Reaction mechanisms of smectite illitization associated with hydrothermal alteration from Ponza Island, Italy

A hydrothermally altered rhyolitic hyaloclastite from Ponza island, Italy, has four alteration zones with unique clay assemblages: (1) a non-pervasive argillic zone characterized by smectite; (2) a propylitic zone with interstratified illite-smectite (I-S) containing 10-85% illite (I): (3) a silicic zone composed of I-S with ≥90% I and pure illite; and (4) a sericitic zone with I-S ranging from 66% I to pure illite. Atomic force microscopy reveals abrupt changes in particle morphology with illitization, including initial changes from anhedral plates to laths and then to euhedral plates and hexagonal plates. I-S particles progressively thicken with illitization and mean particle area (basal plane) remains constant from pure smectite to I-S with 80% I. However, particle area increases from 90 to 100% illite. Computer modeling of I-S structural forms indicates octahedral cation ordering progressively changes from cis-vacant smectite to interstratified cis- and trans-vacant I-S. and then to trans-vacant illite. In addition, polytypes progressively change from 1M_d to 1M, and then to 2M₁ illite. Electron-microprobe and X-ray flouresence analyses show that I-S chemistry progressively changes during illitization, evolving toward a phengitic composition with ~0.89 fixed interlayer K⁴ per O₁₀(OH)₂. Octahedral Mg²⁺ shows little change with illitization, varying from 0.3 to 0.5 cations per O₁₀(OH)₂. The layer charge of smectite is ~0.38 equivalents per O₁₀(OH)₂. On the basis of abrupt changes in morphology and progressive changes in polytype and chemistry, smectite illitization on Ponza involved a dissolution and recrystallization mechanism with multiple stages of nucleation and crystal growth. In this multi-step model, temperature of alteration provided the major control for the layer composition, polytype, and morphology of I-S crystallites. Other factors that may play a secondary role include: K⁺ availability, water-rock ratio, and permeability. Alternatively, the mechanism of I-S and illite formation at Ponza and other hydrothermal environments may occur by direct precipitation of I-S crystallites from rhyolite glass and may not involve progressive reactions of smectite precursors.