Giancarlo Della Ventura

A Rietveld and infrared study of synthetic amphiboles along the potassium-richterite - tremolite join

Abstract Amphiboles were synthesized at 750 ℃, 1 kbar (H2O) for compositions at 20% intervals along the join potassium-richterite-tremolite. Structural variations, site occupancies, and modal analyses of the experimental products (amphibole + minor diopside, quartz, and enstatite) were characterized by Rietveld structure refinement, with final RBragg indices in the range 4-6%, and by infrared spectroscopy in the principal OH-stretching region. Amphibole compositions were determined by (1) site-scattering refinement for the A and M4 sites that are occupied by (K, ⃞) (⃞ = vacancy) and (Na,Ca), respectively; and (2) mass- balance calculations involving the modal analysis and the nominal experimental product composition. These measurements agree within 1% absolute and show close agreement with electron-microprobe compositions for the two samples that we could analyze. Deviations from nominal amphibole composition are up to 19% absolute. The resulting relations between cell dimension and composition are linear. The major change in cell dimensions is a decrease of 0.25 Å in a with increasing tremolite component. The infrared spectra show two principal peaks at 3735 and 3675 cm-1, corresponding to the local arrangements MgMgMg-OH-AK (the Kr band) and MgMgMg-OH-A⃞ (the Kr band), respectively. The relative variation in peak intensity as a function of amphibole composition shows that the molar absorptivities of the two bands are significantly different. The ratio of the molar absorptivities for the two bands is 2.2.

Short-range order in synthetic aluminous tremolites: An infrared and triple-quantum MAS NMR study

Abstract Fourier-transform infrared spectra (FTIR) were recorded on a series of synthetic amphiboles along the join Ca1.8Mg5.2Si8O22(OH)2-Ca1.8(Mg4.2Al)(Si7Al)O22(OH)2. The spectra were fitted by up to six component bands by optimization and non-linear least-squares techniques. 27Al MAS NMR and triple-quantum (3Q) MAS NMR spectra were recorded for the synthetic amphibole Ca1.8(Mg4.8Al0.4)(Si7.6Al0.4)O22(OH)2. The fitted FTIR and NMR spectra show that VIAl occurs at the M2 site and at the M1 or M3 sites; by analogy with previous crystal-structure refinement results on natural amphiboles, VIAl occupancy of M2 and M3 is presumed. The fine structure present in the FTIR spectra indicates that they are also affected by NNN (next-nearest-neighbor) interactions. There are two types of NNN arrangements: (1) SiSi or SiAl at adjacent T1T1 dimers; (2) permutation of Mg/Al over M2M2M3 sites. Discounting those arrangements unlikely on bond-valence grounds, there are two arrangements that give rise to five distinct bands in the infrared spectra. There are two principal conclusions: (1) infrared spectra of amphiboles in the principal OH-stretching region can be affected significantly by NNN effects; (2) the small number of bands due to NNN effects indicates that amphiboles show strong short-range order

On the symmetry and crystal chemistry of britholite: New structural and microanalytical data

Abstract We provide in this paper complete structural and micro-chemical characterization of two britholite samples with compositions (Mn0.04Ca4.75REE4.37Th0.72U0.12)Σ10.00(Si5.57P0.25B0.16)Σ5.98O24(OH0.23F1.77)Σ2.00 (from Latium, Italy) and (Na0.98Ca2.01REE6.97)Σ9.96(Si5.07P0.75)Σ5.82O24(OH0.53F1.47)Σ2.00 (from Los Islands, Guinea). The crystal-chemical formulae were calculated by combining electron-microprobe analyses for intermediate-Z elements (Na, Ca, Mn, Si, P), ion-microprobe analyses for low-Z (H, Li, Be, B, F) and high-Z elements (Ba, Y, REE and actinides), and high-quality (Robs 1.2-2.6) single-crystal structure refinements. Structure refinements indicate that the best approximation to the real symmetry is the P63 space group. In britholite, the lowering of symmetry with respect to the P63/m space group of apatite means that the O3 and O3a atoms are no longer equivalent and allows the tetrahedron to rotate up to ~4° around the Si-O1 bond. Consequently, the O3a atom moves closer to the REE1a site, whereas the O3 atom moves farther from the REE1 site and closer to the REE2 site, which thus assumes a [7+1] coordination. The infrared spectrum of britholite from Latium shows a unique and very broad band in the OH-stretching region at 3437 cm-1, which is consistent with the ordering of trivalent REE cations at the REE2 site. The calculated integral molar absorptivity εi is 23600 L · mol-1⋅cm-2. A remarkable constancy in the unit-cell volume along the whole apatite-britholite compositional range is observed for values of the aggregate ionic radius at the REE sites shorter than 1.12 and longer than 1.15 Å, whereas large variations are observed for intermediate values; this behavior suggests constraints due to the rigidity of the tetrahedral group, that are further enhanced at high symmetry.

