Palagonite – a review

Abstract

Palagonite is the first stable product of volcanic glass alteration. It is a heterogeneous material, usually with highly variable optical and structural properties, ranging from a clear, transparent, isotropic, smooth and commonly concentrically banded material, commonly called "gel-palagonite", to a translucent, anisotropic, slightly to strongly birefringent material of fibrous, lath-like or granular structure, commonly called "fibro-palagonite". The color of palagonite ranges from shades of yellow to shades of brown. Palagonite forms rinds of variable thickness on every mafic glass surface exposed for some time to aquatic fluids. It is formed by either incongruent dissolution or by congruent dissolution of glass with contemporaneous precipitation of insoluble material at the glass–fluid interface. The process of palagonitization is accompanied by extensive mobilization of all elements involved in the alteration process, resulting in the depletion or enrichment of certain elements. The extent and direction of element mobility and the palagonitization process itself (including the rate of palagonitization) depend on a number of different, complex interacting properties: e.g. (1) temperature, (2) the structure of the primary material, (3) the reactive surface area of the primary material, (4) the structure of the precipitating secondary phases, (5) the growth rates of the secondary phases, (6) time, and (7) fluid properties such as fluid flow rates, pH, Eh, ionic strength, and oxygen fugacity. The fluid properties themselves are affected by different hydrogeological properties such as porosity, permeability, and pressure gradients.