Transmission Electron Microscopy Study of Illitization in Pelites from the Iberian Range, Spain: Layer-by-Layer Replacement?

Blanca Bauluz1, Donald R. Peacor2 and Jose Manuel Gonzalez Lopez1
1 Departamento de Ciencias de la Tierra, Cristalografia y Mineralogia, Universidad de Zaragoza, 50.009 Zaragoza, Spain
2 Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109-1063, USA
E-mail of corresponding author: bauluz@posta.unizar.es

Abstract: A sequence of interstratified illite-smectite (I-S) and illite in Paleozoic pelites and metapelites from the Iberian Range, Spain, was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The fine-grained matrix of diagenetic pelites is composed of I-S with sequences of illite- and smectite-like layers in a given sample. The Reichweite (R) values as determined by XRD and averaged over heterogenous I-S layer sequences increase with increasing grade, apparently continuously, in sharp contrast with TEM observations of other published sequences. Changes in I-S sequences along layers are rarely observed. In the higher-grade diagenetic pelites, I-S coexists with illite. Each I-S phase has a composition similar to that of illite, implying unique Al-Si distributions in contrast to smectite and muscovite. Selected area electron diffraction (SAED) patterns of I-S and illite are diagnostic of 1Md polytypism. Anchizonal metapelites consist of larger packets of well-crystallized muscovite, with SAED patterns corresponding to a two-layer polytype.

The continuous sequence of changes studied by TEM in I-S sequences and lateral transitions among these units is consistent with illitization by layer-by-layer replacement, although other processes are possible also. Replacement of individual layers probably occurs via fluids at reaction interfaces, in contrast to solid-state reactions, sensu strictu. The transition from the diagenetic to anchizonal rocks (transition in textures and formation of muscovite-2M1) occurred via dissolution/crystallization, however, presumably by tectonic stress. XRD and TEM data imply a consistent prograde trend in the sequence, the XRD data defining the average, long-range Reichweite ordering sequence, whereas the TEM data define the short-range layer sequences.

Key Words: Analytical Electron Microscopy • I-S • Illite • Illitization Process • Polytypism • Prograde Diagenesis • Scanning Electron Microscopy • Transmission Electron Microscopy

Clays and Clay Minerals; June 2000 v. 48; no. 3; p. 374-384; DOI: 10.1346/CCMN.2000.0480308
© 2000, The Clay Minerals Society
Clay Minerals Society (www.clays.org)