Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 21 February 2019 doi:10.20944/preprints201902.0202.v1 Peer-reviewed version available at Minerals 2019, 9, 178; doi:10.3390/min9030178 1 Article 2 Precipitation of CaCO3 polymorphs from aqueous 3 solutions: the role of pH and sulphate groups 4 Iris Cuesta Mayorga 1,2, José Manuel Astilleros 1,2*, and Lurdes Fernández Díaz 1,2 5 1 Departamento de Mineralogía y Petrología, Universidad Complutense de Madrid, C/ José Antonio Novais 6 2, Madrid 28040, Spain; [email protected] 7 2 Instituto de Geociencias (CSIC, UCM), C/ José Antonio Novais 2, Madrid E‐28040, Spain 8 * Correspondence: [email protected]; Tel.: +34‐913944876 9 10 11 Abstract: In this work we aim to experimentally study the nucleation and growth of CaCO3 phases 12 precipitated from supersaturated aqueous solutions in the presence of varying concentrations of 13 sulphate oxyanion. The experiments were conducted under pH conditions close to neutral (7.6) and 14 considering a wide range of initial (SO42‐)/(CO32‐) ratios (0 to 68) in the aqueous solution. We paid 15 special attention to the evolution of the precipitates during ageing within a time framework of 14 16 days. The mineralogy, morphology and composition of the precipitates were studied by X‐ray 17 diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and EDX 18 microanalysis. The concentration of sulphate ions in the reacted aqueous solution was study by 19 ICPs. The experimental results show that the mineral composition of the precipitate recovered in 20 each run varied with the (SO42‐)/(CO32‐) ratio in the parental solution, which influences the mineral 21 evolution of the precipitates during ageing. We observe that high concentrations of sulphate in the 22 aqueous solution stabilize the vaterite precipitates and inhibit calcite formation. Furthermore, 23 aragonite never precipitates directly form the solution and it is only formed via a dissolution‐ 24 precipitation process in solutions with high (SO42‐)/(CO32‐) ratio after long reaction times. Finally, 25 gypsum only precipitates after long ageing in those aqueous solutions with the highest 26 concentration of sulphate. The reaction pathways during ageing, the morphology of the calcite 27 crystals and the composition of vaterite and calcite are discussed considering both, kinetic and 28 thermodynamic factors. These results show a considerably more complex behavior of the system 29 than that observed in experiments conducted under higher pHs and supersaturation levels and 30 lower (SO42‐)/(CO32‐) ratios in the aqueous phase. 31 Keywords: CaCO3 polymorphs; sulphate; ageing process; aragonite; gypsum 32 33 1. Introduction 34 The presence of calcium carbonate mineral phases is ubiquitous in surface and subsurface 35 environments in the Earth. Calcium carbonate phases in such environments range from amorphous 36 to crystalline and from anhydrous (calcite, aragonite and vaterite, CaCO3) to differently hydrated 37 (monohydrocalcite, CaCO3∙H2O, and ikaite, CaCO3∙6H2O) [1]. In these environments, the only stable 38 phase of calcium carbonate is calcite, which appears as the main constituent of sedimentary carbonate 39 rocks [1‐2] and hard tissues in marine organisms [2‐4]. However, all other phases can also form and 40 often remain metastable over long periods due to kinetic factors and/or the influence of specific 41 chemical species in a variety of bio‐geological contexts [5‐7]. Thus, amorphous calcium carbonate 42 (ACC) plays a main role in the early stages of the formation of carbonate hard tissues, where it forms 43 at far from equilibrium conditions and, following the Ostwald’s step rule, subsequently transforms 44 into other calcium carbonate phases through different reaction pathways [8‐10]. In carbonate hard © 2019 by the author(s). Distributed under a Creative Commons CC BY license. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 21 February 2019 doi:10.20944/preprints201902.0202.v1 Peer-reviewed version available at Minerals 2019, 9, 178; doi:10.3390/min9030178 2 of 15 45 tissues ACC can also remain stabilised in specific locations under the influence of organic molecules 46 and/or inorganic foreign ions like magnesium [11] as a sort of calcium carbonate storage which can 47 eventually be disposable for hard tissue‐damage reparation [12]. The appearance of the hydrated 48 forms of calcium carbonate ikaite and monohydrocalcite is restricted to very specific environments 49 like near‐freezing conditions aqueous environments, where the presence of magnesium or phosphate 50 ions has been proposed to promote their formation [13‐14]. Kinetics effects facilitate the formation of 51 vaterite in natural environments, where this phase represents an intermediate step in the reaction 52 pathway that leads from ACC to calcite following Ostwald’s steps rule [15]. The seasonal formation 53 and temporal stabilization of vaterite in evaporite‐rich geological settings has also been linked to the 54 influence of high concentrations of dissolved sulphate [16]. Finally, aragonite is the second most 55 abundant calcium carbonate phase in surface and subsurface environments after calcite [17]. 