Evolution of Fundamental-Particle Size during Illitization of Smectite and Implications for Reaction Mechanism

J. Środoń1, D.D. Eberl2 and V.A. Drits3
1 Institute of Geological Sciences PAN, Senacka 1, 31002 Kraków, Poland
2 U.S. Geological Survey, 3215 Marine St., Boulder, Colorado 80303-1066, USA
3 Institute of Geology RAN, Pyzhevsky 7, 109017 Moscow, Russia
E-mail of corresponding author: ndsrodon@cyf-kr.edu.pl

Abstract: Area-weighted thickness distributions of fundamental illite particles for samples of illite and illite-smectite from seven locations (including bentonites and hydrothermally altered pyroclastics) were measured by Pt-shadowing technique, by transmission electron microscopy. Most thickness distributions are described by lognormal distributions, which suggest a unique crystallization process. The shapes of lognormal distributions of fundamental illite particles can be calculated from the distribution mean because the shape parameters α and β2 are interrelated: β2 = 0.107α − 0.03. This growth process was simulated by the mathematical Law of Proportionate Effect that generates lognormal distributions. Simulations indicated that illite particles grow from 2-nm thick illite nuclei by surface-controlled growth, i.e., the rate of growth is restricted by how rapid crystallization proceeds given a near infinite supply of reactants, and not by the rate of supply of reactants to the crystal surface. Initially formed, 2-nm thick crystals may nucleate and grow within smectite interlayers from material produced by dissolution of single smectite 2:1 layers, thereby transforming the clay from randomly interstratified (Reichweite, R = 0) to ordered (R = 1) illite-smectite after the smectite single layers dissolve. In this initial period of illite nucleation and growth, during which expandable layers range from 100 to 20%, illite crystals grow parallel to [001]* direction, and the dimensions of the (001) plane are confined to the size of the original smectite 2:1 layers. After nucleation ceases, illite crystals may continue to grow by surface-controlled growth, and the expandable-layer content ranges from 20 to 0%. This latter period of illitization is characterized by three-dimensional growth. Other crystal-growth mechanisms, such as Ostwald ripening, supply-controlled growth, and the coalescence of smectite layers, do not produce the observed evolution of α and β2 and the observed shapes of crystal thickness distributions.

Key Words: Crystal Growth • Fundamental Particle • Illite-Smectite • Illitization Mechanism • Lognormal Distribution • Ostwald Ripening

Clays and Clay Minerals; August 2000 v. 48; no. 4; p. 446-458; DOI: 10.1346/CCMN.2000.0480405
© 2000, The Clay Minerals Society
Clay Minerals Society (www.clays.org)