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 ≥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 1Md to 1M, and then to 2M1 illite. Electron-microprobe and X-ray fluoresence analyses show that I–S chemistry progressively changes during illitization, evolving toward a phengitic composition with ~0.89 fixed interlayer K+ 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.

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