The synthesis and solubility of the copper hydroxyl nitrates: gerhardtite, rouaite and likasite C. H. Y ODER 1, *, E. B USHONG 1 , X. L IU 1 , V. W EIDNER 1 , P. M C W ILLIAMS 1 , K. M ARTIN 2 , J. L ORGUNPAI 2 , J. H ALLER 2 AND R. W. S CHAEFFER 2 1 Department of Chemistry, Franklin Marshall College, Lancaster, PA 17604-3303, USA 2 Department of Chemistry and Biochemistry, Messiah College, Grantham, PA 17027-9800, USA [ Received 6 November 2009; Accepted 12 May 2010 ] ABSTR ACT Syntheses for the three members of the copper hydroxyl nitrate family the polymorphs rouaite and gerhardtite, and likasite are presented along with powder diffraction data and unit-cell parameters. The solubilities, determined in 0.05 M KNO 3 solution after equilibration at 25ºC for 10 days were used to calculate activity-based solubility product constants. The Gibbs energies of formation, obtained from the solubility products, are 653.2B0.7 kJ/mol, 655.1B1.2 kJ/mol and 1506.4B1.1 kJ/mol, for rouaite, gerhardtite, and likasite (Cu 3 NO 3 (OH) 5 ·2H 2 O), respectively. The values for the polymorphs rouaite and gerhardtite validate the observations of Oswald that gerhardtite is the most stable polymorph at room temperature and that the preparation of predominantly rouaite in syntheses carried out at room temperature must be due to the metastability and low rate of conversion to the more stable gerhardtite. K EY WORDS : copper hydroxyl nitrate, rouaite, gerhardtite, likasite, solubilities, Gibbs energies of formation. Introduction T HE copper hydroxyl nitrate family consists of three minerals with two different stoichiometries. Gerhardtite and rouaite are polymorphs with the formulaCu 2 NO 3 (OH) 3 and likasite has the formulaCu 3 NO 3 (OH) 5 ·2H 2 O. All are rare minerals with rouaite only recognized as a mineral species in 2002 (Jambor and Roberts, 2002). Likasite was first reported by Schoef et al (1955) from the Likasi copper mine in the Belgian Congo with the composition given as Cu 6 (OH) 7 (NO 3 ) 2 PO 4 Declercq et al (1977) revised the formula, based on their structure determination, to Cu 3 P 2 H 3 (NO 3 )(OH) 2 ·H 2 O. Effenberger (1986) found no evidence for the presence of phosphorus and determined that the substance is a copper hydroxyl nitrate with the formulaCu 3 NO 3 (OH) 5 ·2H 2 O, recently corrobo- rated by Raman analysis (Frost et al ., 2005). The Cu 2 NO 3 (OH) 3 polymorphs were reported by Gerhardt (1846 a,b ); gerhardtite was first described as orthorhombic by Wells and Penfield (1885). The similarities between gerhard- tite and its polymorph were summarized by Oswald (1961): the two have very similar cell parameters, gerhardtite in the orthorhombic system, the synthetic polymorph in the mono- clinic system. Despite early reports of the synthesis of gerhardtite, summarized by Oswald (1961), many attempts to prepare gerhardtite have resulted in the monoclinic polymorph, although some methods produce small amounts of gerhard- tite. The reaction of copper nitrate with urea at room temperature for 1 y was reported to produce 60% gerhardtite (Oswald, 1961). Oswald also suggests that gerhardtite is stable below 140ºC and that the monoclinic form is stable only above that temperature but that the conversion to the orthorhombic form at room temperature is very * E-mail: claude.yoder@fandm.edu DOI: 10.1180/minmag.2010.074.3.433 Mineralogical Magazine, June 2010, Vol.74(3), pp.433–440 # 2010 The Mineralogical Society slow (Oswald, 1961). The monoclinic polymorph was identified as a mineral from the Roua copper mine (France) in 2001 and described by Sarp et al . (2001) (cf. Jambor and Roberts, 2002). More recent reports of the structure of gerhardtite support the orthorhombic structure (Bovio and Locchi, 1982). Crystal structure determinations of the synthetic monoclinic form, obtained by Effenberger (1983) by hydrothermal reaction of copper nitrate with copper foil, gave space group of P 2 1 . Chernorukov et al . (2005), using a synthetic sample, also obtained space group P 2 1 . The structure of natural rouaite has not been determined, although the powder diffraction pattern given in the ICDD (#01-075-1779) is identical to the theoretical pattern based on the analysis of Effenberger (1983). A very recent report of the solubility of Cu 2 NO 3 (OH) 3 (with no information about the origin or morphology of the compound) in 1 M KNO 3 at 25ºC provided a K sp of 1.3B 0.3 6 10 31 (Sal’nikov et al ., 2008) in reasonable agreement with that reported by Ilcheva and Bjerrum (1976) as 10 31.5 in 2 M methylammonium nitrate and by Bjerrum (1931) a s 4 6 10 33 at 18ºC. Both solubility products refer to the process Cu 2 NO 3 (OH) 3 (s) ? 