Site occupancies in synthetic monoclinic amphiboles: Rietveld structure refinement and infrared spectroscopy of (nickel, magnesium, cobalt)-richterite

Abstract Amphiboles were synthesized at 750 °C, 1 kbar (H2O) on the binary joins (nickel, magnesium)-richterite and (magnesium, cobalt)-richterite. Structural variations and site occupancies were characterized by Rietveld structure refinement, with final RBragg indices in the range 4-9%, and by powder infrared spectroscopy in the principal OH-stretching region. Site-occupancy refinement of Ni-Mg and Mg-Co distributions give the partition coefficients over M1,3 and M2 where KM2+ = (M2+/Mg)M1,3/(M2+/Mg)M2, and M2+ = Ni2+ or Co2+, KdNi = 2.98 ± 0.37 and KdCo = 1.34 ± 0.31. Both Kd values are greater than 1.0, whereas [6]r(Ni2+) < [6]r(Mg) < [6]r(Co2+); this indicates that cation size is not the primary factor affecting the ordering of Ni-Mg and Mg-Co over the octahedral sites. The infrared spectra of intermediate binary compositions show fine structure caused by ordering of Ni- Mg or Mg-Co over the M1,3 sites and by ordering of Na and ⃞ (vacancy) at the A site; thus intermediate compositions show an eight-band spectrum in the principal OH stretching region. Precise band intensities were derived by nonlinear least-squares fitting of Gaussian band shapes to the observed spectra. The relative observed intensities of the combinations of bands 3I₀A + 2I₀B + I₀C and I₀B + 2I₀C + 3I₀D are in accord with the equations of Burns and Strens (1966), indicating that there is no significant variation in molar absorptivity with frequency (energy) for individual bands within a single sample (spectrum). Combined with the results of Skogby and Rossman (1991) on polarized single-crystal infrared spectra of amphiboles, this result suggests that different local configurations of M1,3 cations in amphiboles couple such that the transition probabilities of the associated OH groups are equal.

Synthesis and infrared spectroscopy of amphiboles along the tremolite-pargasite join

Amphiboles along the tremolite-pargasite join have been hydrothermally synthesized at 900°C, 3 kbar. Run products were characterized by optical and electron microscopy, powder X-ray diffraction and infrared spectroscopy. Under the experimental conditions used, there is complete solid-solution between tremolite and pargasite and the variation of cell parameters is linear as a function of composition. The infrared spectra in the principal OH-stretching region are consistent with the predictions of Hawthorne (1997) on local bond-valence grounds, and show strong short-range order of cations at the octahedral sites and strong coupling between Al at T(l) and Na at A.

Near-infrared study of short-range disorder of OH and F in monoclinic amphiboles

Abstract Amphiboles were synthesized along the joins tremolite-fluorotremolite, richterite-fluororichterite and potassic-richterite-potassic-fluororichterite at 750 °C and 1 kbar P(H2O). Infrared spectra of the amphiboles were recorded in the principal OH-stretching region. Amphiboles of the tremolite-fluorotremolite series show one-mode behavior, a single band due to a local MgMgMg-OH-A⃞ (⃞ = vacancy) arrangement; this behavior is consistent with no coupling between NNN (next-nearest-neighbor) O3 anions either through the O3- O3 edge or across the vacant A-site cavity. The amphiboles of the richterite-fluororichterite and potassic-richterite-potassic-fluororichterite series show two-mode behavior, two bands due to the local arrangements MgMgMg-OH-ANa-OH and MgMgMg-OH-ANa-F (and their K equivalents); this behavior is consistent with coupling between NNN O3 anions across the filled A-site cavity through Na or K that occupies the A-site. A mathematical model is developed to describe local (OH,F) ordering in amphiboles as a function of F content. The variation in infrared band intensities is consistent with complete short-range disorder of OH and F in the synthetic amphiboles of the richterite-fluororichterite and potassicrichterite- potassic-fluororichterite series.

