The Link Between Brushite and Gypsum: Miscibility, Dehydration and Crystallochemical Behavior in the Cahpo4.2H2O-Caso4.2H2O System

Crystal Growth & Design

The link between brushite and gypsum: Miscibility, dehydration and crystallochemical behavior in the CaHPO4.2H2O-CaSO4.2H2O system

Journal: Crystal Growth & Design

Manuscript ID: cg-2011-012815.R2

Manuscript Type: Article

Date Submitted by the Author: 09-Nov-2011

Complete List of Authors: Pinto, Andre; Universidade de Lisboa, Geologia Carneiro, Joana; Universidad de Oviedo, Geologia Katsikopoulos, Dionisis; Universidad de Oviedo, Geologia Jimenez, Amalia; Universidad de Oviedo, Geologia Prieto, Manuel; Universidad de Oviedo, Geologia

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1 2 3 The link between brushite and gypsum: Miscibility, dehydration and 4 . . 5 crystallochemical behavior in the CaHPO 4 2H 2O-CaSO 4 2H 2O system 6 7 André J. Pinto, Joana Carneiro, Dionisis Katsikopoulos, Amalia Jiménez* and Manuel Prieto. 8 Dpto. de Geología, Universidad de Oviedo, C/ Arias Velasco s/n, 3305 Oviedo, Spain 9

10 . 11 The present study explores the mixing properties of the Ca(SO 4,HPO 4) 2H 2O solid solution and the 12 role of the “double-salt” Ca 2SO 4HPO 4·4H 2O (ardealite) by means of precipitation experiments 13 carried out in a solution calorimeter at 25ºC. Moreover, the dehydration behavior of a number of 14 solids with different compositions is studied by thermogravimetry and thermo-X-ray diffraction. 15 The experimental results indicate the existence of two (sulfate-rich and phosphate-rich) ranges of 16 solid solution which are separated by two miscibility gaps from a range around the midpoint (~50% 17 molar) composition in which ardealite forms. On the phosphate-rich miscibility range, the structural 18 (020) layers contract with the sulfate content, whereas the interlayer spacing expands. This 19 contraction is consistent with the negative enthalpy of mixing determined from the calorimetric 20 21 data. For the ardealite range of compositions, the strong contraction of the (020) layers resolves in a 22 different stacking sequence (with double b-axis and (040) as elementary stacking layers). Therefore, . 23 ardealite is demonstrated to be not a member of the Ca(SO 4,HPO 4) 2H 2O solid solution, but a nearly 24 stoichiometric compound with specific structural features. The thermogravimetric study indicates a 25 specific dehydration behavior for ardealite, which again supports the idea that this phase is not a 26 member of the solid solution. 27 Ardealite 28 S-rich P-rich 29 solid solution solid solution 30 31 32 30.95 Å 30.95 15.21 Å 15.21 33 Å 15.20 ≈ ≈ 34 ≈ b b b b 35 b 36 37 38 39 25 ºC 40 41 I II III IV V 42 x x x x 0 1 2 3 4 1 43 X 44 br 45 Figure: Scheme of the distribution of phases in the CaSO 4·2H 2O-CaHPO 4·2H 2O system that 46 includes the projection of 101 slices of the structures of ardealite and of the two extremes of the 24 47 Ca(SO 4,HPO 4)·2H 2O solid solution. In the case of ardealite , there are two topologically non- 48 equivalent tetrahedral positions (striped and non-striped tetrahedra). In all cases, the small circles 49 50 represent oxygen atoms from the water molecules. The larger circles represent the calcium atoms. 51 The solid lines separate the hypothetical stability fields of the different phases described in the text. 52 53 *Corresponding author: 54 55 Amalia Jiménez 56 57 C/ Jesus Arias de Velasco s/n. 58 33005 Oviedo, Spain 59 Tel. (+34) 985 109552. Fax. (+34) 985 103 103. 60 [email protected] ACS Paragon Plus Environment 1 Crystal Growth & Design Page 2 of 32

1 2 3 4 The link between brushite and gypsum: Miscibility, dehydration 5 6 . . 7 and crystallochemical behavior in the CaHPO 4 2H 2O-CaSO 4 2H 2O 8 9 10 system 11 12 13 André J. Pinto 1, Joana Carneiro, Dionisis Katsikopoulos, Amalia Jiménez* and Manuel Prieto 14 15 Departamento de Geología, Universidad de Oviedo, C/ Arias Velasco s/n, 3305 Oviedo, Spain 16 17 * Corresponding author: [email protected] 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 1 58 Present address: Andre J. Pinto. Departamento de Geologia, Faculdade de Ciências da Universidade de 59 Lisboa. Edifício C6, Campo Grande, 1749-016 Lisboa, Portugal. 60 ACS Paragon Plus Environment 2 Page 3 of 32 Crystal Growth & Design

1 2 3 ABSTRACT 4 5 . The present study explores the mixing properties of the Ca(SO 4,HPO 4) 2H 2O solid solution and the 6 7 role of the “double-salt” Ca SO HPO ·4H O (ardealite) by means of precipitation experiments 8 2 4 4 2 9 10 carried out in a solution calorimeter at 25ºC. Moreover, the dehydration behavior of a number of 11 12 solids with different compositions is studied by thermogravimetry and thermo-X-ray diffraction. 13 14 The experimental results indicate the existence of two (sulfate-rich and phosphate-rich) ranges of 15 16 solid solution which are separated by two miscibility gaps from a range around the midpoint (~50% 17 18 19 molar) composition in which ardealite forms. On the phosphate-rich miscibility range, the structural 20 21 (020) layers contract with the sulfate content, whereas the interlayer spacing expands. This 22 23 contraction is consistent with the negative enthalpy of mixing determined from the calorimetric 24 25 data. For the ardealite range of compositions, the strong contraction of the (020) layers resolves in a 26 27 different stacking sequence (with double b-axis and (040) as elementary stacking layers). Therefore, 28 29 . 30 ardealite is demonstrated to be not a member of the Ca(SO 4,HPO 4) 2H 2O solid solution, but a nearly 31 32 stoichiometric compound with specific structural features. The thermogravimetric study indicates a 33 34 specific dehydration behavior for ardealite, which again supports the idea that this phase is not a 35 36 member of the solid solution. 37 38 39 40 41 1. INTRODUCTION 42 43 Phosphate minerals are not particularly abundant in the Earth’s crust, but they are of significant 44 45 importance in biogeochemical processes that occur in both marine and terrestrial environments. 46 47 Lamentably, high concentrations of phosphates in natural waters can lead to the increased growth of 48 49 50 green algae, triggering a process known as eutrophication, which usually deteriorates the aquatic 51 52 environment. The excessive use of phosphate-based fertilizers to sustain food production at a global 53 54 scale has recently raised the question of how to recycle our very limited phosphorus resources and 55 56 reduce their environmental impact at the same time 1. The major exploitable reserves of phosphorous 57 58 59 occur in the form of calcium phosphates, mainly minerals of the apatite group, which have been 60 ACS Paragon Plus Environment 3 Crystal Growth & Design Page 4 of 32

1 2 3 extensively investigated because of their relevance in biomineralization, materials and 4 5 2,3 environmental sciences . Another calcium phosphate, brushite CaHPO 4·2H 2O, has received 6 7 increasing attention due to its significance in technological and environmental issues 4-6. Brushite is 8 9 7 10 a major component of kidney and bladder stones and is widely used as a coating for bone implants . 11 12 Moreover, this mineral has been proposed as a precursor of hydroxylapatite Ca 5(PO 4)3OH in many 13 14 natural processes 8, including the formation of bones and teeth 9. In the environment, brushite occurs 15 16 mainly in deposits of phosphorites 10 , caves and other P-rich media, such as fertilized soils 11 . 17 18 12 19 Brushite coexists commonly with gypsum in many of these natural deposits and also in some 20 13 21 industrial wastewater products . In fact, the existence of a relationship between these two minerals 22 23 was first documented by Schadler 14 , who reported the presence of a mineral called ardealite 24 25 Ca 2HPO 4SO 4·4H 2O in cave deposits where brushite and gypsum occur together. However, despite 26 27 this early reference, the factors controlling the co-precipitation of brushite and gypsum from 28 29 30 aqueous solutions and the crystallochemical role of ardealite (particularly the question of whether 31 32 ardealite is a differentiated stoichiometric solid or a member of the Ca 2(HPO 4,SO 4)·4H 2O solid 33 34 solution) require further assessment. 35 36 When two chemically and structurally related compounds precipitate from a supersaturated 37 38 39 aqueous solution, the formation of a solid solution is always a possibility, which holds true for cases 40 41 other than that of cationic substitutions. The formation of solid solutions involving oxyanions like 42 2- 3- 3- 2- 2- 2- 15-17 43 SO 4 , PO 4 , AsO 4 , SeO 4 , CrO 4 , and MoO 4 has been explored by a number of authors due 44 45 to the implications of these systems in nuclear waste disposal and other environmental issues. In the 46 47 case of gypsum, the structural similarity with brushite and pharmacolite CaHAsO .2H O was 48 4 2 49 18 50 considered in depth by Heijnen and Hartman , who presented a comparative study of their crystal 51 19-22 52 morphologies using an A-setting for the three unit cells. More recent papers illustrates how this 53 54 structural similarity favors the development of oriented intergrowths between these three minerals. 55 56 In the case of brushite and gypsum, the possibility of co-precipitation to form solid solution crystals 57 58 59 60 ACS Paragon Plus Environment 4 Page 5 of 32 Crystal Growth & Design