2 Cu 2+ (aq) + NO 3 (aq) + 3 OH (aq) Our objectives in this study were to explore synthetic routes to gerhardtite, rouaite and likasite, and to determine the solubilities of these compounds in order to determine the thermodynamic relationships between them. Experimental Characterization of the compounds was achieved using powder X-ray diffraction (XRD), thermo- gravimetric analysis (TGA), and elemental analyses. Powder XRD patterns were obtained by means of a Phillips 3520 X-ray diffractometer using Cu- K a radiation in conjunction with a monochromator. A step size of 0.02º2 y and a dwell time of 1 s were used over the range 2 to 60º2 y . The instrument was calibrated periodically using a quartz standard. Compounds were identified by matching experimental lines to those of the ICDD PDF-2 Set 1-44 Inorganics (2004) database. The TGA analyses were performed using a Thermal Analysis Q-500 from 25º to 1000ºC at 20ºC/min. Copper analyses were performed by thermal decomposition to CuO. Copper, nitrogen, and hydrogen analyses were als o ob tained from Schwarzkopf Microanalytical laboratory (Woodside, NY) and were in good agreement with the thermal analyses for copper. The synthetic methods can be summarized by the equation n Cu 2+ (aq) + m OH (aq) + NO 3 (aq) ? Cu n NO 3 (OH) m (s) with Na 2 CO 3 or NaOH providing the hydroxide ion. These procedures were adopted from those of Oswald (1961). Rouaite was synthesized by slow addition (~3 ml/min) of asolution of 1.9 g Na 2 CO 3 ·H 2 O in 15 ml of water to a solution of 7.0 g of Cu(NO 3 ) 2 ·2.5H 2 O in 30 ml of water. The solution was heated at 45ºC for 30 min, allowed to cool, and the resulting precipitate was filtered by suction. The residue was washed with distilled water and then allowed to dry overnight at room temperature. Powder diffraction patterns obtained from products of this method were identical to rouaite patterns reported by Effenberger (1983) and published in the ICDD PDF# 01-075-1779. The purest samples of gerhardtite were produced by the so-called double jet method. This consisted of the very slow addition of 20 ml of 0.1 M copper(II) nitrate (0.02 ml/min) and 30 ml of 0.10 M NaOH (0.03 ml/min) into 100 ml of distilled water using two Metrohm model 665 Dosimat autotitrators. The final mixture was stirred for 24 h followed by suction filtration, washing of the precipitate with distilled water, and drying overnight at room temperature. Powder XRD patterns for products from this method generally matched the gerhardtite pattern from ICDD PDF# 00-014-0687 (orthorhombic phase), but also included peaks consistent with varying amounts of rouaite. A sample of gerhardtite from Likasi, Democratic Republic of the Congo, was obtained from the Smithsonian Institute, the XRD pattern of which gave an excellent match to the ICDD PDF# 00-014-0687 pattern. This sample was used in the solubility studies. Likasite was obtained by slow addition (1 ml/min) of 50 ml of 0.1 M NaOH to a mixture of 30 ml of 0.1 M copper(II) acetate and 10 ml of 0.1 M sodium nitrate. The mixture was stirred for 20 h followed by suction filtration, washing of the precipitate with distilled water, and drying overnight at room temperature. Powder diffraction patterns for products from this method were identical to the likasite pattern 434 C. H. YODER ET AL. reported by Deliens (1973) and published in the ICDD PDF# 00-030-0497. The solubility and solubility products were determined in triplicate by weighing ~0.1 g of each solid into a 40 ml screw-top vial with Teflon liner. A 20.0 ml portion of 0.0500 M KNO 3 was added separately to each replicate using a bottle- top repipettor. The vials were agitated at a temperature of 25ºC for a minimum of 10 days in a thermostated water bath/agitator, and the establishment of equilibrium was assessed by measuring solution conductivity as a