Ferro-ferri-hornblende from the Traversella Mine (Ivrea, Italy): occurrence, mineral description and crystal-chemistry

Ferro-ferri-hornblende is a new member of the amphibole supergroup (IMA-CNMNC 2015-054). It has been found in rock specimen from the historical collection of Leandro De Magistris, which was collected at the Traversella Mine (Val Chiusella, Ivrea, Piemonte, Italy). The specimen was catalogued as "speziaite", and contains a wide range of amphibole compositions from tremolite/actinolite to magnesio-hastingsite. The end-member formula of ferro-ferri-hornblende is Avac BCa2 C(Fe2+ 4Fe3+) T(Si7Al) O22 W(OH)2, which requires SiO2 43.41, Al2O3 5.26, FeO 29.66,Fe2O3 8.24 CaO 11.57, H2O 1.86, total 100.00 wt%. The empirical formula derived from electron microprobe analysis and single-crystal structure refinement for the holotype crystal is A(Na0.10K0.13)Σ=0.23 B(Ca1.93Na0.07)Σ=2.00 C(Mg1.16Fe2+ 3.21Mn0.06Fe3+ 0.45Al0.12Ti 0.01)Σ=5.01 T(Si7.26Al0.74)Σ=8.00 O22 W(OH1.89F0.01Cl0.10)Σ=2.00. Ferro-ferri-hornblende is biaxial (–), with α = 1.697(2), β = 1.722(5), γ = 1.726(5) and 2V (meas.) = 35.7(1.4)º, 2V (calc.) = 43.1 o. The unit-cell parameters are a = 9.9307(5), b = 18.2232(10), c = 5.3190(3) Å, β = 104.857(1)°, V = 930.40 (9) Å3, Z = 2, space group C2/m. The a:b:c ratio is 0.545:1:0.292. The strongest eight reflections in the X-ray powder pattern [d values (in Å), I, (hkl)] are: 8.493, 100, (110); 2.728, 69, (151); 3.151, 47, (310); 2.555, 37, (-202); 2.615, 32, (061); 2.359, 28, (-351); 3.406, 26, (131); 2.180, 25, (261). Type material is deposited in the collections of the Museo di Mineralogia, Dipartimento di Scienze della Terra e dell'Ambiente, Università di Pavia, under the catalogue number 2015-01. Sample M/U15285 from the historical collection of Luigi Colomba, presently at the Museo Regionale di Scienze Naturali di Torino, was also checked, and the presence of ferro-ferri-hornblende was confirmed.