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Crystal Structure of Niobium-Rich Lomonosovite with Symmetry P1 from the Khibiny Massif (Kola Peninsula)
Abstract
A new niobium-rich lomonosovite variety with a high degree of ordering of Ti and Nb atoms has been investigated by X-ray diffraction analysis and electron probe microanalysis. Its simplified formula is Na 10 Ti 2 (Nb,Fe,Ti) 2 (Si 2 O 7) 2 (PO 4) 2 O 4. The triclinic unit-cell parameters are a = 5.411(1) Å, b = 7.108(1) Å, c = 14.477(2) Å, α = 99.78(1)°, β = 96.59(1)°, γ = 90.26(1)°, V = 544.94(5) Å 3 , Z = 1, sp. gr. P1. The crystal structure has been refined to the final reliability factor R = 6.3% within the anisotropic approximation of atomic displacements using 3674 reflections with F > 3σ(F). The problem of niobium distribution in minerals with structures of lomonosovite and related types is discussed.
... An ore zone (Khibiny) [28][29][30] and zones of pegmatites and vein formations (Khibiny, Lovozero) usually appear there [31][32][33][34][35][36][37]. There are elevated amounts of rare earth elements (Ce) [31][32][33][34][35][36][37][38]. The share of these elements is variable, but their occurrences sometimes Most of them are central intrusions usually composed of concentric rings of rocks forming the metasomatized contact zone and internal zones of magmatic derivatives. ...
... These sequences are more numerous in large massifs such as Khibiny and Lovozero. An ore zone (Khibiny) [28][29][30] and zones of pegmatites and vein formations (Khibiny, Lovozero) usually appear there [31][32][33][34][35][36][37]. There are elevated amounts of rare earth elements (Ce) [31][32][33][34][35][36][37][38]. ...
... An ore zone (Khibiny) [28][29][30] and zones of pegmatites and vein formations (Khibiny, Lovozero) usually appear there [31][32][33][34][35][36][37]. There are elevated amounts of rare earth elements (Ce) [31][32][33][34][35][36][37][38]. The share of these elements is variable, but their occurrences sometimes cause enrichment in these elements of the rock-forming minerals of the discussed massifs or the form of solid inclusions and separate phases. ...
This manuscript deals with the analysis of significant rare earth elements (REE) minerals such as eudialyte, lorenzenite, loparite, perovskite, titanite, apatite, and carbonates. These minerals are found in the rocks of the Khibiny, Lovozero, Afrikanda, and Kovdor massifs (the Paleozoic hotspot activity in the Kola-Karelian Alkaline Province is estimated at about 100,000 km2). Performed microscopic analyses that demonstrated their structure and optical features (dimming, interference colors, relief). Single-crystal analysis using XRD methods, SEM-EDS, and spectroscopic (FTIR) studies allowed the characteristics of described minerals: Lorenzenite in Lovozero probably crystalized after loparite have small additions of Nb, La, Ce, Pr, and Nd. Loparite and perovskite have the addition of Ce, Nb, and Ta. The same dopants have titanite probably crystalized after perovskite. Calcite in these massifs had the addition of Ce and Sr, the same as in fluorapatite, which was found in these rocks too. All of the analyzed minerals are REE-bearing and can be considered as deposits.
... For the lomonosovite the wide concentration range was shown on the Ti-Nb substitution, up to Nb domination in the one of octahedra [17], in the same it was found no significant presense of divalent LIL cations in the interlayer positions. ...
