The evidence for survival of high-grade diagenesis by dioctahedral vermiculite, as exemplified by upper Triassic rocks of the Tomanová Formation, Tatra Mountains Poland

Abstract

The aim of the present work was to study the clay mineralogy of the Tomanová Formation - a ∼ 65 m thick complex of inland freshwater mudstones and shales of the upper Triassic age occurring in the Tatra Mountains. The formation was chosen for the study because it was assumed that, even though it underwent high-grade diagenesis, it could contain dioctahedral vermiculite-bearing clays. The clays have unique properties concerning the selective adsorption and the fixation of Cs+. Samples used in the present study were collected from outcrops located within Czerwone Żlebki in the Tomanová Valley in the Polish part of the Tatra Mountains. Seven horizons rich in clays were selected for the detailed mineralogical study. Both bulk samples and the clay fractions were analyzed using X-ray diffractometry, Fourier-transform infrared spectroscopy, and optical microscopy. Layer charges of swelling clays separated from the samples were measured using the OD method.All studied samples contained dioctahedral R0 and R1 illite-vermiculite-smectite mixed layered minerals mixed layered minerals (hereafter I-V-S) rich in dioctahedral vermiculite interlayers, kaolinite, quartz, anatase, and rutile. Three of the studied samples contained also berthierine. The swelling clays' measured mean layer charge values were in the vermiculitic range (i.e. 0.64–0.68/O10(OH)2). The common presence of kaolinite having a “poor degree of ordering” in the studied rocks indicated that quite an intense chemical weathering likely occurred within the alimentation area. The vermiculite-bearing clays (i.e. I-V-S) identified in the studied rocks were most likely also the products of the paleo weathering. The I-V-S minerals survived the high-grade diagenesis most likely due to the potassium deficiency in the system preventing the clays from the illitization. Those findings may suggest that deposits of dioctahedral vermiculite-bearing clays can be found even within geological formations that underwent high-grade diagenesis, which is of exploration importance because dioctahedral vermiculite could be possibly used/utilized as a molecular sieve for 137-Cs immobilization in environmental applications.