When mineralogy meets the organic chemistry: delchiaroite, Cu ₃ I(CH ₃ S) ₂ , the first copper iodide–methanethiolate from the Carrara marble quarries, Apuan Alps, Tuscany, Italy

Abstract. The new mineral delchiaroite, Cu3I(CH3S)2, has been discovered in La Piana quarry, Colonnata marble basin, Carrara, Apuan Alps, Tuscany, Italy. It occurs as light-yellow acicular crystals, elongated on [010], up to 0.1 mm in length and is brittle, with a light-yellow streak and a greasy luster. In reflected light, delchiaroite is light gray with abundant internal yellow reflections; anisotropism is moderate, with gray tints. Reflectance values for the four COM wavelengths are (λ (nm) Rmin/Rmax (%)) 470: 20.6/22.2; 546: 20.9/23.4; 589: 20.7/23.3; and 650: 20.1/22.4. Electron microprobe analysis resulted in the following (average of 12 spot analyses – in wt %): Cu 47.27, I 31.09, S 14.90, Ccalc 5.58, Hcalc 1.40, and total 100.24. The empirical formula of delchiaroite, calculated on the basis of Cu + I + S = 6 atoms per formula unit, is Cu3.07I1.01(CH3S)1.92, in accordance with the end-member formula Cu3I(CH3S)2. Delchiaroite is orthorhombic, space group Pmmn, with a=16.924(10), b=4.099(2), c=5.572(3) Å, V=386.5(4) Å3, and Z=2. The crystal structure was refined to R1=0.0962 for 360 unique reflections, with Fo>4σF and 27 refined parameters. It can be described as being formed by electroneutral [Cu3I(CH3S)2]0 {100} layers stacked along a. The origin of delchiaroite is related to the supergene alteration of enargite in vugs of Liassic marbles. Chemically, this mineral belongs to the pseudo-binary system CuI–Cu(CH3S) at a 1:2 ratio. It constitutes the first example of natural iodide–methanethiolate, illustrating the interaction between inorganic and organic chemistry. Its name honors the mineral collector Lorenzo Del Chiaro (b. 1951) for his contribution to the knowledge of the mineralogy of the vugs of the Marble Formation belonging to the Alpi Apuane metamorphic complex.