Using Neutron Diffraction to Determine the Low-Temperature Behavior of Pb2+ in Lead Feldspar

Abstract:

Feldspar minerals comprise 60% of the Earth’s crust, so it imperative that the properties of feldspar be well understood for seismic modeling. The structure of feldspar consists of a three-dimensional framework of strongly-bonded TO₄ tetrahedra formed by the sharing of oxygen atoms between tetrahedra. The main solid solution series found in natural feldspars are alkali NaAlSi₃O₈ –KAlSi₃O₈ and plagioclase CaAl₂Si₂O₈-NaAlSi₃O₈. Recently, efforts have been made to systematically quantify feldspars structural change at non-ambient temperatures by considering only the relative tilts of the tetrahedral framework [1]. This serves as a tool to predict various behaviors of the structure such as the relative anisotropy of unit cell parameters and volume evolution with composition and temperature. Monoclinic feldspars are well predicted by the model [1], but discrepancies still remain between the model predictions and real structures with respect to absolute values of the unit cell parameters. To improve the existing model, a modification must be made to account for the M-cation interaction with its surrounding oxygen atoms.

We have, therefore, chosen to study the structure of Pb-feldspar (PbAl₂Si₂O₈), which provides the opportunity to characterize a monoclinic Al₂Si₂ feldspar containing a large M-site divalent cation using neutron diffraction. Neutron diffraction allows for the characterization of the M-site cation interaction between the oxygen atoms in the polyhedral cage by providing information to accurately determine the atomic displacement parameters..

Lead feldspar was synthesized for this study using the method described in [2], and confirmed to have a monoclinic C2/m space group. In this talk we will present structural determinations and atomic displacement parameters of Pb-feldspar from 10 – 300K generated from Neutron diffraction at the POWGEN beamline at the Spallation Neutron Source at Oak Ridge National lab, and compare our results to those predicted by the tetrahedral tilting model.

[1] Angel, R.J. Ross, N.L, Zhao, J, Sochalski-Kolbus, L., Kruger, H., Schmidt, B.C. (2013) European Journal of Mineralogy, 25: 597-614.
[2] Benna, P., Tribaudino, M., Bruno, E. (1996) American Mineralogist, 81: 1337-1343.