Melting of widely spread in agpaitic rocks titanosilicates: lamprophyllite, barytolamprophyllite and lomonosovite studied at 1 atm. Lamprophyllite and barytolamprophyllite melt incongruently with formation of titanosilicate melt, SrTiO3, TiO2 and Na-Fe or Ba-Mn titanate respectively. Melting temperature decrease from lamprophyllite (~890 °C) to baritolamprophyllite (~840 °C). Crystallization of lamprophyllite from the melt was obtained. Newly formed lamprophyllite always has higher Sr/Ba ratio than coexisting melt. Lomonosovite melts incongruently at 820–866 °C with formation of immiscible titanosilicate and phosphate melts, (Na,Ca)(Ti,Nb)O3, and Na–Fe titanate. The usually fixed in peralkaline rocks evolution from lamprophyllite to baritolamprohyllite, is in accordance with relationship between temperatures of lamprophyllite barytolamprophyllite decomposition. The magmatic crystallization of lamprophyllite group minerals must effectively decrease the Sr/Ba ratio in the melt. The independence of Ba/Sr ratio in lamprophyllite from the vertical position of sample shows that lamrophyllite wasn’t cumulative mineral during crystallization of magma of Lovozero massif. Incongruent melting of titanosilicates serves as model reactions of late-magmatic loparite decomposition. The mineral associations formed by loparite replacement in Lovozero rocks was interpreted as the evidence of influence on loparite phosphate and silicate melts from this point of view.
This review focuses on the results of work on the developing of new methods of nuclear fuel fabrication using microwave radiation, reprocessing of spent nuclear fuel and fractionation of formed high-level waste in the presence of desalting agent, as well as the scientific and technical justification of sodium aluminum iron phosphate glass and mineral-like magnesium potassium phosphate matrix as compounds for disposal of removed radioactive waste (RW). In addition, attention is paid to the study of radionuclide distribution on the minerals of rocks, which are the final safety barriers of the Nizhnekansky granitoid massif as a site of the underground repository for solidified RW.
Kolskyite (Ca square)Na2Ti4(Si2O7)(2)O-4(H2O)(7), is a Group-IV TS-block mineral from the Kirovskii mine, Mount Kukisvumchorr, Khibiny alkaline massif, Kola Peninsula, Russia. The mineral occurs as single, platy crystals 2-40 mu m thick and up to 500 mu m across. It is pinkish yellow, with a white streak and a vitreous luster. The mineral formed in a pegmatite as a result of hydrothermal activity. Associated minerals are natrolite, nechelyustovite, kazanskyite, barytolamprophyllite, hydroxylapatite, belovite-(La), belovite-(Ce), gaidonnayite, nenadkevichite, epididymite, apophyllite-(KF), and sphalerite. Kolskyite has perfect cleavage on {001}, splintery fracture, and a Mohs hardness of 3. Its calculated density is 2.509 g/cm(3). Kolskyite is biaxial negative with alpha 1.669, beta 1.701, gamma 1.720 (lambda 590 nm), 2V(meas.) = 73.6(5)degrees, 2V(calc.) = 74.0 degrees, with no discernible dispersion. It is nonpleochroic. Kolskyite is triclinic, space group P (1) over bar, a 5.387(1), b 7.091(1), c 15.473(3) angstrom, alpha 96.580(4), beta 93.948(4), gamma 89.818(3)degrees, V 585.8(3) angstrom(3). The strongest lines in the X-ray powder-diffraction pattern [d(angstrom)(I)(hkl)] are: 15.161(100)(001), 2.810(19)(121, 1 (2) over bar2), 3.069(12) (005), 2.938(10)(((1) over bar(2) over bar1,120, 1 (2) over bar1), 2.680(9)(((1) over bar(2) over bar3,200,114, (2) over bar 01), 1.771(9) (0 (4) over bar1,040), 2.618(8)(1 (2) over bar3,122), 2.062(7)(221,2 (2) over bar2, (2) over bar(2) over bar3,(2) over bar 22), and 1.600(7)((3) over bar(2) over bar1,(3) over bar 20,320). Chemical analysis by electron microprobe gave Nb2O5 6.96, ZrO2 0.12, TiO2 26.38, SiO2 27.08, FeO 0.83, MnO 2.95, MgO 0.76, BaO 3.20, SrO 5.21, CaO 4.41, K2O 0.79, Na2O 6.75, H2O 13.81, F 0.70, O = F -0.29, sum 99.66 wt.