Octahedral versus tetrahedral coordination of Al in synthetic micas determined by XANES

Abstract We used the JUMBO monochromator at SSRL to measure the Al K-edge X-ray absorption spectra of synthetic micas having variable Al content and occupancy, from 0 to 2/4 in the octahedral M positions, and 0 to ⅔ in the tetrahedral T positions. The measured Al K edges differ markedly, but the differences may have a common explanation: (1) Micas containing ⅓ Al in M or ¼ Al in T have K edges that differ in the energy and intensity of the first two features, which are related to interaction of Al with its first-shell nearest neighbors (O and OH or F). They are nearly identical to the K edges of reference minerals such as albite (tetrahedral Al only) or grossular (octahedral Al only). (2) Micas containing Al in both M and T have K edges that can be interpreted as a weighed combination of the simple edges.

Single-crystal FTIR and X-ray study of vishnevite, ideally [Na6(SO4)][Na2(H2O)2](Si6Al6O24)

Abstract This paper reports a single-crystal FTIR spectroscopic study of vishnevite, ideally [Na6(SO4)] [Na2(H2O)2](Si6Al6O24), a member of the cancrinite group of feldspathoids. The study was done on several crystals from various geological occurrences. Infrared spectra show that most samples, and in particular the specimens from the holotype locality at Vishnevye Mountains (Urals, Russia), contain molecular CO2 as the main carbon species in the structural pores, while the specimens from Loch Borolan (Scotland) were found to be CO3-rich. Polarized-light measurements show that the linear CO2 molecules are oriented perpendicular to the crystallographic c axis. Structure refinement of sample Pi4 from Latium (Italy) shows usual H2O···Na···H2O sequences within the undecahedral cages; however, difference Fourier maps suggest the presence of additional protons in the channels, possibly forming OH groups. The FTIR spectra show three absorptions in the 3800.3200 cm-1 region. The first one, at 3590 cm-1 is strongly polarized for E||c while the second, at 3535 cm-1, behaves almost isotropic. These two bands are assigned to the stretching vibrations of an asymmetric water molecule in the structural cages. The third broad absorption at 3320 cm-1, is predominantly polarized for E||c and is assigned to additional OH groups in the channels. Detailed microspectroscopic mapping showed several samples from Latium (Italy) to be zoned with respect to the CO2/CO3 content, thus pointing to a possible use of the volatile content of these minerals for petrological modeling.

Ti-rich phlogopite from Mt. Vulture (Potenza, Italy) investigated by a multianalytical approach : substitutional mechanisms and orientation of the OH dipoles

Trioctahedral mica samples, collected at Cava St. Antonio (Mt. Vulture, Italy) were studied by combining electron-microprobe and C-H-N elemental analyses, single-crystal X-ray diffraction refinement, Mossbauer and Fourier transform infrared spectroscopies. Electron-microprobe analyses show the crystals to be quite homogeneous with TiO 2 ∼3 wt% and F ranging from 0.42 to 0.59 wt%. Quantitative analyses of H combined with ferric/ferrous ratios from Mossbauer data allowed reliable crystal-chemical formulae to be derived. The results suggest that the entry of both Ti 4+ and Fe 3+ in the structure occurs through R-oxy substitution mechanisms involving deprotonation at O(4). This inference is supported by X-ray structure-refinement results (notably the c cell-parameter, the off-centering of the M2 cation towards O(4), the bond-length distortions of the cis -M2 octahedron) obtained using anisotropic thermal parameters in space group C2/ m . The amount of oxy-substitutions from both electron-microprobe and X-ray data is in agreement with carbon-hydrogen-nitrogen analyses which give an average anion composition (OH 1.25 O 0.65 F 0.10 ). Polarized-light infrared spectroscopy shows a complex OH-stretching spectrum which is composed of several overlapping (at least five) components. These can be assigned to the main octahedral local configurations that are compatible with the chemical composition. Pleochroic Fourier transform infrared spectroscopy measurements done along the principal optical directions show that the O-H bond axis is tilted from [001] and provide the average orientation of the O-H dipole in the structure: O-H ⁁ α ∼ 23° and O-H ⁁ γ ∼ 56°.

Crystal structure of non-metamict Th-rich hellandite-(Ce) from Latium (Italy) and crystal chemistry of the hellandite-group minerals

Abstract Powder neutron diffraction [ISIS Facility (U.K.), POLARIS diffractometer] was used to investigate the effect of elevated pressure on cation partitioning in synthetic Mg0.94Al2.04O4 spinel. The distributions of Mg, Al, and vacancies were studied as a function of pressure, by refinement of the T- and M-site scattering lengths, and determination of the cation partitioning through numerical minimization methods. The partially disordered Mg/Al distribution, which results from the synthesis process, show an increase in ordering between 6 and 18 kbar, where Mg and Al order to the T- and M- sites, respectively. Pressure effectively tends to stabilize MgAl-spinels with a “normal structure,” and this behavior is supported by numerical simulations based on classical electrostatic calculations.