Relative coherent stacking potential of fundamental particles of illite-smectite and its relationship to geological environment

Abstract

Interstratified illite-smectite (I-S) occurring authigenically in diverse earth crust environments reacts toward more illite-rich phases as temperature increases. For that reason, I-S is used for geothermometry when prospecting for hydrocarbons or ore mineral deposits. This study develops the mathematical relations for characterizing the coherent stacking potential of fundamental particles (FP) using the expandability ratio K, where K is defined as (%SMAX –; %SXRD)/%SMAX. The ratio can be applied to differentiating I-S samples from shales, bentonites, and hydrothermal alterations. In particular, patterns on a K vs. T diagram, where T is the average thickness of fundamental particles (FPs), appear to be indicative of the geological conditions related to I-S formation. Shale samples plot in the negative K domain of the diagram, possibly due to the intimate mixing of detrital particles. Both bentonitic and hydrothermal samples display trends of increasing K with T, which suggests the coherent stacking potential progressively decreases as FPs increase in thickness. Hydrothermal samples are more extensively distributed on the diagram than samples from bentonites. This result may reflect differences in particle growth conditions (nutrients and space) between bentonites (short supply) and hydrothermal alterations (good supply).

Authigenic illite within northern and central North Sea oilfield sandstones: evidence for post-growth alteration

“…TEM analyses have shown that the fibrous illite has a complex crystal structure, whereby a 'single' crystal is composed of a stack of crystallites termed fundamental particles (Nadeau, 1985;Kang et al, 2012). A possible explanation of the change in illite composition with depth is that the fundamental particles within the illite fibres continue to thicken after the period of initial growth.…”

“…consists of alumino-silicate layers joined by nonexpandable K-rich layers with a minimum thickness of 2 nm, separated by interfaces that give the mineral expandable properties; see Kang et al (2012). Figure 6 compares whole-illite chemical analyses (Table 2) with the calculated composition of illite in borehole 29/10-2 at 4237À4356 m burial depth for different fundamental particle thicknesses using the model of Yates & Rosenberg (1998).…”