Abstract

Ferro-ferri-hornblende is a new member of the amphibole supergroup (IMA-CNMNC 2015–054). It has been found in a 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 ABCa2C(Fe2+4Fe3+)T(Si7Al) O22W(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=0.23B(Ca1.93Na0.07)Σ=2.00C(Mg1.16Fe2+3.21Mn0.06Fe3+0.45Al0.12Ti 0.01)Σ=5.01T(Si7.26Al0.74)Σ=8.00 O22W(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)°, 2V (calc.) = 43.1o. 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 powder X-ray 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.

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