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Ikranite: Composition and Structure of a New Mineral of the Eudialyte Group R

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Ikranite: Composition and Structure of a New Mineral of the Eudialyte Group R Crystallography Reports, Vol. 48, No. 5, 2003, pp. 717–720. Translated from Kristallografiya, Vol. 48, No. 5, 2003, pp. 775–778. Original Russian Text Copyright © 2003 by Rastsvetaeva, Chukanov. CRYSTAL CHEMISTRY Dedicated to the 60th Anniversary of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences Ikranite: Composition and Structure of a New Mineral of the Eudialyte Group R. K. Rastsvetaeva* and N. V. Chukanov** * Shubnikov Institute of Crystallography, Russian Academy of Sciences, LeninskiÏ pr. 59, Moscow, 119333 Russia e-mail: [email protected] ** Institute of Problems of Chemical Physics in Chernogolovka, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia Received March 14, 2003 Abstract—The crystal structure of a new mineral, ikranite, of the eudialyte group discovered in the Lovozero massif (the Kola Peninsula) was established by X-ray diffraction analysis. The crystals belong to the trigonal system and have the unit-cell parameters a = 14.167(2) Å, c = 30.081(2) Å, V = 5228.5 Å3, sp. gr. R3m. Ikranite is the first purely ring mineral of the eudialyte group (other minerals of this group contain ring platforms of either tetrahedral or mixed types). It is also the first representative of the eudialyte group where Fe3+ prevail over Fe2+ ions. © 2003 MAIK “Nauka/Interperiodica”. A new mineral, ikranite, is named after the Shubni- based on the variations in the chemical composition in kov Institute of Crystallography of the Russian Acad- a number of key positions, among which are both A(1)– emy of Sciences.1 This mineral belongs to the eudialyte A(7) and M(2)–M(4) extraframework positions and family, which includes ring zircono- and titanosilicates some framework positions, such as M(1) and Z. Several of the trigonal symmetry (R3m, R3m , and R3). The minerals of the eudialyte group were discovered with the participation of researchers from the Institute of concept of the composition and atomic structure of eud- Crystallography. These are alluaivite [3], in which Ti in ialyte gradually changed with the accumulation of the the Z framework position is replaced by Zr; feklichevite information beginning with the first structural studies [4], in which Na in the A(4) position is replaced by Ca; published in 1971 [1, 2]. One of these structures was and raslakite [5], in which Ca in the M(1)' position of established at the Shubnikov Institute of Crystallogra- the six-membered ring is orderly replaced by Fe. phy of the Russian Academy of Sciences. At present, the variable complex chemical composition of the min- In the present study, we established the structure of erals of the eudialyte group is described by the follow- ikranite, a new representative of this mineralogical and ing general formula proposed by the Commission on crystallochemical family. The preliminary data on the New Minerals and Mineral Names of the International ikranite structure, corresponding to the anisotropical Mineralogical Association (CNMMN IMA): refinement up to R = 3.7%, were published earlier [6]. [Ä(1)Ä(2)Ä(3)Ä(4)Ä(5)] [M(1)M(1)'] [M(2)M(2a)M Here, we report the results of the repeated refinement of 3 3 several framework positions and characterize the min- (2b)]3Ð6[M(3)M(4)]Z3[Si24O72] · OH2Ð6X2Ð4, where A = Na, K, Sr, REE, Ba, Mn, H O; M(1) and M(1)' = Ca, Mn, eral and its structure in more detail consistent with the 3 modern concept of minerals of the eudialyte group. REE, Na, Fe; M(2) = Fe, Na, Zr, Ta; M(2a) and M(2b) = Ikranite was discovered in the aegirine zone of Mn, Zr, Ti, Na, K, Ba, H3O; M(3) and M(4) = Si, Nb, Ti, W, Na; Z = Zr, Ti; and X = H O, Cl, F, CO , SO , SiO . agpaitic pegmatites from Mount Karnasurt in the north- 2 3 4 4 ern region of the Lovozero alkaline massif (the Kola The letters correspond not only to a set of elements but Peninsula) in the form of 1-cm-large transparent grains also to particular structural positions. According to the with the color varying from yellow to brown-yellow. nomenclature developed by the CNMMN IMA, new This mineral occurs in association with microcline, mineral species in the eudialyte family are singled out lorenzenite, nepheline, lamprophyllite, murmanite, and 1 This name was approved by the Commission on New Minerals arfvedsonite as well as with tetranatrolite and halloysite and Mineral Names of the International Mineralogical Associa- formed at a later stage. Ikranite differs from other rep- tion on March 3, 2003. resentatives of the eudialyte group in its physical