American Mineralogist, Volume 88, pages 167–179, 2003 0003-004X/03/0001–167$05.00 167 INTRODUCTION Hydration and associated crystallographic expansion are important properties of many nanoporous materials, and there have been numerous recent studies of the hydration and swell- ing of, for example, smectite clays (Cases et al. 1997; Yegorov 1997; Prost et al. 1998; Smith 1998; Xu et al. 2000; Young and Smith 2000). In contrast, the hydration and swelling behavior of layered double hydroxides (LDHs, also known as anionic clays and hydrotalcite-like compounds) is much less well un- derstood (Bish 1980; Brindley and Kikkawa 1980; Bish and Livingstone 1981; Drits et al. 1987; Roy et al. 1992; Williams et al. 1999; Wang et al. 2001). LDH phases are finding impor- tant applications as catalysts and filtration and exchange mate- rials (Cavani et al. 1991; Isupov 1999) and are increasingly being recognized as potentially important anion exchange phases in the natural environment (Trolard et al. 1997; Abdelmoula et al. 1998; Ford et al. 1999; Gade et al. 1999; Genin et al. 2001). Their hydration and expansion behavior is closely associated with their anion exchange, adsorption, and electrochemical properties (Drits et al. 1987; Moneyron et al. 1995; Bell et al. 1996; Costantino et al. 1997; Kaneyoshi and Jones 1998). Hydration and swelling are central to understanding LDH interlayer structure and dynamics, and our recent NMR spec- troscopic studies have shown clear correlations between the dynamics and structural environments of interlayer anions and the expansion and contraction of the basal (c-axis) spacings determined under dried and paste conditions and during time- resolved drying (Kirkpatrick et al. 1999; Hou and Kirkpatrick 2000, 2001, 2002; Hou et al. 2000, 2002; Hou 2001). How- ever, the activity of water in the vapor phase (relative humid- ity, RH) was not varied systematically in these experiments, and the previous results do not provide adequately detailed understanding of the expansion or allow for detailed correla- tion with the spectroscopic results. We, thus, present here water vapor gravimetry and powder X-ray diffraction (XRD) data obtained under systematically controlled RH conditions for well-characterized samples of Mg 3 Al and LiAl 2 LDH phases containing a variety of inorganic interlayer anions. The results significantly increase understand- ing of the effects of the anion and the hydroxide layer compo- * E-mail: xhou@uiuc.edu Hydration, expansion, structure, and dynamics of layered double hydroxides XIAOQIANG HOU, 1, * DAVID L. BISH, 2 SHAN-LI WANG, 3 CLIFF T. JOHNSTON, 3 AND R. JAMES KIRKPATRICK 1 1 Department of Geology, University of Illinois at Urbana-Champaign, 245 NHB, 1301 West Green Street, Urbana, Illinois 61801-2919, U.S.A. 2 Earth and Environmental Sciences (EES) Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A. 3 Department of Agronomy, Purdue University, 1150 Lilly Hall, West Lafayette, Indiana 47907-1150, U.S.A. ABSTRACT Water-vapor sorption isotherms, relative humidity (RH) controlled powder X-ray diffrac- tion (XRD) data, and new and previously published multi-nuclear NMR spectroscopic data for a wide range of layered double hydroxides (LDHs) provide greatly increased understanding of the effects of hydration state on the structure and dynamical behavior of interlayer and surface anions and the factors controlling the expansion behavior of this group of minerals. Li,Al and Mg,Al LDH phases containing SO 4 2– , SeO 4 2– , PO 4 3– , HPO 4 2– , MoO 4 2– , ClO 4 – , SeO 3 2– , CO 3 2– , F – , Cl – , Br – , I – , OH – , and NO 3 – were examined. The phases studied can be grouped into three types based on basal spacing expansion, water sorption, and interlayer anion dynamics: Type 1, significantly expandable (1.5–3.0 Å); Type 2, slightly expandable (expansion <0.5 Å) and with significant interlayer water exchange; and Type 3, essentially non-expandable (0–0.2 Å) and with little interlayer water exchange. For Type 1, the fully expanded phases have a two-water layer structure, and the phase transition from one layer to two layers as determined by XRD consistently correlates with a significant step in the water sorption isotherm and with changes in the interlayer structure and dynamics as observed by NMR spectroscopy. For Type-2 phases, only one-water layer structures form, and the interlayer anions may undergo dynamical disordering with increasing RH, as observed by NMR. For both Types 1 and 2, the first water layer does not cause significant basal spacing expansion due to occu- pancy of vacant interstitial sites between the anions by the water molecules. For Type-3 phases, there is little interlayer water sorption because the interlayers are essentially closed due to the small size or planar shape of the anions and their strong electrostatic and hydrogen bonding interaction with the hydroxyl layers. RH has no effect on the structural environments and dynamics of the interlayer anions in this group.