See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/256453543

A temperature-induced reversible transformation between and herbertsmithite

Article in Physics and Chemistry of · September 2013 DOI: 10.1007/s00269-013-0621-5

CITATIONS READS 20 494

6 authors, including:

Mark David Welch Matthew Sciberras Natural History Museum, London 8 PUBLICATIONS 66 CITATIONS 144 PUBLICATIONS 2,370 CITATIONS SEE PROFILE SEE PROFILE

Peter Leverett J. Schlüter Western Sydney University University of Hamburg

81 PUBLICATIONS 1,592 CITATIONS 81 PUBLICATIONS 922 CITATIONS

SEE PROFILE SEE PROFILE

Some of the authors of this publication are also working on these related projects:

Characterization of minerals by vibrational spectroscopy (excluding clay minerals and LDH) View project

Cobalt mineralisation in Australian deposits View project

All content following this page was uploaded by Mark David Welch on 26 May 2014.

The user has requested enhancement of the downloaded file. Phys Chem Minerals DOI 10.1007/s00269-013-0621-5

ORIGINAL PAPER

A temperature-induced reversible transformation between paratacamite and herbertsmithite

Mark D. Welch • Matthew J. Sciberras • Peter A. Williams • Peter Leverett • Jochen Schlu¨ter • Thomas Malcherek

Received: 3 April 2013 / Accepted: 3 August 2013 Ó Springer-Verlag Berlin Heidelberg 2013

Abstract The crystal chemistry of paratacamite has been states are almost identical. The complete reversibility of re-evaluated by studying a crystal from the holotype the transformation establishes that paratacamite of com- specimen BM86958 of composition Cu3.71Zn0.29(OH)6Cl2 position Cu3.71Zn0.29(OH)6Cl2 is thermodynamically stable using single-crystal X-ray diffraction at 100, 200, 300, 353, at ambient temperatures. The nature of the rhombic dis-

393 and 423 K. At 300 K paratacamite has R3 tortion of the M(2)O6 octahedron is discussed by consid- with unit-cell parameters a 13.644 and c 14.035 A˚ and ering two possibilities that are dependent upon the nature exhibits a pronounced subcell, a0 = ‘a and c0 = c, anal- of cation substitution in the interlayer sites. ogous to that of the closely related herbertsmithite,

Cu3Zn(OH)6Cl2. Between 353 and 393 K, paratacamite Keywords Paratacamite Herbertsmithite undergoes a reversible phase transformation to the her- Temperature-induced phase transformation bertsmithite-like substructure, space group R3m, unit-cell Jahn–Teller distortion parameters a 6.839 and c 14.072 A˚ (393 K). The trans- formation is characterised by a gradual reduction in intensity of superlattice reflections, which are absent at 393 Introduction and 443 K. On cooling from 443 to 300 K at *10 K min-1, the superlattice reflections reappear and the The minerals herbertsmithite (Braithwaite et al. 2004) and refined structures (R3) of the initial and recovered 300 K kapellasite (Krause et al. 2006), both of ideal composition Cu3Zn(OH)6Cl2, and haydeeite Cu3Mg(OH)6Cl2 (Malc- herek and Schlu¨ter 2007; Schlu¨ter and Malcherek 2007),

as well as the related synthetic phases Cu4-xMgx(OH)6Cl2 (Chu et al. 2010) and Cu Cd(OH) Cl (McQueen et al. Electronic supplementary material The online version of this 3 6 2 article (doi:10.1007/s00269-013-0621-5) contains supplementary 2011) are examples of spin-‘ frustrated antiferromagnets material, which is available to authorized users. in which an antiferromagnetic state is precluded by an ordered arrangement of non-magnetic elements (Zn and & M. D. Welch ( ) Mg) within the polyhedral structure. Such structures are Mineral and Planetary Sciences Division, Department of Earth Sciences, Natural History Museum, Cromwell Road, London so-called kagome´ phases of great significance for under- SW7 5BD, UK standing quantum spin liquids (Schores et al. 2005; Hel- e-mail: [email protected] ton et al. 2007; Freedman et al. 2010; Chu et al. 2010; Han et al. 2011, 2012; Li and Zhang 2013). This new M. J. Sciberras P. A. Williams P. Leverett School of Science and Health, University of Western Sydney, magnetically frustrated state has many potential applica- Locked Bag 1797, Penrith, NSW 2751, Australia tions including high-temperature superconductors, data e-mail: [email protected] storage and so-called ‘‘quantum-entangled’’ batteries as a new power source greatly superior to lithium-based bat- J. Schlu¨ter T. Malcherek Mineralogisch-Petrographisches Institut, Universita¨t Hamburg, teries (Colman et al. 2008, 2010; de Vries and Harrison Grindelallee 48, 20146 Hamburg, Germany 2007). 123 Phys Chem Minerals

Herbertsmithite has layers comprising 75 % the highly unusual rhombic distortion of one of the two

Cu(OH)4Cl2 octahedra and 25 % vacant octahedra; these interlayer metal positions (Cu2). No additional structural layers share edges with single interlayer Zn(OH)6 octahe- data have been reported for paratacamite, although dra lying directly beneath and above the vacant octahedra. numerous reports of its occurrence have entered the liter- In contrast, kapellasite and haydeeite have layers com- ature (e.g. Smith 1906; Jambor et al. 1996; Pring et al. prising 75 % Cu(OH)4Cl2 octahedra and 25 % Zn(OH)4Cl2 1987; Braithwaite et al. 2004). octahedra, and they have no interlayer octahedra; hydrogen bonding is the only connection between layers. Higher Cu Original structure determination of holotype contents lead to the stabilisation of other closely related paratacamite mineral structures: paratacamite, anatacamite and clinoat- acamite, although the precise compositional phase bound- In the R3m subcell of paratacamite determined by Fleet ary between these structures has yet to be determined. In (1975), the interlayer Cu ion is coordinated by six sym- this paper we focus on the structural chemistry of metry-related OH– ions from adjacent sheets (Fig. 1). The paratacamite. O atom of this OH- anion is disordered over two positions. The structure of paratacamite was solved by Fleet For O1, Cu–O1 = 2.041 A˚ , all trans HO–Cu–OH angles (1975) using a single crystal from the type specimen are constrained to be 180° and the cis HO–Cu–OH angle is (British Museum specimen BM86958) from the Generosa 103.5°. For O2, Cu–O2 = 2.335 A˚ , all trans HO–Cu–OH Mine, Sierra Gorda, Chile, in space group R3 and assuming angles are constrained to be 180° and the cis HO–Cu–OH the formula Cu2(OH)3Cl. It was noted that paratacamite angle is 105.3°.TheOO distances between adjacent O1 has a strong substructure with a0 = ‘a and c0 = c, in space atoms are 2.526 and 3.206 A˚ , while the distances between group R3m. The subcell corresponds to that of herbert- partially occupied O1 and O2 positions are 2.692 and ˚ smithite and gillardite, Cu3Ni(OH)6Cl2 (Clissold et al. 3.462 A. 2007). Braithwaite et al. (2004) reported compositions for Both geometries are highly unusual for six-coordinate various paratacamites (but not the type specimen) and Cu(II) with only OH ligands. Fleet (1975) also reported herbertsmithites containing 6–16 atom% Zn substituting that O1 and O2 (Fig. 1) were fractionally occupied at for Cu. The presence of minor but essential Zn is now 0.76(9) and 0.24(9), respectively. As such, the representa- recognised as a precondition for stabilising the parataca- tion shown in Fig. 1 may be viewed as a superimposition of mite structure. Increasing Zn substitution favours trans- four short Cu–O1 bonds (the higher occupancy) and two formation to the herbertsmithite structure, with loss of the long Cu–O2 bonds. Theoretically, these occupancies 2a superstructure of paratacamite. Thus, a composition- should be 2=3 (short) and 1=3 (long). The result for R3 dependent transformation R3 ? R3m has been recognised, symmetry is the superimposition of three different orien- although the phase boundary has not yet been located. tations of the common (4 ? 2) Jahn–Teller distorted Cu(II) Anatacamite, triclinic P1, has the end-member compo- geometry. The single interlayer site of the R3m substruc- sition Cu2(OH)3Cl (Malcherek and Schlu¨ter 2009; Malc- ture Cu(1) is split in the R3 structure, Cu(1) and Cu(2), and herek and Schlu¨ter 2010). Grice et al. (1996) reported the coordination spheres of these two interlayer sites are clinoatacamite, monoclinic P21/n, as also having the end- member composition Cu2(OH)3Cl. However, the clinoat- acamite structure can accommodate minor amounts of Zn, up to *8 atom% (Jambor et al. 1996). At about one-third Zn occupancy of the interlayer of clinoatacamite, a trigonal phase, assumed to be paratacamite, is stabilised (Jambor et al. 1996). It has been proposed that the substituted Cu(II)-hydroxychloride minerals are related through a series of composition-dependent phase transformations. According to group theory, two different transformational series are possible; P1? C2/m ? P21/c ? R3m, and P1? R3 ? R3m. These possibilities have been discussed by Malcherek and Schlu¨ter (2009), who suggested P1? P21/c (P21/n) ? R3m as the most likely order of trans- formations with increasing substitution for Cu. Fig. 1 The coordination environment of the disordered Cu(OH)6 Aspects of the structure of paratacamite have remained group between the sheets in the R3m subcell of Fleet (1975); Cu is enigmatic since its description by Fleet (1975), principally blue,Oisred. The unit cell is outlined

