Order of [6]Ti4+ in a Ti-rich calcium amphibole from Kaersut, Greenland : a combined X-ray and neutron diffraction study

In order to characterize the role of Ti in the crystal structure of calcium amphiboles with high or even dominant oxo-component, the crystal structure of a Ti-rich calcium amphibole from a gabbro at Kaersut, Greenland, has been refined with single-crystal MoK\u3b1 X-ray intensity data to an R1(F) index of ~0.025, and with single-crystal Laue neutron intensity data to an R1(F) index of ~0.053. The crystal used for X-ray structure refinement was characterized by electron- and ion-microprobe analysis. The site populations of the C-group cations Mg, Fe and Ti were calculated from the refined site-scattering values for the M(1), M(2) and M(3) sites derived by both X-ray and neutron diffraction. Ti is distributed among all the three sixfold coordinated M sites, with a strong preference for the M(1) and M(3) sites, where its main role is maintaining electroneutrality at the deprotonated O(3) site. The pattern of distortion of the M(1), M(2) and M(3) octahedra differs from that in F-free deprotonated or partly deprotonated amphiboles, where Ti4+ does not occur at the M(3) site. The neutron structure refinement provides also a clear picture of the environment of the proton, anisotropic displacement behaviour and potential hydrogen-bonding arrangements. A trifurcated hydrogen-bonding configuration has been identified, with two O(6) and one O(7) oxygen atoms as acceptors of weak hydrogen-bonds

From structure topology to chemical composition. XX. Titanium silicates : The crystal structure of hejtmanite, Ba2Mn4Ti2(Si2O7)2O2(OH)2F2, a Group-II TS-block mineral

The crystal structure of hejtmanite, Ba2Mn4Ti2(Si2O7)2O2(OH)2F2, from Mbolve Hill, Mkushi River area, Central Province, Zambia (holotype material) has been refined on a twinned crystal to R1 = 1.88% on the basis of 4539 [|F| > 4\u3c3|F|] reflections. Hejtmanite is triclinic, C-1, a = 10.716(2), b = 13.795(3), c = 11.778 (2) \uc5, \u3b1 = 90.07(3), \u3b2 = 112.24(3),\u3b3= 90.03(3)\ub0, V = 1612(2) \uc53. Chemical analysis (electron microprobe) gives: Ta2O5 0.09, Nb2O5 1.27, ZrO2 0.65, TiO2 14.35, SiO2 23.13, BaO 26.68, SrO 0.19, FeO 11.28, MnO 15.12, Cs2O 0.05, K2O 0.33, F 3.82, H2Ocalc. 1.63, O = F-1.61, total 97.10 wt.%, where the H2O content was calculated from the crystal-structure refinement, with (OH + F) = 4 apfu. The empirical formula, calculated on the basis of 20 (O + F) anions, is of the form AP2 MO4 MH2 (Si2O7)2(XO)4(XP)2, Z=4: (Ba1.82K0.07 Sr0.02)\u3a31.91(Mn2.33Fe21 1:65Zr0.04Mg0.03)\u3a33.95(Ti1.88Nb0.10Zr0.02)\u3a32(Si2.02O7)2O2[(OH)1.89 F0.11]\u3a32F2. The crystal structure 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 topology of the TS block is as in Group-II TS-block minerals: Ti (+ Nb) = 2 apfu per (Si2O7)2 [as defined by Sokolova (2006)]. In the O sheet, five [6]MO sites are occupied mainly by Mn, less Fe2+ and minor Zr and Mg, with = 2.198 \uc5 (\u3c6 = O,OH), ideally giving Mn4 apfu. In the H sheet, two [6]MH sites are occupied mainly by Ti, with = 1.962 \uc5 (\u3c6 = O,F), ideally giving Ti2 apfu; four [4]Si sites are occupied by Si, with = 1.625 \uc5. TheMH octahedra and Si2O7 groups constitute the H sheet. The two [12]Ba-dominant AP(1,2) sites, with = 2.984 \uc5 (\u3c6 = O, F), ideally give Ba2 apfu. Two XOM(1,2) and two XOA (1,2) sites are occupied by O atoms and OH groups with minor F, respectively, ideally giving (XO)4 = (XM O)2 + (XAO)2=O2(OH)2 pfu. Two XP M(1,2) sites are occupied by F, giving F2 apfu. TS blocks link via a layer of Ba atoms which constitute the I block. Simplified and end-member formulae of hejtmanite are Ba2(Mn,Fe2+)4Ti2 (Si2O7)2O2(OH,F)2 F2 and Ba2Mn4Ti2(Si2O7)2O2(OH)2F2, Z = 4. Hejtmanite is a Mn-analogue of bafertisite, Ba2Fe2+4 Ti2(Si2O7)2O2(OH)2F2

