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Tondiite, Cu 3 Mg(OH) 6 Cl 2 , the Mg-analogue of herbertsmithite

2014; Cambridge University Press; Volume: 78; Issue: 3 Linguagem: Inglês

10.1180/minmag.2014.078.3.08

1471-8022

Thomas Malcherek, Luca Bindi, Maurizio Dini, Maria Rosaria Ghiara, Arturo Molina A. Donoso, Fabrizio Nestola, Manuela Rossi, Jochen Schlüter,

X-ray Diffraction in Crystallography

Abstract Tondiite, with the simplified formula Cu 3 Mg(OH) 6 Cl 2 , occurs as a rare supergene mineral in a phonolitic tephrite from the type locality, Vesuvius volcano, Italy, as well as associated with haydeeite in the Santo Domingo Mine, Arica Province, Chile. It is emerald green to bright green in colour and occurs in irregularly shaped crystals, often with stepped faces. Its calculated density is 3.503 g cm −3 . Tondiite crystallizes with the herbertsmithite structure type, space group R m . Lattice parameters are a = 6.8377(7) Å and c = 14.088(2)Å for the holotype material. The c parameter may vary with Mg/Cu ratio and the presence of impurity atoms. The five strongest lines in the calculated powder diffraction pattern are [ d in Å( I )( hkil )]: 5.459(88)(10 1), 3.419(22)(11 0), 2.764(100)(112 3), 2.266(54)(02 4), 1.706(26)(22 0). Several tondiite crystals have been examined by single-crystal X-ray diffraction and by electron microprobe analysis. The observed Mg content ranges between 0.6 and 0.7 atoms per formula unit. The structural role of Mg is discussed.