123 Phys Chem Minerals

Experimental methods

Chemical analysis

The paratacamite crystal taken from the type specimen (BM86958) and used in the single-crystal XRD study was attached to a glass slide using Crystalbond and was carbon- coated after the series of diffraction experiments ended. Care was taken to ensure that a clean, flat crystal upper surface was presented parallel to the glass slide in order to minimise differential absorption of the electron beam during analysis by electron microprobe. A CAMECA SX100 microprobe operated at 20 kV and 10 nA with a

Fig. 2 The coordination environment of the Cu(OH)6 groups 5 lm beam diameter was used to obtain analyses by between the sheets in the R3 cell of Fleet (1975); Cu is blue,Ois wavelength-dispersive spectrometry. Elemental standards red. The unit cell is outlined used were vanadinite (Cl), pure metal, sulphide (Zn), pure iron metal, pure cobalt metal, pure nickel metal shown in Fig. 2. Cu(1) is bonded to six equivalent OH and forsterite (Mg). Analytical data are given in Table 1. groups (O4) with Cu(1)–O4 = 2.121 A˚ . Trans HO–Cu(1)– The average of five analyses gave the empirical composi-

OH angles are constrained to be 180°, and the cis HO– tion Cu3.73Zn0.29Cl1.95O6.05H6, calculated based on eight Cu(1)–OH angle is 105.4°. Cu(2) is bonded twice (trans)to anions pfu. The sample is rather unstable in the probe, thus three crystallographically independent OH groups with all accounting for the lower analytical totals as well as cation trans HO–Cu(2)–OH angles equal to 180°. Cu(2)– and Cl values deviating from ideal quantities. The structure O1 = 1.933, Cu(2)–O2 = 2.186 and Cu(2)–O3 = 2.204 of paratacamite is composed of sheets of composition ˚ 2– 2? A, respectively, and O1–Cu(2)–O2 = 102.8, O1–Cu(2)– Cu3(OH)6Cl2 linked by M ions lying between them O3 = 101.8 and O2–Cu(2)–O3 = 105.1°, respectively. (Fleet 1975). Therefore, the empirical formula may be

The geometry of the Cu(2)-centred polyhedron is rewritten as Cu3.73Zn0.29(OH)6Cl2. Normalisation based on 2? - anomalous for Cu bonded to six OH ligands. It has a four cations pfu gives Cu3.71Zn0.29(OH)6Cl2. tetragonally compressed and rhombically distorted coordi- nation sphere that, although not unprecedented in the lit- Single-crystal X-ray diffraction erature, is confined to about 10 copper complexes containing chelating ligands and only three containing un- Several crystals of paratacamite from the Generosa mine identates or bridging bidentates (Hathaway 1987). Com- type specimen were examined for optical and diffraction pressed rhombic octahedral geometry for Cu(II) was first quality. The best crystal (0.14 9 0.13 9 0.10 mm) was recognised in the complex b-[Cu(NH3)2Br2], but is usually attached to a non-diffracting, amorphous carbon fibre associated with complexes of mutidentate ligands. It occurs, (0.02 mm diameter), itself glued to a glass fibre base. The for example, in [Cu(dien)2](NO3)2, nitrate and hexafluoro- crystal was mounted on an XcaliburE four-circle diffrac- 2? phosphate salts of [Cu(terpy)2] and [Cu(tach)2](NO3)2 tometer equipped with an Eos 1 K CCD detector (Agilent (dien = 1,4,7-tetraazaheptane, terpy = 2,20:60,200-terpyri- Technologies). A Cryojet cryoheater (Oxford Diffraction) dine, tach = 3-methylamino-1,5-diazapentane). Two other with a liquid-nitrogen supply was used for variable-tem- complexes containing bidentate nitrate or methoxyacetate perature experiments at 100, 200, 300, 353, 393 and 443 K. ions exhibit similar geometries. Hathaway noted that the The tip of the nozzle of the cryoheater was positioned to restricted bite angles of the ligands involved may play a role within 7 mm of the crystal. Monochromatic MoKa radia- in some cases, but no satisfactory explanation for the nature tion (k = 0.71073 A˚ ) at 45 kV and 40 mA was used for all of this kind of Jahn–Teller distortion has been advanced. experiments. Pure x scans with a 1° frame-width and a 40-s This type of geometry has since been noted to exist in the frame-time were used. The data collection strategy was structure of several mineral species (Burns and Hawthorne determined from a 30 min pre-experiment, and collection 1996; Kolitsch and Giester 2000). of each full data set lasted 25 h. For the non-ambient In this paper we present new structural data for parat- experiments, nitrogen flow rates of 6 L min-1 onto the acamite from the type specimen between 100 and 443 K crystal and 4 L min-1 for the shield flow were used. that allow re-evaluation of the relationship between pa- Temperatures were logged continuously and were found to ratacamite and herbertsmithite structures and suggest an be within 2 K of nominal throughout each experiment. A origin of the unusual distortion of the Cu(2) octahedron. 15-min thermal equilibration time was used before each 123 Phys Chem Minerals

Table 1 Electron microprobe Range (wt%) Averagea Empirical composition Normalised compositionb analyses of type paratacamite (BM 86958) CuO 66.42–67.76 67.27(51) 3.73 3.71 ZnO 4.47–6.09 5.33(64) 0.29 0.29 NiO 0–0.04 0.02(2) – – MgO 0–0.02 \0.01 – – FeO 0–0.02 \0.01 – – CoO 0–0.01 \0.01 – – Cl 15.6–15.85 15.69(11) 1.95 2.00 a Standard deviation of the H2O 12.24 6.00 6.00 average value is in parenthesis O:Cl -3.55 b Compositions were Total 97.00 normalised to R(cations) = 4 data collection began. A sphere of data was collected to 34° choice of space group. However, structure solution in R3m h, with 100 % completeness up to 30° h. Intensity data did not reach convergence for all data sets in which su- were corrected for Lorentz, polarisation and absorption perlattice reflections were present. In this space group, the (multiscan) effects and converted to structure factors using structure solution located the position of three O atoms, the program CrysalisRed (Agilent 2012). Unit-cell one of which was in a lower symmetry 36i position and parameters were calculated from reflections having bonded to M(2). All of the O atoms had non-positive def- I [ 7r(I). Structure solution by direct methods and full- inite (NPD) displacement parameters when refined aniso- matrix least-squares refinement on F2 used the program tropically, and it was evident that significantly fewer SHELX-97 (Sheldrick 2008). Neutral scattering factors for superlattice reflections actually fit the R3m model. Evi- Cu, O, Cl and H were taken from International Tables for dently, there is enough information from the superlattice Crystallography, Volume C (1992). reflections to discriminate clearly between R3m and R3 The unit-cell metric of paratacamite for all data sets is symmetry. trigonal (hexagonal). Examination of the unconstrained Structure solution in R3 found all four Cu and both Cl triclinic cells did not indicate departure from this metric atoms. All four O atoms appeared after the first 10 cycles within two standard deviations of all cell parameters. of least-squares refinement. All four H atoms were found Examination of ‘‘pseudo-precession’’ photographs recon- after a further 10 cycles of weighted least-squares refine- structed using CrysalisPro for hk0, h0l and 0kl sections was ment. All non-H atoms were refined anisotropically. made for each data set collected. All hk0 diffraction pat- However, in the 100 K refinement two oxygen atoms (O1 terns contain an array of strong reflections, defined by and O3) had NPD displacement parameters, and the 200 K m{600} and m{220} with m = integer, and corresponding refinement had one NPD oxygen atom (O1). NPD behav- ˚ to a 6.85 9 6.85 9 14 A subcell. However, diffraction iour for the 100 and 200 K refinements appears when H patterns collected at 100–353 K contain many weak atoms are refined. It is a recurrent feature of refinements of reflections at half-integer positions of h and k, defining a R3 paratacamite that one oxygen (O1) has a Ueq of about 2a 9 2a 9 c supercell. Figure 3 depicts hk0 reconstruc- half those of the other three. As temperature decreases, this tions of the initial 300, 353, 393 and return to 300 K data atom inevitably becomes NPD. sets. The 300 and 353 K patterns show the loss and return Information relating to data collections and relevant of superlattice reflections, for which the most obvious of structure refinement details at each temperature is given in these reflections form an array defined by {600} ? n{110} Table 2. Data relating to an R3 and R3m refinement for the with n = odd, and alternate with sublattice reflections 443 K data set are displayed for comparison. Atom coor- (n = even). The 393 and 443 K patterns completely lack dinates and equivalent-isotropic displacement parameters superlattice reflections. are shown in Table 3. Anisotropic displacement parameters There is a gradual reduction in the intensities of super- are given in Table 4 and selected bond lengths and angles lattice reflections from 100 to 353 K. Systematic absences in Table 5. for all data sets are consistent with space groups R3m, R3m, R3, R3 and R32. Structures related to paratacamite, such as herbertsmithite, have R3m symmetry, and so this space Results group was tested for the superstructure in order to elucidate whether or not the occurrence of this unusual rhombic Variations in unit-cell parameters with temperature are distortion of the M(2)O6 coordination sphere is due to the shown in Fig. 4. Both a and c parameters show a smooth

