Halloysite occurrence at the karstified contact of Oligocene sands and Cretaceous calcarenites in Hinnisdael quarries, Vechmaal (NE of Belgium)

1. Introduction Halloysite is a dioctahedral 1:1 clay mineral of the kaolinite group frequently discussed in literature because of its potential for nanotechnological applications (Keeling, 2015; Yuan et al., 2015; Yuan et al., 2016). Its geological occurrence has been primarily linked to soil and weathering environments, by the weathering and alteration of volcanic rocks (Vaughan et al., 2002; Velde & Meunier, 2008), the alteration of clay minerals like montmorillonite or biotite (Hill, 2000; Papoulis et al., 2009) or weathering of feldspars (Sheets & Tettenhorst, 1997; Adamo et al., 2001). Halloysite is also a common mineral constituent in karst and paleokarst environments as a result of acid weathering (Polyak & Guven, 2000; Joussein et al., 2005). In Belgium, halloysite was reported in over 30 localities, almost all with a very similar geological setting, i.e. karstified carbonate substrates filled up by Cenozoic sand deposits (Buurman & Van der Plas, 1968; Dupuis & Ertus, 1995; Goemaere & Hanson, 1997; Nicaise, 1998; Kloprogge & Frost, 1999; De Putter et al. 2002; Bruyere, 2004). A similar geological setting is found inside the underground quarries of Hinnisdael, locally known as “mergelgrotten” (“marl caves”), located in Vechmaal, Limburg province, Belgium (Fig. 1). In two of the Hinnisdael underground quarries, dolines filled with marine sand were intersected and an irregular white clay layer occurs at the contact between the karstified top of the Cretaceous calcareni