ORIGINAL PAPER D. Tunega Æ H. Lischka Effect of the Si/Al ordering on structural parameters and the energetic stabilization of vermiculites – a theoretical study Received: 4 December 2002 / Accepted: 2 July 2003 Abstract The effect of the Si/Al distribution in the tet- rahedral sheets of the vermiculite mineral has been investigated employing density functional theory. The structures of six models for vermiculite with the struc- tural formula (Mg 4 )(Mg 12 )(Si 8 Al 8 )O 40 (OH) 8 Æ24(H 2 O) per unit cell were fully optimized. The models differ by the TÆÆÆMg 2+ ÆÆÆT¢ coordination of the interlayer Mg 2+ cations by two central cations from the adjacent tetra- hedral sheets of the 2:1 vermiculite layers (T,T¢ ¼ Si,Al). We observed the formation of very strong hydrogen bonds between water molecules solvating the interlayer Mg 2+ cations and the surface basal oxygen atoms of the 2:1 layers. The directionality of hydrogen bonds is the major factor determining the layer stacking in the ver- miculite structure. Results showed that the most stable model is that where only silicon atoms in the tetrahedral sheets coordinate all interlayer Mg 2+ cations. Keywords Vermiculite Æ Density functional theory Æ Calculation Introduction Vermiculites are expandable 2:1 clay minerals formed in soils from the weathering of micas and chlorites (Shulze 1989). Their 2:1-layer structure consists of two tetrahe- dral sheets bound to either side of an octahedral sheet via the plane of basal oxygen atoms (see Fig. 1). The outer two planes of the layer are formed from the basal oxygen atoms. The structure of vermiculite layers can be derived from the structure of the layers of the simplest 2:1 phyllosilicates—talc and pyrophyllite. Talc has Mg 2+ cations in all possible octahedral sites and is of a trioctahedral type while pyrophyllite has only 2/3 of the octahedral sites occupied by Al 3+ cations and is of the dioctahedral type. Tetrahedral sheets in both minerals contain only Si 4+ cations. In both cases layers are electrically neutral and held together only by weak dis- persion forces between layers. The 2:1-layer structure of vermiculite minerals differs from talc and pyrophyllite mainly in the chemical composition. A part of tetrahe- dral sites is occupied by trivalent cations (e.g. Al 3+ , Fe 3+ ) instead of Si 4+ (in some cases also isomorphic substitution in the octahedral sheet can occur). This substitution results in an excess of the negative charge per formula unit, which is compensated by cations in the interlayer space, similar to the situation occurring in micas. While in micas the charge per formula is about )1 or more and mainly K + , Na + and Ca 2+ are compen- sating cations, in vermiculites the charge is lower (from - 0.6 to )0.9) and compensating cations are hydrated Mg 2+ and Ca 2+ in most cases. However, in some cases vermiculite structures have excess charge larger than –1.5 (Van Olphen 1969). Forces keeping layers in vermi- culites together are of Coulombic nature supported by strong hydrogen bonds. In contrast to micas, the inter- layer counter ions are exchangeable and can be replaced by various monovalent or divalent cations (e.g. Li + , Na + , K + , Cs + , NH 4 + , Ni 2+ ). The amount of interlayer water in vermiculites is variable and depends on physico- chemical conditions. The interlayer spacing, d, depends on this confined interlayer water (Smalley 1994). The structures of natural Mg-vermiculites are known from the pioneering investigations of Hendricks and Jefferson (1938), Mathieson and Walker (1954) and Shirozu and Bailey (1966). Later experimental work was focused on different cation–water complexes, layer- stacking orders and the structure of interlayer water. A summary of these aspects is given in the review of de la Calle and Suquet (1988). Interlayer structures of the two-layer hydrates of Na- and Ca-vermiculites were Phys Chem Minerals (2003) 30: 517–522 Ó Springer-Verlag 2003 DOI 10.1007/s00269-003-0347-x D. Tunega (&) Austrian Research Centers Seibersdorf, 2444 Seibersdorf, Austria e-mail: Daniel.Tunega@univie.ac.at Fax: +43-1-4277-9527 D. Tunega Æ H. Lischka Institute for Theoretical Chemistry and Structural Biology, University of Vienna, Wa¨hringerstrasse 17, 1090 Vienna, Austria