importance for problems ranging from volcano-climate interactions to the genesis of ore deposits of great economic importance to industrial glass-forming processes and treatment of vitreous waste material from refuse incineration activities. Evaluation of such problems requires new data on the structural environments around P, Cl and S dissolved in silicate melts and potential interactions with metal cations. Direct structural data (local bonding environment) are lacking, due to experimental difficulty in analyzing minor to trace light elements. We collected high quality XANES and EXAFS data on synthetic glasses in order to obtain information on the local chemical and structural environment around the studied elements. Use of synthetic glasses produced under controlled pressure, temperature and oxygen fugacity conditions allowed to understand how the structural and geochemical role of these light elements vary according to the physical and chemical conditions existing in the magma chamber. Data analysis has been accomplished by comparison with well characterized model compounds with known absorber oxidation state and coordination number. Theoretical XANES calculations have further helped in understanding the chemical and structural environment of the studied elements (e.g. cations bonded to [PO4]3- groups or to Cl- and S2- anions) in the synthetic glasses