Revision 1 1 Kaolinization of 2 : 1 type clay minerals with different swelling properties 2 3

23 Kaolinization of 2:1 type clay minerals commonly occurs in the supergene 24 environments of the Earth, which plays critical roles in many geochemical and 25 environmental processes. However, the transformation mechanism involved and the 26 specific behavior of 2:1 type swelling and non-swelling clay minerals during 27 kaolinization remain poorly understood. In this study, laboratory experiments on the 28 kaolinization of montmorillonite (swelling), illite (non-swelling), and rectorite 29 (partially swelling) were carried out to investigate the kaolinization mechanism of 2:1 30 type clay minerals and to evaluate whether swelling and non-swelling layers of 2:1 31 type clay minerals perform differently or not in their kaolinization processes. The 32 results show that montmorillonite, illite, and rectorite in acidic Al3+-containing 33 solutions can be transformed into kaolinite, whereas such transformation is hard to 34 take place in Al3+-free solutions. Part of the Al3+ in the solutions was exchanged into 35 the interlayer spaces of swelling clay minerals at the early stage and resulted in the 36 formation of hydroxy-aluminosilicate (HAS) interlayers, but they show no influence 37 on the transformation process. Interstratified kaolinite-smectite (K-S), kaolinite-illite 38 (K-I), and kaolinite-rectorite (K-R) formed as the intermediate phases during the 39 transformations of the three different precursor minerals, respectively. The results 40 obtained in this study demonstrate that 2:1 type clay minerals, including both swelling 41 and non-swelling ones, can be transformed into kaolinite via a local 42 dissolution-crystallization mechanism, which starts mainly from the layer edges rather 43 than the basal surfaces. Due to different dissolution rates from domain to domain 44 within a precursor mineral particle, the layers with a low dissolution rate become 45 “splints”, while the dissolved elements are concentrated between two “splints”, 46 leading to precipitation of kaolinite along the basal surfaces of precursor minerals. 47 The size and stacking order of the newly formed kaolinite strongly depend on the 48 morphology and property of the precursor minerals. These findings not only are of 49 importance for better understanding the transformation procedures between different 50 clay minerals and the mechanisms involved, but also provide new insights for well 51 understanding mineral-water interactions that are central to all geochemical processes. 52