123 Phys Chem Minerals

Fig. 3 A reconstruction of the pseudo-precession hk0 diffraction patterns of paratacamite at various temperatures. Many superlattice reflections are evident in the diffraction patterns of the two 300 K data sets, whereas in the 353 K pattern, they are very weak (arrows indicate two rows containing more obvious superlattice reflections alternating with sublattice reflections). Superlattice reflections are absent from the 393 K pattern and are recovered on return to 300 K. In the two 300 K patterns, the most obvious superlattice reflections form rows with {600} ? n{110} and n = odd. These superlattice reflections alternate with sublattice reflections having n = even. The three superlattice reflections arrowed in the initial 300 K pattern are (710), (930) and (1150)

nonlinear increase up to 393 K. At 443 K there are sharp octahedra out of these mirror planes are also apparent. The discontinuities in both parameters, with a being unchanged distortion of the M(3)O6 octahedron is also evident in from 393 K and c showing an additional increase. Fig. 5, where it can be seen to violate a mirror plane of The M(1) interlayer site lies at 3m and is coordinated R3m. octahedrally by O4 atoms. The M(2) interlayer site is The variation in M–O and M–Cl bond lengths with coordinated by O1, O2 and O3 atoms in a tetragonally temperature is shown in Fig. 6. Several significant obser- elongated and rhombically distorted octahedral environ- vations can be made from this figure, as follows. ment, which may be described as a (2 ? 2 ? 2) Jahn– 1. The M(1)O octahedron is essentially unaffected by Teller distortion and is discussed further below. The in- 6 heating and the structural transformation. tralayer M(3) and M(4) sites are both coordinated to four 2. The M(2)O octahedron shows a marked change with short equatorial O atoms and two long axial Cl atoms, 6 heating that involves convergence of M(2)–O1 and which is a typical (4 ? 2) Jahn–Teller distorted configu- M(2)–O3 bond lengths with those of M(2)–O2 and ration for octahedrally coordinated Cu2?. The polyhedral M(1)–O4. environments of M(1)- and M(2)-centred octahedra in R3 3. The M(3)O Cl octahedron has a differential response paratacamite at 300 K are shown in Fig. 5. The M(1)O 4 2 6 between 100 and 353 K in which the two M(3)–O2 octahedron shares edges with triplets of M(4)O Cl octa- 4 2 bonds show small increases in length while the M(3)– hedra above and below it. The M(2)O octahedron shares 6 O1 and M(3)–O3 bonds, which are quite different, edges with two triplets above and below it, each com- rapidly converge at the transformation. The M(3)–Cl2 prising one M(4)- and two M(3)-centred octahedra. Mirror bonds converge steadily towards the transformation. planes of the R3m superstructure are broken primarily by 4. For the M(4)O4Cl2 octahedron, the initially very the (2 ? 2 ? 2) distortion of the M(2)O6 octahedron. different M(4)–O1 and M(4)–O3 bond lengths Small rotations (*2°) of the M(1)- and M(2)-centred

123 Phys Chem Minerals

Table 2 Data collection and structure refinement details of type paratacamite (BM 86958) at each experimental temperature 100 K 200 K 300 K initial 353 K 393 K 443 K 443 K 300 K return R3 R3 R3 R3 R3m R3 R3m R3

Unit-cell a (A˚ ) 13.6245(7) 13.6311(6) 13.6440(4) 13.6558(3) 6.8394(1) 6.8396(3) 6.8396(3) 13.6495(4) c (A˚ ) 14.011(1) 14.018(1) 14.0354(7) 14.0428(5) 14.0716(2) 14.0834(7) 14.0834(7) 14.0359(7) Volume (A˚ 3) 2252.3(2) 2255.7(2) 2262.8(1) 2267.9(1) 570.052(15) 570.55(4) 570.55(4) 2264.7(1) h range (°) 2.99–34.35 2.99–34.51 2.99–34.38 2.98–34.45 3.73–33.23 3.73–34.31 3.73–34.31 2.98–34.39 Reflections (I) 18909 19161 18984 18883 4191 4452 4452 18951 Unique reflections (I) 2032 2041 2039 2031 297 519 318 2036 I [ 2r(I) 1648 1578 1554 1391 292 504 313 1520

Rint 0.039 0.037 0.030 0.024 0.019 0.021 0.022 0.031

R1 [I [ 2r(I)] 0.037 0.035 0.030 0.027 0.014 0.016 0.016 0.031

R1 (all) 0.046 0.047 0.043 0.044 0.014 0.017 0.017 0.047

*wR2 (I [ 2r(I)) 0.078 0.078 0.062 0.064 0.034 0.040 0.043 0.059

*wR2 (all) 0.086 0.087 0.071 0.075 0.035 0.040 0.043 0.068 *w coefficient a 0.0249 0.0268 0.0179 0.0227 0.0157 0.0172 0.0234 0.0154 *w coefficient b 29.0307 20.2596 14.2129 8.3876 1.9943 1.1630 1.8437 18.0036 GoF 1.064 1.053 1.084 1.084 1.095 1.181 1.056 1.059 max. shift/r 0.001 0.001 0.001 0.001 0.002 0.000 0.000 0.001

Dqelec (max, min) 1.54, -2.08 0.90, -1.96 0.93, -1.48 0.43, -1.45 0.41, -0.45 0.39, -0.64 0.42, -0.64 0.71, -1.57  1=2 2 2 2 ½RkkFojjFc R½wðFo Fc Þ R ¼ ; wR ¼ 2 1 RjjFo 2 2 R½wFðÞo 2 2 1 max Fo þ2Fc *¼ 2 2 2 ; where P ¼ 3 ½r ðÞFo þðÞaP þbP

converge smoothly towards the transformation. The the intralayer M(3)O4Cl2 and M(4)O4Cl2 octahedra are two M(4)–O4 bond lengths do not change significantly highly Jahn–Teller distorted, their response to heating is an with heating or cooling. One of them is almost homogenous shortening of bonds that does not significantly ˚ unchanged at *1.980 A, while the other bond shows change their degree of angular deviation. As the M(1)O6 a minor contraction at 200 K, followed by a steady octahedron has equal M(1)–O4 bond lengths, the distortion lengthening until convergence is achieved at the registered by its QE is due entirely to deformation of the transformation. The two M(4)–Cl bonds have very O–M–O bond angle, which at 76° differs considerably from

similar lengths and remain almost constant throughout 90°. The higher QE values of the M(2)O6 octahedron in the heating. R3 structure are due to the three very different bond lengths of the (2 ? 2 ? 2) configuration and deformation of the Figure 7 shows the variation with temperature of the O–M–O bond angles. On heating, its QE converges volumes of M(1)-, M(2)-, M(3)- and M(4)-centred octahe- smoothly to 1.054 at the transformation. dra calculated using Xtaldraw (Downs and Hall-Wallace 2003). There is a clear difference in behaviour between interlayer and intralayer octahedra; M(1)O6 and M(2)O6 Discussion octahedra contract on heating, whereas M(3)O4Cl2 and M(4)O4Cl2 octahedra expand. Figure 8 shows the bond angle variances (BAV) of all The structures of paratacamite from 100 to 353 K reported four octahedra and the quadratic elongation (QE) of the here are analogous to that of the R3 superstructure first

