Gabriella Ivaldi

Bond valence vs bond length in O⋯O hydrogen bonds

The total valence of the bonds between acceptors and the H atoms bonded to a water molecule (s d) is taken equal to the bond valence (st) received by the donor 0 atom from coordination bonds. For an OH- group, s d = sr-l. Plots of the bond valence s for H...O bonds as functions of O...O, H...O and O-H distances are fitted with the function s = (R/Ro)-b+ k. The most reliable fitting is obtained for s vs O...O; a minimum value of O...O ~_2.55 A for the hydrogen bonds donated by a water molecule is predicted. Similar curves from the literature are compared and discussed.

X-ray single-crystal structure refinement of an OH-rich topaz from Sulu UHP terrane (Eastern China) – Structural foundation of the correlation between cell parameters and fluorine content

An OH-rich topaz, Al 2 SiO 4 F 1.4 OH 0.6 , occurring in a kyanite quartzite of the Sulu UHP terrane (eastern China) has been optically and chemically (electron microprobe) characterised and its crystal structure refined by single-crystal X-ray diffraction data. Anisotropic refinement converged to R = 0.024 in the Pbnm space group with cell parameters a = 4.6696(8) A, b = 8.8486(17) A, c = 8.3915(22) A. The position of the hydrogen atom has been detected; because the OH → F substitution is less than 50 %, the disorder of H does not lower the overall symmetry. The influence of the OH → F substitution on the lattice parameters is discussed in terms of crystal structure and shown to affect primarily the a and b cell edges; instead c is almost constant. A critical revision of earlier linear correlations of the F content ( w f ) vs the lattice parameters and the availability of new data recently published, allowed to obtain improved regression lines for w f vs a ( r 2 = 0.982) and b ( r 2 = 0.968) cell edges.

First structure determination of an MDO-2 O mica polytype associated with a 1 M polytype

The structure refinement of associated phlogopite-2 O and phlogopite-1 M from the Khibiny massif (Kola Peninsula, Russia) is reported. Crystal data are: α = 5.2781(5), b = 9.141(1), c = 20.124(4) A, Ccmm (20); a = 5.305(2), b = 9.199(2), c = 10.232(4) A, β = 100.03(2)°, C 2/m (1 M ). Least-squares refinement of single-crystal X-ray diffraction data converged to R1 = 0.034 (2 O , 926 independent reflections) and 0.037 (1 M , 677 independent reflections). This is the first structure refinement of an MDO (standard) mica-2 O : two previous structure reports concerned anandite-2 O , which was not a true polytype, having an oP (primitive orthorhombic) lattice not compatible with the C -centred cell common to all mica polytypes. The two phlogopite polytypes show practically the same chemical composition (K 0 . 95Na 0.01 ) (Mg 2.16 Fe 0.34 Ti 0.04 Mn 0.04 Li 0.40 )[Si 3.40 Al 0.60 O 10 ][(OH) 1.35 F 0 .65] and no cation ordering. Both polytypes are affected by stacking disorder, which broadens the non-family reflections ( k ≠ 3n). As a consequence, the diffracted intensities of the two types are measured at different scales and large residues in the difference Fourier maps were observed at ± b /3 along [010], [310] and [310]. These residues ( uroviceffect ) disappear by carrying out the refinement with separate scale factors for the two types of reflections. On the basis of structural, morphology and zoning considerations the formation of the two associated polytypes is attributed to chemical oscillation in the crystallization mean.

Coexisting 3T and 2M1 polytypes of phengite from Cima Pal (Val Savenca, western Alps): Chemical and polytypic zoning and structural characterisation.

