1,721,106 research outputs found
Introduction: The role of modern mineralogy in cultural heritage studies
No full textThis short introduction aims to rethink the role of modern mineralogy and highlights the diverse and important contributions that it may provide in the study of materials and processes relevant to cultural heritage. It is argued that mineralogy lies in a very special position between Earth and materials sciences and that mineralogists have a profound perception of the structural and chemical complexity of natural materials. They possess knowledge of both the ancient and recent geological and physicochemical processes which produced the rawmaterials used by humans, and of the analogue processes used to transform them into artefacts. It is thus highly appropriate that a volume in the EMU series acknowledges some of the recent contributions of mineralogy to the investigation of human history, art and technology
Short-range order in amphiboles: Si and Al at T(1) in edenite
No full textModeling the Al distribution in the amphibole structure has important implications for the geobarometry of igneous and metamorphic processes [1,2,3]. Most thermodynamic analyses assign VIAl only at M(2) andIVAl only at T(1)[4]; however, crystal-structure work has shown that this model is inadequate, at least for calcic amphiboles where VIAl disorders between the M(2) and M(3) sites[5] and IVAl disorders between the T(1) and T(2) sites[6] (at high mg# or high T, respectively). In this work, we report an FTIR study in the OH-stretching region of three amphibole compositions with different Si/Al ratios at the T sites (richterite, fluoro-edenite and pargasite). In the amphibole structure, the H atom forms a hydrogen bond to the closest O(7) atom [7]. When the T(1)-O(7)-T(1) linkages are of the type Si-O(7)-Al, the deficit in bond valence at the O(7) anion must be alleviated via a stronger O(3)-H...O(7) hydrogen bonding, and this feature affects significantly the O-H stretching frequency. Hence careful analysis of the OH-spectra may detect SRO of cations at the T(1) sites in amphiboles.
In synthetic richterite, all the tetrahedra are occupied by Si, and all the T(1)-O(7)-T(1) linkages are of the Si-O(7)-Si type; accordingly, the OH-stretching spectrum consists of a single band at 3730 cm-1, which is assigned to the MgMgMg-OH-ANa:SiSi local configuration (neglecting minor deviations toward tremolite). In pargasite all the T(2) sites are occupied by Si and half of the T(1) sites are occupied by Al, hence all the T(1)-O(7)-T(1) linkages in the structure must be of the Si-O(7)-Al type in order to avoid insufficient bond-strength contribution to the O(7) oxygen atom, and thus all H atoms are involved in an hydrogen bond with the closest O(7) atom. Accordingly, a single band is observed at 3709 cm-1, and is assigned to the local MgMgMg-OH-ANa:SiAl configuration. In edenite, the composition of the T sites is Si7Al1; hence, half of the T(1)-O(7)-T(1) linkages must be of the Si-O(7)-Si type, and half must be of the Si-O(7)-Al type. Hence, there are two possible patterns of order between T(1)Si and T(1)Al, and these must have different spectral expressions in the infrared. The first (“non-clustered”, or “fully ordered”) pattern has Si-O(7)-Si linkages regularly alternating with Si-O(7)-Al linkages. The second pattern has clusters of Si-O(7)-Al linkages alternating with clusters of Si-O(7)-Si linkages (the total number of the different linkages in the two cluster being equal). We can label this second pattern as “clustered pattern”.
In the first model, all OH-stretching bands must be of “pargasite”-type and should be observed at ~ 3709 cm-1 [or at lower wavenumber if cations different from Mg occur at the M(1,3) octahedra]. In the second model, we must observe two bands with almost the same intensity at 3730 (richterite-type) and 3709 cm-1 (pargasite-type) (or at lower wavenumbers in the presence of different octahedral cations). The OH-stretching spectra of edenites systematically show only absorptions at frequency < 3710 cm-1, thus confirming complete short-range order between T(1)]Si and T(1)Al throughout the double-chain.
[1] Spear, F.S. (1981) Am. J. Sc., 281, 697-734.
[2] Hammarstrom, J.M. and Zen, E-an (1986) Am. Min., 71, 1297-1313.
[3] Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., and Sisson, V.B. (1987) Am. Min., 72, 231-239.
