American Mineralogist, Volume 82, pages 517-525, 1997 Na, K, Rb, and Cs exchange in heulandite single crystals: Diffusion kinetics PING YANG,1 JANO STOLZ,1 THOMAS ARMBRUSTER,1 AND MICKEY E. GUNTER2 1Laboratorium fuÈr chemische und mineralogische Kristallographie, UniversitaÈt Bern, Freiestrasse 3, CH-3012 Bern, Switzerland 2Department of Geology and Geological Engineering, University of Idaho, Moscow, Idaho 83844, U.S.A. ABSTRACT Diffusion-exchange kinetics were determined at temperatures between 320 and 425 K 1 for Na in single crystals of natural heulandite, Na0.96Ca3.54K0.09(Al8.62Si27.51O72)´nH2O, and K1,Rb1, and Cs1 for Na1 in Na-exchanged heulandite. Cation diffusion was measured on (010) cleavage plates with the aid of a polarizing microscope. This is possible because the optical properties, in particular the extinction angle, vary with the chemical composition of the channel occupants. This optical method is restricted to platy zeolites of monoclinic or triclinic symmetry where the channels are parallel to the crystal plate. Unlike conven- tional measurements, this method does not require the determination of the surface area of the zeolite. No indication of anisotropic diffusion was observed on (010) plates for any of the exchanged cation pairs. Diffusion coef®cients (D) show that the diffusion rate of Na1 → Ca21 is much slower than those of K1 → Na1,Rb1 →Na1, and Cs1 → Na1, because of the strong Coulombic interaction of Ca21 with the tetrahedral framework compared with an exchange of monovalent species. The activation energy (Ea) and the pre-exponential factor (D0) were calculated from the variation of D with temperature for each diffusing 1 → 1 1 → 1 cation. Comparable values of Ea for K Na and Rb Na suggest a similar diffusion 1 process, while the lower value of Ea, combined with relatively low diffusion rates for Cs → Na1 implies a different diffusion mechanism for Cs1. INTRODUCTION (Na,K)Ca4(Al9Si27O72)´24 H2O and clinoptilolite (Na,K)6 (Al Si O )´20 H O, have been used for the removal of The cation-exchange properties of zeolite minerals 6 30 72 2 1 21 were ®rst observed almost 100 years ago (Breck 1974). radioactive Cs and Sr from low-level waste streams of Since then, there have been many investigations of cat- nuclear power stations and for the extraction of ammo- ion-diffusion mechanisms in natural and synthetic zeo- nium in sewage because of their excellent selectivity of lites. Barrer and Hinds (1953), Barrer et al. (1963, 1967), cation-exchange and absorption (Mumpton 1988; Smyth Barrer and Munday (1971a, 1971b), Ames (1960, 1962), et al. 1990). and Sherry (1966) have studied the cation-exchange prop- Heulandite belongs to the family of platy zeolites. Its erties of many natural zeolites (e.g., analcime, leucite, framework is composed of dense (010) layers formed by chabazite, and clinoptilolite) and many synthetic zeolites four- and ®ve-membered rings of (Si,Al) tetrahedra. (e.g., X, Na-P, K-F). However, in most of these studies These layers are connected by relatively sparse (Si,Al)- the determination of kinetic parameters mainly depended O-(Si,Al) columns. The crystal structure is characterized on monitoring the variation of solution concentrations ac- by three kinds of channels (Fig. 1) de®ned by tetrahedral companied by a determination of the zeolite surface area. rings (unit-cell setting: a 5 17.7, b 5 17.9, c 5 7.4 AÊ , b The latter measurement is the crucial step in this tech- 5 1168). Channels A and B extend parallel to the c axis nique, and surface area measurements may result to errors and are de®ned by ten- and eight-membered rings of tet- in D by three orders of magnitude (Rees and Rao 1966). rahedra, respectively (Fig. 1). Channel C, formed by an- The krypton adsorption method, which is frequently used other eight-membered ring, runs parallel to the a axis to measure the surface area of a zeolite powder, may lead (Koyama and TakeÂuchi 1977) and parallel to the [102] to incorrect results if during the measurement zeolites re- direction (Merkle and Slaughter 1968). All channels lie lease H2O or if krypton enters the structural pores (Barrer parallel to (010) (Fig. 1). The channels, in turn, produce et al. 1963). In addition, experiments using powdered the perfect (010) cleavage found in this family of zeolites. samples do not provide any direct information on the an- Thus at low temperature (, 500 K), signi®cant cation isotropy of exchange kinetics. A simple effective method diffusion in heulandite occurs only parallel to the (010) for the direct observation of cation diffusion in zeolite plane. Diffusion perpendicular to (010) would force cat- single crystals has not been reported to date. ions to penetrate ®ve-membered rings of tetrahedra, Minerals of the heulandite structure type, heulandite which seems unlikely for Na1,K1,Rb1, and Cs1. 