M(2)O6 octahedron as functions of temperature. These determined by Fleet (1975), although with all four H atoms geometrical parameters were calculated using the formu- also located. At 393 and 443 K, the structure is that of the lation of Robinson et al. (1971) as implemented in Xtal- average substructure, again very similar to the R3m struc- draw. The quadratic elongation of M(1)-, M(3)- and M(4)- ture of Fleet (1975). However, refinements of this sub- centred octahedra is invariant with temperature, having structure in R3 and R3m give comparable results (Table 2), values of 1.054, 1.074 and 1.075, respectively. In contrast, and there is no clear evidence of a split oxygen position the quadratic elongation of the M(2)O6 octahedron shows a reported by Fleet (1975). There is no indication of residual smooth, nonlinear decrease in the transformation. While peaks on Fo–Fc maps existing near the O atom position.

123 Phys Chem Minerals

Table 3 Atomic coordinates and isotropic thermal parameters of type paratacamite (BM86958) at each temperature 100 K (R3) 200 K (R3)

xyzUeq xyzUeq

M(1) 0 0 ‘ 0.0053(1) 0 0 ‘ 0.0076(1) M(2) ‘‘‘0.0051(1) ‘‘‘0.0075(1) M(3) 0.41416(3) 0.32839(2) 0.33148(2) 0.00659(9) 0.41437(2) 0.32881(2) 0.33136(2) 0.00919(9) M(4) 0.41089(2) 0.57757(2) 0.33340(2) 0.00598(9) 0.41105(2) 0.57773(2) 0.33331(2) 0.00851(9) Cl1 0 0 0.19376(8) 0.0078(2) 0 0 0.19375(8) 0.0111(2) Cl2 0.50221(5) 0.50221(5) 0.19358(5) 0.0073(1) 0.50208(5) 0.50207(5) 0.19365(5) 0.0111(2) O1 0.5558(2) 0.6196(2) 0.4009(1) 0.0040(3) 0.5562(1) 0.6199(1) 0.4009(1) 0.0059(3) O2 0.5592(2) 0.4324(2) 0.3944(2) 0.0151(4) 0.5594(2) 0.4328(2) 0.3944(2) 0.0172(4) O3 0.3634(2) 0.4282(2) 0.3848(2) 0.0122(4) 0.3637(2) 0.4284(2) 0.3853(2) 0.0145(4) O4 0.0683(2) 0.1269(2) 0.3944(2) 0.0151(4) 0.0681(2) 0.1266(2) 0.3942(2) 0.0165(4) H1 0.579(5) 0.662(4) 0.428(4) 0.027(9) 0.585(4) 0.678(4) 0.432(3) 0.031(8) H2 0.583(4) 0.400(4) 0.421(4) 0.027(9) 0.586(4) 0.402(4) 0.416(4) 0.031(8) H3 0.316(4) 0.401(5) 0.409(4) 0.027(9) 0.310(4) 0.398(4) 0.409(3) 0.031(8) H4 0.097(4) 0.182(5) 0.417(4) 0.027(9) 0.096(4) 0.183(4) 0.412(4) 0.031(8) Initial 300 K (R3) 353 K (R3)

xyzUeq xyzUeq

M(1) 0 0 ‘ 0.0093(1) 0 0 ‘ 0.0116(1) M(2) ‘‘‘0.00915(9) ‘‘‘0.01136(9) M(3) 0.41479(2) 0.32965(2) 0.33163(2) 0.01104(8) 0.41542(2) 0.33083(2) 0.33215(2) 0.01340(8) M(4) 0.41191(2) 0.57858(2) 0.33339(2) 0.01054(8) 0.41333(2) 0.58003(2) 0.33331(2) 0.01313(8) Cl1 0 0 0.19388(7) 0.0142(2) 0 0 0.19401(6) 0.0170(2) Cl2 0.50169(4) 0.50172(4) 0.19380(4) 0.0139(1) 0.50115(4) 0.50113(4) 0.19402(4) 0.0169(1) O1 0.5569(1) 0.6205(1) 0.4003(1) 0.0091(3) 0.5586(1) 0.6219(1) 0.3990(1) 0.0145(3) O2 0.5602(2) 0.4335(1) 0.3948(2) 0.0181(4) 0.5611(1) 0.4344(1) 0.3948(1) 0.0201(3) O3 0.3653(2) 0.4295(1) 0.3869(1) 0.0174(4) 0.3677(1) 0.4314(1) 0.3895(1) 0.0211(3) O4 0.0674(2) 0.1266(2) 0.3945(2) 0.0182(4) 0.0661(1) 0.1265(1) 0.3948(1) 0.0199(3) H1 0.588(3) 0.679(3) 0.426(3) 0.031(7) 0.591(3) 0.683(3) 0.427(3) 0.039(7) H2 0.591(3) 0.399(3) 0.420(3) 0.031(7) 0.593(3) 0.403(3) 0.419(3) 0.039(7) H3 0.308(3) 0.399(3) 0.410(3) 0.031(7) 0.308(3) 0.402(3) 0.414(2) 0.039(7) H4 0.098(3) 0.185(4) 0.414(3) 0.031(7) 0.098(3) 0.188(3) 0.420(3) 0.039(7)

393 K (R3m) 443 K (R3m)

xyz Ueq xyz Ueq

M(1) 0 0 ‘ 0.0117(1) 0 0 ‘ 0.0132(1) M(2) ‘ 0 0 0.01383(9) ‘ 0 0 0.0153(1) Cl1 0 0 0.19405(5) 0.0177(1) 0 0 0.19427(6) 0.0200(2) O1 0.2069(1) 0.7932(1) 0.0616(1) 0.0202(3) 0.2068(1) 0.4136(3) 0.0616(1) 0.0215(3) H1 0.137(3) 0.275(7) 0.085(2) 0.044(10) 0.141(3) 0.283(7) 0.085(2) 0.035(10)

Final 300 K (R3)

xyzUeq

M(1) 0 0 ‘ 0.0096(1) M(2) ‘‘‘0.00938(9) M(3) 0.41484(2) 0.32970(2) 0.33166(2) 0.01134(8) M(4) 0.41197(2) 0.57866(2) 0.33330(2) 0.01081(8)

123 Phys Chem Minerals

Table 3 continued

Final 300 K (R3)

xyzUeq

Cl1 0 0 0.19382(7) 0.0146(2) Cl2 0.50172(5) 0.50171(5) 0.19383(4) 0.0141(1) O1 0.5572(1) 0.6208(1) 0.4002(1) 0.0097(3) O2 0.5601(2) 0.4335(2) 0.3945(2) 0.0185(4) O3 0.3653(2) 0.4296(2) 0.3871(2) 0.0184(4) O4 0.0673(2) 0.1266(2) 0.3945(2) 0.0179(4) H1 0.584(4) 0.678(4) 0.432(3) 0.037(7) H2 0.588(4) 0.403(4) 0.421(3) 0.037(7) H3 0.308(4) 0.401(4) 0.410(3) 0.037(7) H4 0.102(4) 0.191(4) 0.418(3) 0.037(7)

