DETERMINATION OF HYDROGEN POSITIONS IN PREHNITE USING POWDER NEUTRON DIFFRACTION

Neutron diffraction has become a well-established technique for structural studies of earth materials, especially for the determination of atomic positions of hydrogen in minerals. Many previous neutron diffraction studies on fluoro-apophyllite (Stähl, 1993), laumontite (Stähl and Artioli, 1993), hydroxyl-clinohumite (Berry and James, 2001), palygorskite (Giustetto and Chiari, 2004), amphibole (Iezzi et al, 2005) and others, used deuterated samples to aid in the identification of the location of the hydrogen atoms. However, in many minerals, the exchange rate of hydrogen/deuterium (H/D) is unknown and there is a risk that incomplete exchange may result in nearly contrast-matched H:D (2:1) values, making H(D) invisible to neutrons. Recent powder neutron diffraction studies on leucophoenicite (Welch et al, 2002) and chrondrodite (Berry and James, 2002) have shown that it is possible to locate the hydrogen in undeuterated samples.

In this study, powder neutron diffraction data was collected on a natural sample of prehnite, Ca₂Al(AlSi₃O₁₀)(OH)₂, from Mali, Africa, using the neutron time-of-flight diffractometer POLARIS at the ISIS facility, Rutherford-Appleton Laboratory, U.K. POLARIS is a medium-resolution, high-count-rate diffractometer (Smith et al., 1994). 2.7 g of powdered sample was loaded for data collections at room temperature and 2 K. Rietveld structure refinement was performed using the program GSAS (Larson and Von Dreele 1994) and data from two detector banks (90° and 145°) were refined simultaneously. The Pncm structure reported by Detrie et al. (2006) was used as a starting model with unit cell parameters of a=4.629(5) Å, b=5.488(5) Å, and c=18.503(5) Å. Difference-Fourier maps are being constructed to determine the position of the hydrogen atoms. The role of hydrogen in linking the octahedral sheets of the structure and the strength and directionality of hydrogen bonding in prehnite will be discussed. This study will also provide a more comprehensive basis for understanding hydrogen behavior in hydrous minerals.