“Palagonite,” which was first used to describe altered hyaloclastite deposits from Palagonia, Mount Iblei, Sicily [Von Waltershausen, 1845], forms rinds on mafic glass surfaces exposed to aquatic fluids (e.g., pillow rims, hyaloclastite particles, fractures, vesicle walls), commonly leaving islands of fresh glass embedded in palagonite rinds of varying thickness. Palagonite is commonly considered as the first replacement product of mafic glass alteration [Furnes, 1975; Hay and Iijima, 1968b; Moore, 1966; Peacock, 1926; Staudigel and Hart, 1983; Thorseth et al., 1991]. The formation, composition, and evolution of palagonite thus partly control the rate of glass alteration as well as the elements effectively released during glass dissolution and thereby the development of other secondary mineral assemblages.
 Peacock , in the first comprehensive petrographic study of palagonite, distinguished two main palagonite varieties, a classification scheme still used today: (1) yellow, transparent, isotropic, clear, commonly concentrically banded “gel palagonite” and (2) yellow-brown, translucent, slightly anisotropic, slightly to strongly birefringent, fibrous, lath-like, or granular “fibro-palagonite,” which develops during more advanced steps of palagonitization on the outer surface of gel palagonite. Until now the term palagonite was commonly used both as a general term for any hydrous alteration product of mafic glass and also for the crystalline material (e.g., smectite) evolving from the palagonite itself [Furnes, 1984; Jakobsson, 1972; Jercinovic et al., 1990; Thorseth et al., 1991]. Also widespread is the usage of the term palagonitized glass usually referring to the glass alteration rim as a whole. Kinetic and thermodynamic modeling as well as mass balance calculations necessitate, however, the exact differentiation of different secondary products developed during alteration in a water-rock system. Even so, palagonite is a metastable product phase, like other metastable phases (e.g., opal); it is formed under specific thermodynamic conditions, making a more restricted use of this term feasible. Consequently, we here use the term palagonite only for the amorphous alteration product (Figure 1) and are only going to refer to the terms gel- and fibro-palagonite in the context with other studies. Thus, as soon as crystals form, a two-phase system is established consisting of palagonite and the crystalline material, mainly clay minerals. In this regard the process of glass palagonitization in this study is considered based on the evolutionary aspects of a thermodynamically unstable, gel-like, amorphous, aging material. Aging is synonymous with crystallization and crystal growth processes in gels. In practice (Figure 1), however, use of the term palagonite in a comprehensive sense is advocated because different aging steps of palagonite cannot be identified without the use of detailed analytical methods.
 Palagonite's mineralogical nature is still poorly defined. Hay and Iijima [1968b] proposed that palagonite is composed of montmorillonite and mixed-layer mica montmorillonite. According to Furnes , palagonite consists of kaolinite, illite, mixed-layer clay minerals, or zeolites. Honnorez  suggested that palagonite is a mixture of altered, hydrated, and oxidized glass with authigenic minerals (clays, zeolites) and proposed to abandon the term completely. Palagonite was also interpreted to be composed of some smectite variety and minor amounts of zeolites and oxides following from a combination of detailed analytical methods with stoichiometric considerations [Daux et al., 1994; Eggleton and Keller, 1982; Staudigel and Hart, 1983; Zhou et al., 1992].
 Numerous studies have focused on the chemical composition of palagonite and the chemical changes occurring during palagonitization; see Honnorez , Fisher and Schmincke , and N. A. Stroncik and H.-U. Schmincke (submitted manuscript, 2001) for reviews. In general, the chemical composition of palagonite and its parent glasses differ significantly from each other. Even a homogeneous parent glass may result in palagonite with pronounced intragrain variations; the reasons for which remain unknown. The estimated extent and direction of element mobility accompanying the glass to palagonite transformation also varies and depends also, in the absence of a kinetic model for the glass alteration process, on the method of calculation [Bednarz and Schmincke, 1989; Furnes, 1984; Hay and Iijima, 1968b; Staudigel and Hart, 1983].
 Important for the chemical composition of palagonite and the element mobilities accompanying the palagonitization process is, of course, also the mode of its formation. It is generally accepted today that some type of dissolution-precipitation, either incongruent or congruent dissolution, is responsible for the alteration of sideromelane to palagonite [Berger et al., 1987; Crovisier et al., 1987; Daux et al., 1994; Jercinovic et al., 1990; Thorseth et al., 1992, 1991, 1995a; Zhou and Fyfe, 1989]. This is indicated by the physical characteristics of alteration products, reaction rates, and element mobilities. Besides this general agreement on a dissolution-precipitation process being responsible for the formation of palagonite from glass, the mechanisms controlling these processes, especially the precipitation of palagonite, are still not fully understood. Lately, the influence of microorganisms on the process of glass alteration has also been emphasized in a number of studies [Furnes et al., 1996; Staudigel et al., 1995, 1998; Thorseth et al., 1992, 1995a, 1995b]. Bacteria and microorganisms create a local microenvironment through the waste of their metabolic products. These fluids have either an acidic or basic pH, depending on the type of bacteria. An acidic pH basically results in incongruent glass dissolution, whereas a basic one results in congruent glass dissolution, leaving large pits on the glass surface. Overall microbial activity enhances the dissolution rate of volcanic glass [Staudigel et al., 1995, 1998]. Microbial alteration also results in the formation of authigenic phases and is accompanied by redistribution of elements [Drewello and Weissmann, 1997; Furnes et al., 1996; Staudigel et al., 1995, 1998; Thorseth et al., 1992, 1995a, 1995b]. Although the role of biological activity in glass alteration in general is well established, its role in glass palagonitization is not. Until now no biotic glass alteration experiment has resulted in the formation of palagonite, and the link between palagonite precipitation and biotic activity, considering natural samples, has not been established to full satisfaction. Although there are a number of natural samples showing features interpreted as indicative of microbial activity (e.g., micropits, microchannels, remnants of DNA [Fisk et al., 1998; Furnes et al., 1996; Thorseth et al., 1992, 1995a, 1995b; Torsvik et al., 1998]), a large number of palagonite samples do not show these features.
 Here a combination of electron microprobe (EMP), infrared photometry (IRP), atomic force microscopy (AFM), X-ray diffraction (XRD), and conventional optical methods was used to “model” the mineralogical and chemical evolution of palagonite and the impact of palagonite evolution on the overall element budget of a water/rock system.