Paratacamite-(Mg), Cu₃(Mg,Cu)Cl₂(OH)₆; a new substituted basic copper chloride mineral from Camerones, Chile

Abstract

Paratacamite-(Mg) (IMA 2013-014), Cu 3 (Mg,Cu)Cl 2 (OH) 6 , is the new Mg-analogue of paratacamite. It was found near the village of Cuya in the Camarones Valley, Arica Province, Chile. The mineral is a supergene secondary phase occurring in association with anhydrite, atacamite, chalcopyrite, copiapite, dolomite, epsomite, haydeeite, hematite, magnesite and quartz. Paratacamite-(Mg) crystals are rhombs and thick to thin prisms up to 0.3 mm in size exhibiting the forms {201} and {001}. Twinning by reflection on {101} is common. The mineral is transparent with a vitreous lustre, with medium to deep-green colour and light-green streak. Mohs hardness is 3À3Ý, the tenacity is brittle and the fracture is conchoidal. Paratacamite-(Mg) has one perfect cleavage on {201}. The measured and calculated densities are 3.50(2) and 3.551 g cm À3 , respectively. The mineral is optically uniaxial (À) with e = 1.785(5) and o > 1.8 and slight pleochroism: O (bluish green) > E (green). Electron-microprobe analyses provided the empirical formula Cu 3 (Mg 0.60 Cu 0.38 Ni 0.01 Mn 0.01 )Cl 2 (OH) 6 . The mineral is easily soluble in dilute HCl. Paratacamite-(Mg) is trigonal, R3, with cell parameters a = 13.689(1), c = 14.025(1) Å , V = 2275.8(3) Å 3 and Z = 12. There is a pronounced sub-cell corresponding to a' & Ýa, c' & c in space group R3m. The eight strongest lines in the X-ray powder diffraction pattern are [d obs Å (I)(hklThe structure was refined to R 1 = 0.039 for 480 F o > 4sF reflections. Refinement using interlayer Mg-Cu site scattering factors indicated that Mg is distributed statistically between both interlayer octahedra M1O 6 and M2O 6 . A comparison of the distortions associated with M1O 6 and M2O 6 octahedra suggest that the sample is near the upper compositional limit for stability of the R3 phase.

Structural complexity and crystallization: the Ostwald sequence of phases in the Cu2(OH)3Cl system (botallackite–atacamite–clinoatacamite)

“…4. The condensation of chains by sharing common Cu corners leaves one of the Cu atoms not bonded to Cl, and this is the position that is substituted by Zn, Mg and Ni in other atacamite group minerals [44][45][46][47][48][49].…”

Structural and compositional variations of basic Cu(II) chlorides in the herbertsmithite and gillardite structure field

“…Specimens of the basic Cu(II) chlorides were obtained from the Mineralogical Museum, Hamburg, Germany, and from several private collections for compositional and crystallographic analysis. The authors analysed samples of paratacamite from the British Museum, London, UK (specimen BM86958), paratacamite-(Mg) from the Natural History Museum of Los Angeles County, USA (specimen 64041), and paratacamite-(Ni) from the Western Australian Museum, Western Australia, Australia (specimen WAM M365.2003), in this study, but full data of the analyses appear in the separate publications Welch et al, (2014), Kampf et al (2013a), and Sciberras et al (2013), respectively. Additional analyses of these samples are included in this paper.…”

“…Paratacamite, Cu 3 (Cu,Zn)(OH) 6 Cl 2 , trigonal, space group R3 (Smith 1906;Frondel 1950;Fleet 1975;Welch et al, 2014), is a member of the substituted basic Cu(II) chloride group of minerals. Two newly described paratacamite congeners, paratacamite-(Ni), Cu 3 (Ni,Cu)(OH) 6 Cl 2 (Sciberras et al, 2013) and paratacamite-(Mg), Cu 3 (Mg,Cu)(OH) 6 Cl 2 (Kampf et al, 2013a), are characterised by extensive substitution for Cu in the interlayer sites. Jambor et al (1996) reported that clinoatacamite, Cu (OH) 3 Cl, monoclinic, space group P2 1 /n, structurally transforms to a trigonal phase, assumed to be paratacamite, when 2-3 wt% Zn or Ni occupies its structure.…”

“…The associated solid-solution series is apparently continuous and extends to the minerals herbertsmithite, Cu 3 Zn(OH) 6 Cl 2 (Braithwaite et al, 2004), gillardite, Cu 3 Ni(OH) 6 Cl 2 (Colchester et al, 2007;Clissold et al, 2007), leverettite, Cu 3 Co(OH) 6 Cl 2 (Kampf et al, 2013b) and tondiite, Cu 3 Mg(OH) 6 Cl 2 (Malcherek et al, 2014) (isostructural, trigonal, space group R3 m), depending upon the nature of the dominant substituting cation. This R3 m structure corresponds to a pronounced substructure inherent in paratacamite (Fleet, 1975;Kampf et al, 2013a;Sciberras et al, 2013;Welch et al, 2014) and may be considered as the aristotype model for the group of basic Cu(II) chlorides (Malcherek and Schlüter, 2009). This group has received much attention in recent years due to their structure induced magnetic properties, as they are so-called "frustrated antiferromagnets" (Schores et al, 2005;Helton et al, 2007;Freedman et al, 2010;Chu et al, 2010;Han et al, 2011Han et al, , 2012Li and Zhang, 2013).…”

“…The boundary between the R3 and R3 m phases is difficult to quantify due to the very similar powder X-ray diffraction patterns of the minerals (Jambor et al, 1996;Braithwaite et al, 2004;Kampf et al, 2013a;Sciberras et al, 2013). The superstructure reflections of paratacamite may only be quantifiable using single-crystal diffraction methods (Kampf et al, 2013a;Sciberras et al, 2013;Welch et al, 2014). Braithwaite et al (2004) suggested an upper compositional limit for the stability of paratacamite of ca 50% interlayer occupancy of Zn, which implies a destabilisation of the herbertsmithite structure below this threshold.…”