1 2 2- 2- 23-24 3 with anionic SO 4 /HPO 4 substitution was pointed out by a number of authors , but an 4 5 evaluation of the extent and character of this solid solution has yet to be performed. 6 7 Brushite and gypsum have nearly identical unit cells, although due to the protonation of one of 8 9 10 the oxygen atoms in the phosphate ion and to a different configuration of the water molecules, the 11 12 spatial group (for a discussion on the unit-cell choice, see section 3.1) of brushite is A1a1 (No. 9), 13 14 while gypsum crystallizes in A12/ a1 (No. 15). The relation between the two crystal structures is 15 16 reflected in the presence, or lack, of a 2-fold rotation axis along [010] and a symmetry center. As a 17 18 19 consequence, brushite can grow onto gypsum following two alternative epitaxial orientations 20 21 21 related by a 2-fold rotation axis . Ardealite crystallizes in the same spatial group than brushite (No. 22 23 9) with similar a and c parameters (see section 3.1), but the b-axis periodicity is considered to be 24 25 about twice (~30.95 > 2 ×15.180 Å) as long as that of brushite. 26 27 28 Like gypsum and brushite, natural and synthetic ardealites have a layered structure parallel to 29 30 (010). However, the occurrence of 0 k0 X-ray reflections only with k = 4 n suggests that the 31 32 periodicity along the b-axis in ardealite occurs at four times the elementary stacking layer 25 , 33 34 whereas in gypsum and brushite the repetition occurs every two layers. In fact, most evidence 35 36 indicates that ardealite cannot be considered an intermediate member of the Ca(SO ,HPO ).2H O 37 4 4 2 38 39 solid solution, but a virtually stoichiometric double salt with specific structural features. The 40 2- 2- 41 presence of some SO 4 /HPO 4 ordering involving alternating layers along the b-axis is, however, 42 43 more than doubtful. Several differences in the structural parameters (structure factor multiplicities, 44 45 mean T-O bond lengths, etc.) concerning the tetrahedral [TO ] groups in alternate (040) layers were 46 4 47 25 48 identified in a synthetic ardealite-like phase . However, given the small value of these differences, 49 25 2- 2- 50 Sakae and co-workers considered the SO 4 /HPO 4 distribution to be essentially random. In the 51 52 model used by these authors, all of the contrasting layers are chemically analogous, but the mode of 53 54 stacking occurs in such a way that the repeating period includes four layers. The question of 55 56 25 57 whether or not the compounds synthesized and natural ardealites are identical is not completely 58 59 resolved, but it does seem likely. 60 ACS Paragon Plus Environment 5 Crystal Growth & Design Page 6 of 32

1 2 3 Dehydration studies can offer an additional tool to further understand the differences between 4 5 ardealite-like phases and gypsum-brushite solid-solution members. Since Linck and Jung 26 first 6 7 described the nature of the gypsum dehydration products, the dehydration behavior of natural 8 9 27, 28 10 gypsum, phosphogypsum, and synthetic gypsum has been widely studied in the literature . It is 11 12 worth noting the essential consistency among the existing data, despite the wide variety of 13 14 techniques applied to these studies. After past discussion 29,30 regarding the possible formation of 15 16 several intermediate hydrates with a number of water molecules other than 0.5 per formula unit, 17 18 31 19 Putnis and co-workers gathered in situ evidence for the formation of only the hemihydrate 20 21 bassanite CaSO 4·0.5H 2O using infrared spectroscopy combined with thermogravimetry. The 22 23 structure of the low-temperature (T < 383 K) dehydration products of gypsum (CaSO 4·0.5H 2O and 24 25 32 γ-CaSO 4) was studied using time-of-flight neutron powder diffraction data . Clarifying the pioneer 26 27 33,34 28 studies , these authors conclude that, even though bassanite presents an overall structure very 29 30 similar to γ-CaSO 4 (P6222, S.G. No. 180), its diffraction patterns are consistent not with a trigonal, 31 32 but with a monoclinic (I2, S.G. No.5) structure. Nevertheless, there are only minor arrangements in 33 34 the crystal structure when bassanite dehydrates at low temperature to form γ-CaSO (also known as 35 4 36 37 the soluble anhydrite or III phase). 38 39 In contrast with gypsum, the dehydration behavior of brushite has not elicited a similar 40 41 abundance of references 35,36 . Moreover, with the exception of a recent paper by Frost and co- 42 43 workers 37 describing the thermal stability of a natural ardealite sample, there is no detailed study on 44 45 46 the dehydration behavior of intermediate members in the gypsum-brushite joint. In this framework, 47 48 the objectives of the present work were to (i) explore the mixing properties and assess the extent of 49 . 50 the Ca(SO 4,HPO 4) 2H 2O solid solution at ambient temperature, (ii) evaluate the presence of 51 52 miscibility gaps and the crystallochemical role of ardealite in the gypsum-brushite joint, and (iii) 53 54 55 describe the dehydration behavior of a number of solid phases with different compositions in this 56 57 series. With this aim, we carried out a combined study involving solution calorimetry, 58 59 thermogravimetry, thermo-XRD in solids formed via precipitation experiments. 60 ACS Paragon Plus Environment 6 Page 7 of 32 Crystal Growth & Design

1 2 3 2. EXPERIMENTAL METHODS 4 5 2.1. Solution calorimetry 6 7 The enthalpy of mixing of different members of the Ca(SO ,HPO ).2H O solid solution was 8 4 4 2 9 38,39 10 studied by solution calorimetry following a protocol previously described in the literature . 11 12 Precipitation experiments were performed in the Dewar flask of a PARR6755 solution calorimeter 13 14 equipped with a PARR 6772 high-precision thermometer. The Dewar flask was filled with 100 ml 15 16 2- 2- of a series of aqueous solutions containing dissolved Na 2SO 4-H3PO 4 with different SO 4 /HPO 4 17 18 19 ratios (Table 1). These aqueous solutions were adjusted to pH 5.5 with the addition of NaOH and 20 21 were left to cool for three hours at 25ºC until the heat of dilution of the NaOH had completely 22 23 dissipated. Next, a glass cell containing 10 ml of a 1 M CaCl 2 aqueous solution sealed with a 24 25 detachable Teflon cap was immersed in the Dewar flask and continuously rotated by an external 26 27 electric motor. As soon as thermal equilibrium was reached, the glass cell was opened, and the 28 29 30 reactants were mixed in the Dewar flask, where precipitation occurred. The changes in temperature 31 32 associated with the precipitate formation were measured and recorded by a PARR 6772 precision 33 34 thermometer. All experiments were performed in quintuplicate in a thermostated cabinet (25 ± 0.5 35 36 ºC) using deionized (MilliQ) water and analytical grade reagents. 37 38 39 Table 1. Initial concentrations of Na 2SO 4 and H 3PO 4 used in the experiments 40 41 Sample Na 2SO 4(M) H3PO 4 (M) 42 C1 0.50 - 43 44 C2 0.50 0.10 45 C3 0.50 0.15 46 47 C4 0.50 0.20 48 C5 0.50 0.25 49 50 C6 0.50 0.30 51 C7 0.50 0.35 52 53 C8 0.50 0.40 54 55 C9 0.25 0.50 56 C10 0.20 0.50 57 58 C11 0.15 0.50 59 C12 - 0.50 60 ACS Paragon Plus Environment 7 Crystal Growth & Design Page 8 of 32

