Hydrothermal alteration of a saponitic bentonite: mineral reactivity and evolution of surface properties J. CUEVAS*, A. GARRALO Â N, S. RAMI Â REZ AND S. LEGUEY Departamento Quõ Âmica Agrõ Âcola, Geologõ Âa y Geoquõ Âmica, Universidad Auto Ânoma de Madrid, Cantoblanco s/n, 28049 Madrid, Spain (Received 29 November 1999; revised 17 August 2000) ABSTRACT: Saponitic bentonite mined in the Magan deposit (Toledo, Spain), has been classified as a suitable clay barrier in the storage of high level radioactive waste. Several hydrothermal alteration assays have been carried out in Teflon reactors at 45, 60, 90, 120, 175 and 2008C for periods of up to 1 y. The mineral components of bentonite are stable below 1758C. At and above this temperature, the accessory sepiolite transforms into a monomineral phase of saponitic composition. The texture of the clay also changes. A rise in temperature above 1208C causes a decrease in the proportion of the <2 mm size-fraction, a reduction of BET and total surface areas and an increase in the relative volume of micropores (<20 A Ê ). This process has been interpreted as the formation of granular aggregates that preserve a micropore network. This new arrangement of the aggregates produces a significant reduction in the free swelling volume. KEYWORDS: bentonite, saponite, hydrothermal alteration, surface properties. Saponite is a frequent mineral in Miocene lacustrine sediments of the Neogene Madrid Basin, where some beds are exploited for bentonite (Galan et al., 1986; Ordon Äez et al., 1991; Garcõ Âa del Cura et al., 1996). Gala Ân et al. (1986) studied the bentonite deposits of Cerro del Monte and reported the presence of saponitic magnesian smectites along with sepiolite, illite, carbonates and detrital minerals. Saponite probably formed via authigen- esis in lakes and ponds, and sepiolite in shallower areas during periods of desiccation. (Leguey et al., 1985; Pozo et al., 1992). Recently, De Santiago et al. (1998), showed by TEM-EDX the close textural relationship between saponite and an altered illite- intermediate phase in these materials. Saponite is characterized by a high thermal stability and dehydroxylation temperatures between 800 and 8508C (Iiyama & Roy, 1963; Kawano & Tomita, 1991). This makes it a good candidate as a sealing material in the storage of high level radioactive waste (HLRW) since this requires smectite to be stable at ~1008C for a long period of time (Pusch, 1977, 1992; Brookins, 1984). The potential use of bentonites from the Madrid Basin in the storage of HLRW has been investigated by Cuevas et al. (1993) who concluded that some of these materials had valuable physical and chemical properties meeting criteria expected for the engineered barrier system (EBS) design (Grauer, 1994; Pusch, 1994). In natural environments, saponite can transform at high temperatures into chlorite through the formation of corrensite (regularly interstratified chlorite-smectite). In the laboratory, corrensite has been synthesized at 350À5008C during the hydro- thermal transformation of saponites without any evidence of the formation of random interstratified chlorite-smectite (Roberson et al., 1999). During Clay Minerals (2001) 36, 61±74 # 2001 The Mineralogical Society * E-mail: jaime.cuevas@uam.es