Publication details
Dacostaite, K(Mg2Al)[Mg(H2O)6]2(AsO4)2F6 ⋅ 2H2O, a new fluoride–arsenate mineral from the Cetine di Cotorniano Mine (Tuscany, Italy)
| Authors | |
|---|---|
| Year of publication | 2025 |
| Type | Article in Periodical |
| Magazine / Source | European Journal of Mineralogy |
| MU Faculty or unit | |
| Citation | |
| web | https://doi.org/10.5194/ejm-37-39-2025 |
| Doi | http://dx.doi.org/10.5194/ejm-37-39-2025 |
| Keywords | CRYSTAL-STRUCTURE; BOND-VALENCE; BLACK RANGE; NEW-MEXICO; VICANITE-(CE); BOROSILICATE; GLADSTONE; DURANGITE; DISTRICT; MEMBER |
| Description | The new mineral dacostaite, K(Mg2Al)[Mg(H2O)6]2(AsO4)2F6?·?2H2O, has been discovered in the Cetine di Cotorniano Mine, Chiusdino, Siena, Tuscany, Italy. It occurs as thin, colourless-to-white pseudo-hexagonal micaceous crystals up to 0.5?mm in size. The streak is white, and the lustre is silky. The cleavage is perfect on {001}. The empirical formula of dacostaite, based on (As?+?P)?=?2 atoms per formula unit, is (K0.56Ca0.04Na0.03?0.37)?1.00 (Al1.54Mg1.38Cu0.03Zn0.03)?2.98 [Mg(H2O)6]2 [(As0.99P0.01)O4]2 [F4.46(OH)1.46O0.08]?6.00?·?2H2O (Z=2). Dacostaite is monoclinic, with a space group of C2/m and a=12.474(5), b=7.198(3), c=13.724(6)?A, ß=99.518(13)°, and V=1215.3(8)?A3. The crystal structure was solved using single-crystal X-ray diffraction data and refined to R1=0.0927 for 1022 unique reflections with Fo?>?4? (Fo). The crystal structure of dacostaite can be described as formed by heteropolyhedral {001} layers and isolated Mg(H2O)6 groups connected by H bonds. In the type material, dacostaite is associated with quartz, sulfur, gypsum, and a pharmacosiderite-like mineral in a small cavity of silicified limestone. Its genesis is related to the activity of oxidized (Al,F)-rich fluids during the late-stage evolution of the Sb ore deposit formerly exploited at the Cetine di Cotorniano Mine. |