Crystal-chemistry and short-range order of fluoro-edenite and fluoro-pargasite: a combined x-ray diffraction and ftir spectroscopic approach

In this study we address the crystal-chemistry of a set of five samples of F-rich amphiboles from the Franklin marble (USA), using a combination of microchemical (EMPA), SREF, and FTIR spectroscopy methods. The EMPA data show that three samples fall into the compositional field of fluoro-edenite whereas two samples are enriched in high-charged C cations, and must be classified as fluoro-pargasite. Mg is by far the dominant C cation, Ca is the dominant B cation (with BNa in the range 0.00-0.05 apfu, atoms per formula unit), and Na is the dominant A cation, with A0 (vacancy) in the range 0.07-0.21 apfu; WF is in the range 1.18-1.46 apfu. SREF data show that: TAl is completely ordered at the T(1) site; the M(1) site is occupied only by divalent cations (Mg and Fe2+); CAl is disordered between the M(2) and M(3) sites; ANa is ordered at the A(m) site, as expected in F-rich compositions. The FTIR spectra show a triplet of intense and sharp components at ~ 3690, 3675, and 3660 cm-1, which are assigned to the amphibole, and the systematic presence of two very broad absorptions at 3560 and 3430 cm-1. These latter are assigned, on the basis of polarized measurements and FPA imaging, to chlorite-type inclusions within the amphibole matrix. Up to eight components can be fitted to the spectra; band assignment based on previous literature on similar compositions shows that CAl is disordered over the M(2) and M(3) sites, thus supporting the SREF conclusions based on the bond distance analysis. The measured frequencies are typical of O-H groups pointing toward Si-O(7)-Al tetrahedral linkages, thus allowing to characterize the SRO (short-range-order) of TAl in the double chain. Accordingly, the spectra show that in the fluoroedenite/pargasite structure, the T cations, Si and Al, are ordered in such a way that Si- O(7)-Si linkages regularly alternate with Si-O(7)-Al linkages along the double chain.Comment: 38 pages, 10 figures - in press. Mineralogical Magazine, special issue for the international year of crystallography (2013) in pres

FTIR imaging in diffusion studies: CO2 and H2O in a synthetic sector-zoned beryl

In this work we investigate the strongly inhomogeneous distribution of CO2 and H2O in a synthetic beryl having a peculiar hourglass zoning of Cr due to the crystal growth. The sample was treated at 800°C, 500 MPa, in a CO2-rich atmosphere. High-resolution FESEM images revealed that the hourglass boundary is not correlated to physical discontinuities, at least at the scale of tens of nanometers. Polarized FPA-FTIR imaging, on the other side, revealed that the chemical zoning acts as a fast pathway for carbon dioxide diffusion, a feature never observed so far in minerals. The hourglass zone boundary may be thus considered as a structural defect possibly due to the mismatch induced by the different growth rates of each sector. High-resolution synchrotron-light FTIR imaging, in addition, also allows enhancement of CO2 diffusion along the hourglass boundary to be distinguished from diffusion along fractures in the grain. Therefore, FTIR imaging provides evidence that different diffusion mechanisms may locally combine, suggesting that the distribution of the target molecules needs to be carefully characterized in experimental studies. This piece of information is mandatory when the study is aimed at extracting diffusion coefficients from analytical profiles. Combination of TOF-SIMS and FPA data shows a significant depletion of type II H2O along the hourglass boundary, indicating that water diffusion could be controlled by the distribution of alkali cations within channels, coupled to a plug effect of CO2. © 2015 Della Ventura, Radica, Bellatreccia, Cavallo, Cinque, Tortora and Behrens