%; H2O was determined from structure solution and refinement. The empirical formula was calculated on 25 (O + F) apfu: (Na1.93Mn0.04Ca0.03)(Sigma 2)(Ca0.67Sr0.45Ba0.19K0.15)(Sigma 1.46)(Ti2.93Nb0.46Mn0.33Mg0.17Fe2+ Zr-0.10(0.01))(Sigma 4)Si4.00O24.67H13.60F0.33, Z = 1. Simplified and ideal formulae are as follows: (Ca, square)(2)Na2Ti4(Si2O7)(2)O-4(H2O)(7) and (Ca square)Na2Ti4(Si2O7)(2)O-4(H2O)(7). The FTIR spectrum of the mineral contains the following bands: 3300 cm(-1) (very broad) and 1600 cm(-1) (sharp). The crystal structure was solved by direct methods and refined to an R-1 index of 8.8%. The crystal structure of kolskyite is a combination of a TS (titanium-silicate) block and an I (intermediate) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block exhibits linkage and stereochemistry typical for Group IV [Ti (+ Mg + Mn) = 4 apfu] of Ti-disilicate minerals. In the H sheet in kolskyite, Si2O7 groups link to [6]-coordinated Ti octahedra. In the O sheet, Ti-dominant and Na octahedra each form brookite-like chains. There is one peripheral A(P) site occupied mainly by Ca (less Sr, Ba, and K) at 68%. The I block consists of H2O groups and A(P) atoms. The I block is topologically identical to those in the kazanskyite and nechelyustovite structures. The mineral is named after the Kola Peninsula (Kolskyi Poluostrov in Russian). The chemical formula and structure of kolskyite were predicted by Sokolova & Cmara (2010); this is the first correct prediction of a TS-block mineral.
The crystal structure of vuonnemite, Na11Ti4+Nb2(Si2O7)2(PO4)2O3(F,OH) from llímaussaq, Greenland, a 5.4984(6), b 7.161(1), c 14.450(2) Å, α 92.60(1), β 95.30(1), γ 90.60(1), V 565.9(2) Å3, and from the Kola Peninsula, Russia, a 5.4970(4), b 7.1630(6), c 14.437(2) Å, α 92.55(1), β 95.30(1), γ 90.61(1)°, V 565.4(1) Å3, has been solved by direct methods in the space group P-1 (Z = 1) to a residual R value of ~2%. Vuonnemite is a phosphate-bearing sorosilicate. The structure is layered along [001], and consists of (1) a layer of dominantly edge- and face-sharing coordination polyhedra for Na and P atoms, (2) a layer with corner-linked Si2O7 dimers and NbO6 octahedra, reinforced with Naφ8 hexagonal bipyramids via edge-sharing, and (3) a close-packed layer of NaO6 and TiO6 octahedra. For all samples of vuonnemite analyzed, the (Mn + Ca) content matches the Ti content in excess of 1.0 Ti apfu. The substitution (Mn + Ca)2+ + Ti4+ = Nb5+ + Na+ operates, whereby Ti in excess of 1 apfu is disordered onto the Nb site, and (Mn + Ca) populates the Na(2) and Na(3) sites. The compositional range for vuonnemite is now extended from near end-member compositions of (Mn + Ca)0 Na11 Ti1 Nb2 (Si2O7)2 (PO4)2 O3 (F,OH) to (Mn + Ca)0.33 Na10.67 Ti1.33 Nb1.67 (Si2O7)2 (PO4)2 O3 (F,OH). Correlation of mineral chemistry, crystal structure and unit-cell parameters allows prediction of many of the structural features of epistolite, the only member of the lomonosovite group for which a successful structure analysis has not been done.
