Dolomite dissolution: An alternative diagenetic pathway for the formation of palygorskite clay

Abstract

Palygorskite is a fibrous, magnesium-bearing clay mineral commonly associated with Late Mesozoic and Early Cenozoic dolomites. The presence of palygorskite is thought to be indicative of warm, alkaline fluids rich in Si, Al and Mg. Palygorskite has been interpreted to form in peritidal diagenetic environments, either as a replacement of detrital smectite clay during a dissolutionprecipitation reaction or solid-state transformation, or as a direct precipitate from solution. Despite a lack of evidence, most diagenetic studies involving these two minerals posit that dolomite and palygorskite form concurrently. Here, petrological evidence is presented from the Umm er Radhuma Formation (Palaeocene-Eocene) in the subsurface of central Qatar for an alternative pathway for palygorskite formation. The Umm er Radhuma is comprised of dolomitized subtidal to peritidal carbonate cycles that are commonly capped by centimetre-scale beds rich in palygorskite. Thin section, scanning electron microscopy and elemental analyses demonstrate that palygorskite fibres formed on both the outermost surfaces of dissolved euhedral dolomite crystals and within partially to completely dissolved dolomite crystal cores. These observations suggest that dolomite and palygorskite formed sequentially, and support a model by which the release of Mg 2+ ions and the buffering of solution pH during dolomite dissolution promote the formation of palygorskite. This new diagenetic model explains the co-occurrence of palygorskite and dolomite in the rock record, and provides valuable insight into the specific diagenetic conditions under which these minerals may form.

Early and pervasive dolomitization by near‐normal marine fluids: New lessons from an Eocene evaporative setting in Qatar

“…Dolomite dissolution obviously followed dolomitization, and the presence of authigenic palygorskite ( Fig. 7A; see also Ryan et al, 2019) and calcite ( Fig. 8) in the dissolved cores of many dolomite crystals indicates that dolomite dissolution is concurrent with or pre-dates these phases.…”

“…The fifth diagenetic event was the formation of palygorskite, which was previously interpreted by Ryan et al (2019) to be a replacement of smectite that in turn formed from Mg liberated by the dissolution of dolomite. This clay occurs both as an intercrystalline cement and as an intracrystalline cement inside of partially to completely dissolved dolomite cores.…”

“…Mineralogical characterization of core plugs was carried out in two separate ways, as similarly described in Ryan et al (2019). Bulk mineralogy for all 391 core plugs was quantified by Core Laboratories using their standard X-ray diffraction (XRD) procedure.…”

“…A palaeogeographic map of the eastern half of the Arabian Peninsula (modified fromZiegler, 2001), along with an inset map of Qatar (modified fromRyan et al, 2019) with locations of cores 1, 2 and 3.…”

Dologrus: The impact of meteoric‐water‐controlled diagenesis following early‐marine dolomitization

“…Meteoric diagenesis is also important to the evolution of matrix-scale poresystems in rocks that have undergone dolomitization. Examples include 'dedolomitization' or the replacement of dolomite by calcite (Frank, 1981;Kupecz et al, 1993;Nader et al, 2008;Rameil, 2008;Vandeginste & John, 2012) commonly in the presence of calcium sulphate dissolution (Von Morlot, 1847;Bischoff et al, 1994;Arenas et al, 1999;Hauck et al, 2018) and the formation of hollow dolomite rhombs (Jones et al, 1989;Youssef, 1997;Ryan et al, 2019). Dolomite dissolution in the meteoric realm has also been implicated in the formation of dolomitic sediment variably referred to as 'pulverulent chalk' (Blank & Tynes, 1965), 'pulverulite' (Rose, 1972;Chafetz & Butler, 1980;Kahle, 2012), 'crushed dolomite' (Zogovi c, 1966), 'flour dolomite' (Ji et al, 2004), 'dolomite powder' (Machel et al, 2012), 'dolomite silt' (Hajna, 2003), 'dolomite sand' (James et al, 1993;Richter et al, 2018) and 'disintegrated dolomite' (Richter et al, 2018).…”