Randomly oriented phengite flakes, up to 15 cm across and 5 cm thick, occur in a metamorphic dyke at Cima Pal (upper Val Savenca, western Alps). The phengite formed in the Sesia zone at quartz-eclogite-facies conditions ( T ∼ 550°C, P ∼ 16–18 kbar) in the presence of hydrous fluid. A greenschist-facies retrogression (∼ 4–5 kbar and 500°C) locally altered the eclogite-facies assemblage. The mica flakes show yellowish and greenish zoning. This zoning is superposed by a structural zoning of 3 T and 2 M 1 (more abundant) polytypes. Single-crystal X-ray diffraction data were collected and the structures refined from the following samples. \[\mathrm{(K_{0.90}Na_{0.05})\ (Al_{1.51}Mg_{0.32}Fe_{0.18}Ti_{0.03})\ [Si_{3.40}Al_{0.60}O_{10}](OH)_{2}\ [yellowish\ 3\mathit{T};\ \mathit{a}\ 5.220(1),\ \mathit{c}\ 29.762(9)\ {\AA}]}\] , \[\mathrm{(K_{0.95}Na_{0.01})\ (Al_{1.40}Mg_{0.34}Fe_{0.27}Ti_{0.03})\ [Si_{3.50}Al_{0.50}O_{10}](OH)_{2}\ [greenish\ 3\mathit{T};\ \mathit{a}\ 5.228(1),\ \mathit{c}\ 29.73(1)\ {\AA}]}\] , \[\mathrm{(K_{0.98}Na_{0.02})\ (Al_{1.55}Mg_{0.24}Fe_{0.21}Ti_{0.02})\ [Si_{3.38}Al_{0.62}O_{10}](OH)_{2}\ [yellowish\ 2\mathit{M}_{1};\ \mathit{a}\ 5.2132(8),\ \mathit{b}\ 9.051(2),\ \mathit{c}\ 19.937(5)\ {\AA},\ {\beta}\ 95.76(2){^\circ}]}\] , and \[\mathrm{(K_{0.99}Na_{0.02})\ (Al_{1.42}Mg_{0.33}Fe_{0.24}Ti_{0.04})\ [Si_{3.45}Al_{0.55}O_{10}](OH)_{2}\ [greenish\ 2\mathit{M}_{1};\ \mathit{a}\ 5.225(1),\ \mathit{b}\ 9.057(2),\ \mathit{c}\ 19.956(6)\ {\AA},\ {\beta}\ 95.73(2){^\circ}]}\] . Tetrahedral cation order is absent in both polytypes, although partial octahedral order is present in the 3 T polytype. Colour and limited chemical zoning is attributed to a fluctuation in the fluid-phase composition. The 2 M 1 polytype with disordered pattern of octahedral cations was probably crystallised from the pre-existing 3 T polytype by a solid state reaction. The transformation occurred during the sharp decompression stage leading to the greenschist facies, which triggered nucleation and growth of the 2 M 1 polytype.

Shirokshinite, K(NaMg2)Si4O10F2, a new mica with octahedral Na from Khibiny massif, Kola Peninsula: descriptive data and structural disorder

Shirokshinite, K(NaMg 2 )Si 4 O 10 F 2 , is the analogue of tainiolite, K(LiMg 2 )Si 4 O 10 F 2 , with the M 1 octahedron fully occupied by Na instead of Li. It was found in the Kirovskii underground apatite mine (Kukisvumchorr Mountain, Khibiny massif, Kola Peninsula, Russia) as a late hydrothermal mineral in a small hyperalkaline pegmatite embedded in ristschorrite. Shirokshinite is associated with microcline, kupletskite, aegirine, natrolite, lorenzenite, calcite, remondite-(Ce), donnayite-(Y), mckelveyite-(Y) and galena. Crystals are usually skeletal and coarse hexagonal [001] prismatic. Shirokshinite is transparent to translucent, colourless to pale greyish, hardness Mohs9 ∼2.5; D(calc) = 2.922 g/cm 3 . Optically biaxial (-), α = 1.526(1), β = 1.553(2), γ = 1.553(2); 2V meas = −5(5)°, 2V calc = −0°; Y = b , Z ∼ a , Xc = 3(2)°. The IR spectrum of shirokshinite is unique even if close to that of tainiolite: in particular, the presence of Na + instead of Li + shifts some bands towards low-frequencies. Single-crystal diffraction data (Mo K α-radiation) gave a = 5.269(2), b = 9.092(9), c = 10.198(3) A, β = 100.12(7)°, Z = 2, 1 M -polytype, space group C 2/ m. Structure anisotropic refinement converged R = 0.13 for 715 observed reflections. Evidence of stacking faults in the structure is discussed and compared with the so called Ďurovic effect. The very little ditrigonal distortion in spite of the large dimension of the Na octahedron is discussed in comparison with tainiolite. A critical revision of old published data indicating octahedral Na in micas shows that this hypothesis was biased by the low quality of the chemical analyses.

Deprotonation and order-disorder reactions as a function of temperature in a phengite 3T (Cima Pal, western Alps) by neutron diffraction and Mössbauer spectroscopy.

The results on deprotonation and cation partitioning as a function of temperature in the tetrahedral and octahedral sites of a phengite 3 T , (K 0.94 Na 0.02 ) ∑0.96 (Al 1.43 Mg 0.33 Fe 0.25 ) ∑2.01 (Si 3.47 Al 0.53 ) ∑4 O 10 (OH) 2 , from Cima Pal (western Alps) are presented and discussed. They have been obtained by a synergic approach based on neutron powder diffraction from 293 K to 873 K, and on Mossbauer spectroscopy at ambient conditions, of the sample before and after heating. Order is observed at room temperature in both tetrahedral (Si fully occupies T 1) and octahedral (Al almost fills M 2) sites. Upon heating, deprotonation [mainly according to the reaction 2[(OH) - + Fe 2+ ] → 2O 2- + 2Fe 3+ + H 2 ↑], Fe oxidation and inter-site cation re-ordering reactions take place. Al moves by ~ 0.1 atoms from M 2 to M 3, and is replaced by Mg and Fe; Fe 2+ and Fe 3+ occupy different M -sites, after cooling, whereas Fe 2+ was partitioned on two sites, one site shared with Fe 3+ , before heating.