In view of new data on the chemical composition and structure of a series of natural layered silicates containing three-layer blocks and their corresponding microporous minerals with related fragments, their structural features; the structural conditionality of their properties; their transformation in the presence of isomorphism, decationation, and hydration; and the problems related to typomorphism and some other questions urgent for modern mineralogy have been considered. The structures are described in terms of threelayer modules of different forms and dimensions that are present in these structures. The composition and design of the general structural base of related series—three-layer modules forming layered or linear radicals—are also discussed. The minerals of these families differ in chemical composition, symmetry, and unit-cell parameters, as well as in the topological features associated with the type of condensation of the Si tetrahedra into layers or ribbons and the degree of condensation of M polyhedra in the central part of blocks: from layers and octahedral ribbons of different widths to chains and isolated polyhedra.
Hydrogen-bearing vuonnemite from the Shkatulka hyperagpaitic pegmatite (the Lovozero alkaline massif, Kola Peninsula) was studied by single-crystal X-ray diffraction. The triclinic unit-cell parameters are as follows: a = 5.4712(1) , b = 7.1626(1) , c = 14.3702(3) , alpha = 92.623(2)A degrees, beta = 95.135(1)A degrees, gamma = 90.440(1)A degrees, sp. gr. P1, R = 3.4%. The Na(+) cations and H(2)O molecules are ordered in sites between the packets. The water molecules are hydrogen bonded to the PO(4) tetrahedra.
Several structural families reported in this book can be described by using concepts of modular crystallography (Thompson 1978; Veblen 1991; Merlino 1997; Ferraris et al. 2004). This chapter presents three groups of microporous minerals emphasizing the modular aspects of their crystal structures and the role that modularity plays in correlating different structures as well as structure and properties, namely aspects aimed at an engineering of microporous materials (cf. Rocha and Lin 2005).
The description of a crystal structure as an edifice consisting of complex building modules that occur also in other structures implicitly leads to identify features that are common to a group of compounds. This kind of group can often be expressed as a series of structures that are collinear in composition and cell parameters, information that may be crucial to model unknown structures related to the series, as illustrated by some examples in this chapter.
Biopyriboles (Fig. 1⇓) represent a first and now classical example of modular structures established by Thompson (1978). He showed that the structures of micas, pyroxenes and amphiboles share, according to different ratios, the same modules of mica ( M ) and pyroxene ( P ) and are members of a polysomatic series MmPp . The ideal chemical composition and cell parameters of the members of the series are linear functions of the ratio m / p . The classification of biopyriboles as members of a polysomatic series, a type of series belonging to the wider category of the homologous series (cf. Ferraris et al. 2004), and the consequent discovery of the multiple-chain-width biopyriboles jimthompsonite and chesterite (Veblen and Buseck 1979) dramatically proved the predictive power of these series in terms of structure characterization and modeling. The modeling of carlosturanite (Mellini et al. 1985) and of other modular structures …
