Synthesis of Smectite Clay Minerals: a Critical Review

-Smectites are one of the most important groups of phyllositicates found in soils and sediments, and certainly one of the most difficult to study. New information about the formation mechanisms, impact of structural features on surface properties, and long-term stability of smectites can best be gained from the systematic study of single-phase specimens. In most instances, these specimens can only be obtained through synthesis under controlled conditions. Syntheses of smectites have been attempted (1) at ambient pressure and low-temperature (< 100~ (2) under moderate hydrothermal conditions (100-1000~ pressures to several kbars), (3) under extreme hydrothermal conditions (>1000~ or pressures >10 kbars), and (4) in the presence of fluoride. Of these approaches, syntheses performed under moderate hydrothermal conditions are the most numerous and the most successful in terms of smectite yield and phasepurity. Using hydrothermal techniques, high phase-purity can be obtained for beidellites and several transition-metal smectites. However, synthesis of montmorillonite in high purity remains difficult. Starting materials for hydrothermal syntheses include gels, glasses, and other aluminosilicate minerals. The presence of Mg 2+ seems to be essential for the formation of smectites, even for phases such as montmorillonite which contain low amounts of Mg. Highly crystalline smectites can be obtained when extreme temperatures or pressures are used, but other crystalline impurities are always present. Although the correlation between synthesis stability fields and thermodynamic stability fields is good in many instances, metastable phases are often formed. Few studies, however, include the additional experiments (approach from underand over-saturation, reversal experiments) needed to ascertain the conditions for formation of thermodynamically stable phases. Thorough characterization of synthetic products by modern instrumental and molecular-scale techniques is also needed to better understand the processes leading to smectite formation. Key Words--Beidellite, Fluoride, Hectorite, Hydrothermal, Montmorillonite, Nontronite, Phyllosilicate, Saponite, Sauconite, Stevensite. I N T R O D U C T I O N Clay minerals form an important group of the phyllosil icate family o f minerals, which are dist inguished by layered structures composed of po lymer ic sheets of S i t 4 te t rahedra l inked to sheets o f (A1, Mg , Fe) (O,OH) 6 octahedra. The geochemica l importance of clay minerals stems f rom their ubiquity in soils and sediments, high specific surfaces, and ion-exchange properties. Consequently, clay minerals tend to dominate the surface chemist ry of soils and sediments. Furthermore, these properties have g iven rise to a wide range of industrial applications throughout history. Pr imit ive peoples used clay minerals for ceramic figures and pot tery as early as 25,000 years B.E (Shaikh and Wik, 1986). Today, clay minerals are important materials with a great variety of applications in ceramics, nanocomposi te materials, oil drill ing, waste isolation, and the metal and paper industries. Chemica l applications of clay minerals include their use as adsorbents, decolor iz ing agents, ion exchangers, and molecular-sieve catalysts (Fowden et al., 1984; Murray, 1995). Despi te their importance, the clay minerals fo rm a difficult group to study due to their small size, variable structural composi t ion, and relat ively slow rate of format ion and alteration. N e w informat ion about the format ion mechanisms, variat ion in surface propert ies with structure, composi t ion, and solution composi t ion, and the longterm stability of clay minerals in surficial and hydrothermal envi ronments may be gained f rom studies of synthetic clay minerals. In particular, the rapid and rel iable synthesis of single-phase end-member specimens for characterizat ion and reference purposes will greatly accelerate progress in this area. Smecti tes are one o f the largest and most important classes of the phyl losi l icate c lay-mineral group. They are c o m m o n in temperate soils and, because of their cat ion exchange capacit ies (CEC) and very high specific surfaces, tend to domina te the cationic adsorpt ion chemis t ry of these soils. Their small particle size (typical ly < 1 p~m) and large aspect ratio also al low them to strongly inf luence the physical properties o f soils and sediments. These chemica l and physical propert ies explain the suitability o f clays as catalysts and as pond liners for isolat ion of hazardous wastes. Smal l size and highly variable composi t ion, however, have led to considerable uncertainty regarding the origin and thermodynamic stability o f smectites. As r ev iewed by Borchardt (1989, p. 689-703) , considerable ev idence Copyright 9 1999, The Clay Minerals Society 529 530 Kloprogge, Komarneni, and Amonette Clays and Clay Minerals suggests that they are of detrital origin (i.e., a stage in the weathering of micas and chlorites to kaolinite and gibbsite) and not thermodynamically stable phases in soils and sediments, yet some evidence also exists suggesting that at least some of the end-member phases form authigenically in sediments under ambient conditions. Because natural specimens typically contain impurities, questions related to formation and stability are best answered by study of synthetic, fully characterized, single-phase specimens. Although a number of