1 2 3 4 5 2.2. Characterization of solids and aqueous solutions 6 7 The precipitates formed in the Dewar flask were extracted from the aqueous solution using 0.65 8 9 10 m filters (Millipore) and left to dry at room temperature to avoid dehydration of the formed 11 12 phases. Next, the solids were characterized by powder X-ray diffraction (XRD). The diffraction 13 14 patterns were scanned from 5º to 80º (2 θ) with a step size of 0.02º on a Philips X’Pert Pro automatic 15 16 17 diffractometer using CuK α radiation. Between successive measurements, the diffractometer was 18 19 calibrated using a silicon (Si) external standard. The diffractograms were studied using X’Pert 20 21 HighScore Plus (©2008, PANalytical B.V.) to index the main reflections and calculate the cell 22 23 parameters of each precipitate. Obviously, this method allows for recognizing the cases in which 24 25 26 not a single phase but a mixture of different solid phases was formed. Moreover, the compositional 27 28 homogeneity of the solids was checked by studying the full width at half maximum (FWHM) 29 30 values of the main reflections. 31 32 The aqueous solutions were analyzed with an iCAP 6000 (Thermo) Inductively Coupled Plasma 33 34 - Atomic Emission Spectrometer (ICP-AES). The mass and composition, expressed in terms of the 35 36 37 brushite mole fraction ( Xbr = 1 −Xgy ) of each homogeneous (single-phase) precipitate, were 38 39 determined from the initial and final (before and after precipitation) concentrations of sulfate and 40 41 phosphate in the aqueous solution. Furthermore, representative samples of each precipitate were 42 43 analyzed using a JEOL-6100 scanning electron microscope (SEM) equipped with an INCA Energy 44 45 46 200 microanalysis system (EDS), and the results were found to be in good agreement with the 47 48 values derived from the solution compositions. However, the compositions derived from the ICP- 49

50 AES analyses were more accurate (SD of Xbr ≈ ±0.01) than the EDS measurements and thus were 51 52 chosen for subsequent calculations. 53 54 55 2.3 Thermogravimetry and Thermo-XRD 56 57 Thermogravimetric analyses (TG) of the precipitates obtained in the calorimeter were carried 58 59 out using a Metter Toledo Me/TG thermal analyzer. Only the precipitates consisting of a single 60 ACS Paragon Plus Environment 8 Page 9 of 32 Crystal Growth & Design

1 2 3 phase were studied. All measurements were performed in the 293–650 K temperature range at a 4 5 −1 heating rate of 5 K·min in an inert N2 atmosphere. The initial mass of each sample was ~10 mg. 6 7 8 The temperature precision of the equipment was ±0.25 K, and the weight precision was ±1 g. 9 10 The dehydration of the single-phase precipitates was also studied by in situ thermo-XRD using 11 12 CuK α radiation on a PANalytical X’PERT PRO powder diffractometer equipped with a PIXcel 13 14 solid-state detector and a high temperature chamber (Anton Paar HTK 1200 N). The diffractograms 15 16 17 were scanned over a 5 < 2 θ < 40º range with a step size of 0.013º and a time step of 23.97 s. All 18 19 measurements were performed in air by collecting the diffraction patterns at fixed temperatures 20 21 (typically at 25 or 10 K increments) between 298 and 723 K. The heating rate between two 22 23 consecutive scans was 5 K·min −1. The phase transformations were simply detected by the 24 25 26 appearance and disappearance of characteristic reflections in the XRD patterns. 27 28 3. RESULTS AND DISCUSSION 29 30 3.1. Composition and mineralogical features of the precipitates 31 32 The powder XRD study revealed that the precipitates obtained in experiments C3, C4, and C8 33 34 35 (see Table 1) were not composed of a single solid phase, but rather of a mixture of two phases. For 36 37 the rest of the experiments, the precipitates consisted of a single, compositionally homogeneous 38 39 solid phase. A number of these precipitates (C2, C9, C10, and C11) were confirmed to be 40 41 intermediate members of the Ca(SO 4,HPO 4)·2H 2O solid solution, while precipitates C5, C6, and C7 42 43 were identified as an ardealite-like (Ca SO HPO .4H O) phase. Finally, as expected, the 44 2 4 4 2 45 46 diffractograms of precipitates C1 and C12 (see Table 1) matched the reference diffraction patterns 47 48 of pure gypsum (PDF 33-0311) and brushite (PDF 009-0077), respectively. 49 50 Figure 1 shows the indexed diffraction patterns of gypsum, brushite and intermediate members 51 52 of the Ca(SO ,HPO )·2H O solid solution using an A-centered unit cell setting 18 . These authors used 53 4 4 2 54 55 A12/ a1 and A1a1 space group settings for describing the structure of gypsum and brushite, as this 56 57 choice of unit cell enables the coincidence of the structural and the morphological c-axis of gypsum. 58 59 60 ACS Paragon Plus Environment 9 Crystal Growth & Design Page 10 of 32

1 2 3 20000 4 120 5 020 6 Intensity(counts) 7 8 140 15000 9 10 11 12 13 14 10000 15 16 040 17 C1 18 C2 19 5000 C9 20 C10 21 C11 22 23 C12 0 24 5 10 15 20 25 30 35 40 45 50 25 2Theta (°) 26 27 Figure 1. Powder diffraction patterns of gypsum (C1), brushite (C12) and several intermediate 28 members (C2, C9, C10, and C11) of the Ca(SO ,HPO )·2H O solid solution. The reflections have 29 4 4 2 30 been indexed using an A-centered unit cell setting (see text). 31 32 33 The relationships between both crystal structures have been widely described in a previous 34 35 paper 21 . Gypsum crystallizes in the centrosymmetric point group 2/m (without polar directions in its 36 37 38 structure) and brushite crystallizes in the non-centrosymmetric point group m. Therefore, the space 39 40 group of brushite ( A1a1) is a subgroup of that of gypsum ( A12/ a1). Using these space group 41 42 settings, the main reflections in the diffractogram of pure gypsum (C1) can be indexed as 020, 120, 43 44 040, and 140, (at d-spacings of ~7.60, 4.29, 3.80, and 3.07 Å, respectively). The main reflections 45 46 47 are similar in the case of pure brushite (C12), with spacings that are slightly different (the unit cell 48 49 volume is slightly smaller) from those of gypsum (~7.58, 4.24, 3.80, and 3.05 Å, respectively). By 50 51 comparing diffractograms C2, C9, C10, and C11 with those of the pure end-members, it is clear that 52 . 53 these samples correspond to intermediate members of the Ca(SO 4,HPO 4) 2H 2O solid solution (see 54 55 Fig. 1). For instance, diffractogram C2 shows reflections at ~7.62, 4.28, 3.81, and 3.07 Å, with the 56 57 58 020 spacing slightly larger than that of pure gypsum. Similarly, diffractograms C9, C10, and C11 59 60 are similar to that of brushite. The highest difference occurs in the case of C9, which shows ACS Paragon Plus Environment 10 Page 11 of 32 Crystal Growth & Design

1 2 3 spacings significantly larger (~7.66, 4.26, 3.81, and 3.06 Å) than brushite. The precipitates were 4 5 checked for compositional homogeneity by considering the broadening of a given reflection with 6 7 respect to the width of the corresponding reflections in the diffractograms of the pure end- 8 9 16 10 members . Working with 020, i.e., the reflection with the largest d-spacing, the FWHM value was 11 . . 12 0.097 (º2 θ) for pure CaSO 4 2H 2O and 0.124 (º2 θ) for pure CaHPO 4 2H 2O. 13 14 The diffractograms of intermediate members of the solid solution show excess broadening with 15 16 respect to the linear trend determined by the FWHM values of the end-members, but the deviations 17 18 19 are relatively small. The maximum deviation (0.029º2 θ) occurs for the 020 reflection in C9. 20 21 Therefore, compositional heterogeneities (due either to the presence of relatively S- and P-enriched 22 23 microdomains within each single crystal or to the coexistence of microcrystals with different 24 25 26 compositions), if they exist, do not seem to be very significant. Table 2 shows the lattice parameters 27 28 calculated (using X’Pert HighScore Plus) from these diffractograms. While the unit cell dimensions 29 30 a and c are smaller in gypsum (C1), the parameter b and the cell volume are smaller in brushite 31 32 (C12). Moreover, the evolution of the lattice parameters with composition is not monotonic. As we 33 34 will see later, the b-axis is larger for intermediate compositions than for both end-members, and the 35 36 37 unit cell dimensions do not change in a simple way along a and c. 38 39 Table 2. Calculated unit cell parameters ( A unit- 40 cell setting) of the samples identified as members 41 of the Ca(SO 4,HPO 4)·2H 2O solid solution. 42 43 Vcell 44 Sample a (Å) b(Å) c(Å) 3 45 (Å ) 46 C1 5.675 15.214 6.284 114.06 495.45 47 48 C2 5.681 15.231 6.285 114.07 496.56 49 C3ª 5.686 15.239 6.287 114.20 496.91 50 51 C4 a 5.710 15.278 6.296 115.52 495.71 52 C8 a 5.823 15.281 6.341 118.79 494.57 53 54 C9 5.820 15.268 6.327 118.44 494.38 55 C10 5.846 15.215 6.338 118.78 494.16 56 57 C11 5.839 15.207 6.341 118.65 494.12 58 C12 5.832 15.203 6.357 118.78 494.01 59 60 ACS Paragon Plus Environment 11 Crystal Growth & Design Page 12 of 32