The crystal structures of lomonosovite, ideally Na10Ti4(Si2O7)2(PO4)2 O4, a = 5.4170(7) Å, b = 7.1190(9) Å, c = 14.487(2) Å, α = 99.957(3)º, = 96.711(3)º, = 90.360(3)º, V = 546.28(4) Å3, Dcalc. = 3.175 g cm−3, and murmanite, ideally Na4Ti4(Si2O7)2O4(H2O)4, a = 5.3875(6) Å, b = 7.0579(7) Å, c = 12.176(1) Å, α = 93.511(2)º, = 107.943(4)º, = 90.093(2)º, V = 439.55(2) Å3, Dcalc. = 2.956 g.cm−3, from the Lovozero alkaline massif, Kola Peninsula, Russia, have been refined in the space group Pbar1 (Z = 1) to R values of 2.64 and 4.47%, respectively, using 4572 and 2222 observed |Fo ≥ 4F| reflections collected with a single-crystal Bruker AXS SMART APEX diffractometer with a CCD detector and Mo-Kα radiation. Electron microprobe analysis gave empirical formulae for lomonosovite (Na9.50Mn0.16Ca0.11)Σ9.77(Ti4+2.83Nb0.51Mn2+0.27Zr0.11Mg0.11Fe2+0.10Fe3+0.06Ta0.01)Σ4.00(Si2.02O7)2(P0.98O4)2(O3.50F0.50)Σ4, Z = 1, calculated on the basis of 26(O+F) a.p.f.u., and murmanite (Na3.32Mn0.15Ca0.21K0.05)Σ3.73(Ti4+3.08Nb0.51Mn2+0.18Fe3+0.15Mg0.07Zr0.01)Σ4.00(Si1.98O7)2(O3.76F0.24)Σ4(H2O)4, Z = 1, calculated on the basis of 22(O+F) a.p.f.u., with H2O determined from structure refinement and Fe3+/(Fe2++Fe3+) ratios obtained by Mössbauer spectroscopy. The crystal structures of lomonosovite and murmanite are a combination of a titanium silicate (TS) block and an intermediate (I) block. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral), and is characterized by a planar cell based on translation vectors, t1 and t2, with t1 ∼5.5 and t2 ∼7 Å and t1 ^ t2 close to 90°. The TS block exhibits linkage and stereochemistry typical for Group IV (Ti = 4 a.p.f.u.) of the Ti disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to Ti polyhedra of the O sheet adjacent along t1. In murmanite and lomonosovite, the invariant part of the TS block is of composition Na4Ti4(Si2O7)2O4. There is no evidence of vacancy-dominant cation sites or (OH) groups in the O sheet of lomonosovite or murmanite. In lomonosovite, the I block is a framework of Na polyhedra and P tetrahedra which gives 2[Na3 (PO)4] p.f.u. In murmanite, there are four (H2O) groups in the intermediate space between TS blocks. In lomonosovite, TS and I blocks alternate along c. In murmanite, TS blocks are connected via hydrogen bonding. The H atoms were located and details of the hydrogen bonding are discussed.
The crystal structures of four betalomonosovite samples from peralkaline pegmatites of the Khibiny alkaline complex, Kola Peninsula, Russia, were studied using single-crystal X-ray diffraction. The samples represent different chemical and structural varieties of betalomonosovite. The suggested general formula of betalomonosovite, based on the empirical data known to date, is Na5 + xTi4[Si2O7]2[PO3(OH)]2 − y[PO2(OH)2]yO2[(OH,F)2 − zOz], where 0 ≤ x ≤ 2, 0 ≤ y ≤ 1 and 0 ≤ z ≤ 1. Betalomonosovite is a transformation product of lomonosovite, Na10Ti4(Si2O7)2(PO4)2O4, due to the leaching of Na and protonation of O atoms of phosphate groups and Ti–O–Ti bridges. Distinctive features of betalomonosovite are: (a) the presence of H+ in the form of species-defining acid phosphate anion(s) [PO2(OH)2] ± [PO3(OH)]; (b) the presence of OH- anion at the Ti–O–Ti bridges in the O sheet of the HOH block and c) deficiency of Na (as compared to lomonosovite) in an amount related to the degree of protonation. A hypothetical end-member product of lomonosovite alteration with the simplified formula Na4Ti4[Si2O7]2[PO2(OH)2]2O2(OH)2 has not been reported in Nature yet. The ways and the degree of alteration may vary significantly, resulting in significant variations in both crystal structure and chemical composition, including the Na content, the degree of the substitution of O by OH, unit-cell dimensions and splitting of the P, O and/or Na sites.
