Mixed-layer corrensite-chlorites and their formation mechanism in the glauconitic sandstone-clayey rocks (Riphean, Anabar Uplift)

Abstract

The paper presents the first detailed mineralogical, structural, and crystal-chemical characteristics of the mixed-layer corrensite-chlorites from the glauconitic sandy-clayey rocks that make up the bottom (0.10 m) of a basal member (1.50 m) of the lower subformation of the Yusmastakh Formation (Riphean, Anabar Uplift, North Siberia). Like the overlying mudstones (1.40 m) in the basal member, these rocks are generally transformed up to the deep catagenesis level and included in a thick dolomite sequence. In mudstones represented by the dioctahedral micas, the corrensite-type minerals are observed as traces. The real structure of the studied mixed-layer phases represents an alternation of corrensite (Cor) and chlorite (Ch) layers with a distinct tendency of segregation ( R = 1) and ratio of layers Cor: Ch = 0.60: 0.40 in the well-crystallized corrensite-type minerals and 0.70: 0.30 in the poorly crystallized varieties. Among these mineral formations, the high-Mg (hereafter, Mg) and magnesian-ferruginous (Mg-Fe) mixedlayer corrensite-chlorites prevail, whereas the high-Fe (Fe) and ferruginous-magnesian (Fe-Mg) varieties are rare. Synthesis of the corrensite-type microcrystals of different sizes took place after the formation of glauconite globules, probably, at the reductive stage of late diagenesis. Two possible structural mechanisms are proposed for the formation of the mixed-layer corrensite-chlorites, preference being given to the two-stage version. The first stage is marked by the formation of corrensite proper with a structure periodic along the layer normal; the second stage, by the successive replacement of smectite interlayers by the brucite-type sheets, probably, due to the further evaporitization that promotes the replacement of a part of smectite interlayers in the corrensite component of the structure and the stabilization of the newly formed brucite-type sheets.