The glauconite–Fe‐illite–Fe‐smectite problem: a critical review

Abstract

Iron silicate minerals are a significant component of sedimentary systems but their modes of formation remain controversial. Our analysis of published data identifies end‐member compositions and mixtures and allows us to recognize controls of formation of different mineral species. The compositional fields of glaucony, Fe‐illite, Fe–Al smectites are determined in the M+/4Si vs. Fe/Sum of octahedral cations (M+ = interlayer charge). Solid solutions could exist between these phases. The Fe–Al and Fe‐rich clay minerals form two distinct solid solutions. The earliest phases to be formed are Fe–Al smectites or berthierine depending on the sedimentation rate. Reductive microsystems appear in the vicinity of organic debris in unconsolidated sediments. The Fe is incorporated first in pyrite and then in silicates after oxidation. Potassium ions diffuse from the sea‐water–sediment interface. If not interrupted, the diffusion process is active until reaction completion is reached, i.e. formation of Fe‐illite or glauconite or a mineral assemblage (berthierine–nontronite) according to the available Al ion amounts in the microsystem. Mixed‐layer minerals are formed when the diffusion process is interrupted because of sedimentation, compaction or cementation. Despite the common belief of their value as palaeoenvironment indicators, these minerals can form in a variety of environments and over a period of millions of years during sediment burial.

Sediment Effects on the Preservation of Burgess Shale-Type Compression Fossils

“…Many Cretaceous greensands, for example, appear to be derived from volcanic sources (Jeans et al 1982), and it is clear that glauconite forms from a wide range of precursor minerals, including kaolinite (Meunier and El Albani 2007). Evidence of intense, large-scale continental weathering through the Cambro-Ordovician interval (Avigad et al 2005;Peters and Gaines 2012) implies significant inputs of Al-rich clays to the global oceans at this time, potentially contributing to both the spike in both shallow-water glauconite and BST preservation.…”

“…As a class, glauconite-Fe-illite-Fe-smectite represents a broad continuum of iron-rich, redox-sensitive clay minerals that form during marine diagenesis (Meunier and El Albani 2007). Iron-rich clays have previously been invoked to explain BST preservation through the adsorption and deactivation of degradative enzymes (Butterfield 1995) but might alternatively be viewed as evidence of enhanced marine Fe 2+ availability, leading to the secondary stabilization of otherwise-labile substrates (cf.…”

Conditions and mechanism for the formation of iron-rich Montmorillonite in deep sea sediments (Costa Rica margin): Coupling high resolution mineralogical characterization and geochemical modeling

“…These authors suggest that glauconitization can be effective at 60-500 m water depth, because at greater water depths, this process is inhibited by low temperature and low sedimentation rates. Meunier and El Albani (2007) recently illustrated that the glauconitization processes depends on the nature of the sediment and its porosity. In the present study, we suggest that the formation of authigenic iron-rich montmorillonite is related to early diagenetic processes in deep-water sediments (3000 m).…”

Clay mineralogy, organic carbon burial, and redox evolution in Proterozoic oceans

“…2). Glauconite is generally thought to form by the recrystallization of a precursor material (usually Fe-rich) under typically reducing conditions, and this reaction must occur near the sediment-water interface in contact with seawater derived K to allow K 2 O incorporation and the formation of glauconitic mica (Rousset et al, 2004;Meunier and El Albani, 2007). The precursor may form as an intermediate phase during early diagenesis and glauconitization, or it may be composed of detrital material that reacted with ambient chemistry in the sediment.…”