The new mineral calciomurmanite, (Na,□)2Ca(Ti,Mg,Nb)4[Si2O7]2O2(OH,O)2(H2O)4, a Na-Ca ordered analogue of murmanite, was found in three localities at Kola Peninsula, Russia: at Mt. Flora in the Lovozero alkaline complex (the holotype) and at Mts. Eveslogchorr (the cotype) and Koashva, both in the Khibiny alkaline complex. Calciomurmanite is a hydrothermal mineral formed as a result of late-stage, low-temperature alteration (hydration combined with natural cation exchange) of a high-temperature, anhydrous phosphate-bearing titanosilicate, most likely lomonosovite and/or betalomonosovite, in the peralkaline (hyperagpaitic) rocks. The holotype sample is associated with microcline, aegirine, lorenzenite and fluorapatite, whereas the cotype sample occurs with microcline, aegirine, lamprophyllite, tsepinite-Ca and tsepinite-K. The mineral occurs as lamellae up to 0.1 × 0.4 × 0.6 cm, sometimes combined in fan-shaped aggregates up to 3.5 cm. Calciomurmanite is pale brownish or purple; the streak is white. The lustre is nacreous on cleavage surface and greasy on broken surface across cleavage. The (001) cleavage is perfect, mica-like; the fracture is stepped. Dmeas = 2.70(3), Dcalc = 2.85 g cm−3. The mineral is optically biaxial (−), α = 1.680(4), β = 1.728(4), γ = 1.743(4), 2Vmeas = 58(5)°. The IR spectrum is reported. The chemical composition (wt%, electron-microprobe data, H2O by the Alimarin method) is: Na2O 5.39, K2O 0.30, CaO 7.61, MgO 2.54, MnO 2.65, FeO 1.93, Al2O3 0.85, SiO2 30.27, TiO2 29.69, Nb2O5 6.14, P2O5 0.27, H2O 11.59, total 99.23. The empirical formula of the holotype sample, calculated on the basis of Si + Al = 4 apfu, is: Na1.34Ca1.04K0.05Mg0.49Mn0.29Fe0.21Nb0.36Ti2.85(Si3.87Al0.13)Σ4O16.40(OH)1.60(PO4)0.03(H2O)4.94. Calciomurmanite is triclinic, P-1, a = 5.3470(6), b = 7.0774(7), c = 12.146(1) Å, α = 91.827(4), β = 107.527(4), γ = 90.155(4)°, V = 438.03(8) Å3 and Z = 1. The strongest reflections of the X-ray powder pattern [d,Å(I)(hkl)] are: 11.69(100)(001), 5.87(68)(011, 002), 4.25(89) (−1−11, −111), 3.825(44)(−1–12, 003, −112), 2.940(100)(−1−21, −121), 2.900(79)(004, 120). The crystal structure was solved by direct methods from single-crystal low-temperature (200 K) X-ray diffraction data and refined to R = 0.0656 for the holotype and 0.0663 for the cotype. The structure is based on a three-sheet HOH block: an octahedral (O) sheet containing alternating chains of NaO6 and TiO6 octahedra and two heteropolyhedral (H) sheets consisting of Si2O7 groups, TiO6 octahedra and CaO8 polyhedra. H2O molecules occupy two sites in the interlayer space.
The crystal structure of vigrishinite, an epistolite-group heterophyllosilicate with essential Zn, has been reinvestigated; the ideal end-member formula is revised to Zn2Ti4-x(Si2O7)2O2(OH,F,O)2(H2O,OH,[ ])4 with x,1. Structure models of Zn-exchanged forms of murmanite after 5 and 24 hour experiments with 1N ZnSO4 solution at 90 C have been obtained from single-crystal X-ray diffraction data. The structural formulae are [B1]Ca0.04{Na1.22(Ti1.19Mn0.60Nb0.21)}{[A1,A2]Zn1.03(Ti1.64Nb0.36)[Si2O7]2}O2(O,OH)2(H2O)4 for vigrishinite and {Na1.14(Ti1.45Mn0.50Nb0.05)}{[A1]Ca0.77[A2]Zn0.13(Ti1.85Nb0.15)[Si2O7]2}O2(OH,O)2(H2O)4 and [B1]Zn0.15[B2]Ca0.18{Na1.06(Ti1.32Mn0.60Nb0.08)}{[A1]Zn0.70[A2]Ca0.12(Ti1.74Nb0.26)[Si2O7]2}O2(OH,O)2(H2O)4 for the 5 and 24 hour Zn-exchanged forms of murmanite, respectively (braces give successively the contents of the octahedral O and heteropolyhedral H sheets). The triclinic (P1) unit-cell parameters are respectively: a = 8.7127(17), 8.871(3), 8.748(2) A˚ ; b = 8.6823(17), 8.844(6), 8.724(2) A˚ ; c = 11.746(2), 11.734(6), 11.675(3) A˚ ; a = 91.481(4), 92.75(3), 92.503(13); b = 98.471(4), 97.60(4), 97.846(13); g = 105.474(4), 106.23(2), 