A New Mineral Ferrisanidine, K [Fe3+ Si3o8], the First Natural Feldspar
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minerals Article 3+ A New Mineral Ferrisanidine, K[Fe Si3O8], the First Natural Feldspar with Species-Defining Iron Nadezhda V. Shchipalkina 1,*, Igor V. Pekov 1, Sergey N. Britvin 2,3 , Natalia N. Koshlyakova 1, Marina F. Vigasina 1 and Evgeny G. Sidorov 4 1 Faculty of Geology, Moscow State University, Vorobievy Gory, Moscow 119991, Russia; igorpekov@mail.ru (I.V.P.); nkoshlyakova@gmail.com (N.N.K.); vigasina@geol.msu.ru (M.F.V.) 2 Department of Crystallography, St Petersburg State University, University Embankment 7/9, St Petersburg 199034, Russia; sbritvin@gmail.com 3 Nanomaterials Research Center, Kola Science Center of Russian Academy of Sciences, Fersman Str. 14, Apatity 184209, Russia 4 Institute of Volcanology and Seismology, Far Eastern Branch of Russian Academy of Sciences, Piip Boulevard 9, Petropavlovsk-Kamchatsky 683006, Russia; mineral@kscnet.ru * Correspondence: estel58@yandex.ru Received: 10 November 2019; Accepted: 9 December 2019; Published: 11 December 2019 3+ Abstract: Ferrisanidine, K[Fe 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. D is 2.722 g cm 3. The calc · − 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 3+ (K0.97Na0.03)S1.00(Si3.03Fe 0.94Al0.02)S3.99O8. 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 3 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) Å . Space group is C2/m. The crystal structure of ferrisanidine is based on the sanidine-type “ferrisilicate” 3+ framework formed by disordered [SiO4] and [Fe O4] tetrahedra. Keywords: ferrisanidine; new mineral; sanidine; feldspar group; iron in feldspar; crystal structure; fumarole; Tolbachik volcano; Kamchatka 1. Introduction High content of iron is extremely rare for feldspar-group minerals. In most cases, the content of Fe2O3 in natural feldspars is not higher than 3 wt. % and all Fe-rich samples of minerals of this group belong to potassic feldspars [1,2]. The K-feldspar with distinct content of Fe2O3 (2.5–3.0 wt. %) was first described by Alfred Lacroix in 1912 from granitic pegmatites at Madagascar. Its optical properties are close to those of sanidine [3]. Fe-enriched (6.3 wt. % Fe2O3) potassic feldspar was described from manganese-rich skarns of the famous Långban deposit, Filipstad, Sweden [4]. “Sanidine” 3+ with 13.7 wt. % Fe2O3 and the empirical formula (K0.95Na0.03)[Si3.08Fe 0.50Al0.40O8] was reported by [5] from lamproites of Cancarix, Spain. Other researchers [6] published data on zoned crystals of sanidine with zones containing up to 18.4 wt. % Fe2O3 from leucite- and sanidine-bearing lavas of Leucite Hills, Wyoming, USA. The empirical formula of the most Fe-rich zone in these crystals is Minerals 2019, 9, 770; doi:10.3390/min9120770 www.mdpi.com/journal/minerals Minerals 2019, 9, 770 2 of 17 3+ (K0.96Na0.04)S1.00[(Si3.02Fe 0.70Al0.20Mg0.05Ti0.03)S4.00O8]. In both these cases, an Fe-dominant (Fe > Al in atom proportions) feldspar occurs as thin rims on sanidine crystals. However, no X-ray diffraction (XRD)Minerals data 2019 for, natural9, x FOR PEER samples REVIEW of Fe-dominant feldspars were published and, therefore,2 of nothing 18 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. However, no X-ray is known. diffraction (XRD) data for natural samples of Fe-dominant feldspars were published and, therefore, Unlikenothing natural on crystallography Fe-rich feldspars, of thes synthetice minerals ferri-feldspars and the distribution are well-studied. of Si, Al and TheFe in first their experimental crystal 3+ data revealingstructures is the known. possibility of Fe presence in the feldspar-type crystal structure were obtained 3+ by [7], whoUnlike synthesized natural so-calledFe-rich Fe-orthoclase.feldspars, synthetic Later theferri-feldspars investigation are ofwell-studied. the KAlSi3O 8The–KFe firstSi 3O8 3+ 3+ solid-solutionexperimental series data and revealing transitions the possibility between of monoclinic Fe presence and in the triclinic feldspar-type forms of crystal K[Fe structureSi3O8] were were the focusobtained of many by studies [7], who [8– 13synthesized]. so-called Fe-orthoclase. Later the investigation of the KAlSi3O8– KFe3+Si3O8 solid-solution series and transitions between monoclinic and triclinic3+ forms of K[Fe3+Si3O8] In this paper, we describe a natural feldspar chemically close to KFe Si3O8 found in fumarole sublimateswere the at thefocus Tolbachik of many studies volcano, [8–13]. Kamchatka, Russia. This