Abstract
Ferrisanidine, K[Fe3+Si3O8], the first natural feldspar with species-defining iron, is an analogue of sanidine bearing Fe3+ 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. Dcalc is 2.722 g·cm−3. The chemical composition of ferrisanidine (wt. %, electron microprobe) is: Na2O 0.25, K2O 15.15, Al2O3 0.27, Fe2O3 24.92, SiO2 60.50, in total 101.09. The empirical formula calculated based on 8 O apfu is (K0.97Na0.03)Ʃ1.00(Si3.03Fe3+0.94Al0.02)Ʃ3.99O8. The crystal structure of ferrisanidine was studied using the Rietveld method, the final R indices are: Rp = 0.0053, Rwp = 0.0075, R1 = 0.0536. Parameters of the monoclinic unit cell are: a = 8.678(4), b = 13.144(8), c = 7.337(5) Å, β = 116.39(8)°, V = 749.6(9) Å3. Space group is C2/m. The crystal structure of ferrisanidine is based on the sanidine-type “ferrisilicate” framework formed by disordered [SiO4] and [Fe3+O4] tetrahedra.
Highlights
High content of iron is extremely rare for feldspar-group minerals
We describe a natural feldspar chemically close to KFe3+Si3O8 found in fumarole as an analogue of sanidine with Fe3+ instead of Al
The synthetic analogue of ferrisanidine is biaxial these optical data correspond to the mineral well
Summary
High content of iron is extremely rare for feldspar-group minerals. In most cases, the content of. (K0.96 Na0.04 )Σ1.00 [(Si3.02 Fe3+ 0.70 Al0.20 Mg0.05 Ti0.03 )Σ4.00 O8 ] In both these cases, an Fe-dominant (Fe > Al in atom proportions) feldspar occurs as thin rims on sanidine crystals. REVIEWof Fe-dominant feldspars were published and, of (XRD)Minerals data 2019, for natural samples nothing on crystallography of these minerals and the distribution of Si, Al and Fe in their crystal structures. Al in atom proportions) feldspar occurs as thin rims on sanidine crystals. Diffraction (XRD) data for natural samples of Fe-dominant feldspars were published and, Unlike. Sublimates at the Tolbachik volcano, Kamchatka, Russia Museum of the Russian Academy of Sciences, Moscow, Russia, with catalogue number 96732
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