Aluminum-for-iron substitution, hydrogen bonding, and a novel structure-type in coquimbite-like minerals

A single-crystal XRD study of coquimbite specimens from Vulcano Island, Italy, and from non-volcanic occurrences has confirmed the existence of a marked difference in the occupancy of the metal sites by Fe and Al. For compositions with a 1:1 Al:Fe ratio, a novel structure-type corresponding to the new mineral aluminocoquimbite was found, which demonstrates the different behavior of the Al and Fe in forming such complexes with H2O and sulfate ions, respectively. The hydrogen-bond geometry was determined, and a peculiar cyclohexane-like arrangement of the H2O molecules that do not coordinate the metal atoms has also been observed.