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From structure topology to chemical composition. VIII. Titanium silicates : the crystal chemistry of mosandrite from type locality of Låven (Skådon), Langesundsfjorden, Larvik, Vestfold, Norway
No full textThe crystal structure of mosandrite, ideally Na2 Ca4REETi (Si2O7)2 OF3, a = 7.4184(8), b = 5.6789(6), c = 18.873(2) Å, β = 101.410(2)°, V = 779.35(5) Å3, space group P21/c, Z = 2, Dcalc = 3.363 g.cm-3, from the type locality, Låven (Skådön), Langesundsfjorden, Larvik, Vestfold, Norway, has been refined to R1 = 6.33% on the basis of 1113 unique reflections Fo 5 4sF . Electron microprobe analysis gave the empirical formula Na1.99 (Ca3.93 Sr0.02) ∑3.95 (Ce0.41 La0.16 Nd0.1 3Pr0.04 Sm0.02 Dy0.01 Y0.13) ∑0.90 (Ti0.864+ Nb0.08 Zr0.05) ∑0.99 (Si2 O7)2 (F1.20 O0.80) ∑2.00 F2, Z = 2, calculated on the basis of 18 (O + F) a.p.f.u.. The crystal structure of mosandrite is a framework of TS (titanium silicate) blocks. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block in mosandrite exhibits linkage and stereochemistry typical for Group I (Ti = 1 a.p.f.u.) Ti-disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to the trans edges of a Na polyhedron of the O sheet. The O sheet cations give Na(NaCa)Ti (4 a.p.f.u.). The TS blocks link via common vertices of (Si2O7) groups and common vertices and edges of Ca-dominant MH and AP polyhedra. Two adjacent TS blocks are related by the glide plane cy. Composition and topology of the TS block in mosandrite and rinkite are identical. The crystal structure of mosandrite from the type locality is topologically and chemically identical to that of rinkite from the type locality of Kangerdluarssuk, Greenland
New data on the crystal-chemistry of fluoborite by means of SREF, SIMS, and EMP analysis
No full textThe crystal structure of fluoborite [Mg3F3(BO3)] was refined by Dal Negro and Tadini (1974) who provided a complete structural model. Previously, Takeuchi (1950) had refined an OH-dominant fluoborite (OH ~70%), but the limited quantity of data (extracted from two Weissenberg-Buerger photographs) did not permit the location of H atoms. Dal Negro and Tadini (1974) also could not locate H atoms because they used a crystal with near end-member composition. We have located the H bond in an OH-dominant fluoborite from the Betic Cordilleras (SE Spain). Excellent quality X-ray data on two crystals of fluoborite allowed discovery and refinement of the H position in this mineral. Electron microprobe (EMP) and secondary-ion mass spectrometry (SIMS) analyses of the light elements H, B, and F have resulted in the formulation of special procedures to obtain accurate, highquality quantitative data, which are presented in this paper. EMP, SIMS, and crystal structure refinement (SREF) data are in a good agreement. Linear equations are also presented to calculate the F content directly from cell parameters
The structure of bornemanite, a Group III Ti-disilicate mineral from Lovozero alkaline massif, Kola Peninsula, Russia
No full textThe crystal structure of bornemanite, ideally Na6BaTi2Nb(Si2O7)2(PO4)O2(OH)F, Ti+Nb = 3 apfu (atom per formula unit), a 5.4587(3), b 7.1421(5), c 24.528(2)Å, α 96.790(1), β 96.927(1), γ 90.326(1)o, V 942.4(2) Å3, sp. gr. P⎯1, Z = 2, Dcalc. 3.342 g.cm-3, from Lovozero alkaline massif, Kola Peninsula, Russia, has been solved and refined to R1 6.36% on the basis of 4414 unique reflections (Fo > 4σF). The crystal structure of bornemanite is as predicted by Sokolova [1]. Sokolova [1] established the relation between structure topology and chemical composition for 24 minerals with the TS (titanium-silicate) block, a central trioctahedral (O) sheet and two adjacent (H) sheets containing different polyhedra including (Si2O7) groups, and divided these minerals into four groups characterized by different topology and stereochemistry of the TS block. Each group of structures has a different linkage of H and O sheets in the TS block and a different arrangement of Ti (= Ti + Nb) polyhedra. In Groups I, II III and IV, Ti equals 1, 2, 3 and 4 apfu, respectively. Group III includes lamprophyllite, barytolamprophyllite, nabalamprophyllite, innelite, vuonnemite and epistolite. In bornemanite, the TS block exhibits the stereochemistry of Group III: Ti occurs in the H and O sheets, two (Si2O7) groups link to trans edges of a Ti octahedron in the O sheet. The O sheet cations give Na3Ti (4 apfu) in accord with Group III. The TS block has two different H sheets, H1 and H2, where (Si2O7) groups link to [5]Ti and [6]Nb polyhedra and there are two peripheral sites, which are occupied by Ba and Na, respectively. The crystal structure of bornemanite is a combination of a TS block and an I (intermediate) block. There are two I blocks: the I1 block is a layer of Ba atoms; the I2 block consists of Na polyhedra and (PO4) tetrahedra. The I1 and I2 blocks are topologically and chemically similar to the intermediate blocks in barytolamprophyllite and vuonnemite,
respectively. [1] Sokolova (2006)Can. Min. 44, 1273-1330
From structure topology to chemical composition. X. Titanium silicates : the crystal structure and crystal chemistry of nechelyustovite, a group III Ti-disilicate mineral (vol 73, pg 753, 2009)
Full text linkHT-XRD study of synthetic ferrian magnesian spodumene: the effect of site dimension on the P21/c ↔ C2/c phase transition.