1 2 3 (a) The lattice parameters of the precipitates C3, C4, and C8 4 were calculated using exclusively the reflections assigned to intermediate members of the solid solution (see text). 5 6 7 The diffractograms of precipitates C5, C6 and C7 have some peculiarities that allow us to 8 9 identify them as an ardealite-type phase. In these three cases, the largest spacing in the diffraction 10 11 12 pattern occurs at ~7.75 Å, a value significantly larger than those of the end-members brushite and 13 14 gypsum (at ~7.58 and ~7.60 Å, respectively). This reflection is rather sharp, appears isolated (see 15 16 Fig. 2) and can be attributed to the typical 040 spacing of natural ardealite and “ardealite-like” 17 18 synthetic compounds 23 . The occurrence of 0 k0 reflections with k = 4 n only (040, 080, and 0160 at 19 20 ~7.75, 3.88, and 1.94 Å) indicates that the periodicity along the b-axis occurs at four times the 21 22 23 elementary stacking layer, as described previously. Some typical broad peaks caused by the 24 25 occurrence of several reflections at very close 2 θ angles (e.g. 112 and 111 at 28.85º and 28.86º), as 26 27 28 well as the presence of peaks at ~2.81, 2.54, and 2.45 Å (indexed as 2 21, 200, and 062 using a 29 30 C1c1-setting), are also typical of ardealite. Moreover, there are several reflections in C5, C6 and C7 31 32 that are missing in the pattern calculated by Sakae and co-workers 25 but are usually observed in 33 34 natural ardealite 40 . 35 36 In the structure of the synthetic ardealite obtained by Sakae and co-workers 25 , there are two 37 38 39 types of tetrahedral positions that occur in alternating (040) layers that are linked by hydrogen 40 41 bonds from water molecules. In a first approach, these two different tetrahedral positions could be 42 43 considered to be occupied alternatively by phosphorous and sulfur. However, the single-crystal X- 44 45 2- 2 ray data indicate that the SO 4 /HPO 4 distribution is not ordered. In the model by Sakae and co- 46 47 25 48 workers all of the elementary contrasting layers are chemically analogous and thus comparable to 49 50 the contrasting layers of brushite and gypsum. However, the mode of stacking occurs in such a way 51 52 that the repeating period includes four layers. Here, the relatively sharp 040 reflection in 53 54 diffractograms C5, C6, and C7 allows us to play down the occurrence of compositional 55 56 heterogeneities and support the existence of an ardealite-type layering in these precipitates with a 57 58 . 59 composition close to Ca 2SO 4HPO 4 4H 2O. Note that if we choose a < c (5.721 < 6.250 Å) for the 60 ACS Paragon Plus Environment 12 Page 13 of 32 Crystal Growth & Design

1 2 3 unit cell of ardealite, the space group setting will be C1c1, whereas if we choose a > c (6.250 > 4 5 5.721 Å), the space group will be A1a1. This is the inverse of the case of brushite and illustrates 6 7 again that, despite the structural similarities, the ardealite structure 25 does not correspond to a 8 9 10 member of the Ca(SO 4,HPO 4)·2H 2O solid solution. Moreover, despite the double b-axis parameter 11 12 (lower translational symmetry) the ardealite structure cannot be considered a superstructure of the 13 14 “basic” brushite structure. 15 16 Table 3 shows the composition, expressed as the brushite mole fraction ( Xbr ), of the phases 17 18 19 obtained in the whole set of precipitation experiments. These compositions were determined from 20 21 the difference between the initial and final concentrations of sulfate and phosphate in the aqueous 22 23 solution. The composition was also checked by SEM-EDS microanalysis of the solids, and the 24 25 results were in good agreement (the deviation was less than 0.03 Xbr ) with those calculated from 26 27 analyses of the aqueous solution. The nature of the solid phases involved is also shown in Table 3. 28 29 30 In the case of experiments C3, C4, and C8, the precipitates were heterogeneous mixtures whose 31 32 diffractograms show two reflections in the region of largest spacings (see Fig. 2). The first 33 34 reflection (at ~7.75 Å) corresponds to the 040 spacing of an ardealite-like compound, and the 35 36 second can be attributed to the 020 spacing (~7.60 Å) of a member of the Ca(SO ,HPO ).2H O 37 4 4 2 38 39 solid-solution. Thus, as shown in Table 3, two phases (a disordered solid solution and an ardealite- 40 41 type compound with Xbr ≈ 0.5) were simultaneously obtained from these parent solutions. The set of 42 43 reflections unequivocally assigned to the solid solution can be used to refine their lattice parameters 44 45 using X’Pert HighScore Plus. These parameters have been included in Table 2 (see C3, C4, and C8) 46 47 48 together with those of the homogeneous precipitates (C1, C2, C9, C10, C11, and C12). 49 50 Unfortunately, the composition of the solid-solution phase in the C3, C4 and C8 mixtures is 51 52 uncertain, as the chemical analyses involve two phases. Moreover, the unit cell dimensions do not 53 54 vary in a linear way with composition and cannot be used to make more than a very rough 55 56 57 estimation. 58 59 60 ACS Paragon Plus Environment 13 Crystal Growth & Design Page 14 of 32

1 2 3 Table 3. Composition and nature of the phases obtained in the precipitation experiments. 4 5 Sample X (± 0.02) Phase(s) 6 br 7 C1 0 gypsum end-member 8 9 C2 0.09 solid solution 10 C3* ? ardealite + s.s. 11 12 C4* ? ardealite + s.s. 13 C5 0.46 ardealite 14 15 C6 0.56 ardealite 16 C7 0.59 ardealite 17 18 C8* ? ardealite + s. s. 19 C9 0.87 solid solution 20 21 C10 0.91 solid solution 22 C11 0.93 solid solution 23 24 C12 1.00 Brus. end-member 25 (a) The composition of the solid-solution phase (s.s.) 26 in the C3, C4 and C8 mixtures could not be measured 27 (see text). A rough estimation from the unit cell 28 dimensions is given in text. 29 30 31 (020) 32 solid solution 33 34 35 36 37 38 39 40 C2 41 42 C3 43 44 C4 (020) 45 solid solution 46 C8 47 48 (040) 49 ardealite 50 C5 51 52 11 12 53 13.5 14.0 2Theta (°) 54 55 Figure 2. Detail of the region of largest spacings in the powder diffraction patterns of C2, C3, C4 56 and C5. In C2, the reflection 020 appears isolated. C3, C4 and C8 are mechanical mixtures of 57 ardealite with a Ca(SO 4,HPO 4)·2H 2O solid-solution member (reflections 040 and 020, respectively). 58 59 In C5, only an ardealite phase appears (reflection 040). 60 ACS Paragon Plus Environment 14 Page 15 of 32 Crystal Growth & Design

1 2 3 The appearance of two ranges of “mechanical mixtures” denotes the existence of two miscibility 4 5 gaps, one on each side of the “ardealite-range” of compositions. This further supports the idea of 6 7 X 8 attributing a different nature to the solids with compositions br ≈ 0.5. The homogeneous solids 9 10 (0.46 ≤ Xbr ≤ 0.59) obtained in C5, C6, and C7 cannot be considered members of the solid solution, 11 12 as their diffractograms match with a (040)-layered ardealite-type compound 25 . Unfortunately, while 13 14 the presence of two gaps is clearly demonstrated in experiments C3, C4, and C8, the equilibrium 15 16 17 miscibility limits are difficult to establish. In the Xbr < 0.5 range, the solid solution in the C4 18 19 mixture can be expected to be the most phosphate-rich member obtained in the present experiments. 20 21 According to the general trend of the cell parameters (see Table 2), this sample could correspond to 22 23 compositions of approximately X ≈ 0.15. However, as the precipitation experiments were carried 24 br 25 26 out at a high supersaturation, a solid solution with this composition could be metastable, and the 27 28 thermodynamic range of miscibility could be narrower. Similarly, in the Xbr > 0.5 range, the most 29

