Valletta mine: a treasure trove of new minerals in the Western Alps.

The Valletta Fe-Mn deposit is located in the Briançonnais Zone of the Cottian Alps. It is the remains of an old mining operation, probably an open pit, at 2536 m asl, active in 1455 in the Saluzzo Marquisate. For more than 5 centuries it was abandoned until a recognition exploration by G.C. Piccoli, who noticed the presence of mineral phases in quartzite and quartz veins difficult to classify. As in previous occasions, the collaboration among mineral collectors and academia researchers led to a detailed study of the mineral phases in the collected samples and soon it was evident that a forgotten and unknown treasure had been unearthed. Since then we have recognized a great number of new mineral compositions and its characterization has led to new approved mineral species, grandaite Sr2Al(AsO4)2(OH) (space group P21/m, a 7.5764(5), b 5.9507(4), c 8.8050(6) Å, β 112.551(2)°, V 366.62(4) Å3, Z 2; IMA-CNMNC no. 2013-059, [1]) and braccoite NaMn2+5[Si5As5+O17(OH)](OH) (triclinic, space group P¯1, a 9.7354(4), b 9.9572(3), c 9.0657(3) Å, α 92.691(2)°, β 117.057(4)°, γ 105.323(3)°, V 740.37(4) Å3, Z 2 ; IMA 2013-093 [2]). But many others are on the run: we have observed a large solid solution among vanadates and arsenates of the arsenbrackebuschite group, including compositional changes among Ba, Sr and Ca members and proposals for new mineral species are being submitted to IMA-CNMNC. Within these series we have observed a triclinic modification (triclinic, space group P¯1, a 6.1187(3), b 9.3705(4), c 14.2503(9) Å, α 99.914(5)°, β 112.433(5)°, γ 89.983(4)°, V 785.39(7) Å3, Z 4). We have also recognized unknown Mn-Fe oxides that deserve appropriate studies. The description of the mineral treasure trove has become an exciting experience. [1] Cámara F., Ciriotti M.E., Bittarello E., Nestola F., Massimi F., Radica F., Costa E., Benna P., and Piccoli G.C. (2014). Mineralogical Magazine (in press). [2] Cámara F., Bittarello E., Ciriotti M.E., Nestola F., Radica F. and Marchesini M. (2013). Braccoite, IMA 2013-093. CNMNC Newsletter No. 18, December 2013, page 3256; Mineralogical Magazine, 77, 3249-3258. doi:10.1180/minmag.2013.077.8.15.