Mixed-layer illite-smectite (I/S) in recent massive sulfide deposits from a seamount near the East Pacific Rise has been studied by X-ray powder diffraction (XRD) and scanning and transmission electron microscopy (SEM, TEM). The I/S fills pore space between quartz and sulfides, and XRD and TEM analyses indicate that the I/S contains ∼25%-45% smectite, with R1-type ordering (ISISIS). Microprobe and analytical electron-microscope analyses indicate a rectorite-like composition, K being the dominant interlayer cation. The occurrence of the I/S within a hydrothermally precipitated sulfide deposit, the pore-filling texture, and the internal structure of the I/S are all consistent with direct precipitation from hydrothermal solutions. Direct precipitation of I/S from solution has two important implications. First, some sericite in ancient volcanogenic massive sulfide deposits may have similarly originated as hydrothermal precipitates rather than as alteration products, with implications for mobility and transport of Al. Second, direct precipitation of R1-ordered I/S from fluid implies that during diagenetic evolution of smectite to illite, the sequence of smectite → random I/S → ordered I/S → illite may not be followed.

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