New insights into the crystal chemistry of agardite-(Ce): refinement of the crystal structure, hydrogen bonding, and epitaxial intergrowths with the Sb-analogue of auriacusite

Agardite-(Ce) from Clara Mine, Schwarzwald, Germany, has been investigated by means of electron microprobe analysis, single-crystal X-ray analysis, XANES spectroscopy and IR spectroscopy. Hexagonal unit-cell parameters are: a = 13.598(6), c = 5.954(3) Å; V = 953.5(2) Å 3 ; space group P 6 3 / m . The structure has been solved and refined to final R 1 = 3.87%, w R 2 = 5.02 for 786 I > 3 σ ( I ). Hydrogen atoms have been localized. The crystal-chemical formula is ( Z = 2): A (1) (Ce 0.82 Ca 0.14 Sr 0.04 ) Σ1.00 A (2) (Ca 0.03 Ce 0.02 ) Σ0.05 [Cu 5.75 (Fe 3+ , Mn) 0.20 ] Σ5.95 [ T (1) (AsO 4 ) 2.96 T (2) (SbO 4 ) 0.04 )] Σ3.00 (OH) 5.96 O 0.04 ·3H 2 O. Hydrogen bonding in agardite-series minerals has been characterized for the first time. IR spectra of agardite-(Ce) and agardite-(Nd) from Lavrion used for comparison, as well as structural data indicate the presence of isolated H + cations that do not form strong covalent bonds with coordinating O atoms. Agardite-(Ce) from Clara Mine forms epitaxial growths with the Sb-analogue of auriacusite. The latter mineral was characterized by EDS analyses; its typical empirical formulae are Ca 0.06 Ce 0.04 Fe 3 + 1.06 Cu 0.89 [(SbO 4 ) 0.58 (AsO 4 ) 0.38 (SiO 4 ) 0.04 ] Σ1.00 (O,OH) and Ca 0.075 Ce 0.04 Fe 3 + 0.93 Cu 0.97 [(SbO 4 ) 0.59 (AsO 4 ) 0.35 (SiO 4 ) 0.06 ] Σ1.00 (O,OH). The formation of uniaxial growths of the Sb-analogue of auriacusite and agardite-(Ce) is caused by the close values of their c parameters (for auriacusite s.s . c = 5.9501(5) Å). Three-valence state of iron and five-valence of antimony in both minerals has been validated by means of Fe K - and Sb L 2,3 -edge XANES spectroscopy.