Water in Feldspar Minerals and in Glasses

Water is a minor constituent of most minerals typically considered to be anhydrous, such as feldspar.  Water can play a major role in such minerals in terms of influencing mineral strength, melting temperature, and electrical properties.  My interest in water in anhydrous minerals is based on the goal of using quantitative analysis of water in common anhydrous minerals such as feldspars to derive information about the amount of water that was present in the magma when the feldspar crystallized.  This would be very useful in terms of understanding magmas that produced igneous rocks that no longer—or never did—host melt inclusions that can be directly analyzed for their volatile concentrations.  Analyses of water in minerals is done using infrared radiation generated by the UV-IR synchrotron at the National Synchrotron Light Source at Brookhaven National Laboratory, on Long Island.  The brightness of the synchrotron-generated beam, and its high resolution provide the ability to collect maps of water concentration across portions of crystals and glasses.  These images provide the opportunity to form hypotheses about the behavior of water that then can be tested by quantitative analysis.  So far I have done work in collaboration with Darby Dyar from Mount Holyoke College, and with Nebosja Marinkovic of the National Synchrotron Light Source, on melt inclusion-bearing anorthoclase megacrysts from Mount Erebus, Antarctica, donated by Nelia Dunbar, on K-feldspar megacrysts from the Lexington batholith of central Maine (which were the subject of the master’s thesis of Kathleen Williamson), on rapakivi feldspars from the Tunk Lake pluton (the subject of the master’s thesis of Helle Gylling), on volcanic rocks of the Purico complex of Chile, donated by Bob Trumbull, and on flow-banded rhyolite from the Bartolo Mountain volcanic vent of southern Arizona.   This work has yielded information on the partitioning of water between anorthoclase and phonolitic melt, on the heterogeneity of water concentration in glass, on the diffusion behavior of water in phonolitic glass, on relative water concentrations of crystals that form in succession in felsic magmas, and on the control exerted by water on producing volcanic textures such as flow banding.  Anna Keskula-Snyder is now working on an M.S. thesis project focusing on the distribution of water between melt and crystals in a series of hybrid rocks in volcanic centers along a fissure zone in the Veidivotn fissure swarm of southeastern Iceland.