Claraite, (Cu,Zn)15(AsO4)2(CO3)4(SO4)(OH)14·7H2O: redefinition and crystal structure

Abstract

Since the beginning of the 2000s, several authors reported the occurrence of As and S as chemical components of the hydrated copper hydroxy-carbonate claraite, originally described with the formula (Cu,Zn) 3 (CO 3 )(OH) 4 ·4H 2 O. Owing to the lack of knowledge about the crystal structure of this mineral, the structural role played by these chemical elements was unknown. The crystal structure of claraite has now been solved from single-crystal X-ray diffraction data by using a specimen from the marble quarries of Carrara, Apuan Alps, Tuscany, Italy. Electron-microprobe analyses gave (in wt% – average of eight spot analyses): SO 3 4.00, As 2 O 5 13.16, CuO 52.64, ZnO 9.03, CO 2(calc) 9.08, H 2 O (calc) 12.56, total 100.47. On the basis of 15 (Cu + Zn) and 45 O atoms per formula unit, the chemical formula of claraite could be written as (Cu 12.85 Zn 2.15 ) Σ15.00 (AsO 4 ) 2.22 (CO 3 ) 4 (SO 4 ) 0.97 (OH) 13.40 ·6.83H 2 O, ideally (Cu,Zn) 15 (AsO 4 ) 2 (CO 3 ) 4 (SO 4 )(OH) 14 ·7H 2 O. Raman spectrometry shows bands related to bending and stretching vibrations of AsO 4 and SO 4 groups, as well as the stretching mode of CO 3 groups and O–H bonds. The unit-cell parameters of claraite are a = 10.3343(6) A, b = 12.8212(7) A, c = 14.7889(9) A, α = 113.196(4)°, β = 90.811(4)°, γ = 89.818(4)°, V = 1800.9(2) A 3 , space group P 1 ¯ . The crystal structure has been refined to R 1 = 0.111 on the basis of 6956 reflections with F o > 4σ( F o ) and 363 refined parameters. Claraite shows a layered structure, with {0 0 1} heteropolyhedral layers formed by Cuϕ 5 and Cuϕ 6 polyhedra as well as AsO 4 and CO 3 groups. These layers are stacked along c through edge-sharing Cu 2 ϕ 10 and Cu 2 ϕ 8 dimers, the former being decorated by corner-sharing SO 4 groups hosted within intra-framework channels together with H 2 O groups. Claraite is the only known mineral showing the simultaneous occurrence of essential AsO 4 , CO 3 , and SO 4 groups.