196,226 research outputs found
MOSANDRITE: STRUCTURAL AND CRYSTAL-CHEMICAL RELATIONSHIPS WITH RINKITE
No full textemical (EPM data), TG–DSC studies and structural investigations indicate that mosandrite presents a particular chemical
composition (low Ca, Na and F contents, high amount of H2O), unit-cell parameters (a 7.398, b 5.595, c 18.662 Å, b 101.37°,
V 757.29 Å3; space-group symmetry P21/c; Z = 2), and a rinkite-type structure characterized by a low occupancy of the M(2) and M(3) sites
PORTITE DISCREDITED = NATROLITE AND NEW DATA ON SCHNEIDERITE (= LAUMONTITE)
No full textThe sample of portite studied by D'Achiardi (1873), a fragment of the holotype of Meneghini & Bechi (1852), was rediscovered in some of the oldest collections of the Natural History Museum of Pisa University. X-ray and optical data demonstrate that portite is identical to natrolite, thus allowing us to discredit the mineral name portite. This proposal is approved by the I.M.A, Commission on New Minerals and Mineral Names. Moreover, X-ray data show ''schneiderite'' to be identical to laumontite
Phase transition at high pressure in Cu2CO3(OH)(2) related to the reduction of the Jahn-Teller effect
No full textHydroxycarbonates with the general formula Me-2(CO3)(OH)(2) are widely used materials in industrial processes and are widespread in nature. The Cu term, malachite, Cu2CO3(OH)(2), is monoclinic, P2(1)/a. Substitution of Cu2+ with other bivalent cations such as Mg, Zn, Fe, Cu or Ni is possible and leads to a different structure type, rosasite, P2(1)/a or P2(1)/b11 in the same cell setting as malachite. Rosasite structure is topologically similar to malachite, but the symmetry elements are oriented differently with respect to structural units. The stability of the malachite-like structure (MS) compared with the rosasite-like structure (RS) has been suggested to be related to the Jahn-Teller effect in CuO6 coordination polyhedra. For this reason the hypothesis of the phase transition of malachite, Cu2CO3(OH)(2), to a rosasite structure at high pressure, as a result of the reduced Jahn-Teller effect, has been tested and confirmed by powder and single-crystal diffraction structural studies: above 6 GPa the malachite structure is no longer stable and transforms to a RS structure. RS Cu2CO3(OH)(2) is 3% more dense than malachite and the bulk modulus is remarkably higher, 80 (2) GPa compared with 48 (4) GPa. The longer apical Cu-O bonds in the distorted Me1 octahedral site are progressively shortened with increasing pressure, revealing a decrease in the Jahn-Teller effect at high pressure. The transition has a first-order character, is reversible with a significant hysteresis, and there is no evidence of any intermediate phase between the two structures. We then have further evidence that in the Me-2(CO3)(OH)(2) compounds, the two main structural types, MS and RS, are closely related. The former structure is stabilized only when Cu is the prevalent cation in the octahedral sites, and it can transform directly to the RS as a function of thermodynamic changes
Distinct domains in guarinite from Monte Somma, Italy: crystal structures and crystal chemistry.