30 sulfate-rich member occurs in C8 and could correspond to compositions of approximately Xbr ≈ 31 32 0.85. Finally, the ardealite-type 040 spacings observed for C3, C4 and C8 fall in the 7.76-7.75 Å 33 34 35 range, which indicates that the compositions observed for these (stable or metastable) ardealite-type 36 37 phases do not exceed the limits 0.46 ≤ Xbr ≤ 0.59. 38 39 3.2. Excess volume of mixing 40 41 42 Figure 3a displays the value of the crystallographic parameter b for the solids obtained in the 43 44 experiments compiled in Table 2. The data-point labels indicate the solid composition expressed as 45 46 mole fraction of brushite ( Xbr ). As shown in this figure, for intermediate compositions, all data fall 47 48 above the straight line that connects the values corresponding to the pure gypsum and brushite end- 49 50 members (C1 and C12). At first sight, this result seems to indicate the existence of a positive excess 51 52 ex 53 volume of mixing ( ∆V ) in the solid solution. The excess volume of mixing is defined as the 54 55 difference between the molar volume of the solid solution ( VSS ) and the molar volume of a 56 57 mechanical mixture ( VMM ) of the end-members with the same composition ( Xbr ) according to the 58 59 expression: 60 ACS Paragon Plus Environment 15 Crystal Growth & Design Page 16 of 32

1 2 3 ex V = VSS −[Vbr X br +Vgy X gy ] (1) 4 5 6 where the quantity in brackets corresponds to VMM and the terms Vbr and Vgy are the molar volumes 7 8 of pure brushite and gypsum, respectively. Clearly, Xgy = 1-Xbr is the molar fraction of the gypsum 9 10 component in the solid solution. The molar volume of the solid solution (VSS ) can be determined 11 12 from the unit cell volume ( V ) using the expression: 13 cell 14 1 15 V = V N (2) 16 SS 4 cell Avogadro 17 18 where the factor 1/4 arises from the four formula units contained in the unit cells of gypsum and 19 20 brushite. The molar volumes of the brushite and gypsum end-members (74.37 and 74.59 cm 3 mol -1) 21 22 ex 23 were also derived from the corresponding unit cell parameters. The values of VSS , VMM , and ∆V 24 25 calculated in this way are shown in Table 4 where it is obvious that the excess volume of mixing is 26 27 clearly positive, but very small. In reality, the expansion of the unit cell along the b axis observed 28 29 for intermediate compositions is accompanied by a significant deformation of the unit cell shape on 30 31 the (010) plane. The unit cell parameters a and c vary in an irregular way with the composition 32 33 34 (increase for some ranges and decrease for others), and the β-angle is significantly higher for the P- 35 36 rich members than for the S-rich members. 37

38 ? 15.55 0.09 39 0.56 32.6 0 solid solution 1 (a) 0.46 0.59 0.91 0.93 40 ?

15.50 ) 41 2 ? 0.87 ardealite (½ b) 32.4 42 15.45

43 32.2 15.40 44

(Å) 45 (Å) 15.35 32.0 46 b 15.30 ? ? 47 0.87 0.46 0.59 31.8 0.56 ? 48 15.25 0.09 solid solution 0 0.91 0.93 1 (010) 2D-Unit-Cell Area (Å ardealite 49 31.6 15.20 (b) 50 51 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10C11C12 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10C11C12 52 Experiment Experiment 53 54 Figure 3. a) Values of the unit cell parameter b for the complete experimental series. The labels near 55 the data points indicate the solid composition ( X ). The three data points encircled by a dashed line 56 br 57 correspond to the ardealite phase obtained in experiments C5, C6, and C7 and represent one-half of 58 the repeating period along the b-axis (see text). The rest of the data correspond to different members 59 60 ACS Paragon Plus Environment 16 Page 17 of 32 Crystal Growth & Design

1 2 3 of the Ca(SO 4,HPO 4)·2H 2O solid solution. b) Area of a two-dimensional unit cell on (010). Again, 4 the data points encircled by a dashed line correspond to an ardealite-type phase. 5 6 7 8 Figure 3a also displays (encircled by a dashed line) three data points corresponding to 9 10 experiments C5, C6, and C7. These three values represent one-half of the repeating period along the 11 12 b-axis (2×d040 ) and have been plotted to illustrate the dilatation of the interlayer spacing in ardealite 13 14 ex 15 as compared to the solid-solution members. It is worth noting that the values of V obtained using 16 17 Eqn. (1) for experiments C5, C6, and C7 are clearly negative (≈ -0.35 cm 3/mol). These values are 18 19 meaningless, as they correspond to a solid phase whose structure is not equivalent to that of the 20 21 22 solid solution. However, they still indicate that, in ardealite, while the interlayer spacing is 23 24 expanded, the (040) layers are dramatically contracted when compared with the equivalent (020) 25 26 layers of pure gypsum and brushite. This phenomenon is illustrated in Fig. 3b, which displays the 27 28 area of a two-dimensional unit cell on (010) as a function of composition. As shown in this figure, 29 30 the incorporation of sulfate ions into the brushite structure leads to a contraction of the (020) layers. 31 32 33 In contrast, on the sulfate-rich range, the incorporation of phosphate substituting for sulfate initially 34 2- 2- 35 produces a positive deviation in the 2D unit cell area. Therefore, the SO 4 /HPO 4 substitution does 36 37 not produce a symmetric effect on both miscibility ranges. Finally, precipitates C5, C6 and C7 38 39 exhibit a strongly differentiated behavior and are plotted below to form a separated population 40 41 42 labeled as ardealite in Fig. 3b. 43 44 3.3. Enthalpy of precipitation and enthalpy of mixing 45 46 In a similar way to Vex , the excess enthalpy of mixing ( Hex ) is given by the difference 47 48 between the enthalpy of the solid solution (H ) and the enthalpy ( H ) of a mechanical mixture of 49 SS MM 50 51 the end-members with the same composition. The excess enthalpy of mixing can be determined 52 53 from the calorimetric data (see section 2.1) obtained during the precipitation experiments, according 54 55 to the expression below 39 : 56

57 ex ppt ppt 58 H = HSS - H MM (3) 59 60 ACS Paragon Plus Environment 17 Crystal Growth & Design Page 18 of 32

1 2 3 ppt where HSS stands for the measured enthalpy of precipitation of a solid solution of composition Xbr 4 5 ppt 6 and H MM is the enthalpy of precipitation of a compositionally equivalent mechanical mixture of the 7 8 pure brushite and gypsum end-members, given by: 9

10 ppt ppt ppt 11 H MM = H br X br + Hgy X gy (4) 12 13 In this equation, H ppt and H ppt are the measured enthalpies of precipitation of the pure brushite and 14 br gy 15 ppt 16 gypsum, respectively. Table 4 displays the values of the precipitation enthalpy ( H ) obtained from 17 18 the calorimetric measurements carried out during experiments C1, C2, C9, C10, C11, and C12. 19 20 Each value corresponds to the average of five replicate experimental runs. The relative standard 21 22 ppt 23 deviation (RSD) in H was approximately ±6%, which is a relatively small margin of error. As can 24 25 be observed in Table 4, while the enthalpy of precipitation of gypsum is significantly lower than 26 27 that of brushite, the values obtained for the intermediate compositions of the solid solution are all 28 29 close to 2.5 kJ/mol. As a consequence, the excess enthalpy of mixing calculated according to Eqn. 30 31 32 (3) is clearly negative for the phosphate-rich members ( Xbr > 0.85) and slightly positive in the 33 34 sulfate-rich ( Xbr < 0.15) miscibility range. The values corresponding to C3, C4 and C8 are not 35 36 displayed in Table 4 because these experiments involve an assemblage of two phases. Similarly, the 37 38 values for C5, C6, and C7 have been excluded from Table 4 because they correspond to ardealite- 39 40 ex 41 type phases. Obviously, we can use Eqn. (3) to calculate the values of H for these precipitates 42 43 (≈−1.6 kJ·mol -1), but the physical meaning is uncertain because ardealite has a different structure, 44 45 enthalpy and entropy than a compositionally equivalent solid solution. 46 47 Table 4 Excess volume ( Vex ) and enthalpy ( Hex ) of mixing. The molar volumes of the 48 49 precipitated solid solutions ( Vss) and those of a mechanical mixture ( VMM ) of the end- members with the same composition are also shown. The last two columns display the 50 ppt 51 enthalpy of precipitation ( H ) and excess enthalpy of mixing obtained from the 52 calorimetric measurements. 53 X br V V Vex Hppt Hex 54 Experiment SS MM (± 0.02) (cm 3/mol) (cm 3/mol) 3 (kJ/mol) 55 (cm /mol) (kJ/mol) 56 C1 0 74.590 74.590 - 1.09 ± 0.13 - 57 58 C2 0.09 74.757 74.570 0.186 2.51 ± 0.41 0.92 59 C9 0.87 74.429 74.402 0.028 2.62 ± 0.39 -3.13 60 ACS Paragon Plus Environment 18 Page 19 of 32 Crystal Growth & Design