From structure topology to chemical composition. XXIII. Revision of the crystal structure and chemical formula of zvyaginite, Na2ZnTiNb2(Si2O7)2O2(OH)2(H2O)4, a seidozerite-supergroup mineral from the Lovozero alkaline massif, Kola peninsula, Russia

The crystal structure and chemical formula of zvyaginite, ideally Na2ZnTiNb2(Si2O7)2O2(OH)2(H2O)4, a lamprophyllite-group mineral of the seidozerite supergroup from the type locality, Mt. Malyi Punkaruaiv, Lovozero alkaline massif, Kola Peninsula, Russia have been revised. The crystal structurewas refined with a new origin in space group C1, a = 10.769(2), b = 14.276(3), c = 12.101(2) \uc5, \u3b1 = 105.45(3), \u3b2 = 95.17(3), \u3b3 = 90.04(3)\ub0, V = 1785.3(3.2) \uc53, R1 = 9.23%. The electron-microprobe analysis gave the following empirical formula [calculated on 22 (O + F)]: (Na0.75Ca0.09K0.04 \u25a11.12)\u3c32 (Na1.12Zn0.88Mn0.17Fe2+ 0.04 \u25a10.79)\u3c33 (Nb1.68Ti1.25Al0.07)\u3c33 (Si4.03O14)O2 [(OH)1.11F0.89]\u3c32(H2O)4, Z = 4. Electron-diffraction patterns have prominent streaking along c 17and HRTEM images show an intergrowth of crystalline zvyaginite with two distinct phases, both of which are partially amorphous. The crystal structure of zvyaginite is an array of TS (Titanium-Silicate) blocks connected via hydrogen bonds between H2O groups. The TS block consists of HOH sheets (H = heteropolyhedral, O = octahedral) parallel to (001). In the O sheet, the [6]MH(1,5) sites are occupied mainly by Ti, Zn and Na and the [6]MO(2,3) sites are occupied by Na at less than 50%. In the H sheet, the [6]MH(1,2) sites are occupied mainly by Nb and the [8]AP(1) and [8]AP(2) sites are occupied mainly by Na and \u25a1. The MH and AP polyhedra and Si2O7 groups constitute the H sheet. The ideal structural formula is Na Nb2NaZnTi(Si2O7)2O2(OH)2(H2O)4. Zvyaginite is a Zn-bearing and Na-poor analogue of epistolite, ideally (Na)Nb2Na3Ti(Si2O7)2O2(OH)2(H2O)4. Epistolite and zvyaginite are related by the following substitution in the O sheet of the TS-block: (Na+ 2 )epi\u2194Zn2+ zvy +\u25a1zvy. The doubling of the t1 and t2 translations of zvyaginite relative to those of epistolite is due to the order of Zn and Na along a (t1) and b (t2) in the O sheet of zvyaginite

Fluorapophyllite-(Cs), CsCa₄(Si₈O₂₀)F(H₂O)₈, a new apophyllite-group mineral from the Darai-Pioz Massif, Tien-Shan, Northern Tajikistan