No full texttract“Guarinite” is a typical accessory mineral of the Monte Somma syenite; it belongs to the cuspidine group and displays a domain structure. “Guarinite” contains up to three distinct domains, corresponding to three different ways to connect disilicate groups and walls of octahedra; these may all be simultaneously present in the same crystal. The domains present in the crystals of “guarinite” have cell type I, II and IV, according to the classification scheme proposed for cuspidine-group minerals. Domain IV is the most common, and domain I is the rarest; domain II may occur as the only domain, whereas domain I occurs only in association with domain IV, which invariably predominates. So far, the actual structure of the various domains had not been defined. EPMA and single-crystal structural studies indicate that domain I of “guarinite” displays space group P1, with a 10.973(2), b 10.306(1), c 7.367(3) Å, a 90.03(3), b 109.63(3), g 90.11(2)°, with a crystal-chemical formula Ca3(Ca0.72
Zr0.28)S1.00(Zr0.86M0.14)S1.00(Ca0.59Mn0.25Fe0.16)S1.00(Na1.20Ca0.76)S1.96(Si1.98O7)2(F2.88O1.12)S4.00, where M represents Nb, Ti, Al, Sr, and REE. Domain I is isostructural with hiortdahlite II, and its crystal structure was refined to a final R of 0.072. Domain II of “guarinite” displays space group P1211, with a 10.836(1), b 10.270(1), c 7.296(1) Å, b 109.13(3)°, with a crystal-chemical formula Ca3Zr(Nb0.56Fe0.15Mn0.10Ti0.10Zr0.09)S1.00(Ca0.72Mn0.18M0.10)S1.00(Na0.77Ca0.23)S1.00(Na0.80Ca0.22)S1.02(Si2O7)2(O2.17
F1.83)S4.00, where M represents Al, Mg, Sr and Y. Domain II is isostructural with wöhlerite, and its crystal structure was refined to a final R of 0.045. Domain IV of “guarinite” adopts space group P1, with a 10.970(2), b 10.943(2), c 7.365(1) Å, a 109.63(2), b 109.65(2), g 83.39(1)°, with a crystal-chemical formula Ca4Zr(Ca0.31Mn0.25Fe0.16Zr0.14M0.14)S1.00(Na1.20
Ca0.76)S1.96(Si1.98O7)2(F2.88O1.12)S4.00, where M represents Nb, Ti, Al, Sr and REE. Domain IV is isostructural with hiortdahlite I, and its crystal structure was refined to a final R of 0.067. One should note that the refinements of domain I and domain IV (both twinned) have been carried out on the same crystal. EMPA and SEM studies show the presence of chemically homogeneous crystals as well as crystals with distinct chemical zoning due to a wide variation of the major elements Nb, Ca, Na, F, and pointing to the possible coupled substitution Nb5+ + 2Na+ + O2– →← 3Ca2+ + F– as one of the main mechanisms of chemical variation
I minerali delle scorie ferrifere etrusche di Baratti
No full textVengono presentati dati chimici, cristallografici e morfologici per 42 distinte specie mineralogiche, principalmente cloruri, solfati e carbonati dei piombo e/o rame, identificate nelle scorie ferrifere etrusche di Baratti. Alcune caratteristiche e ricorrenti sequenze paragenetiche sono spiegate in funzione della variazione di parametri chimici quali pH e attività del cloro nella soluzione di cristallizzazione. Dallo studio in diffrattometria di polveri, ottico ed in fluorescenza-X delle scorie metallurgiche, si deducono temperature di lavoro e provenienza del minerale trattato nei processi metallurgici
The mineralogical collections of the Natural History Museum of Pisa University. International Mineralogical Association
No full text40Ar/39Ar dating of tephra in blue ice (Northern Victoria Land, Antarctica): prospects and limitations.
No full textTephra layers are commonly embedded within ice in Antarctica. They represent powerful
qualitative and quantitative stratigraphic markers of ice ages and dynamics, as well as a
chronometer for all palaeoclimatic and palaeoenvironmental information that ice might contain.
The 40Ar/39Ar method was used to date tephra sampled in northern Victoria Land, in the
productive Frontier Mountain meteorite trap and in nearby regions, in an attempt to constrain
the age of the ice field. The first results were not conclusive because both laser total-fusion ages
and laser step-heating ages varied widely, an indication of sample contamination and/or biased
original samples due to the presence of parentless 40Ar. The youngest recorded ages for each
layer can be considered an upper limit for the age of the tephra and its enclosing ice. They are
of the order of 150-170 ka for Frontier Mountain, in good agreement with the limit indicated by
meteorite terrestrial ages, and 60-80 ka for Lichen Hills and Oona Clif
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