MAKING DIOPSIDE CRYSTALS AND GLASS UNDER CONTROLLED OXIDATION STATES: AN ADVENTURE IN EXPERIMENTAL PETROLOGY

Diopside glass and crystals doped with vanadium were synthesized as calibration “standards” for a developing analytical procedure.  The valence state of V in mineral phases occurring in minettes and similar rock types is determined by X-ray absorption near-edge spectroscopy (XANES) and used as a proxy for the oxygen fugacity of the minette magmas.  Calibration curves have been prepared for V-doped glass and olivine, but not pyroxenes. 

Seven glasses of mole composition CaO – MgO – 2SiO₂ with 1 wt% V were synthesized by melting oxide powders at 1425º C in air, equilibrating for 6 hours at 1300º C, and then quenching the melts to room temperatures to freeze in the liquid structure.  Six crystalline samples were made in a similar manner but were equilibrated at 1425 C for 6 hours and cooled at a rate of 2 º C per minute from 1425º to 1300º C.  The samples were made under oxidizing and reducing conditions (atmospheres of air, CO₂ and CO/CO₂).

About 0.1 g of each experimental charge was ground into powder and a titration was completed to determine the concentration of each V valence state present.  Additionally, a thin slice of each charge was cut and polished for optical spectroscopy.  Spectra of these glass and crystal samples were obtained on a visible/near infrared spectrophotometer from 300 nm to 2600 nm.

The titrations and spectroscopy allow determination of the oxygen fugacity of the samples.  These samples, created under known experimental conditions, will be used to create a calibration curve for XANES work at the APS synchrotron.  Once the calibration curve for diopsides is complete, the oxygen fugacity for naturally occurring samples can be determined.  XANES allows us to determine the relative amounts of the various oxidation states present in the samples by the size and position of a small “hump” just below the V absorption edge.