Crystal structure of natrite, gamma-Na2CO3

Abstract: A new occurrence of natrite,g-Na 2 (CO 3 ), was discovered as monominerallenses (more than 20cm in size) in pegmatites of Mt. Koashva, Khibiny alkalinemassif, Kola peninsula, Russia. Electron microprobe, XRD, IR and optical data fornatrite of this new occurrence are given. A single-crystal fragment isolated frompolysynthetic twinned material was used for collecting X-ray single-crystal data:l(Mo K a)=0.71073A, C 2/ m,a =8.905(4), b =5.237(3), c =6.045(2)A,b=101.32(3)E , 1290 reflections. Satellite reflections on oscillation photographs indicatethe presence of modulation in the structure of natrite. The average structure con-vergedto R =0.106; models with ( i ) all atomic positions split and ( ii ) anharmonicthermal displacement parameters applied to unsplit atoms converged to R =0.036and 0.029 respectively. The structure of natrite contains [001] columns formed byface sharing Na octahedra and further 7-fold coordinated Na polyhedra; the col-umns are connected by CO 3 groups.Key words: natrite, crystal structure, carbonate, modulated structure, Khibiny.

Crystal structure of kristiansenite: a case of class IIB twinning by metric merohedry

Abstract The structure of the new disilicate kristiansenite, Ca2ScSn(Si2O7)(Si2O6OH), has been solved and refined from a crystal polysynthetically twinned by metric merohedry. The Bravais lattice is mC, with parameters a = 10.028(1), b = 8.408(1), c = 13.339(2) Å, α = 90.01(1), β = 109.10(1), γ = 90.00(1)°, but the space-group type is C1 (Z = 4). The twin law is m´, and the two components of the twin have nearly identical volumes: as a consequence, the Laue group of the twin is practically 2/m. By taking into account the twinning, an anisotropic refinement of the structure in C1 converged to R1 = 0.0242 for 259 refined parameters and 4862 observed reflections. The effects of the twinning by metric merohedry and of the volume ratio of the components on the symmetry of the diffraction pattern are discussed. The triclinic structure approximates within about 0.1 Å the monoclinic symmetry, the lower symmetry resulting mainly from cation ordering. Kristiansenite represents a new type of silicate structure and the first known case with the presence of protonated and normal disilicate groups at the same time. The disilicate groups and the other polyhedra centred on cations lie on different alternating (101) planes.

Correlation between crystallization pressure and structural parameters of phengites

Abstract Linear correlations of lnP vs. the thickness, t, of the TOT mica module and vs. the ditrigonal distortion, α, have been established by analyzing a population of 43 natural phengites for which estimates of the crystallization pressure P independent of the Si content are available. Synthetic phengites are not included because they behave as a separate population. The equations for the resultant regression lines are lnP = -0.35(3)α + 5.0(3) (R = 0.90, 26 observations) and lnP = -29(2)t + 293(22) (R = 0.90, 43 observations). These linear correlations are evidence of the links involving P, chemical composition, structural parameter c (i.e., t), and α. In turn, there is a linear correlation between t and α: t = 0.0110(9)α + 9.859(7) (R = 0.92, 26 observations). The linear correlations of lnP, t, and α vs. Si content have equations: lnP = 4.2(4)Si - 12(1) (R = 0.86, 43 observations), t = -0.13(1)Si + 10.37(4) (R = 0.87, 43 observations), and α = -12.7(9)Si + 50(3) (R = 0.94, 26 observations). The observed dispersion of the data are mainly due to the uncertainty in the pressures estimated from mineral assemblages. The possibility of calculating P from only a measurement of c neither overcomes this type of uncertainty nor pretends to be competitive with petrologic geobarometers, but is does open the possibility of estimating the crystallization pressure of nanoscale phengite relics (e.g., by transmission electron diffraction).

Charge distribution from least-energy structure in Ca - Mg orthosilicates

Calcium-olivine, γ-Ca2SiO4, larnite, β-Ca2SiO4, merwinite, Ca3Mg(SiO4)2, and monticellite, CaMgSiO4, are considered. According to a rigid oxyanion scheme, eulerian orientation angles of the SiO4 tetrahedra and translation coordinates of Ca and Si atoms are specified as structural variables τk. All derivatives of the static energy (Born model) contain atomic charges and repulsive parameters as unknowns; the minimum energy conditions ∂EL/∂τk=0 yield 34 equations which are solved by a least-squares method. The set of energy parameters fitting structural properties of all four phases together is: zCa=1.50, zo=−1.10 e, rCa=1.05, ρ=0.25 Å; the Mg charge was fixed at 1.38 e, from a previous study on forsterite. An average shift of 0.04 Å is observed between experimental and least-energy calculated atomic positions. Results are compared with those of Mg2SiO4, where the fit was based both on thermoelastic and on structural properties. If no charge values were fixed “a priori”, just ratios between charges could be determined by fitting them to structural data only.