new mineral was named ferrisanidine In this paper, we describe a natural feldspar chemically close to KFe3+Si3O8 found in fumarole as an analogue of sanidine with Fe3+ instead of Al. Both the new mineral and its name have been sublimates at the Tolbachik volcano, Kamchatka, Russia. This new mineral was named ferrisanidine approved by the IMA Commission on New Minerals, Nomenclature and Classification, IMA No. as an analogue of sanidine with Fe3+ instead of Al. Both the new mineral and its name have been 2019-052.approved The typeby the specimen IMA Commission is deposited on New in the Minera collectionls, Nomenclature of the Fersman and Classification, Mineralogical IMA Museum No. of the Russian2019-052. Academy The type ofspecimen Sciences, is deposited Moscow, in Russia, the collection with catalogue of the Fersman number Mineralogical 96732. Museum of the Russian Academy of Sciences, Moscow, Russia, with catalogue number 96732. 2. Background Information: Crystal Structure and Nomenclature of Potassic Feldspars 2. Background Information: Crystal Structure and Nomenclature of Potassic Feldspars Feldspar-type crystal structures are well-studied. Crystal chemistry of feldspars is described in many publicationsFeldspar-type including crystal severalstructures handbooks are well-studied. [1,2,14 Crystal–16]. The chemistry feldspar of structuralfeldspars is archetype described in unites many publications including several handbooks [1,2,14–16]. The feldspar structural archetype unites compounds with the general formula AT4O8. Until the present work, the species-defining constituents in validcompounds feldspar-group with the minerals general formula were as AT follows:4O8. Until tetrahedrally the present work, coordinated the species-definingT = Si, Al, B,constituents and As5+ and in valid feldspar-group minerals were as follows: tetrahedrally coordinated T = Si, Al, B, and As5+ and extra-framework cations A = Na, K, Rb, NH4, Ca, Sr, and Ba. extra-framework cations A = Na, K, Rb, NH4, Ca, Sr, and Ba. The crystal structure of feldspars is based on a three-dimensional framework composed by The crystal structure of feldspars is based on a three-dimensional framework composed by corner-sharing TO tetrahedra. The main structural unit is a chain of four-membered rings, called as corner-sharing4 TO4 tetrahedra. The main structural unit is a chain of four-membered rings, called as “double“double crankshaft”. crankshaft”. These These rings rings consist consist of tetrahedraof tetrahedraT(1) T(1) and andT (2)T(2) (in (in ordered ordered species, species, TT10,10, T1m,, T20 and TT220m). and The T2 doublem). The crankshaftsdouble crankshafts repeat repeat throughout throughout space space by cby-translations c-translations and and are are linked linked with one anotherone as another shown as in shown Figure in1 Figure. 1. Figure 1. Two double-crankshaft chains parallel to the a axis in the crystal structure of ferrisanidine: the leftFigure figure 1. isTwo a schematic double-crankshaft illustration chains of crankshaftparallel to the chains a axis where in the eachcrystal line structure becomes of ferrisanidine: a branch joining the left figure is a schematic illustration of crankshaft chains where each line becomes a branch joining nodes, in which tetrahedrally coordinated T atoms located, of a four-connected net; the right figure is a nodes, in which tetrahedrally coordinated T atoms located, of a four-connected net; the right figure is projection of double-crankshaft chains parallel to the a axis with outlined TO4 tetrahedra and K atoms; a projection of double-crankshaft chains parallel to the a axis with outlined TO4 tetrahedra and K the loweratoms; figure the lower displays figure the displays projection the projection of the ferrisanidine of the ferrisanidine crystal crystal structure structure along along the a theaxis. a axis. Minerals 2019, 9, 770 3 of 17 The extra-framework A cations are located in the interstices, on the (010) mirror plane in monoclinic feldspars or near the plane in triclinic feldspars. All feldspars have topological monoclinic symmetry C2/m which can be lowered to C1 or I2/c in the fully T-ordered species. Ordering at tetrahedral sites T causes an appearance of several polymorphs of potassic feldspar, ideally K[AlSi3O8]. Sanidine (C2/m) is a disordered species; if the Al content at T(1) is < 0.50 atoms per formula unit (apfu), the term high sanidine is used, while the variety with Al content at T(1) between 0.50 and 0.75 apfu is named low sanidine.