clay minerals including kaolins and various smectites have been synthesized during the past 50 years, most of this synthetic work is empirical in nature (Giiven, 1988). The most common synthetic approach is to treat starting materials (mostly solid phases such as other aluminosilicates, glasses, gels) having a composition close to the desired clay by heating in water or under hydrothermal conditions [i.e., 100-1000~ autogenous water pressure (the pressure reached when excess water is heated in a confined volume) or higher]. A major difficulty is that the reactions rarely go to completion unless hydrothermal conditions are employed and, even then, a mixture of products is typically obtained rather than a pure clay mineral. The purpose of this review is to summarize and evaluate smectite synthesis techniques in terms of the crystallinity and phase-purity of the products obtained. Information about smectite (and clay-mineral) formation mechanisms that can be gleaned from the synthesis studies will also be presented. The review begins with a brief overview of the classification scheme of the smectite group. The main part of the review is divided into four sections that describe approaches to smectite synthesis under different sets of conditions (i.e., low temperature and pressure, hydrothermal, very high temperature and pressure, and the presence of fluoride). These four sections are further divided into subsections that cover synthesis of individual smectite minerals. The review concludes with a critical discussion of the approaches to smectite synthesis and product characterization outlined in the previous four sections. Portions of this review were presented in Klopproge (1998). CLASSIFICATION OF SMECTITES Smectites form a group in the class of minerals known as phyllosilicates or layer silicates. Other groups in this class include the micas, kaolins, vermiculites, chlorites, talc, and pyrophyllite. The phyllosilicate structure consists of layers in which planes of oxygen atoms coordinate to cations such as Si, A1, Mg, and Fe to form two-dimensional "sheets". The coordination of cations in adjacent sheets typically alternates between tetrahedral and octahedral. Tetrahedral sheets, which commonly contain Si, A1, and Fe 3§ consist of hexagonal or ditrigonal rings of oxygen tetrahedra linked by shared basal oxygens. The apical oxygens of these tetrahedra help form the base of octahedral sheets having brucite-like or gibbsite-like structures and commonly containing Mg, A1, Li, Fe 2§ and Fe 3+. A regular repeating assemblage of sheets (e.g., tetrahedral-octahedral or tetrahedral-octahedraltetrahedral) is referred to as a layer. Smectites, micas, vermiculites, talc, and pyrophyllite are characterized by a 2:1 layer structure in which two tetrahedral sheets form on either side of an octahedral sheet through sharing of apical oxygens. As the apical oxygens from the tetrahedral sheet form ditrigonal or hexagonal rings, one oxygen from the octahedral sheet is located in the center of each ring and is protonated to yield a structural hydroxyl. In 2:1 phyllosilicates, isomorphous substitution of cations having different valences can lead to charge imbalances within a sheet. These may be partly balanced by the opposite type of charge imbalance in the adjacent sheet (e.g., a positively charged octahedral sheet may offset some of the negative charge associated with a tetrahedral sheet). The net charge imbalance on a 2:1 layer, if it occurs, is negative. This charge is referred to as the layer charge of the mineral and is balanced by larger cations (e.g., Na § K § Ca z*, and Mg 2+) that coordinate to the basal surfaces of the tetrahedral sheets from adjacent layers. Because these charge-balancing cations are located between adjacent 2:1 layers they are referred to as "interlayer cations". The 2:1 phyllosilicates are distinguished chiefly on the basis of their layer charge. Kaolins, on the other hand, form a 1:1 layer structure consisting of a single tetrahedral sheet linked to a single octahedral sheet through apical oxygens. As isomorphous substitution, if present, is very low in kaolins, adjacent 1:1 layers are linked by hydrogen bonding between the hydroxylated surface of the octahedral sheet and the basal oxygens of the tetrahedral sheet from the adjacent layer. No interlayer cations are present in the 1:1 structure. The chlorite structure is in many ways a hybrid of the 2:1 and 1:1 structures. Although it includes a regular 2:1 layer, the layer charge is balanced by a positively charged sheet of hydroxylated cations in octahedral coordination (i.e., a brucite-like or gibbsite-like sheet) thus simulating, in a gross sense, the alternating octahedral-tetrahedral structure of a 1:1 phyllosilicate. The bonding between the interlayer hydroxylated sheet and the 2:1 layer, however, is distinct from that of the 1:1 minerals in that the oxygen atoms in the hydroxylated sheet are not shared with the basal oxygen atoms of the tetrahedral sheets on the adjacent 2:1 layers. A further designation among the phyllosilicates is made based on type and location of the cations in the oxygen framework. In half of a unit cell composed of ten oxygen atoms and two hydroxyl groups, there are four tetrahedral and three octahedral sites. A phyllosilVol. 47, No. 5, 1999 531 Synthesis of smectites Table 1. Classification of selected common hydroxylated phyllosilicates. Layer Interlayer Layer OctahedralMineral type species charge sheet type name Formula