The anisotropic displacements associated with the single O Jahn–Teller distortion. We emphasise that for holotype pa- atom of this R3m substructure appear reasonably elongated ratacamite, the superstructure cannot be refined satisfactorily and are discussed further below. The disappearance of su- in space group R3m, but refines very well in R3. perlattice reflections between 353 and 393 K, together with In all paratacamite-related phases with R3m subcells, clear discontinuities in cell parameters and their smooth such as gillardite and herbertsmithite, the minor substituent nonlinear variations from 100 to 353 K, is consistent with a cation occurs within the interlayer site. This ordering structural phase transformation. The reappearance of su- scheme was determined from considerations of bond perlattice reflections on cooling from 443 to 300 K demon- lengths and site geometry, knowing that the M(1) interlayer strates that the transformation is reversible. The lattice site coordinated with OH- only lacks the Jahn–Teller parameters and structure of the high-temperature phase distortion of the M(2)O6 octahedron, which is a charac- correspond closely to those of the related mineral herbert- teristic of a site with a high Cu2? content. In the R3 su- smithite (R3m), a 6.834(1) and c 14.075(2) A˚ (Braithwaite percell of holotype paratacamite, Zn may be located in one et al. 2004). The structures determined at 300 and 393 K are or both interlayer sites, M(1) and M(2). shown in Fig. 9. The transformation of paratacamite to The X-ray scattering factors of Cu (Z = 29) and Zn herbertsmithite between 353 and 393 K involves a fourfold (Z = 30) are too similar to allow refinement of M(1) and reduction in unit-cell volume and a corresponding change in M(2) site occupancies in holotype paratacamite. Its empirical point group from C3i (3) to D3d (3m). To achieve the trans- composition is very close to Cu3.75Zn0.25(OH)6Cl2, which formation, the most pronounced atomic displacements occur could correspond to a structure in which all Zn is ordered at with the O positions (Table 3). From the geometrical chan- and fully occupies M(1) at 3b, leading to the observed uni- ges of M octahedra as a function of temperature (Figs. 5, 6, 7, form coordination geometry of this octahedron. In the R3

8), it is clear that the M(1)O6 octahedron is a rigid, almost superstructure, the (2 ? 2 ? 2) coordination of the M(2)O6 invariant, feature of the R3 paratacamite structure. The octahedron at 9d, which in the fully ordered structure would 2? M(2)O6 octahedron changes considerably on heating as it be filled by Cu , could be generated by long-range crystal- becomes progressively less distorted, leading to conver- structure constraints that distort the common (4 ? 2) Jahn– gence upon the M(1)O6 configuration at the transformation. Teller configuration. However, a (2 ? 2 ? 2) Jahn–Teller In contrast to the different behaviour of M(1)- and M(2)- distortion (sensu stricto) from Cu2? octahedra with OH- centred interlayer octahedra, the M(3)- and M(4)-centred ligands is unprecedented. intralayer octahedra experience comparable expansions. We have no direct evidence for the distribution of Zn

Structurally, it appears that the variable nature of the M(2)O6 between M(1) and M(2) in holotype paratacamite. However, octahedron is a key feature of paratacamite. our recent study of a natural Mg-substituted paratacamite of

formula Cu3(Mg0.60Cu0.38Ni0.01Mn0.01)(OH)6Cl2 and space The (2 ? 2 ? 2) Jahn–Teller distortion group R3 provides indirect evidence of statistical disorder of the Mg over M(1) and M(2), with 60 % Mg at both interlayer The M(2)O6 coordination environment of paratacamite dis- sites (Kampf et al. 2013). plays a tetragonally elongated and rhombically distorted The various possible Jahn–Teller configurations were octahedron, which may be described as the rare (2 ? 2 ? 2) investigated by Eby and Hawthorne (1993) and Burns and

123 Phys Chem Minerals

Table 4 Anisotropic displacement parameters of type paratacamite (BM 86958) at each temperature 100 K (R3) U11 U22 U33 U23 U13 U12

M(1) 0.00394(19) 0.00394(19) 0.0080(3) 0 0 0.00197(9) M(2) 0.00434(18) 0.00356(18) 0.0072(2) 0.00047(13) 0.00043(13) 0.00189(14) M(3) 0.00676(15) 0.00250(14) 0.00931(17) -0.00133(10) -0.00052(11) 0.00143(11) M(4) 0.00309(14) 0.00266(14) 0.01012(17) 0.00007(10) 0.00029(10) -0.0001(1) Cl1 0.0069(2) 0.0069(2) 0.0096(5) 0 0 0.00344(12) Cl2 0.0057(2) 0.0063(2) 0.0096(3) -0.00014(18) 0.00001(18) 0.00268(19) O1 0.0032(7) 0.0003(7) 0.0047(8) 0.0030(5) 0.0026(6) -0.0021(6) O2 0.0128(9) 0.0055(8) 0.0246(12) 0.0015(7) -0.0102(8) 0.0028(7) O3 0.0067(8) 0.0001(7) 0.0227(11) -0.0049(7) 0.0079(7) -0.0036(6) O4 0.0051(8) 0.0140(10) 0.0259(13) -0.0133(9) -0.0025(8) 0.0044(7) 200 K (R3) U11 U22 U33 U23 U13 U12

M(1) 0.00689(19) 0.00689(19) 0.0091(3) 0 0 0.00344(9) M(2) 0.00752(18) 0.00615(17) 0.0088(2) 0.00086(13) 0.00073(13) 0.00340(14) M(3) 0.00932(15) 0.00500(14) 0.01214(17) -0.00197(10) -0.00072(10) 0.00275(11) M(4) 0.00558(13) 0.00511(14) 0.01329(17) 0.00034(10) 0.00000(10) 0.00151(10) Cl1 0.0107(3) 0.0107(3) 0.0119(5) 0 0 0.00536(13) Cl2 0.0096(2) 0.0109(2) 0.0126(3) -0.00014(18) 0.00006(18) 0.00484(19) O1 0.0058(6) 0.0017(6) 0.0068(7) 0.0032(5) 0.0024(6) -0.0007(5) O2 0.0137(8) 0.0076(8) 0.0279(12) 0.0016(7) -0.0098(8) 0.0035(7) O3 0.0099(8) 0.0015(7) 0.0254(11) -0.0041(7) 0.0085(7) -0.0022(6) O4 0.0081(8) 0.0135(9) 0.0273(12) -0.0118(8) -0.0030(7) 0.0050(7)

Initial 300 K (R3) U11 U22 U33 U23 U13 U12

M(1) 0.00966(17) 0.00966(17) 0.0084(3) 0 0 0.00483(8) M(2) 0.01034(16) 0.00876(16) 0.00834(17) 0.00082(12) 0.00083(12) 0.00476(13) M(3) 0.01130(13) 0.00738(12) 0.01351(15) -0.00222(9) -0.00072(9)l 0.00398(10) M(4) 0.00873(12) 0.00815(12) 0.01415(15) 0.00087(9) -0.00009(9) 0.00377(10) Cl1 0.0146(2) 0.0146(2) 0.0132(4) 0 0 0.00730(12) Cl2 0.0135(2) 0.0151(2) 0.0128(2) -0.00010(17) -0.00005(17) 0.00695(19) O1 0.0086(6) 0.0051(6) 0.0102(7) 0.0011(5) 0.0007(5) 0.0009(5) O2 0.0156(8) 0.0108(7) 0.0263(10) 0.0021(6) -0.0089(7) 0.0054(6) O3 0.0137(7) 0.0057(6) 0.0278(10) -0.0009(6) 0.0111(7) 0.0012(6) O4 0.0112(7) 0.0161(8) 0.0266(10) -0.0112(7) -0.0028(7) 0.0064(6)

353 K (R3) U11 U22 U33 U23 U13 U12

M(1) 0.01281(16) 0.01281(16) 0.0091(3) 0 0 0.00641(8) M(2) 0.01321(15) 0.01205(15) 0.00883(16) 0.00051(10) 0.00056(10) 0.00633(12) M(3) 0.01356(12) 0.01054(12) 0.01542(14) -0.00200(8) -0.00066(8) 0.00552(9) M(4) 0.01229(12) 0.01185(12) 0.01564(14) 0.00109(8) -0.00048(8) 0.00632(9) Cl1 0.0182(2) 0.0182(2) 0.0143(4) 0 0 0.00911(11) Cl2 0.0175(2) 0.0186(2) 0.0144(2) -0.00037(15) -0.00026(15) 0.00888(18) O1 0.0138(6) 0.0103(6) 0.0175(7) -0.0029(5) -0.0014(5) 0.0045(5) O2 0.0173(7) 0.0144(7) 0.0277(9) 0.0042(6) -0.0075(6) 0.0073(6) O3 0.0186(7) 0.0117(6) 0.0310(9) 0.0032(6) 0.0140(6) 0.0061(6)