prop- 1063-7745/03/4805-0717 $24.00 © 2003 MAIK “Nauka/Interperiodica” 718 RASTSVETAEVA, CHUKANOV Research Institute of Synthesis of Mineral Raw Materi- als). According to the results of Mössbauer spectros- copy, 84.56% of iron in ikranite is in the trivalent state, which is in good agreement with the chemical-analysis data. The presence of two close doublets of Fe3+ ions in the Mössbauer spectrum seems to be indicative of weak splitting of the Fe3+ position. The IR spectrum of ikranite (Fig. 1) has broad bands in the frequency range from 3000 to 3300 cmÐ1 (corre- 500 1000 1500 3000 3500 ν ν cmÐ1 sponding to vibrational frequencies 3(E) and 1(A1) of + Ð1 the H3O groups) and an intense band at 473 cm 3+ Fig. 1. IR spectrum of ikranite. (attributed to vibrations of an [Fe O6] octahedron) absent in the spectra of other minerals of the eudialyte group. Main stretching vibrations of the tetrahedral erties and some compositional characteristics, in partic- rings are observed as an intense band at 1000 cmÐ1, ular, by the presence of trivalent iron and deficit of whereas the band due to additional SiO4 tetrahedra is sodium. rather weak (the spectrum shows only the shoulder at The parameters of the trigonal unit cell are: a = 930 cmÐ1). Therefore, the IR spectrum of ikranite dif- 14.167(2) Å, c = 30.081(2) Å, V = 5228.5 Å3, sp. gr. fers radically from the spectra of other minerals of the R3m. According to the results of microprobe analysis, eudialyte group and can serve as a reliable diagnostic the empirical formula of ikranite (for 24 Si atoms) can characteristic. Apparently, the replacement of [4]Fe2+ by be written as Na7.56(H3O)6.64K0.27Ca3.31Sr0.46Ce0.27La0.11 · [6]Fe3+ is responsible for the change from lilac-red color 2+ 2+ 3+ (eudialyte) to brown-yellow color (ikranite). Nd0.03Mn1.41 Fe0.16 Fe1.77 Zr3.33Ti0.14Nb0.06Si24O72Cl0.74 · 2.64H2O. A total of 631 X-ray diffraction reflections were col- lected from a 0.4 × 0.4 × 0.15-mm single crystal on a The ikranite crystals are optically positive with no = 1.612(1) and = 1.615(2) and often show a weak KM-4 diffractometer (MoKα radiation, graphite mono- ne θ λ anomalous biaxiality. The densities measured by bal- chromator, sin / < 0.481). All the computations were ancing mineral grains in heavy liquids and calculated made using the AREN program package [7]. from the empirical formula are 2.82(1) and 2.83 g/cm3, The atomic coordinates, equivalent thermal parame- respectively. Ikranite was also studied by Mössbauer ters, and occupancies of the positions are listed in spectroscopy on a YaGRS-4 spectrometer in a mode of Table 1. Since the positions of silicon atoms of the tet- permanent acceleration using a 57Co source in a rhod- rahedral rings and positions of the framework O atoms ium matrix (analyst V.V. Korovushkin, All-Russia were published in [6], these data are omitted from a Zr Zr Fe Fe Fe Fe Ca Ca Ca Ca Zr Zr Zr Zr Zr Fe Ca Ca Fe Ca Ca Ca Ca Ca Ca Zr Zr b Fe Fe Fe Fe Ca Ca Ca Ca Zr Zr Zr Zr Zr Fe Ca Ca Fe Ca Ca Ca Ca Ca Ca Zr Zr Fig. 2. Framework of the ikranite structure projected onto the (001) plane. CRYSTALLOGRAPHY REPORTS Vol. 48 No. 5 2003 IKRANITE: COMPOSITION AND STRUCTURE OF A NEW MINERAL 719 Table 1. Coordinates, occupancies of the key positions, and equivalent thermal parameters of the atoms in the ikranite structure 2 Position x/a y/b z/c Beq, Å Q q Zr 0.3332(1) 0.1666(1) 0.1669(1) 1.86(8) 9 1 M(1) 0.2626(1) 0.0012(2) 0.0004(1) 2.21(6) 18 1 M(2a) 0.4907(3) 0.5094(2) 0.0016(2) 4.6(1) 9 0.70(1) M(2b) 0.5324(9) 0.4676(9) –0.003(1) 9.3(7) 9 0.30(2) M(3a) 0.3334 0.6667 0.0306(6) 1.9(5) 3 0.15(1) M(3b) 0.3334 0.6667 0.0859(8) 1.7(7) 3 0.30(2) M(4a) 0.3334 0.6667 0.2271(4) 2.2(3) 3 0.15(1) M(4b) 0.3334 0.6667 0.2875(2) 2.8(3) 3 0.30(2) A(1a) 0.1110(3) 0.2220(5) 0.1528(3) 2.2(2) 9 0.66(1) A(1b) 0.0808(6) 0.1616(9) 0.1663(5) 3(1) 9 0.34(5) A(2a) 0.5681(2) 0.4319(2) 0.1730(2) 1.3(2) 9 0.70(1) A(2b) 0.5450(8) 0.4550(8) 0.1789(7) 2.6(5) 9 0.30(1) A(3a) 0.4648(6) 0.2324(4) 0.0484(2) 1.4(1) 9 0.82(2) A(3b) 0.492(4) 0.246(2) 0.035(2) 1.2(3)* 9 0.18(2) A(4a) 0.2359(4) 0.4718(3) –0.0459(2) 2.5(2) 9 0.80(2) A(4b) 0.266(3) 0.533(2) –0.054(1) 1.9(5)* 9 0.20(1) A(5a) 0.391(1) 0.609(1) 0.159(1) 4(1) 9 0.23(4) A(5b) 0.185(1) 0.593(1) 0.164(1) 4(1) 9 0.45(4) OH(1) 0.3334 0.6667 0.137(3) 7(1) 3 0.30(2) OH(2) 0.391(1) 0.609(1) 0.159(1) 4(1) 9 0.23(4) OH(3) 0.259(1) 0.518(2) 0.3307(7) 6.5(5) 9 0.70(5) OH(4) 0.420(1) 0.580(1) –0.001(1) 7.1(5) 9 0.70(4) Cl 0.3334 0.6667 0.150(1) 5.3(9) 3 0.26(2) H2O(1a) 0 0 0.2250(9) 1.5(4) 3 0.7(1) H2O(1b) 0 0 0.199(2) 1.4(3) 3 0.3(1) H2O(2) 0.6667 0.3334 0.0999(6) 2.5(6) 3 1 Note: The composition of the M(1) position is Ca3.3Mn1.4REE0.5Na0.45Sr0.35.
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