Interstratified illite-smectites (I/S) and illite-smectite-vermiculites (ISV) representing both hydrothermal and diagenetic transformations and having different degrees of structural order were investigated for cis-trans occupancy in the octahedral sheet by X-ray diffraction (XRD) and by differential thermal analysis (DTA) in combination with evolved water analysis (EWA) using an infrared detector. By XRD, the amounts of cis (w cv) and trans (w tv) vacant 2:1 layers were determined for the three-dimensionally ordered samples using both the WILDFIRE simulation program and calculations based on positions of the 11l and 11 reflections. Based on the EWA curves, the I/S and ISV could be divided into threē l groups having (1) one strong and one or more weak EWA peaks; (2) two well-resolved peaks; and (3) a complex EWA curve. The amounts of cis-and trans-vacant sites were determined by peak fitting of the total dehydroxylation curve. The complex EWA curves were, however, in addition split into separate dehydroxylation processes during a stepheating technique. If the EWA peaks below and above 600 ЊC were attributed to trans vacant (tv) and cis vacant (cv) octahedra, respectively, the w cv values determined by XRD and by EWA were in agreement. For the three-dimensionally ordered minerals, both XRD and EWA should be used, whereas the EWA method can be applied to the structurally disordered samples having no diagnostic 11l reflections. Accordingly, a combination of XRD and EWA for the determination of w cv and w tv supports an evaluation of the mechanism of illitization in various geological environments. Thus, significant changes in w cv and w tv during illitization are likely due to a dissolution-precipation, whereas almost constant values indicate a solid-state transformation.

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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 eubedral 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 cisvacant smectite to interstratified cis-and trans-vacant l-S, and then to trans-vacant illite. In addition, polytypes progressively change from 1M d to 1M, and then to 2MI 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 O~0(OH)2. Octahedral Mg 2+ shows little change with illitization, varying from 0.3 to 0.5 cations per O~0(OH)2. The layer charge of smectite is --0.38 equivalents per OI0(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.

Dioctahedral phyllosilicates from an altered rhyolitic hyaloclastite located at Ponza Island, Italy, were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples are from a sequence previously characterized by X-ray diffraction (XRD) methods, indicating that a complete range of illitization accompanies alteration. Backscattered electron (BSE) images, obtained from ion-milled samples, show that samples partly retain the original textures since clay minerals pseudomorph lapilli fragments and preserve vesicular texture. The lowest-grade sample studied contains obsidian clasts partially replaced by smectite. As the alteration grade increases, illitization proceeds with formation of interstratified illite-smectite (I-S), zeolites, illitic phases, feldspars and quartz. The most altered sample contains illite, mica and quartz. Lattice-fringe images show that following the formation of smectite, illitization takes place through the formation of (R=1) I-S, highly illitic I-S and illite with mica; (R=1) I-S is the only ordered interstratified I-S. The BSE and TEM images of Ponza samples show irregular cavities filled with euhedral dioctahedral clay minerals and the zeolite mordenite, providing direct evidence for neocrystallization from a fluid. Chemical compositions by analytical electron microscopy (AEM) support the sequence described. Selected area electron diffraction (SAED) patterns indicate the predominance of 1 Md polytypism both in I-S and illitic phases, and the coexistence in the more altered samples of 1Md illite and a 2-layer mica polytype (probably 2M1), without the intermediate 1M polytype generally assumed to exist in prograde sequences. Previous XRD studies indicated progressive change from cis-vacant, turbostratically stacked smectite, to interstratified cis- and trans-vacant, 1Md I-S, to trans-vacant, 1M illite, and then to 2M1 illite in Ponza Island samples. We observed a clear correlation between the chemical compositions as determined by AEM and the proportion of cis-vacant determined by XRD, suggesting that the octahedral cation distributions change in the studied samples with increasing degree of illitization.