123 Phys Chem Minerals

Table 4 continued

353 K (R3) U11 U22 U33 U23 U13 U12

O4 0.0145(7) 0.0177(7) 0.0280(9) -0.0111(6) -0.0042(6) 0.0084(6)

393 K (R3m) U11 U22 U33 U23 U13 U12

M(1) 0.01347(15) 0.01347(15) 0.0082(2) 0 0 0.00674(7) M(2) 0.01373(12) 0.01172(14) 0.01536(14) 0.00196(10) 0.00098(5) 0.00586(7) Cl1 0.0195(2) 0.0195(2) 0.0138(3) 0 0 0.00976(10) O1 0.0167(4) 0.0167(4) 0.0262(6) -0.0054(3) 0.0054(3) 0.0078(5)

423 K (R3m) U11 U22 U33 U23 U13 U12

M(1) 0.01523(16) 0.01523(16) 0.0092(2) 0 0 0.00762(8) M(2) 0.01515(13) 0.01285(15) 0.01707(16) 0.00221(10) 0.00111(5) 0.00642(8) Cl1 0.0220(2) 0.0220(2) 0.0157(3) 0 0 0.01101(11) O1 0.0180(4) 0.0191(7) 0.0279(6) 0.0106(6) 0.0053(3) 0.0096(3)

Final 300 K (R3) U11 U22 U33 U23 U13 U12

M(1) 0.01017(18) 0.01017(18) 0.0083(3) 0 0 0.00508(9) M(2) 0.01064(17) 0.00897(17) 0.00840(18) 0.00077(13) 0.00078(13) 0.00480(14) M(3) 0.01164(13) 0.00791(13) 0.01361(15) -0.00210(10) -0.00072(10) 0.00423(10) M(4) 0.00912(13) 0.00850(13) 0.01432(15) 0.00085(10) -0.00021(10) 0.00403(10) Cl1 0.0153(3) 0.0153(3) 0.0133(4) 0 0 0.00763(13) Cl2 0.0138(2) 0.0153(2) 0.0129(2) -0.00022(18) -0.00045(18) 0.0071(2) O1 0.0093(7) 0.0046(6) 0.0117(7) 0.0010(5) 0.0014(6) 0.0008(6) O2 0.0161(8) 0.0114(8) 0.0268(11) 0.0026(7) -0.0088(7) 0.0059(7) O3 0.0141(8) 0.0064(7) 0.0301(11) -0.0014(7) 0.0115(8) 0.0016(6) O4 0.0118(8) 0.0167(9) 0.0253(11) -0.0116(8) -0.0033(7) 0.0071(7)

Hawthorne (1996) for a host of Cu(II)-bearing compounds. as a slightly compressed sphere with its maximum princi- The latter concluded that the occurrence of a (2 ? 2 ? 2) pal axis almost perpendicular to the M(2)–O1 bond axis. distortion may be explained by a dynamic Jahn–Teller Burns and Hawthorne (1996) suggested that a more or less effect as the octahedron continually shifts between two spherical thermal ellipsoid indicates a static bond. How- configurations of the (4 ? 2) Jahn–Teller geometry. As a ever, the M(2)–O1 bond length, which is the shortest of the consequence, the detection of a (2 ? 2 ? 2) octahedral coordination sphere, increases significantly with an coordination sphere using X-ray diffraction is a result of a increase in temperature. Heating to 353 K yielded similar time lapse average position of the atoms involved. Fig- anisotropic displacement values and orientation for all O ure 10 displays the M(2)O6 coordination environment in atoms. After the transformation to the R3m structure, the the structure of paratacamite before and after the structural maximum principal axis of the single crystallographic O transformation. The analogous interlayer octahedron, thermal ellipsoid is again subparallel to the M(1)–O bond M(1)O6, in the 393 K structure is also displayed, and all direction. atoms are pictured with anisotropic thermal ellipsoids. The For a statistical (random) distribution of nine Cu and maximum principal axis of the trans O2 and O3 ellipsoids three Zn between both interlayer sites in paratacamite, a is subparallel with the direction of the M(2)–O bond axis in superimposition of 25 % occupancy of Zn in M(2) at paratacamite. In contrast, the O1 thermal ellipsoid appears 9d (Zn–O *2.1 A˚ , as in herbertsmithite, Braithwaite

123 Phys Chem Minerals

Table 5 Selected bond lengths and angles of type paratacamite (BM86958) at each temperature Temperature 100 K 200 K Initial 300 K 353 K 393 K 423 K Final 300 K Space group R3 R3 R3 R3 R3mR3mR3

Interlayera M(1)–O4 2.106(3) 2.107(2) 2.106(2) 2.1037(19) 2.1055(16) 2.106(2) 2.106(2) O4–M(1)–O4 103.91(9) 104.10(9) 104.02(8) 103.91(7) 103.88(6) 103.77(7) 103.99(8) M(2)–O1 1.9803(17) 1.9832(16) 1.9963(15) 2.0224(15) 2.0014(16) M(2)–O2 2.104(3) 2.105(2) 2.103(2) 2.1045(19) 2.106(2) M(2)–O3 2.282(2) 2.276(2) 2.249(2) 2.2041(19) 2.247(2) O1–M(2)–O2 101.88(8) 101.99(8) 102.24(7) 102.70(6) 102.27(7) O1–M(2)–O3 104.36(8) 104.20(7) 104.15(7) 103.95(6) 104.05(7) O2–M(2)–O3 106.03(8) 105.91(7) 105.59(7) 105.08(6) 105.58(7) Intralayer M(3)[2]–O3[1] 1.955(2) 1.9573(19) 1.9570(17) 1.9633(15) 1.9840(7) 1.9860(9) 1.9601(18) M(3)–O2 1.973(2) 1.974(2) 1.9792(18) 1.9792(15) 1.9768(19) M(3)–O2 1.976(2) 1.976(2) 1.9793(17) 1.9810(15) 1.9788(19) M(3)–O1 1.9930(19) 1.9940(18) 1.9962(16) 1.9952(15) 1.9966(17) M(3)[2]–Cl2[1] 2.7347(7) 2.7412(7) 2.7499(6) 2.7585(6) 2.7820(5) 2.7817(6) 2.7502(7) M(3)–Cl2 2.8177(7) 2.8124(7) 2.8065(6) 2.7962(6) 2.8066(7) O3(1)–M(3)[2]–O2[1] 177.10(10) 177.29(9) 177.72(8) 178.44(7) 180.00(8) 180.00(10) 177.78(9) O2–M(3)–O1 178.72(9) 178.76(8) 178.80(7) 178.95(7) 178.89(8) O2–M(3)–O2 97.93(15) 97.99(14) 98.10(12) 98.14(10) 98.08(13) O2[1]–M(3)[2]–O1[1] 80.92(9) 80.86(9) 80.73(8) 80.85(7) 81.73(9) 81.79(12) 80.87(8) O3[1]–M(3)[2]–O1[1] 96.57(8) 96.64(8) 97.17(7) 97.66(7) 98.27(9) 98.21(12) 97.03(8) Cl2[1]–M(3)[2]–Cl2[1] 176.781(19) 176.923(19) 177.449(17) 178.261(15) 180.00(2) 180.00(3) 177.475(19) Cl2[1]–M(3)[2]–O2[1] 96.16(7) 96.17(7) 96.29(6) 96.70(5) 97.50(3) 97.41(4) 96.40(6) Cl2[1]–M(3)[2]–O1[1] 84.28(6) 84.18(5) 83.86(5) 83.42(4) 82.50(3) 82.59(4) 83.85(5) Cl2–M(3)–O3 97.70(7) 97.51(6) 97.43(6) 97.36(5) 97.39(6) M(4)–O3 1.940(2) 1.943(2) 1.953(2) 1.966(1) 1.955(2) M(4)–O4 1.970(2) 1.967(2) 1.971(2) 1.976(2) 1.972(2) M(4)–O4 1.981(2) 1.981(2) 1.9839(18) 1.9836(16) 1.9835(19) M(4)–O1 1.9975(19) 2.0012(18) 1.9981(16) 1.9943(15) 2.0000(17) M(4)–Cl1 2.7735(9) 2.7760(8) 2.7767(8) 2.7774(7) 2.7786(8) M(4)–Cl2 2.7780(7) 2.7777(7) 2.7790(6) 2.7779(6) 2.7790(7) O3–M(4)–O4 175.86(10) 176.23(9) 176.67(8) 177.72(7) 176.83(9) O3–M(4)–O4 97.05(10) 97.15(10) 97.47(9) 97.74(8) 97.43(9) O3–M(4)–O1 83.61(9) 83.49(8) 82.94(8) 82.35(7) 83.03(8) O4–M(4)–O1 176.84(8) 176.71(8) 177.24(7) 178.01(6) 177.24(8) O4–M(4)–O1 97.45(9) 97.51(9) 97.85(8) 98.16(7) 97.75(8) Cl1–M(4)–Cl2 176.130(17) 176.261(16) 176.843(15) 177.807(13) 176.881(16) Cl1–M(4)–O1 94.52(6) 94.53(5) 94.96(5) 95.64(4) 94.95(5) Cl1–M(4)–O3 101.42(6) 101.16(6) 100.67(5) 99.66(5) 100.52(6) Cl1–M(4)–O4 82.51(7) 82.41(7) 82.50(6) 82.51(5) 82.49(6) Cl1–M(4)–O4 82.32(7) 82.18(6) 82.28(6) 82.38(5) 82.29(6) Other O2[1]–Cl1[1] 3.065(2) 3.068(2) 3.0688(19) 3.0728(17) 3.0785(14) 3.0774(18) 3.072(2) O1–Cl2 3.0574(19) 3.0584(18) 3.0659(17) 3.0714(15) 3.0643(18) O3–Cl2 3.080(2) 3.077(2) 3.0800(18) 3.0765(16) 3.0784(19) O4–Cl2 3.064(2) 3.071(2) 3.0726(19) 3.0745(17) 3.074(2) a Atom labels in curved brackets correspond to R3 structures, while atom labels in square brackets correspond to R3m structures