1 2 3 C10 0.91 74,395 74.393 0.002 2.65 ± 0.37 -3.31 4 C11 0.93 5 74.429 74.389 0.003 2.57 ± 0.12 -3.54 6 C12 1.00 74.373 74.373 - 6.46 ± 1.46 - 7 8 9 10 The negative value of the excess enthalpy of mixing in precipitates C9, C10, and C11 is in 11 12 agreement with the contraction of the (020) layers, shown in Fig. 3b. Both effects seem to indicate 13 14 2- 2- that, in the phosphate-rich range, the substitution of HPO 4 by SO 4 increases the bond interaction 15 16 17 within these layers. In the simplest case, a negative enthalpy of mixing correlates with a negative 18 19 excess volume of mixing (i.e., with a contraction of the structure) and implies a tendency toward 20 21 alternation (ordering) of the substituting ions 41 . Here, such a tendency can be expected to occur 22 2- 2- 23 within each (020) layer and does not imply SO 4 /HPO 4 ordering in alternating layers as the layer 24 25 spacing expands. Moreover, the ardealite-type structure determined by Sakae and co-workers 25 does 26 27 2- 2- 28 not exhibit any sign of SO 4 /HPO 4 ordering. The existence of a negative enthalpy of mixing does 29 30 not necessarily imply the stability of the ordered phase due to its lower entropy. Moreover, ordering 31 32 is frequently precluded by the crystallization kinetics. In practice, the strong contraction of the (020) 33 34 layers seems to resolve in a different stacking sequence (with double b-axis parameter) for the 35 36 37 ardealite range of compositions. 38 39 3.4. Phase distribution scheme 40 . 41 Although a precise determination of the degree of non-ideality of the Ca(SO 4,HPO 4) 2H 2O solid 42 43 solution was not possible, the existence of two miscibility gaps, one on each side of the ardealite 44 45 range of compositions, is clearly related to the tendency to form a “double salt”. Further, the 46 47 48 existence of these two gaps reinforces the idea that ardealite is not a member of the brushite- 49 50 gypsum solid solution. The fact that slightly different diffraction patterns have been observed for 51 52 ardealite-type phases is not strange. In layered compounds with complex stacking sequences like 53 54 ardealite, the presence of stacking faults and/or different polytypes 42 is always a possibility and 55 56 57 would explain the observed phenomena. 58 59 60 ACS Paragon Plus Environment 19 Crystal Growth & Design Page 20 of 32

1 2 3 The occurrence of intermediate, ordered phases coexisting with miscibility gaps is frequent in 4 5 nature. Such is the case of the formation of ordered dolomite CaMg(CO 3)2 as an intermediate phase 6 7 (with double c-axis parameter) in the CaCO -MgCO system. In these cases, ordering and unmixing 8 3 3 9 10 are closely related and interdependent phenomena, and their relative kinetics is decisive in defining 11 12 the result at specific conditions. These systems have certain similarities with the CaSO 4·2H 2O- 13 25 2- 2- 14 CaHPO 4·2H 2O system, even though the available structural data suggest that the SO 4 /HPO 4 15 16 distribution is random. A crucial point is that the ardealite structure shows a (040) layering that 17 18 19 involves two topologically different types of tetrahedral positions. This stacking sequence only 20 21 appears for the midterm composition, probably as a result of the strong contraction of the stacking 22 2- 2- 23 layers. Finally, the observation that the SO 4 /HPO 4 substitution does not produce symmetric 24 25 effects (see Fig. 3b and Hex values in Table 4) on both miscibility ranges is not strange. Brushite 26 27 and gypsum are not completely isostructural due to the different orientation of the tetrahedra and to 28 29 30 the presence of the acidic hydrogen in the phosphate group. The brushite structure exhibits a chain 31 2- 32 of hydrogen bridges that connects adjacent HPO 4 tetrahedra along [101], which is clearly absent in 33 2- 2- 34 gypsum. Thus, on the phosphate-rich side, the substitution of HPO 4 by SO 4 will involve the 35 36 cancelation of some H-links in these chains and a local, energetically favorable reorientation of the 37 38 39 tetrahedra. 40 41 Figure 4 displays a schematic model that qualitatively represents the phase relationships in the 42 43 gypsum-brushite system. The scheme defines the compositional fields where either single phases or 44 45 assemblages of two phases exist. A single solid solution phase is stable within two compositional 46 47 regions, I and V, whereas ardealite is stable within III. There are two compositional intervals (II and 48 49 50 IV) where assemblages of ardealite and the solid solution are stable. The first, (II), corresponds to a 51 52 mixture of an ardealite phase with composition x2 and a solid solution with composition x1. The 53 54 second, (IV), corresponds to the assemblage of an ardealite phase with composition x3 and a solid 55 56 solution with composition x . As can be observed, the regions defined in Fig. 4 correspond to the 57 4 58 59 compositional fields defined by the precipitation experiments presented in this work (Table 3). 60 ACS Paragon Plus Environment 20 Page 21 of 32 Crystal Growth & Design

1 2 3 However, the extent of these experimental regions most likely does not reflect the true stability 4 5 limits, as precipitation occurred far from equilibrium, and the obtained limits could involve 6 7 metastable compositions. Figure 4 also displays a projection of a 101 slice of the crystal structures 8 9 10 of ardealite as well as a phosphate-rich and a sulfate-rich member of the solid solution in which the 11 12 different repeating periods along the b-axis can be observed. 13 14 Ardealite 15 16 S-rich P-rich 17 solid solution solid solution 18 19 20 21 22 23 24 15.21Å Å 30.95 15.20Å ≈ ≈ ≈

25 b b b 26 27 28 29 30 31 32 33 25 ºC 34 I II III IV V 35 36 x x x x 0 1 2 3 4 1 37 X 38 br 39 40 Figure 4. Scheme of the distribution of phases in the CaSO 4·2H 2O-CaHPO 4·2H 2O system. The 41 figure also shows the projection of 101 slices of the structures of ardealite (Ca 2SO 4HPO 4·4H 2O) and 42 24 of the two extremes of the Ca(SO 4,HPO 4)·2H 2O solid solution. In the case of ardealite , there are 43 two topologically non-equivalent tetrahedral positions (striped and non-striped tetrahedra in the 44 figure). In all cases, the small circles represent oxygen atoms from the water molecules. The larger 45 46 circles represent the calcium atoms. Hydrogen atoms are not represented. The solid lines separate 47 the hypothetical stability fields of the different phases described in the text. 48 49 50 51 3.5. Dehydration behavior of the solid phases in the gypsum-brushite joint 52 53 54 Although previous observations and the available structural data clearly support the view that 55 56 ardealite is not a member of the Ca(SO 4,HPO 4)·2H 2O solid solution, studies of the dehydration 57 58 behavior of the solids obtained in these experiments can provide further insight into this matter. 59 60 ACS Paragon Plus Environment 21 Crystal Growth & Design Page 22 of 32

1 2 3 With this aim, Figures 5 through 9 show the results of a dehydration study carried out by combining 4 5 thermogravimetric and thermo-XRD techniques. 6 7 Figure 5 displays the thermogravimetric (TG) (weight loss as a function of temperature) and the 8 9 10 derivative thermogravimetric (DTG) curves obtained for gypsum (Fig. 5a), brushite (Fig. 5b), a 11 12 sulfate-rich member (Fig. 5c), and a phosphate-rich member (Fig. 5d) of the Ca(SO 4,HPO 4)·2H 2O 13 14 solid solution. There is a reduction of approximately 21% of the initial weight in all of the samples, 15 16 which corresponds to the total loss of two water molecules per formula unit. However, the DTG 17 18 19 curves show that this process occurs in a different way for brushite-rich and gypsum-rich members 20 21 of the solid solution. As expected, the dehydration of the pure-gypsum sample C1 involved an 22 23 initial loss of ~75% of water molecules (Fig. 5a), which was attributed to formation of bassanite 24 25 (CaSO 4·0.5H 2O). This initial loss was followed by complete dehydration to form anhydrite ( γ- 26 27 28 CaSO 4) at approximately 150ºC. In contrast, the pure-brushite sample C12 (Fig. 5b) dehydrates in a 29 30 single step at ~125ºC to form monetite (anhydrous CaHPO 4). This temperature is lower than the 31 32 typical values (150–180ºC) reported in the pioneering XRD 35 , which could be due to the effect of 33 34 using different heating rates, grain sizes, and impurities on the kinetics of the dehydration process. 35 36 37 a) b) 38 25 0.0008 25 0.0020

39 (DTG)(DTG) dw/dT

20 (DTG)(DTG) dw/dT 20 0.0006 0.0015 40 15 TG 15 41 DTG 0.0004 TG 0.0010 -2H O 42 10 2 10 -2H 2O DTG 43 0.0002 0.0005 5 5 % weightweight lossloss (TG) (TG)