No full textAn investigation of matrix effects in the analysis of fluorine in humite-group minerals by EMPA, SIMS, and SREF
No full textAccurate determination of F in minerals is a difficult task even when high F concentrations are present. Fluorine usually is determined by means of electron micro-probe analysis (EMPA) standardized on non-silicate-matrix compounds (e.g., fluorite), and some previous work has revealed the difficulties in determining F at high concentrations such as found in the humite-group minerals. Moreover, when both single-crystal structure refinement (SREF) and EMPA are available for the same crystal, the two estimates do not always agree. On the other hand, the secondary ion mass spectrometry (SIMS) technique is not easily applied at high F concentrations due to the existence of matrix effects related to the chemical composition and structure of the sample as well as to the concentration of the element itself. We tested the agreement among these analytical techniques in the estimation of high F contents and propose an analytical procedure for the analysis of fluorine. Our results indicate that careful selection of working conditions for EMPA of F together with appropriate correction, can yield accurate fluorine concentrations in minerals. Fluorine data extracted from refined site occupancies are systematically overestimated. New accurate working curves have been worked out for SIMS analysis of F taking Si and Mg, in turn, as the reference element for the matrix. Humite-group minerals show SIMS matrix effects on the order of similar to 10%. In analyzing fluoborite in the most unfavorable cases, the difference in Ion Yield (F/Mg) between "disoriented" humite-group minerals and "oriented" fluoborite samples can reach similar to 27%. Finally, a lower than expected IY(F/Si) from the F/Si working curve (made with humite minerals) is shown by topaz, which can be ascribed to chemical matrix effects, as well as to the covalent-type bonding between F and the major element in the matrix (Al)
From structure topology to chemical composition. X. Titanium silicates : The crystal structure and crystal chemistry of nechelyustovite, a group III Ti-disilicate mineral
Full text linkThe crystal structure of nechelyustovite, ideally Na4Ba2Mn1.5white medium square2.5Ti5Nb(Si2O7)4O4(OH)3F(H2O)6, a 5.447(1) Å, b 7.157(1) Å, c 47.259(9) Å, α 95.759(4)°, β 92.136(4)°, γ 89.978(4)°, V 1831.7(4) Å3, space group P1-,Z=2,Dcalc. 3.041gcm -3, from Lovozero alkaline massif, Kola Peninsula, Russia, has been solved and refined to R1 = 13.9% on the basis of 1745 unique reflections (Fo > 15σF). Electron microprobe analysis yielded the empirical formula (Na4.21Mn2+1.11Ca0.46white medium square1.22)Σ7.00 (Ba1.28Sr0.50K0.30white medium square0.92)Σ3.00(Ti4.14Nb1.43Mn2+0+.33Fe3+0+.06Al0.04)Σ6.00(Si8.03O28)O3.99[(OH)2.94F1.06]Σ4.00 (H2O)6.01, Z = 2, calculated on the basis of 42 (O + F) a.p.f.u., H2O and OH are calculated from structure refinement (H2O = 6 p.f.u.; F + OH = 4 p.f.u.). The crystal structure of nechelyustovite 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 III (Ti = 3 a.p.f.u.) of Ti-disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to the trans edges of a Ti octahedron of the O sheet. There are two distinct TS blocks of the same topology, TS1 and TS2, that differ in the cations of the O sheet, [(Na1.5Mn1white medium square0.5)Ti] and [(Na2Mn0.5white medium square0.5)Ti] (4 a.p.f.u.) respectively. The TS1 and TS2 blocks have two different H sheets, H1,2 and H3,4, where (Si2O7) groups link to [5]- and [6]-coordinated (Ti,Nb) polyhedra respectively. There are three peripheral sites, AP(1 3), occupied mainly by Ba (less Sr and K) at 96, 86 and 26% and one peripheral site AP(4) occupied by Na at 50%. There are two I blocks: the I1 block is a layer of Ba atoms; the I2 block consists of H2O groups and AP(3) atoms. TS blocks alternate with I blocks or link through hydrogen bonds (as in epistolite). There is a sequence of four TS blocks and three I blocks per the c cell parameter: TS2I1TS1I2TS1I1TS2
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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