0003±004X/97/0506±0517$05.00 517 518 YANG ET AL.: ALKALI EXCHANGE IN HEULANDITE FIGURE 1. A schematic representation of the cation sites and alignment of channels in heulandite viewed down the b axis. The A and B channels are shown parallel to c and contain the A2 and B4 cation sites (normally called M1 and M2) and the new cations site II-1. The C channel is parallel to a and contains the C3 cation (normally called M3). In a recent paper Yang and Armbruster (1996) deter- FIGURE 2. Photomicrograph taken with crossed polarizers showing Rb1 exchange into a Na-exchanged heulandite at 330 mined the crystal structures of Na-, K-, Rb-, and Cs-ex- K after 24 h. The diameter of the crystal is approximately 350 changed varieties of heulandite studied by single-crystal mm. The light rim of the crystal shows the Rb-exchanged zone, X-ray diffraction data at 100 K. Three general cation po- approximately 96 mm, and the dark boundary indicating that the sitions, II-1, C3, and B4 (Fig. 1), were found in all alkali- exchange process had not yet been completed. Note also that the cation-exchanged heulandite samples. For Rb- and Cs-ex- center of the crystal is not at perfect extinction but is a few changed crystals, the additional cation site A2 also occurs degrees off. This slightly off-extinction position induces no error (Fig. 1). The letter of the cation position represents the in the measurement of t but allows for better visual distinction channel (de®ned above) where the cations were located, of the dark boundary. and the Roman numeral represents a cage. II-1 indicates a site close to the wall of cage II, which is formed by two ten-membered A-rings and two eight-membered diffusion kinetics in heulandite can also be monitored C-rings (Yang and Armbruster 1996). For all exchanged with a polarizing microscope, provided that the ex- samples, the highest cation concentration was located at changed cations lead to a variation of birefringence or C3, near the center of an eight-membered ring formed by extinction angle on (010). (Si,Al)O4 tetrahedra. This cation site nomenclature was Relative differences in the extinction angles were used developed by Yang and Armbruster (1996) to correlate a herein to perform a systematic quantitative study of alkali cation's location to a channel (i.e., A, B, or C) or a cage group ions exchanged into heulandite single crystals. type. Past cation site designation (Gunter et al. 1994) Thus diffusion in heulandite could be directly observed would relate Na1 (5 M1) to A2, Ca2 (5 M2) to B4, K3 with a polarizing microscope by placing the non-ex- (5 M3) to C3, while the II-1 site would be in a similar changed core at extinction and measuring the non-extinct location to that of Pb5 (Gunter et al. 1994). exchanged rim to determine the depth of cation diffusion Tiselius (1934) showed that the rate of hydration and (Fig. 2). dehydration of heulandite single crystals under conditions of controlled vapor-pressure and temperature can be EXPERIMENTAL METHODS quantitatively measured by the change of optical birefrin- As starting material, coarse-grained heulandite from gence and extinction angle on (010) crystal plates. Gunter Nasik, India (Sukheswala et al. 1974) with the composi- et al. (1994) reported that the birefringence, d (010), is tion Na0.96Ca3.54K0.09(Al8.62Si27.51O72)´nH2O was used. The 0.0010 for a natural Ca-rich heulandite, 0.0026 for a large single crystals were ground and sieved to an ap- Na-exchanged heulandite, and 0.0145 for a Pb-exchanged proximate grain size of 0.1±0.5 mm. The exchange-dif- heulandite. With b 5 Z, the extinction angle (X : c) fusion experiment of Na1 with natural Ca-rich heulandite changes from 78 (non-exchanged) to 198 (Na-exchanged) was carried out by heating heulandite samples in 2M to 118 (Pb-exchanged). Thus the birefringence and optical NaCl solution. A te¯on-coated autoclave was used for orientation of the samples before and after exchange var- high temperature exchange experiments (. 373 K). The ied dramatically. These experiments indicate that cation- other exchange-diffusion experiments were performed by YANG ET AL.: ALKALI EXCHANGE IN HEULANDITE 519 two-dimensional channel system where the channels are parallel to the crystal plates. Exchanged Na, K, Rb, and Cs end-member heulandite samples used for measurement of optical properties and composition were prepared as described above (Yang and Armbruster 1996) but exchange times were lengthened and higher temperatures were used in the corresponding alkali-chloride solution. The composition of the natural heulandite and the exchanged samples were determined with a CAMECA SX50 electron microprobe operating at 20kV and 20 nA beam current and defocused beam di- ameter of about 20 mm to prevent sample destruction and loss of channel cations.