44 % weightweight lossloss (TG) (TG) 45 0 0.0000 0 0.0000 46 0 50 100 150 200 250 0 50 100 150 200 250 47 Temperature (ºC) Temperature (ºC) 48 c) d) 49 25 0.0020 25 0.00075 50 dw/dT (DTG) dw/dT

20 (DTG) dw/dT 20 0.00060 51 0.0015 15 0.00045 52 15 TG TG 0.0010 DTG 53 -2H O -2H 2O 10 2 DTG 10 0.00030 54 0.0005 5 5 0.00015

55 %weight loss (TG) % weight loss (TG) 0.00000 56 0 0.0000 0 57 0 50 100 150 200 250 0 50 100 150 200 250 58 Temperature (ºC) Temperature (ºC) 59 60 ACS Paragon Plus Environment 22 Page 23 of 32 Crystal Growth & Design

1 2 3 4 Figure 5. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves determined for 5 6 gypsum (a), brushite (b), a sulfate-rich (c) and a phosphate-rich (d) member of the 7 Ca(SO 4,HPO 4)·2H 2O solid solution. 8 9 10 The solid-solution sample representative of sulfate-rich compositions (C2) exhibits a similar 11 12 13 behavior to that observed for gypsum (Fig. 5c), with completion of the dehydration process at 14 15 ~160ºC. The only difference with respect to pure gypsum is that the two dehydration steps are less 16 17 individualized (see the small shoulder on the right side of the curve) so that the DTG curve simply 18 19 reflects a decrease in the rate of dehydration after the initial loss of 75% of the water molecules. In 20 21 contrast, the dehydration of the phosphate-rich sample C10 (Fig. 5d) occurs in a single step, as it 22 23 24 does in brushite, although at significantly higher temperatures (~200ºC) than in the case of the pure 25 26 brushite end-member. Moreover, there is a weight loss of approximately 7.5% before the 27 28 dehydration peaks of the DTG curve (see the left side of the TG curve in Fig. 5d). From these 29 30 observations, one can assume that the sulfate and the phosphate-rich members of the solid solution 31 32 33 tend to emulate the dehydration behavior of the respective pure end-members, albeit with some 34 35 differences. Specifically, it seems that the presence of sulfate ions dramatically increases the 36 37 dehydration temperature of brushite. 38 39 The TG and DTG curves displayed in Fig. 6 correspond to the dehydration of the ardealite-type 40 41 phase obtained in C5. As can be observed, for ardealite, the completion of the water loss occurs at 42 43 44 higher temperatures (~250ºC) than in the case of brushite and gypsum. Moreover, the process 45 46 occurs in two steps, each involving ~50% loss of water molecules. Both of these observations 47 48 support the idea that ardealite exhibits a distinctive behavior and cannot be considered a member of 49 50 the Ca(SO ,HPO )·2H O solid solution. The nature of the phases resulting from the two dehydration 51 4 4 2 52 53 steps, however, requires further research. Toward this aim, a thermo-XRD study is likely the best 54 55 tool to complement the macroscopic TG-DTG analyses and to identify the dehydration products. 56 57 58 59 60 ACS Paragon Plus Environment 23 Crystal Growth & Design Page 24 of 32

1 2 3 4 30 0.0003

5 25 (DTG)dw/dT 6 7 20 0.0002 8 TG 15 9 DTG 10 10 0.0001 11 5

12 weight% loss(TG) 13 0 0.0000

14 0 50 100 150 200 250 300 350 400 15 16 Temperature (ºC) 17 18 19 20 Figure 6. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves determined for 21 the ardealite sample C5. 22 23 24 25 Fig. 7 displays a series of diffractograms obtained at different temperatures, starting from the 26 . 27 sulfate-rich member of the Ca(SO 4,HPO 4) 2H 2O solid solution synthesized in experiment C2 (Xbr = 28 29 0.09). This member shows a gypsum-type structure (see Table 2 in section 3.1), with displacement 30 31 2- 32 of the reflections toward smaller 2θ angles than in the case of pure gypsum caused by the SO 4 33 2- 34 /HPO 4 substitution. When the temperature increases up to ~110–120ºC, some new peaks in the 35 36 diffraction pattern occur together with the pre-existing gypsum peaks. These new reflections can be 37 38 identified as belonging to bassanite (CaSO .0.5H O) and/or γ-anhydrite (γ-CaSO ). With increasing 39 4 2 4 40 41 temperature, the typical reflections of gypsum progressively disappear so that, in agreement with 42 43 the TG-DTG observations, at 150ºC, the dehydration process is virtually completed, and only 44 45 metastable γ-anhydrite seems to be present. It is worth noting that the crystal structures of bassanite 46 47 and γ-anhydrite show only slight differences related to the presence (or not) of water molecules 32,43 . 48 49 50 Therefore, discrimination between both phases is not straightforward using conventional powder 51 52 XRD. However, we assume that the diffractogram taken at 150ºC corresponds to γ-anhydrite, as the 53 54 TG-DTG curves indicate that complete dehydration is attained at this temperature. No sign of any 55 56 2- 2- phosphate phase has been detected, which seems to indicate that SO 4 /HPO 4 substitution can still 57 58 59 occur in the anhydrous residuum. 60 ACS Paragon Plus Environment 24 Page 25 of 32 Crystal Growth & Design

1 2 3 ϒ-anh ϒ-anh 4 175 ºC -anh 5 ϒ-anh ϒ 6 7 Bas / -anh Bas / ϒ-anh 120 ºC ϒ Bas / ϒ-anh 8 9 10 Gy Gy 11 Gy 12 13 Gy 14 Gy 15 25 ºC 16

17 10 15 20 25 30 35 18 2Theta (°) 19 Figure 7. Powder XRD patterns obtained during the dehydration of a sulfate-rich member of the 20 21 Ca(SO 4, HPO 4)·2H 2O solid solution (C2). 22 23 24 25 Figure 8 depicts a series of XRD patterns that correspond to the dehydration products of a 26 27 phosphate-rich member (C10) of the Ca(SO ,HPO ).2H O solid solution ( X = 0.91). The first 28 4 4 2 br 29 30 diffractogram (taken at 25ºC) can be attributed to a brushite-type structure (see section 3.1) with a 31 32 slight displacement of the reflections toward smaller 2θ angles than those corresponding to the pure 33 34 brushite end-member. At ~150ºC, two peaks (at d ≈ 3.37 and 3.35 Å) develop to occur together with 35 36 37 the brushite reflections. This doublet clearly corresponds to the most intense peaks of monetite 38 39 (CaHPO 4), which indicates that dehydration (accompanied by a gradual decrease of crystallinity) 40 41 begins at this temperature. Beyond 170ºC, the brushite peaks progressively lose expression, and the 42 43 monetite reflections become dominant to be the only detectable in the diffractograms taken at 44 45 46 temperatures greater than 190ºC. These results reveal a notable similarity between the dehydration 47 . 48 mechanisms of brushite and of phosphate-rich members of the Ca(SO 4,HPO 4) 2H 2O solid solution. 49 50 Because the identified dehydration products of these precipitates consist only of monetite (no 51 52 2- 2- sulfate phase has been detected), some SO 4 /HPO 4 substitution can be expected in the structure of 53 54 monetite. An alternative would be the concentration of sulfate in an anhydrous residuum of low- 55 56 57 crystallinity, which would simply contribute to an increased background of the diffractograms. 58 59 60 ACS Paragon Plus Environment 25 Crystal Growth & Design Page 26 of 32

1 2 3 180 ºC Mo Mo 4 5 6 7 8 Mo 9 150 ºC 10 11 Br Br 12 Br 13 14 15 25 ºC 16 17 10 15 20 25 30 35 18 2Theta (°) 19 Figure 8. Powder XRD patterns obtained during the dehydration of a phosphate-rich member of the 20 21 Ca(SO 4, HPO 4)·2H 2O solid solution (sample C10). 22 23 24 25 The thermal-XRD evolution of the ardealite-type phase obtained in experiment C5 is shown in 26 27 28 Fig. 9, where the typical 040 reflection can be observed. Evidence of dehydration does not appear 29 30 before 150ºC, when new reflections corresponding to a bassanite-type (CaSO 4·0.5H 2O) and/or an 31 32 anhydrite-type (γ-CaSO 4) phase begin to develop. From 150 to 190ºC, the intensity of the ardealite 33 34 reflections decreases in a gradual way, and at 200ºC, metastable ( γ-CaSO 4) anhydrite is the only 35 36 37 phase that is identifiable in the diffractograms. Dehydration is accompanied by an important 38 39 decrease in crystallinity, which becomes evident by the low intensities and high FWHM values 40 41 observed in the diffractograms of the dehydration product. It is worth noting that crystalline 42 43 phosphate solids were not detected. Two incipient reflections reminiscent of the typical doublet 44 45 observed in monetite were detected in some diffractograms (see Fig. 9), but this occurrence cannot 46 47 48 be considered significant given the high phosphate content (50 mole percent) of ardealite. Overall, 49 50 the thermal-XRD data are consistent with a dehydration process that proceeds toward an anhydrite- 51 52 type material embedded in an almost amorphous residuum. The low degree of crystallinity would 53 54 favor the incorporation of phosphate substituting for sulfate in the molecular framework of this 55 56 57 dehydration product. Above ~250ºC, the diffractograms develop two incipient reflections at d ≈ 58 59 3.50 and 2.85 Å. Finally, as the temperature increases from 300 to 450ºC, these two reflections gain 60 ACS Paragon Plus Environment 26 Page 27 of 32 Crystal Growth & Design