The temperature behaviour of water in leucite

Naturally leucite crystallizes in the cubic phase, with space group Ia3d (Peacor, 1968). On cooling below T = 625°C it undergoes a phase transition to a tetragonal I4/a form (Mazzi et al. 1976); there are indications, however, that an additional tetragonal phase is stable over a narrow temperature interval (Lange et al. 1986). Palmer et al. (1997) have shown that the displacive phase transition to tetragonal symmetry is due to twisting of tetragonal prisms of corner-linked (Al,Si)O4 tetrahedra about [001] and a collapse of the [111] structural channels with concomitant volume reduction. Although nominally anhydrous (NAM), leucite typically contains significant amounts of water; this feature was reported for samples from Roccamonfina (Balassone et al., 2006) and the Alban Hills volcano (Della Ventura et al., 2008). Della Ventura et al. (2008) have shown in addition that H2O may be significantly zoned, thus providing a potential tool to monitor the evolution of the magmatic conditions with time. More recently, Martucci et al. (2011) studied the dehydration of synthetic B-substituted leucite (KBSi2O6) by synchrotron powder diffraction and concluded that the structural modifications accompanying the tetragonal cubic transition is associated with the migration of H2O molecules through the [111] channels. We relate here a single-crystal high-T in situ FTIR study of a set of natural inclusion-free leucite phenocrystals occurring within lava flows, pyroclastic rocks or ejecta in the Roman Comagmatic Province. The spectra show broad absorptions in the 4000-3000 cm-1 region consisting of overlapping components around 3604, 3500 and 3250 cm-1. Interestingly, two different types of spectra are observed in the H2O stretching region, indicating that water molecules may be trapped in leucite in two different environments (hereafter “type I” and “type II”). These different H2O types are systematically associated with samples from different volcanic areas, thus suggesting a possible role of the petrological conditions (pressure, temperature) of crystallization on the H2O entrapment in leucite. FTIR-FPA images show significant H2O zoning across the samples; crystals with homogeneously-distributed water were selected for the dehydration experiments, done using a Linkam T600 heating stage fitted under a NicPlan FTIR microscope at University Roma Tre. The evolution of the water loss as a function of T was monitored by measuring the principal H2O water absorption. The data indicate a continuous water loss with a break in the trend; in “type I” leucite the slope change occurs at ~ 500°C, and dehydration is complete at T > 600°C, probably close to the transition temperature. In “type II” leucite, the slope change occurs at ~ 350-400°C, and dehydration is complete at ~ 500°C. This behaviour is compared with isostructural materials like analcime or pollucite. Mazzi, F., Galli, E., and Gottardi, G. (1976) Am. Mineral., 61, 108-115. Peacor, D.R (1968) Z. Kristall., 127, 213-224. D.C. Palmer, M.T. Dove, R.M. Ibberson, B.M. Powell (1997) Am. Mineral. 82, 16-29. G. Balassone, A. Beran, G. Fameli, C. Amalfitano, C. Petti (2006) N. Jahr. Miner. Abh., 182, 149-156 G. Della Ventura, F. Bellatreccia, M. Piccinini (2008) Am. Mineral., 93 1538-1544. Lange, R.A., Carmichael, LS.E., and Stebbins, IF. (1986) Am. Mineral., 71, 937-945

New crystal-chemical and structural data of dietrichite, ideally ZnAl₂(SO₄)₄•22H₂O, a member of the halotrichite group

New crystal-chemical and structural data of a sample of dietrichite, ideally ZnAl2(SO4)4·22H2O, from the pyrite mine of Boccheggiano, Grosseto, Italy, are reported. This sample, unlike holotype dietrichite, is very close to the ideal chemical composition, in fact combined ICP and thermogravimetry indicate a formula (Zn0.98Fe0.07)Al1.91(SO4)4.03·21.88H2O based on 38O. The crystal structure has been refined by the Rietveld method on transmission X-ray powder diffraction data (Rp = 4.13%, Rwp = 5.44%, RB = 4.66%). Dietrichite is monoclinic P21/c, Z = 4, a = 6.1757(2), b = 24.262(1), c = 21.206(1) Å, = 100.436(3)°. The structure of dietrichite consists of one ZnO(H2O)5 octahedron, two independent Al(H2O)6 octahedra, and four independent SO4 tetrahedra per asymmetric unit. The only direct connection between polyhedra is by sharing of an oxygen atom, O(16), between S(4) and Zn. The structure contains 22 water molecules, 17 of which are octahedrally co-ordinated with Zn and Al cations whereas the remaining five molecules are only linked via hydrogen bonds to O or other H2O molecules. Hexagonal channels, running along [100], originate from a regular alternation of one ZnO(H2O)5 octahedron, two Al(H2O)6 octahedra, and three SO4 tetrahedra. Within the structure two types of channels may be identified, the first one containing three and the second two H2O molecules. Band positions of the IR spectrum of dietrichite are consistent with those of reference data