A New Mineral Ferrisanidine, K[Fe3+Si3O8], the First Natural Feldspar with Species-Defining Iron

Ferrisanidine, K[Fe 3+Si 3O 8], the first natural feldspar with species-defining iron, is an analogue of sanidine bearing Fe 3+ instead of Al. It was found in exhalations of the active Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Fissure Tolbachik Eruption, Tolbachik volcano, Kamchatka Peninsula, Russia. The associated minerals are aegirine, cassiterite, hematite, sylvite, halite, johillerite, arsmirandite, axelite, aphthitalite. Ferrisanidine forms porous crusts composed by cavernous short prismatic crystals or irregular grains up to 10 μm × 20 μm. Ferrisanidine is transparent, colorless to white, the lustre is vitreous. D calc is 2.722 g·cm ？3. The chemical composition of ferrisanidine (wt. %, electron microprobe) is: Na 2O 0.25, K 2O 15.15, Al 2O 3 0.27, Fe 2O 3 24.92, SiO 2 60.50, in total 101.09. The empirical formula calculated based on 8 O apfu is (K 0.97Na 0.03) ？1.00(Si 3.03Fe 3+ 0.94Al 0.02) ？3.99O 8. The crystal structure of ferrisanidine was studied using the Rietveld method, the final R indices are: R p = 0.0053, R wp = 0.0075, R 1 = 0.0536. Parameters of the monoclinic unit cell are: a = 8.678(4), b = 13.144(8), c = 7.337(5) ？, β = 116.39(8)°, V = 749.6(9) ？ 3. Space group is C2/ m. The crystal structure of ferrisanidine is based on the sanidine-type “ferrisilicate” framework formed by disordered [SiO 4] and [Fe 3+O 4] tetrahedra. View Full-Tex