function of time. After this equilibration time, the solutions were analysed for dissolved copper content with a Varian model AA240FS double-beam atomic absorption spectrometer. The pH of each solution was measured with a Fisher model 310 pH meter equipped with a standard glass-membrane probe. The undissolved solid for each sample was analysed with a Rigaku Miniflex II powder X-ray diffractometer for phase identification to determine if any reaction had occurred while in contact with its solution. The total dissolved copper and pH data were entered into Visual MINTEQ (version 2.52) software, which modeled the speciation and activities of all species in the system. Solubility products were then calculated based on the activities of Cu 2+ , NO 3 , and OH calculated by Visual MINTEQ. Several samples of each compound were allowed to remain in contact with its solution for up to 24 days and the solutions were then re-analyzed. The Visual MINTEQ database contains a large number of species, such as Cu(NO 3 ) 2 , Cu(OH) 2 , Cu(OH) 3 , Cu(OH) 4 2 , Cu 2 + , Cu 2 (OH) 2 2 + , Cu 2 OH 3 + , Cu 3 (OH) 4 2+ , CuOH + , and so on. Activity coeffi- cients were calculated within Visual MINTEQ using the Davies equation as follows: log g i ¼ A Z 2 i ffiffiffi I p 1 þ ffiffiffi I p B I where g i is the activity coefficient of species i, I is the ionic strength, Z i the charge of the species, A is the Debye-Hu ̈ckel coefficient, which takes the value of 0.51 at 25ºC, and B is the so-called Davies B parameter, which was assigned a value of 0.30. The applicability of the Davies equation is limited to solutions of small or intermediate ionic strength, such that, for most systems, the Davies equation is likely to give acceptable results at I < 0.3. Estimates of solubilities were made using the simple salt approximation of lattice energy (Yoder and Rowand, 2006) The simple salt method, which has been shown to produce lattice energies within 1% of the experimental (Born-Haber) values, was used to obtain the lattice enthalpy (Yoder and Flora, 2005). Enthalpies of hydration of the gaseous ions were obtained using the values of Marcus (1987). The absolute entropy was approximated using Latimer’s entropy contributions (Latimer, 1952). The Gibbs energy changes for reactions were obtained using standard values of Gibbs energies of formation in the state indicated (at 25ºC, infinite dilution for aqueous solution) from Wagman et al . (1982). The value for likasite of formulaCu 3 NO 3 (OH) 5 ·2H 2 O was obtained by using the value of 2 6 242.4 kJ/mol to correct for the two waters of hydration (Jenkins and Glasser, 2004). Results and discussion Syntheses The relationship between rouaite or its polymorph gerhardtite and likasite in aqueous solution can be expressed by the equation: 3Cu 2 NO 3 (OH) 3 (s) + OH (aq) ? 2Cu 3 NO 3 (OH) 5 (s) + NO 3 (aq) The equilibrium constant for this reaction can be estimated using K sp values for the two compounds approximated using the simple salt method (Yoder and Rowand, 2006). The esti- mated K sp values for rouaite (and gerhardtite) and likasite are 10 34 and 10 58 , respectively, which produce an equilibrium constant for the equation above of 10 14 . Consequently, the hydroxide ion concentration must be kept small and the nitrate concentration large for the preparation of rouaite/ gerhardtite rather than likasite. For the prepara- tion of likasite, the hydroxide concentration should be relatively larger and the nitrate concentration smaller. The use of carbonate with copper nitrate for the preparation of rouaite produces smaller concentrations of hydroxide and greater than the rouaite stoichiometric ratio of nitrate to copper. The addition of NaOH solution to a solution of copper acetate and sodium nitrate in the proper stoichimetric ratio for likasite results in greater hydroxide and smaller nitrate concentrations. The use of a hundred-fold more dilute NaOH solution resulted in a compound the powder diffraction pattern of which was in reasonable agreement with that of likasite (Cu 3 (OH) 3 (NO 3 ) 5 (H 2 O) 2 ), ICDD C U HYDROXYL NITRATE POLYMORPHS 435 #01-075-1485, but differed from the pattern of the compound prepared at the greater hydroxide concentration in the presence of peaks at 5.46 and 