New arsenate minerals from the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. XVIII. Khrenovite, Na3Fe3+2(AsO4)3, the member with the highest sodium in the alluaudite supergroup

Abstract The new alluaudite-group mineral khrenovite with the ideal, end-member formula Na 3 Fe 3+ 2 (AsO 4 ) 3 was found in the Arsenatnaya fumarole, Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with aphthitalite-group sulfates, badalovite, calciojohillerite, nickenichite, johillerite, tilasite, svabite, achyrophanite, ozerovaite, pansnerite, arsenatrotitanite, anhydrite, sanidine, hematite, cassiterite, rutile and pseudobrookite. Khrenovite occurs as coarse prismatic crystals up to 0.2 × 0.3 × 0.8 mm and their clusters up to 1 mm across. It is transparent, honey-coloured, red-, orange- or yellow-brown, with vitreous lustre. Khrenovite is brittle, cleavage was not observed. D calc is 4.257 g cm –3 . Khrenovite is optically biaxial (+), α = 1.825(7), β = 1.834(7), γ = 1.845(7) and 2V meas. = 80(10)°. The chemical composition (wt.%, electron-microprobe) is: Na 2 O 11.47, K 2 O 1.23, CaO 0.18, MgO 0.01, MnO 4.10, CuO 4.27, ZnO 1.99, Al 2 O 3 0.17, Fe 2 O 3 21.12, SiO 2 0.08, P 2 O 5 0.01, V 2 O 5 0.10, As 2 O 5 56.03, SO 3 0.02, total 100.78. The empirical formula calculated on the basis of 12 O apfu is (Na 2.26 K 0.16 Ca 0.02 Mn 0.35 Cu 0.33 Zn 0.15 Al 0.02 Fe 3+ 1.62 ) Σ4.91 (As 2.98 Si 0.01 V 0.01 ) Σ3.00 O 12 . Khrenovite is monoclinic, C 2/ c , a = 12.2394(7), b = 12.7967(5), c = 6.6589(4) Å, β = 112.953(7)°, V = 960.37(10) Å 3 and Z = 4. The crystal structure was solved from single-crystal X-ray diffraction data with R 1 = 0.0287. Khrenovite is isostructural with other alluaudite-group minerals. Its structural formula simplified to the species-defining constituents is A (1) Na A (2)’ Na M (1) Na M (2) Fe 3+ 2 ( T AsO 4 ) 3 . The mineral is named in honour of the Russian volcanologist and geologist Anatoly Petrovich Khrenov (1946–2016).