56 Aragonite also is a main component of carbonate biominerals, often coexisting with calcite in the 57 same hard tissue [4,18‐19]. The main role played by high Mg2+/Ca2+ ratios in aqueous environments 58 as the determining factor in the crystallization and stabilization of aragonite has been long known 59 [5]. This role has been attributed to the inhibitory effect of dissolved magnesium on the growth of 60 calcite [20‐23]. Changes in Mg2+/Ca2+ ratio in seawater have been proposed to explain recurrent calcite‐ 61 aragonite switches in marine calcium carbonate biominerals along the Earth’s history [24]. 62 Consequently, the influence of different foreign ions in the crystallization of calcium carbonate and, 63 specifically, in its polymorph selection, has long been a main topic of research in carbonates 64 mineralogy. Understandably, most research efforts have been focused on shedding light on the role 65 of magnesium and, to a lesser extent, that of other cations like Sr2+ or Co2+ [5,23,25‐27]. However, 66 much less attention has been paid to the influence of different anions on calcium carbonate 67 polymorph selection, even though there exist strong evidences that this influence, especially that of 68 tetrahedral oxyanions, could also be most relevant [28]. Indeed, as mentioned above, phosphate 69 oxyanions have been linked to the formation of ikaite [13]. A combined role of sulphate and Mg2+ in 70 the switch from calcite to aragonite seas has been claimed on both, geochemical and experimental 71 evidences [29]. The notion that sulphate (SO42‐), chromate (CrO42‐) and selenate (SeO42‐) promote the 72 formation of vaterite and inhibit its transformation into calcite is supported by overwhelming 73 experimental evidence [30‐38]. Furthermore, this notion has been given theoretical support by 74 computational studies that relate the effect of tetrahedral oxyanions on calcium carbonate polymorph 75 selection to thermodynamic effects arising from the different impact that their incorporation 76 substituting carbonate groups has in the structure of the three calcium carbonate polymorphs calcite, 77 aragonite and vaterite [28,31]. This incorporation produces a most significant disruption of the local 78 structure of aragonite. Although less so, this disruption still is very marked in calcite, while vaterite 79 structure is the least disturbed by the isomorphic incorporation of tetrahedral oxyanions. 80 In an earlier work, the specific influence of the presence of sulphate oxyanions in the 81 crystallization medium on the formation and evolution upon ageing of calcium carbonate precipitates 82 was studied at high pHs (10.9) by conducting batch experiment [31]. High pH conditions are 83 restricted to limited geological settings such as hyper alkaline lakes in connection to tectonic rifts [39]. 84 Even though other authors have studied the influence of sulphate ions on the precipitation of calcium 85 carbonate under lower pHs [37‐38], these studies only explored a narrow range of SO42‐/CO32‐ ratio 86 conditions and a comprehensive study of close to neutrality conditions is still missing. In this study 87 we aim to improve our current understanding of the influence of sulphate oxyanions in calcium 88 carbonate formation and evolution by filling this gap. We present here the results of precipitation 89 experiments conducted under circa neutral conditions (initial pH = 7.6), closer to those commonly 90 found in surface and subsurface waters, and considering a wide range of initial SO42‐/CO32‐ ratios (0 91 to 68) in the aqueous solution. Furthermore, we study the evolution of the precipitates upon ageing 92 within a time framework of 14 days, regarding its mineralogy and the morphology and composition 93 of the different phases that constitute the precipitate at each stage. Different reaction pathways during 94 ageing are unraveled depending on the initial (SO42‐)/(CO32‐) ratio in the solution. These pathways are 95 discussed considering both, kinetic and thermodynamic arguments. Our goal is to draw general Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 21 February 2019 doi:10.20944/preprints201902.0202.v1 Peer-reviewed version available at Minerals 2019, 9, 178; doi:10.3390/min9030178 3 of 15 96 conclusions on the influence of sulphate oxyanions and the (SO42‐)/(CO32‐) ratio in aqueous