105.875(13); V = 845.0(3), 872.7(8), 845.9(4) A˚ 3. Our data (1) confirm that vigrishinite was formed as a result of natural ion-exchange of murmanite Na4Ti4(Si2O7)2O4 � 4H2O with Zn2+ in low-temperature solutions; (2) prove that direct transformation of lomonosovite Na4Ti4(Si2O7)2O4 � 2Na3PO4 into vigrishinite, without prior leaching of Na+ and PO43– from the former and formation of murmanite, is unlikely; (3) suggest the following ion-exchange mechanism: during the early stage Na+ leaches into the solution whereas Ca2+, a common admixture in murmanite, migrates into one of the sites in the H sheet, leaving another site vacant for further entry of Zn; in the next stage Zn2+ enters the emptied site in the H sheet and, in small amount, into the interlayer whereas Ca2+ partly moves from the H sheet into the interlayer; (4) show the transformation of the murmanite-type unit cell (P1; V �440 A˚ 3) into the vigrishinite-type cell with a and b parameters corresponding to the ab face diagonals of the former during the first stage of the exchange with Zn, due to ordering in the H sheet. New findings of vigrishinite in two pegmatites of the Lovozero massif, Kola peninsula, Russia (at Severnyi open pit, Mt. Alluaiv and at Pegmatite #60, Mt. Karnasurt) in the same setting as at the type locality, i.e. only in close contact with cavities after dissolved sphalerite, show that the ion exchange in epistolite-group heterophyllosilicates is not uncommon in Nature. Our data indicate there is a continuous solid solution between murmanite and vigrishinite.
The paper presents the first data on the crystal chemistry of the cation-exchanged forms of layered titanosilicates belonging to the epistolite group. It was found that these heterophyllosilicates have high exchange capacity and selectivity for cations of chalcophile elements (Ag, Cu and Zn) and could be considered as potentially novel raw materials or, more likely, as possible prototypes of cation-selective synthetic microporous materials. The crystal structures of Ag- and Cu-exchanged forms of lomonosovite and Ag-exchanged form of murmanite were studied by single-crystal X-ray diffraction. The topology of the main structural unit, the HOH block, remains unchanged in cation-exchanged forms as compared to the initial lomonosovite Na4Ti4 (Si2O7)2O4 · 2Na3PO4 (P-1) and murmanite Na4Ti4(Si2O7)2O4 · 4H2O (P-1). In Ag-exchanged murmanite, Ag cations occupy two crystallographically non-equivalent positions: one in the heteropolyhedral (H) sheet and another one in the octahedral (O) sheet corresponding to the positions of Na in initial murmanite. The crystal structure of the Ag-exchanged form of lomonosovite is characterized by an increased unit-cell parameter c and doubled parameter b as compared to initial lomonosovite. Ten large-cation sites statistically occupied by Ag and Na correspond to the Na sites in the initial lomonosovite: six in the interlayer space, two in the H sheet and two in the O sheet. Silver significantly replaces Na in sites in the interlayer space and in the O sheet, whereas sites in the H sheet are less affected by the ion exchange. Unit-cell parameter c of the Cu-exchanged form of lomonosovite decreases by 3.58 % as compared to the initial lomonosovite whereas a and b remain almost the same. The Cu cations occupy two crystallographically independent positions. The Cu(1) site, corresponding to the Na(3) site in the initial lomonosovite, is located in the interlayer space. The low-occupancy Cu(2) site is located on the inversion centre in the O sheet; this site is vacant in the initial lomonosovite. The Cu(2) site is surrounded by six O atoms forming an octahedron distorted due to the Jahn-Teller effect.
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