123 Phys Chem Minerals

13.69 13.68 13.67 13.66

(Å) 13.65 a 13.64 13.63 13.62 14.10

14.08

14.06 ) Å (

c 14.04

14.02

14.00

2280

) 2270 3 (Å V 2260 Fig. 5 Polyhedral environments of M(1) and M(2) interlayer octahe- dra of R3 paratacamite. Cl atoms shown as green spheres. O atoms 2250 100 200 300 400 500 are omitted for clarity. M(1)–M(3), M(2)–M(3) and M(2)–M(4) shared edges are O–O T (K)

Fig. 4 open Variation in unit-cell parameters with temperature. The The anisotropic O2 and O3 displacement ellipsoids and circle is the cell parameter upon return to 300 K. The initial and return to 300 K data points of the c-axis overlap. The unit-cell thevariablenatureofM(2)–O1 and –O3 bond lengths a parameter of the 393 and 443 K structures is doubled for with changes in temperature suggest that this octahedron comparison is dynamically distorted. A similar interpretation can be made concerning the O4 et al. 2004) and 75 % Cu (Jahn–Teller distorted as in atom, which coordinates the non-tetragonally distorted clinoatacamite with Cu–O *2.29, 2.05 and 1.99 A˚ ; M(1)O6 octahedron of the R3 structure, and the O1 atom of Grice et al. 1996) gives the average detectible bond the R3m structure, both of which show a degree of anisotropy lengths of 2.24, 2.06 and 2.02 A˚ , which are similar to with the maximum principal axis of the ellipsoid directed those of the initial and final 300 K structures (Table 5). subparallel to the bond. By the same reasoning, with 25 % The relatively spherical anisotropic O1 ellipsoid may occupancy of Zn in M(1) at 3b, accompanied by three dif- indicate that two different orientations of Jahn–Teller ferent orientations of the common (4 ? 2) Jahn–Teller dis- distorted Cu(OH)6 octahedra occur in this site. The O1 tortion (25 % occupancy of the three orientations), the atoms would become the pivotal short Jahn–Teller bond average detectible M(1)–O bond length would be ca 2.1 A˚ , of both orientations, while the other two directions using the same values as before. This compares well with the alternate between the long and short bond distances. The 300 K data in Table 5. The rigid nature of this octahedron elongated anisotropic thermal ellipsoids of the O atoms with changes in temperature might indicate that the proposed involved would therefore be the result of either a three orientations of the Jahn–Teller effect are static, rather dynamic (4 ? 2) Jahn–Teller distortion of Cu(OH)6 than dynamic. If the M(1)O6 octahedron is dynamically octahedra or from a distribution of two orientations of distorted, then the three symmetry-related orientations of the static (4 ? 2) Jahn–Teller distorted Cu(OH)6 octahedra. Jahn–Teller effect would be energetically equivalent.

123 Phys Chem Minerals

2.30 2.00

M(2)-O(3) 1.99 O(1) 2.20

1.98 M(1)-O(4) O(22) 2.10 M(2)-O(2) -O (Å)

(3)-O (Å) (3)-O 1.97 O(2) M M

2.00 1.96 O(3) M(2)-O(1) M(1), M(2) M(3)

1.90 1.95 100 200 300 400 500 100 200 300 400 500 T (K) T (K)

2.01 O(1) 2.82 M(3)-Cl(2) 2.00

1.99 2.80 O(4) 1.98 M 2.78 (4)-Cl(2) 1.97 -Cl (Å) (4)-O (Å) (4)-O 1.96 O(42) M M(4)-Cl(1)

M 2.76

1.95 O(3) 2.74 1.94 M(4) M-Cl M(3)-Cl(2) 1.93 2.72 100 200 300 400 500 100 200 300 400 500 T (K) T (K)

Fig. 6 Variations in M–O and M–Cl bond lengths of paratacamite on convergence shown at 393 K relates to a transition between 353 and heating from 100 to 443 K. Error bars on bond lengths are shown 393 K. Variation in the M(1)–O(4) bond length is shown as a dashed when these are larger than data symbols. Lines connecting data points line are intended as guides to the eye only, and it should be noted that the

Origin of the phase transformation in type paratacamite The driving force behind the phase transformation is a

temperature-induced reduction in the M(2)O6 octahedral The change in cell parameters and polyhedral behaviour rhombic distortion as it converges upon the configuration with heating is consistent with a steady convergence upon a of the temperature-invariant M(1)O6 coordination sphere. new structure above 353 K. The loss of the paratacamite The responses of intralayer M(3)- and M(4)-centred octa- superstructure above this temperature may indicate that the hedra are primarily determined by deformation of the superstructure reflections are a direct result of atomic dis- M(2)O6 octahedron, with which they share edges. Assum- placements from the R3m aristotype structure, particularly ing a statistical distribution of Zn between these sites as concerning O atoms of the M(2)O6 octahedron. The fact well as the proposed model of superimposed dynamically that we could not refine the superstructure in R3m but Jahn–Teller distorted octahedra with non-tetragonally dis- succeeded easily in R3 indicates that the superlattice torted octahedra, it may be inferred that there is an energy reflections, although much weaker than the sublattice activation barrier associated with the generation of three reflections, contain enough information associated with the configurations of equally occupied (4 ? 2) Jahn–Teller

(2 ? 2 ? 2) distorted M(2)O6 octahedron to enable the distorted octahedra, such as the configuration observed at correct structure to be identified. M(1).

123 Phys Chem Minerals

11.80 220

) M(2) 2 11.70 M(2) 215 ) 3 M(1)

(Å 11.60 210 V BAV (degrees 11.50 205 M(1)

11.40 ) 66 2 M(4)

65 14.20 M(3) BAV (degrees ) 3 64

(Å M(4)

V 14.10 M(2) 1.060 M(3)

14.00 QE 100 200 300 400 500 T(K)

1.050 Fig. 7 Volume changes of M(1), M(2), M(3) and M(4) octahedra with 100 200 300 400 500 temperature T (K)

Fig. 8 Bond angle variance (BAV) of M(1), M(2), M(3) and M(4) octahedra and quadratic elongation (QE) of M(2) with temperature The reversible nature of this phase transformation estab- lishes that paratacamite is a thermodynamically stable phase below 353 K, for the composition Cu3.71Zn0.29(OH)6Cl2. Finally, the high-temperature transformation is in agreement transformation. The presence of significant amounts of non- with the proposed series P1? R3 ? R3m. Although there tetragonally distorted Zn(OH)6 octahedra may hinder a was no direct evidence of a triclinic distortion at low tem- transformation to P1. It must also be emphasised that the peratures, the observed NPD displacement behaviour of O1 state of Cu–Zn ordering over M(1) and M(2) sites in type and O3 atoms at 100 K may indicate the onset of a structural paratacamite remains to be determined.