11.24º2 y The TGA of the compound obtained using the more dilute solution also indicated the presence of more water of hydration. It was found that both compounds are hygroscopic and it is assumed that the two compounds probably differ only in terms of the number of waters of hydration. It also appears that likasite is considerably more likely to decompose to copper oxide, even upon gentle heating, than rouaite. The preparation of phase-pure gerhardtite was largely unsuccessful, although gerhardtite was obtained along with varying amounts of rouaite using the ‘double jet’ method described above. The following procedures were attempted but resulted in either rouaite or CuO: (1) reaction of copper nitrate with either NaOH or Na 2 CO 3 at 10ºC; (2) reaction of copper nitrate with either NaOH or Na 2 CO 3 in ethanol; (3) cooling rouaite to 80ºC for 3 months; (4) dissolution of Cu(NO 3 ) 2 in aqueous NH 3 , followed by evapora- tion yielded the ammonia complex, which, upon gentle heating produced rouaite; (5) the use of aragonite as an orthorhombic seeding agent in the reaction of copper nitrate with either NaOH or Na 2 CO 3 , (6) the reaction of copper nitrate with urea at 50ºC for 4 days and at room temperature for 3 months; (7) heating of rouaite to 50ºC both with and without a small amount of nitric acid for 3 days; (8) the application of 2.8 6 10 5 kPaof pressure for 0.5 h (gerhardtite is slightly more dense than rouaite); and (9) the reaction of CuO with copper(II) nitrate at room temperature and 80ºC for 3 7 days. Powder XRD Samples were routinely characterized with powder XRD as described above. Typical diffraction patterns for rouaite and likasite are given in Figs 1 and 2. In both cases, the patterns match reference patterns from the International Center for Diffraction Data (ICDD) powder diffraction file (PDF-2) database with figures of merit regularly <2.0 as calculated by JADE 8 software (Materials Data, 2006). This indicates the reproducible synthesis of relatively phase-pure rouaite and likasite. The patterns have significant and easily identified differences including a large peak below 10º2 y for likasite, which arises from large d spacings introduced by the waters of hydration and large peaks at 12.8, 25.8 and 33.6º2 y for rouaite. Diffraction patterns for products from attempted gerhardtite syntheses were less consistent and generally revealed a mixture of rouaite and gerhardtite. Figure 3 represents a relatively pure gerhardtite product from the ‘double jet’ synthetic method as indicated by peaks unique to gerhard- tite, including atriplet of pea ks from 33 to 35º2 y , a F IG . 1. Powder XRD pattern from the synthesized rouaite. The lines represent ICDD PDF pattern #01-075-1779. 436 C. H. YODER ET AL. triplet centred on 58º2 y , and peaks at ~41.4 and 47.3º2 y . These experimental XRD patterns are superimposed in Fig. 4 for ease of comparison. Unit-cell parameters were determined with MDI JADE 8 software using profile-fitting (Pearson VII) for peak location and whole- pattern-fitting cell refinement. The results of our analysis along with data published in the ICDD PDF-2 database are provided in Table 1. Solubilities and Gibbs energies of formation The activities as determined for each species that appears in the solubility product expression for each of the three compounds are given in Tables 2 4. The activity-based solubility products of rouaite, gerhardtite, and likasite were determined as: 1.9 (0.2) 6 10 36 , 8.2 (3.2) 6 10 37 and 2.3 (0.2) 6 10 57 , respectively. Several equilibra- F IG . 3. Powder XRD pattern of the gerhardtite-rich product from the ‘double jet’ method. The lines represent ICDD PDF pattern #01-084-0599 (gerhardtite) and pattern #01-075-1779 (rouaite). F IG . 