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A single crystal X-ray study of a sulphate-bearing buttgenbachite, Cu₃₆Cl_7.8(NO₃)_1.3(SO₄)_0.35(OH)_62.2.5.2H₂O, and a re-examination of the crystal chemistry of the buttgenbachite–connellite series

¹ School of Chemistry, University of Sydney, NSW 2006, Australia
² Minerals and Materials Group, School of Science, Food and Horticulture, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 1797, Australia

^* E-mail: p.leverett{at}uws.edu.au

The single-crystal X-ray structure of a sulphate-bearing buttgenbachite, Cu₃₆Cl_7.8(NO₃)_1.3 (SO₄)_0.35(OH)_62.2.5.2H₂O, from Likasi, Democratic Republic of Congo, has been determined at 100 and 288 K. The basic framework of the structure is the same as has been previously reported for buttgenbachite, except for the identification of a hydrogen-bonded chloride ion (occupancy 0.6) at the origin instead of a Cu ion with partial occupancy. The nature of nitrate positional disorder along channels in the c direction and how this relates to the presence of other species such as chloride ions and water molecules and, most importantly, sulphate ions has been elucidated. One nitrate ion, with an occupancy of 0.18, lies at 2/3,1/3,1/4 and shares the site with a chloride ion (occupancy 0.30) and also a sulphate ion (occupancy 0.09); a second nitrate, with an occupancy of 0.24, lies at 2/3,1/3,0.084 and shares the site with a water molecule (occupancy 0.06). As a result, a formula of Cu₃₆Cl_7.8(NO₃)_1.3 (SO₄)_0.35(OH)_62.2.5.2H₂O is obtained. Re-refinement of deposited data for a supposed connellite crystal from the Toughnut mine,Tombstone, Arizona gives a related, but different, pattern of anion substitution. No sulphate could be detected in the structure and it is evident that this structure refers to buttgenbachite. A nitrate nitrogen atom and a chloride ion are disordered at 2/3,1/3,1/4, the overall site being fully occupied. A chloride ion with ~0.5 occupancy is sited at the origin and the formula Cu₃₆Cl_7.9(NO₃)_1.1(OH)₆₃.4H₂O is indicated. Re-refinement of a deposited data set for another buttgenbachite crystal from the Likasi mine reveals a partially occupied nitrate centred at 2/3,1/3, z and a partially occupied chloride at 2/3,1/3,1/4. Either 0.5Cl^–, OH^–, H₂O or H₃O⁺ is located at the origin. If the latter is the case, the stoichiometry for this buttgenbachite is Cu₃₆Cl_6.5(NO₃)_1.5(OH)₆₄.5.5H₂O. The present study has highlighted the fact that a range of compositions for buttgenbachite exists, depending on the pH and relative activities of chloride, nitrate and sulphate ions in solutions from which the mineral crystallizes.

KEYWORDS: buttgenbachite, connellite, solid solution, substitution, X-ray structure