David Virgo

Throughout his career, David Virgo's major field of concentration has been the use of crystal, chemical, and other structural information to characterize properties and processes in rock-forming minerals and silicate melts. His early work focused on intercrystalline partitioning between coexisting feldspar phases in metamorphic rocks and on intracrystalline equilibria, namely the distribution of iron cations between crystallographically distinct sites in ferromagnesian minerals. These initial interests remain a top priority in his research today. In a series of classic studies, he has shown how Fe-Mg order-disorder equilibria in pyroxenes and amphiboles can be used as a powerful means to determine temperature-time paths of metamorphic and igneous rocks on the Earth and the Moon. This interest has led him to projects using thermodynamic calibrations of heterogeneous phase equilibria between coexisting minerals in xenoliths entrained in basaltic and kimberlitic magmas to clarify the oxidation state of the Earth's upper mantle. An extension of these studies, to include the crystallographic controls of Fe³⁺ and Fe²⁺ in lower mantle phases, aims at modeling the oxidation state of the lower mantle. Both these latter studies provided unique answers to fundamentally important questions concerning the Earth's early evolution.

Virgo also devoted a considerable portion of his research time to experimental studies of the structure of silicate glasses and to the relationships between structure and derivative physical and chemical properties. He was an integral part of the extensive effort at the Geophysical Laboratory on silicate melt structure beginning around 1979, when he demonstrated how Mössbauer and Raman spectroscopic data can be used to determine the cationic and anionic speciation in silicate glasses. In these pioneering studies, he advocated the concept of coexisting structural units in melts, a concept that at the time was not fashionable, but that today is in accord with the extensive body of spectroscopic data of silicate glasses and liquids.

Virgo's current research concerns experimental studies of the iron oxidation mechanism in magmatic hydrous minerals such as micas, amphiboles, and allanites. He believes that these studies will lead to a new magmatic geohygrometer. Thus far his work has led to important predictions regarding the water contents of Martian magmas, which in turn have raised questions concerning the possibility of prior existence of life on Mars.