1 2 3 definition (particularly the one at 3.50 Å) and become clearly indicative of the presence of β- 4 5 CaSO 4, the stable structural variant of anhydrite. In fact, the two most intense reflections (020 and 6 7 012) in the β-CaSO reference diffractogram occur exactly at 3.4988 Å and 2.8494 Å, respectively. 8 4 9 10 This finding seems to indicate a gradual transformation from metastable anhydrite ( γ-CaSO 4) to 11 12 stable anhydrite ( β-CaSO 4) when the temperature increases to within this range. 13 14 15 16 400 ºC β-anh 17 β-anh 18 19 Bas / -anh 20 150 ºC Bas / ϒ-anh Mo ϒ 21 22 23 Ard 24 25 26 Ard 27 Ard Ard 28 Ard 29 25 ºC 30 31 10 15 20 25 30 35 32 2Theta (°) 33 34 Figure 9. Powder XRD patterns obtained during the dehydration of ardealite Ca 2SO 4HPO 4·4H 2O 35 (sample C5). 36 37 38 The previous results indicate the existence of three different dehydration behaviors in the 39 40 41 CaSO 4·2H 2O-CaHPO 4·2H 2O system. For both the sulfate-rich and the phosphate-rich member of 42 43 the solid solution, the macroscopic (TG-DTG) study and the XRD data match rather well. However, 44 45 for ardealite, the correlation between the TG-DTG curves and the sequence of phases observed in 46 47 the thermo-XRD study is not straightforward. Whereas the DGT curve indicates that the water loss 48 49 occur in two steps, distinguishing two singular events (each equivalent to a 50% water loss) from 50 51 52 the sequence of diffractograms taken at increasing temperatures proves impossible. Moreover, 53 54 although the ardealite reflections vanish completely at 200ºC, according to the DTG curves, the 55 56 dehydration proceeds to a temperature of 250ºC, and the sample continues to lose weight beyond 57 58 this temperature. A reasonable speculation could be the preferential dehydration of the gypsum 59 60 ACS Paragon Plus Environment 27 Crystal Growth & Design Page 28 of 32

1 2 3 CaSO 4·2H 2O component of ardealite to form bassanite, γ-anhydrite and a low-crystallinity residuum 4 5 . that is rich in the brushite CaHPO 4 2H 2O component. In a second step (see Fig. 6), this residuum 6 7 could dehydrate to form a CaHPO -rich phase of low crystallinity. Finally, further loss of water 8 4 9 40 10 molecules (at T > 250ºC) could occur by means of the following reaction : 11

12 2CaHPO 4 + 2γCaSO 4 → Ca 2P2O7 (amorphous ) + 2βCaSO 4 + H2O (5) 13 14 15 In this reaction, an amorphous Ca 2P2O7 phase would become the target store for most of the initial 16 17 phosphorous content of ardealite. 18 19 20 21 4. CONCLUSIONS AND FUTURE WORK 22 23 Although the crystal chemistry and mixing properties in the gypsum-brushite joint need further 24 25 26 clarification, the foremost significance of our findings is the consistency of the conclusions obtained 27 28 through different methods of analysis. For example, the existence of a negative enthalpy of mixing 29 30 in the phosphate-rich miscibility range is consistent with the contraction of the (020) layers with 31 32 2- respect to the pure brushite. Both effects point toward a reinforcement of the structure when SO 4 33 34 2- 35 substitutes for HPO 4 , which in turn is in agreement with the significant increase of the dehydration 36 37 temperature (compare Figs. 5b and 5d) observed in the thermogravimetry experiments. At the end, 38 39 the contraction of the (020) layers resolves in a different stacking sequence (with a double b-axis 40 41 parameter) for the ardealite range of compositions. This structural change confers a special nature to 42 43 ardealite, which cannot be considered a member of the Ca(HPO ,SO )·2H O solid solution. The 44 4 4 2 45 46 TG-DTG curves indicated a specific dehydration behavior for the ardealite samples, which again 47 48 supports the guess that this phase is not a member of the solid solution. Finally, the conclusion that 49 50 the tendency to order does not imply an HPO 4/SO 4 alternation in the stacking layers is revealing. 51 52 Though all the stacking layers are chemically identical, the lack of equivalence between tetrahedral 53 54 55 positions of alternating layers appears to be a stacking (topological) effect. 56 57 There are, however, several issues that require further study. Whereas the existence of a 58 59 tendency to HPO 4/SO 4 ordering within each (040) layer has been proved, ardealite appears to be 60 ACS Paragon Plus Environment 28 Page 29 of 32 Crystal Growth & Design

1 2 3 disordered. Doubt persists whether this disorder is a thermodynamic or a kinetic effect. Moreover, 4 5 the mechanism of contraction (the local rearrangement of tetrahedra when sulfate substitute for 6 7 phosphate) remains unknown. Molecular simulations and single-crystal diffraction could help to 8 9 10 answer these questions. Finally, an evaluation of the geochemical significance of the gypsum- 11 12 brushite system would require determining the solubility of ardealite and different members of the 13 14 solid solution. Future work will address these matters. 15 16 17 18 19 AKNOWLEDGEMENTS 20 21 This research was supported by the Marie Curie European Network “Mineral Nucleation and 22 Growth Kinetics” (European Commission grant UE-MRTN-CT- 2006- 035488) and by the Spanish 23 24 Ministry of Education and Science (grant CGL2010-20134-CO2-02). We thank two anonymous 25 26 referees and A. V. Mudring for the constructive comments that have helped to improve the overall 27 quality of our paper. 28 29 30 31 Supporting Information. SEM image and EDS spectra of a representative solid formed via 32 33 precipitation experiments. This material is available free of charge via the Internet at 34 35 http://pubs.acs.org . 36 37 38 REFERENCES 39 40 41 (1) Stipp, S. Elements 2008 , 4, 75-76. 42 43 (2) Wang, L.; Nancollas, G.H. Chem. Rev. 2008 , 108 , 4628-4669. 44 45 (3) Oelkers, E.H.; Valsami-Jones, E. Elements 2008, 4, 83-87. 46 47 (4) Tortet, L.; Gavarri, J.R.; Musson, J.; Nihoul, G.; Sarychev, A.K. J. Solid State Chem. 1999 , 141 , 48 49 50 392-403. 51 52 (5) Valsami-Jones, E. (2001) Mineral. Mag. 2001 , 65 , 611-620. 53 54 (6) Kordlaghari, M.P.; Rowell, D.L. Geoderma , 2006, 132 , 105-115. 55 56 (7) Arsic, J.; Kaminski, D.; Poodt, P.; Vlieg, E. Phys. Rev. B 2004 , 69 , 245406. 57 58 (8) Francis, M.D.; Webb, N.C. Calcif. Tissue Res., 1971 , 6, 335-342. 59 60 ACS Paragon Plus Environment 29 Crystal Growth & Design Page 30 of 32

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1 2 3 4 5 6 7 For Table of Contents Use Only 8 9 10 11 12 The link between brushite and gypsum: Miscibility, dehydration and 13 . . 14 crystallochemical behavior in the CaHPO 4 2H 2O-CaSO 4 2H 2O system 15 16 André J. Pinto, Joana Carneiro, Dionisis Katsikopoulos, Amalia Jiménez* and Manuel Prieto. 17 Dpto. de Geología, Universidad de Oviedo, C/ Arias Velasco s/n, 3305 Oviedo, Spain 18 19 20 21 22 The mixing properties of the Ca(SO 4,HPO 4)·2H 2O solid solution indicate the existence of two 23 ranges of solid solution separated by two miscibility gaps from a range of composition in which the 24 "double salt" ardealite, Ca 2SO 4HPO 4·4H 2O, forms. Calorimetric, crystallographic and dehydration 25 data indicate that ardealite is not a solid-solution member, but a nearly stoichiometric compound 26 with specific structural features. 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment 32