Fate of swelling clay minerals during early diagenesis: a case study from Gdańsk Bay (Baltic Sea)

The aim of the study was to recognize the early diagenetic transformations of clay minerals likely taking place in the brackish environment of Gdańsk Bay (Baltic Sea). The Vistula River loads and sediments of the Vistula delta front and prodelta were studied. The mineral compositions of the clay fractions were determined by X-ray diffractometry. The average layer charge (LC) of the expandable interlayers was determined using the O-D vibrational spectroscopy method. The major element content of the studied clays was determined by inductively coupled plasma optical emission spectrometry. The <0.2 μm clay fraction, separated from the river sediments, contained illite-smectite mixed layered minerals, rich in high-charge, dioctahedral smectite (Ilt-Sme), illite, and kaolinite. The same clay fraction, separated from the delta-front sediments, was also composed mainly of Ilt-Sme, illite, kaolinite, and hydroxy-interlayered minerals. The <0.2 μm clay fraction from the prodelta sediments was depleted in Ilt-Sme and enriched in illite and chlorite, relative to the clays from both the river and the delta-front sediments. The LCs (0.45 to 0.56 per formula unit) were higher for clays from the river and the delta front sediments, relative to the clays from the prodelta. The <0.2 μm clay fractions from the prodelta sediments were enriched in MgO, Fe2O3, and K2O, relative to the fine clay fraction from the river. The results indicated that the smectite component of Ilt-Sme, deposited by the Vistula in Gdańsk Bay, underwent chloritization and likely illitization. The chloritization most likely proceeded via formation of hydroxy-interlayers within the smectite. Illite-like minerals, formed at the expense of the smectite with high LC, due to selective adsorption and fixation of K+ from seawater.