2. Powder XRD pattern from the synthesized likasite. The lines represent ICDD PDF pattern #01-07501485. C U HYDROXYL NITRATE POLYMORPHS 437 tions were performed for longer time periods in order to verify the establishment of equilibrium. These solubility products are in reasonable agreement with those obtained by the simple salt approximation, which are 10 34 for Cu 2 NO 3 (OH) 3 and 10 58 for Cu 3 NO 3 (OH) 5 (Yoder et al ., 2010). The experimental solubility products were converted to Gibbs energies of formation using tabulated thermodynamic data for the ions in aqueous solution (Wagman et al ., 1982). The solubility products for both rouaite and gerhardtite are at least a factor of 10 3 less than those reported by Sal’nikov et al . (2008), Ilchevaa nd Bjerrum (1976) and Bjerrum (1931). This difference may be due to the different media and concentrations employed (e.g. Ilchevaa nd Bjerrum (1976) used 2 M methylammonium nitrate), the use of different techniques for the evaluation of the equilibria involved (Sal’nikov et al . (2008) used dilatometry), or the purity of the compounds. Ilchevaa nd Bjerrum (1976) provided the solubility product for synthetic gerhandtite, which, based on our work, is probably rouaite. The Gibbs energies of formation obtained from the solubility products are 653.2B0.7 kJ/mol, 655.5B1.2 kJ/mol, and 1506.4B1.1 kJ/mol, for rouaite, gerhardtite, and likasite (Cu 3 NO 3 (OH) 5 ·2H 2 O), respectively. The values for the polymorphs rouaite and gerhardtite validate the observations of Oswald that gerhard- tite is the more stable polymorph at room temperature and that the preparation of predomi- nantly rouaite in syntheses carried out at room temperature must be due to its metastability and F IG . 4. Powder XRD patterns for: rouaite (upper), gerhardtite (middle), and likasite (lower). T ABLE 1. Crystallographic parameters for copper(II) hydroxynitrates. Unit-cell parameters Rouaite sample Rouaite PDF# 01-075-0779 Gerhardite sample Gerhardtite PDF# 01-84-0599 Likasite sample Likasite PDF# 00-030-0497 a (A ̊ ) 5.605(1) 5.605 6.079(1) 6.087 5.816(1) 5.829 b (A ̊ ) 6.091(1) 6.087 13.868(3) 13.813 6.779(1) 6.772 c (A ̊ ) 6.949(1) 6.929 5.598(1) 5.597 21.737(3) 21.691 b (º) 94.66 94.48 90 90 90 90 V (A ̊ 3 ) 236.42 235.68 471.92 470.59 855.21 856.23 Z 2 2 4 4 4 4 438 C. H. YODER ET AL. low rate of conversion to the more stable gerhardtite. Acknowledgements The authors are indebted to Karen Burger of Franklin Marshall College and to Steve Funck of Messiah College for technical assistance and to the Keystone Innovation Zone (Seed Assistance grant), the Franklin Marshall College Lucille and William Hackman Endowment, the Messiah College Steinbrecher Summer Research Program, and the American Chemical Society Petroleum Research Fund, for support. References Bjerrum, J. (1931) Copper ammonium complex salts. I. Determination of the equilibrium constants of the copper-ammine ions by means of ammonia-tension measurements, and by means of the solulbility data of a basic copper nitrate (gerhardtite). Kongelige Danske Videnskabernas Selskab.Math. -fysiske Meddelelser , 11 , 58. Bovio, B. and Locchi, S. (1982) Crystal structure of the orthorhombic basic copper nitrate, Cu 2 (OH) 3 NO 3 Journal of Crystallographic and Spectroscopic research , 12 , 507 517. Chernorukov, N.G., Mikhailov, Yu.N., Knyazev, A.V., Kanishcheva, A.S. and Bulanov, E.N. (2005) T ABLE 2. Activities and solubility product for gerhardtite. Replicate [Cu 2+ ] (mol/l) [NO 3 ] (mol/l) pH a Cu 2+ a OH a NO 3 Calc K sp 1 1.24E-05 0.010 5.93 8.77E-06 8.55E-09 9.27E-03 4.46E-37 2 2.20E-05 0.010 5.88 1.56E-05 7.62E-09 9.27E-03 9.94E-37 3 2.19E-05 0.010 5.88 1.57E-05 7.63E-09 9.27E-03 1.01E-36 Mean 8.16E-37 S 3.21E-37 T ABLE 3. Activities and solubility product for rouaite. Replicate [Cu 2+ ] (mol/l) [NO 3 ] (mol/l) pH a Cu 2+ a OH a NO 3 Calc K sp 1 4.04E-05 0.010 5.81 2.86E-05 6.50E-09 9.26E-03 2.08E-36 2 3.70E-05 0.010 5.82 2.62E-05 6.67E-09 9.26E-03 1.88E-36 3 3.54E-05 0.010 5.83 2.51E-05 6.75E-09 9.26E-03 1.79E-36 Mean 1.92E-36 S 1.50E-37 T ABLE 4. Activities and solubility product for likasite. Replicate [Cu 2+ ] (mol/l) [NO 3 ] (mol/l) pH a Cu 2+ a OH a NO 3 Calc K sp 1 3.57E-05 0.010 5.83 2.53E-05 6.73E-09 9.26E-03 2.07E-57 2 3.93E-05 0.010 5.82 2.78E-05 6.55E-09 9.26E-03 2.41E-57 3 3.95E-05 0.010 5.82 2.80E-05 6.54E-09 9.26E-03 2.42E-57 Mean 2.30E-57 S 2.01E-58 C U HYDROXYL NITRATE POLYMORPHS 439 Synthesis of trihydroxonitratodicopper(II) and re- finement of its crystal structure, Zhurnal Neorganicheskoi Khimii , 50 , 775 778. Declercq, J.P., Germain, G. and Piret, P. (1977) C o m p o s i t i o n a n d s t r u c t u r e o f l i k a s i t e , Cu 3 P 2 H 3 (NO 3 )(OH) 2 ·H 2 O. 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