Reaction Mechanisms of Smectite Illitization Associated with Hydrothermal Alteration from Ponza Island, Italy

Robert F. Ylagan1, †, Stephen P. Altaner1 and Antonio Pozzuoli2
1 University of Illinois, Department of Geology, 1301 W. Green, Urbana, Illinois 61801 USA
2 Università degli Studi di Napoli ‘Federico II’, Dipartimento di Geofisica e Vulcanologia, Largo S. Marcellino 10 80138, Napoli, Italy
Present address: ExxonMobil Upstream Research Co., P.O. Box 2189, Houston, Texas 77252-2189 USA. E-mail of corresponding author: bob.f.ylagan@exxon.sprint.com

Abstract: 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 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.

Key Words: Atomic Force Microscopy • Chemical Analysis • Illite • Polytype • Smectite • XRD

Clays and Clay Minerals; December 2000 v. 48; no. 6; p. 610-631; DOI: 10.1346/CCMN.2000.0480603
© 2000, The Clay Minerals Society
Clay Minerals Society (www.clays.org)