[4] Graham, C.M. and Navrotsky, A. (1986) Contrib. Mineral. Petrol., 93, 18-32.
[5] Oberti, R., Hawthorne, F.C., Ungaretti, L., and Cannillo, E. (1995a) Can. Min., 33, 867-878.
[6] Oberti, R., Ungaretti, L., Cannillo, E., Hawthorne, F.C., and Memmi, I. (1995b) Eur. J. Min., 7, 1049-1063.
[7] Della Ventura, G., Hawthorne, F.C., Robert, J.-L., Delbove, F., Welch, M.D., Raudsepp, M. (1999) Eur. J. Min., 11, 79-94
The crystal chemistry of gismondines: the non-existence of K-rich gismondines
No full textMicroprobe analyses and unit cell dimensions are tabulated for gismondines from 17 localities. Their chemical composition varies only slightly from the stoichiometric formula Ca4Al8Si8O32.18H2O. The Si/(Al + Si) ratio varies from 0.514 to 0.542; Ba, Fe and Mg are always absent and K, Na and Sr are low or absent. The K-rich analyses in the literature are explained by the frequent occurrence of phillipsite-gismondine intergrowths. No correlation between the relatively constant cell dimensions and chemical composition was found. The strongest correlation is between Si/(Al + Si) and (Na + K)/(Na + K + Ca). The narrow range of chemical variation in this zeolite is attributed to the ordered (Si,Al) distribution. TG and DTG curves are given for one gismondine.-R.A.H
Thermoelasticity, cation exchange, and deprotonation in Fe-rich holmquistite: Toward a crystal-chemical model for the high-temperature behavior of orthorhombic amphiboles
No full textThe thermoelastic behavior of a crystal of Fe-rich holmquistite with crystal-chemical formula A(K0.01Na0.01)B(Li1.88Mg0.10Na0.02)C(Mg1.68Fe1.422+ Mn0.022+ Al1.88)TSi8.00O22W[(OH)1.97F0.03] was studied by single-crystal X‐ray diffraction at temperatures up to 1023 K, where isothermal annealing in air for 160 h yielded the loss of 0.85 H apfu coupled with oxidation of M1Fe. A complex pattern of cation exchanges was deciphered by comparing structure refinements done before and after annealing. Li migration from the M4 to M3 site is responsible for nonlinearity of the c parameter around 600 K during the first annealing. Cooling of the partially deprotonated crystal to room temperature (RT) showed discontinuities in trends of the b and c parameters around 820-800 K, which cannot be ascribed to a phase transition and can be explained by a rearrangement of the structural units affecting the geometry of the M4 polyhedron. Such discontinuities have never been observed in amphiboles before and must be related to dimensional constraints deriving from the peculiar composition of this amphibole, which contains the smallest possible homovalent constituents, i.e., BLi, CAl, and TSi. The calculated thermoelastic parameters are: Fe-rich holmquistite: αa = 1.36(2)×10-5; αb = 0.55(1)×10-5; αc = 1.5(1)×10-5 - 6.7(9)×10-9; αV = 3.5(3)×10-5 - 0.8(3)×10-8 (polynomial); 2.58(6)×10-5 (linear); partially deprotonated Fe-rich holmquistite: αa = 1.324(9)×10-5 (RT-1023 K); αb = 0.60(1)×10-5 (RT-773 K); αc = 0.68(2)×10-5 (RT-773 K); αV = 2.59(2)×10-5 (RT-773 K). Fe-rich holmquistite is much stiffer than all the previously studied orthorhombic Pnma and Pnmn amphiboles. The results of this work allow progress toward a general model that may explain how the amphibole structure responds to non-ambient conditions, and allows the release of water in diverse geological environments
Fluoronyboite from Jianchang (Su-Lu, China) and nyboite from Nybo (Nordfjord, Norway): A petrological and crystal-chemical comparison of these two high-pressure amphiboles
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The photochemical reaction between 1,4-dicyanonaphthalene and methylbenzenes. Electron transfer and formation of benzylic radicals
No full textHigh-T studies of orthorhombic amphiboles: the dehydrogenation process and ist effect on cation ordering and thermal expansivity in gedrite
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