123 Phys Chem Minerals

Fig. 9 Polyhedral structure representation for the initial 300 K R3( left) and 393 K R3m (right) structures. M(3) and M(4) octahedra are blue, M(1) octahedra are orange, M(2) octahedra are yellow, Cl atoms are green, O atoms are red, and H atoms are white. The unit cell is outlined. Both structures are viewed down the a-axis

Initial R3 353 KR3 393 K R3m Final R3 300 K 300 K O2 O2 O2

O1 O1 O1 O1 O3 O3 O3

Fig. 10 The M(2) octahedral coordination environment of parataca- in the direction of atomic displacement (trans atoms) with respect to mite during the heating and cooling cycle. Non-H atoms are displayed the previous temperature with anisotropic thermal ellipsoids (probability 85 %). Arrows point

Supplemental materials Burns PC, Hawthorne FC (1996) Static and dynamic Jahn-Teller effects in Cu2? oxysalt minerals. Can Mineral 34:1089–1105 Chu S, McQueen TM, Chisnell R, Freedman DE, Mu¨ller P, Lee YS, Lists of structure factors are deposited with the journal. Nocera DG (2010) A Cu2? (S = 1/2) kagome´ antiferromagnet: Crystallographic information files (CIF’s) for all seven MgxCu4-x(OH)6Cl2. J Am Chem Soc 132:5570–5571 structure refinements are deposited and can be obtained Clissold ME, Leverett P, Williams PA (2007) The structure of from http://www.fiz-karlsruhe.de/request_for_deposited_ gillardite, the Ni-analogue of herbertsmithite, from Widgiemool- tha, Western Australia. Can Mineral 45:317–320 data.html, quoting the CSD number 426448. Colman RH, Ritter C, Wills AS (2008) Towards perfection: Kapellasite, Cu3Zn(OH)6Cl2, a new model S = 1/2 kagome´ Acknowledgments The authors would like to thank The Natural antiferromagnet. Chem Mater 20:6897–6899 History Museum (London) for loan of the type specimen of parat- Colman RH, Sinclair A, Wills AS (2010) Comparisons between acamite BM86958. We also thank the reviewers Joel Grice and Uwe haydeeite, a-Cu3Mg(OD)6Cl2, and kapellasite, a-Cu3Z- Kolitsch for their helpful comments on this manuscript. n(OD)6Cl2, isostructural S = 1/2 kagome´ magnets. Chem Mater 22:5774–5779 de Vries MA, Harrison A (2007) Physical chemistry: model’s reputation restored. Nature 408:908–909 References Downs RT, Hall-Wallace M (2003) The American mineralogist crystal structure database. Am Mineral 88:247–250 Agilent (2012) CrysAlis PRO and CrysAlis RED. Agilent Technol- Eby RK, Hawthorne FC (1993) Structural relationships in copper ogies, Yarnton oxysalt minerals. I. Structural hierarchy. Acta Crystallogr Braithwaite RSW, Mereiter K, Paar WH, Clark AM (2004) Herbert- B49:28–56 smithite, Cu3Zn(OH)6Cl2, a new species, and the definition of Fleet ME (1975) The crystal structure of paratacamite, Cu2(OH)3Cl. paratacamite. Mineral Mag 68:527–539 Acta Crystallogr B31:183–187

123 Phys Chem Minerals

Freedman DE, Han TH, Prodi A, Mu¨ller P, Huang Q-Z, Chen Y-S, Kolitsch U, Giester G (2000) The crystal structure of faustite and its Webb SM, Lee YS, McQueen TM, Nocera DG (2010) Site copper analogue turquoise. Mineral Mag 64:905–913 specific X-ray anomalous dispersion of the geometrically Krause W, Bernhardt H-J, Braithwaite RSJ, Kolitsch U, Pritchard R frustrated kagome´ magnet, herbertsmithite, ZnCu3(OH)6Cl2. (2006) Kapellasite, Cu3Zn(OH)6Cl2, a new mineral from Lau- J Am Chem Soc 132:16185–16190 rion, Greece, and its crystal structure. Mineral Mag 70:329–340 Grice JD, Szyman´ski JT, Jambor JL (1996) The crystal structure of Li YS, Zhang QM (2013) Structure and magnetism of S = 1/2 clinoatacamite, a new polymorph of Cu2(OH)3Cl. Can Mineral kagome antiferromagnets NiCu3(OH)6Cl2 and CoCu3(OH)6Cl2. 34:73–78 J Phys: Condens Matter 25:026003 Han TH, Helton JS, Chu S, Prodi A, Singh DK, Mazzoli C, Mu¨ller P, Malcherek T, Schlu¨ter J (2007) Cu3MgCl2(OH)6, and the bond- Nocera DG, Lee YS (2011) Synthesis and characterisation of valence parameters of the OH–Cl bond. Acta Crystallogr single crystals of the spin-1/2 kagome-lattice antiferromagnets B63:157–160 ZnxCu4-x(OH)6Cl2. Phys Rev B 83:100402 Malcherek T, Schlu¨ter J (2009) Structures of the pseudo-trigonal Han TH, Helton JS, Chu S, Nocera DG, Rodriguez-Rivera JA, polymorphs of Cu2(OH)3Cl. Acta Crystallogr B65:334–341 Broholm C, Lee YS (2012) Fractionalized excitations in the Malcherek T, Schlu¨ter J (2010) Anatacamite from La Vendida mine, spin-liquid state of a kagome´-lattice antiferromagnet. Nature Sierra Gorda, Atacama desert, Chile: a triclinic polymorph of 492:406–410 Cu2(OH)3Cl. Neuse Jahrb Miner Abh 187:307–312 Hathaway BJ (1987) Copper. In: Wilkinson G, McCleverty JA, McQueen TM, Han TH, Freedman DE, Stephens PW, Lee YS, Gillard RG (eds) Comprehensive coordination chemistry, vol 5. Nocera DG (2011) CdCu3(OH)6Cl2: a new layered hydroxide Pergamon Press, Oxford, pp 534–774 chloride. J Solid State Chem 184:3319–3323 Helton JS, Matan K, Shores MP, Nytko EA, Bartlett BM, Yoshida Y, Pring A, Snow MR, Tiekink ERT (1987) Paratacamite from South Takano Y, Qiu Y, Chung J-H, Nocera DG, Lee YS (2007) Spin Australia. Trans R Soc S Aust 3:127–128 dynamics of the spin-1/2 kagome lattice antiferromagnet Robinson K, Gibbs GV, Ribbe PH (1971) Quadratic elongation: a ZnCu3(OH)6Cl2. Phys Rev Lett 98:107204–107208 quantitative measure of distortion in coordination polyhedral. International Tables for Crystallography, Volume C (1992) Mathe- Science 172:567–570 matical, physical and chemical tables, 1st edn. Edited by A.J.C. Schlu¨ter J, Malcherek T (2007) Haydeeite, Cu3MgCl2(OH)6, a new Wilson. Kluwer Academic Publishers, Dordrecht mineral from the Haydee mine, Salar Grande, Atacama desert. Jambor JL, Dutrizac JE, Roberts AC, Grice JD, Szyman´ski JT (1996) Chile Neuse Jahrb Miner Abh 184:39–42 Clinoatacamite, a new polymorph of Cu2(OH)3Cl, and its Schores MP, Nytko EA, Bartlett BM, Nocera DG (2005) Structurally relationship to paratacamite and ‘‘anarakite’’. Can Mineral perfect S = 1/2 kagome´ antiferromagnet. J Am Chem Soc 34:61–72 127:13462–13463 Kampf AR, Sciberras MJ, Leverett P, Williams PA, Malcherek T, Sheldrick GM (2008) A short history of SHELX. Acta Crystallogr Schlu¨ter., Welch MD, Dini M (2013) Paratacamite-(Mg), A64:112–122 Cu3(Mg,Cu)Cl2(OH)6; a new substituted basic copper chloride Smith GFH (1906) Paratacamite, a new oxychloride of copper. mineral from Camerones, Chile. Mineral Mag (Submitted) Mineral Mag 14:170–177

123

View publication stats