Fluorapophyllite-(Cs) (IMA 2018-108a), ideally CsCa4(Si8O20)F(H2O)(8), is an apophyllite-group mineral from the moraine of the Darai-Pioz glacier, Tien-Shan, Northern Tajikistan. Associated minerals are quartz, pectolite, baratovite, aegirine, leucosphenite, pyrochlore, neptunite, fluorapophyllite-(K), and reedmergnerite. Fluorapophyllite-(Cs) is a hydrothermal mineral. It is colorless and has a vitreous luster and a white streak. Cleavage is perfect; it is brittle and has a stepped fracture. Mohs hardness is 4.5-5. D-meas. = 2.54(2) g/cm(3), D-calc. = 2.513 g/cm(3). Fluorapophyllite-(Cs) is unixial (+) with refractive indices (lambda = 589 nm) omega = 1.540(2), epsilon = 1.544(2). It is non-pleochroic. Chemical analysis by electron microprobe gave SiO2 48.78, Al2O3 0.05, CaO 22.69, Cs2O 10.71, K2O 1.13, Na2O 0.04, F 1.86, H2Ocalc. 14.61, -O=F2 -0.78, sum 99.09 wt.%; H2O was calculated from crystal-structure analysis. The empirical formula based on 29 (O + F) apfu, H2O = 8 pfu, is (Cs0.75K0.24)Sigma(0.99)(Ca3.99Na0.01)Sigma(4)(Si8.01Al0.01)Sigma 8.02O20.03F0.97(H2O)8, Z = 2. The simplified formula is (Cs,K)(Ca,Na)(4)(Si,Al) 8 O20F(H2O)(8). Fluorapophyllite-(Cs) is tetragonal, space group P4/mnc, a 9.060(6), c 15.741(11) angstrom, V 1292.10(19) angstrom(3). The crystal structure has been refined to R-1 = 4.31% based on 498 unique (F-o > 4 sigma F) reflections. In the crystal structure of fluorapophyllite-(Cs), there is one [4] T site occupied solely by Si,,T-O. = 1.615 angstrom. SiO4 tetrahedra link to form a (Si8O20)(8-)sheet perpendicular to [001]. Between the Si-O sheets, there are two cation sites: A and B. The A site is coordinated by eight H2O groups [O(4) site], A-O(4) = 3.152(4) angstrom; the A site contains Cs(0.75)K(0.24)A(0.01 square 0.01), ideally Cs apfu. The Cs-O bond length of 3.152 angstrom is definitely larger than the K-O bond length of 2.966-2.971 angstrom in fluorapophyllite-(K), KCa4(Si8O20)F(H2O)8. The [7]B site contains Ca3.99Na0.01, ideally Ca-4 apfu; < B-phi > = 2.417 angstrom (phi = O, F, H2O). The Si-O sheets connect via A and B polyhedra and hydrogen bonding; two H atoms have been included in the refinement. Fluorapophyllite-(Cs) is isostructural with fluorapophyllite-(K). Fluorapophyllite-(Cs) is a Cs-analogue of fluorapophyllite-(K)

Rubidium-rich feldspars and associated minerals from the Luolamäki pegmatite, Somero, Finland

Rubidium feldspar occurs near the core zone of the highly fractionated petalite-subtype Luolamäki granitic pegmatite in intimate intergrowth with other feldspars which are part of a characteristic sequence of alteration of pollucite. Pods of pollucite are cut by 5-20 cm-wide veins of albite, petalite, non-perthitic microcline, lepidolite and quartz, by thinner veins of fine-grained micas and spodumene, and are replaced by metasomatic adularia. Grains of rubidium feldspar occur as a potentially ordered phase in the vein microcline in association with earlier-exsolved albite, and also as late thin (< 5 μm) veinlets. Rubidium feldspar also occurs as a potentially disordered phase which crystallized along with metasomatic adularia. Both generations of (Rb,K)-feldspar have a similar compositional range, close to the join KAlSi3O8-RbAlSi3O8, typically with up to ~21 wt.% Rb2O (~70 mol.% Rbf) and with minor Cs, but neglible Na, Ca, Fe or P. Extreme compositions have 26.0 wt.% Rb2O (89.0 mol.% Rbf) and 1.26 wt.% Cs2O (2.8 mol.% Csf). The diffuse compositional gradients from microcline to rubicline are consistent with a solid-state exsolution origin, followed by fluid-assisted textural coarsening which generates distinct phase boundaries. In contrast, metasomatic adularian (Rb,K)-feldspar was precipitated at low temperature (250-150°C) and fine-scale zoning with variable K/Rb is preserved as a growth feature

From structure topology to chemical composition. XII.Titanium silicates : the crystal chemistry of rinkite, Na2Ca4REETi(Si2O7)2OF3

