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

Robert F. Ylagan, Stephen P. Altaner, Antonio Pozzuoli

Research output: Contribution to journalArticlepeer-review


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 1Md 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.

Original languageEnglish (US)
Pages (from-to)610-631
Number of pages22
JournalClays and Clay Minerals
Issue number6
StatePublished - Dec 2000
Externally publishedYes


  • Atomic force microscopy
  • Chemical analysis
  • Illite
  • Polytype
  • Smectite
  • XRD

ASJC Scopus subject areas

  • Water Science and Technology
  • Soil Science
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)


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