Rinkite, ideally Na 2Ca 4 REETi(Si 2O 7) 2OF 3, is a common mineral in alkaline andperalkaline rocks. The crystal structures of five rinkite crystals from three alkaline massifs: Ilimaussaq, Greenland; Khibiny, Kola Peninsula, Russia andMont Saint-Hilaire, Canada, have been refinedas two components related by the TWIN matrix (-1 0 0, 0-1 0, 1 0 1) (Mo-K\u3b1 radiation). The crystals, a = 7.4132-7.4414, b = 5.6595-5.6816, c = 18.8181-18.9431 \uc5, \u3b2 = 101.353-101.424(2)\ub0, V = 776.1-786.7 \uc5 3, space group P2 1/c, Z = 2, D calc = 3.376-3.502 g cm 3, were analysedusing an electron microprobe subsequent to collection of the X-ray intensity data. Transmission electron microscopy confirmed the presence of pseudomerohedral twinning in rinkite crystals. The crystal structure of rinkite is a framework of TS (titanium silicate) blocks. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block in rinkite exhibits linkage andstereochemistry typical for Group I (Ti = 1 a.p.f.u.) of Ti disilicate minerals: two H sheets connect to the O sheet such that two (Si 2O 7) groups link to the trans edges of a Na polyhedron of the O sheet. The crystal chemistry of rinkite and nacareniobsite-(Ce) is discussed

From structure topology to chemical composition. XI. Titanium silicates : Crystal structures of innelite-1T and innelite-2M from the Inagli massif, Yakutia, Russia, and the crystal chemistry of innelite

The crystal structures of two polytypes of innelite, ideally Ba 4Ti2Na2M2+Ti(Si2O 7)2[(SO4)(PO4)]O2[O(OH)] where M2+ = Mn, Fe2+, Mg, Ca: innelite-1T, a 5.4234(9), b 7.131(1), c 14.785(3) \uc5, \u3b1 98.442(4), \u3b2 94.579(3), \u3b3 90.009(4)\ub0, V 563.7(3) \uc53, space group P1, D calc. = 4.028 g/cm3, Z = 1; and innelite-2M, a 5.4206(8), b 7.125(1), c 29.314(4) \uc5, \u3b2 94.698(3)\ub0, V 1128.3(2) \uc53, space group P2/c, D calc. = 4.024 g/cm 3, Z = 2, from the Inagli massif, Yakutia, Russia, have been refined to R values of 8.99 and 7.60%, respectively. Electron-microprobe analysis gave the empirical formula for innelite as (Ba3.94Sr0.06) S 4.00(Na2.16Mn2+0.38Fe 2+0.17Mg0.15Ca0.10\u25a1 0.04)\u3a33(Ti2.97Nb0.02Al 0.02)\u3a33.01Si4.01(S1.02P 0.81\u25a10.17)\u3a32H1.84O 25.79F0.21 which is equivalent to(Ba3.94Sr 0.06)\u3a34.00(Ti1.97Nb0.02Al 0.02)\u3a32.01(Na2.16Mn2+0.38Fe2+0.17Mg0.15Ca 0.10\u25a10.04)\u3a33Ti(Si 4.01O14)[(SO4)1.02(PO 4)0.81(OH)0.51H2O 0.17]O2[(OH0.99F0.21) \u3a31.20O0.80], calculated on the basis of 26 (O + F) anions, with H2O calculated from structure refinement. The crystal structure of innelite 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) and exhibits linkage and stereochemistry typical for Ti-disilicate minerals of Group III (Ti = 3 a.p.f.u.): twoH sheets co nnect tothe O sheet such that two(Si2O7) groups link to the trans edges of a Ti octahedron of the O sheet. The I block contains T sites, statistically occupied by S and P, and Ba atoms. In the structures of innelite-1T and innelite-2M, TS blocks are related by an inversion centre and a c y glide plane, respectively. HRTEM images show a coherent intergrowth of the two polytypes, together with an as-yet unidentified 3c10 \uc5 phase

Strategies for quantification of light elements in minerals by SIMS : H, B and F

Using a large set of silicate crystals, characterized by Structure REFinement (SREF), Electron Probe Micro-Analysis (EPMA) and Secondary Ion Mass Spectrometry (SIMS), and mounted with known crystallographic orientation [1], we propose a new SIMS quantification for H, B and F (from ppm level to several wt.%), using 27Al+ and 44Ca+, in turn, as the reference isotope for the matrix, and propose suitable calibration standards to obtain accurate results. The final SIMS data are then compared to those obtained using Si as the reference element, with those available from EMPA (B and F), and with the crystallographic constraints derived from SREF investigation. The results of this study can be extended to the measurement of light elements in complex silicate or non-silicate samples