1,721,507 research outputs found
Crystal structure study of a cobaltoan dolomite from Kolwezi, Democratic Republic of Congo
No full textA structural study has been undertaken on a cobaltoan dolomite, with chemical formula CaMg0.83Co0.17(CO3)2 (calcium magnesium cobalt dicarbonate), from Kolwezi, Democratic Republic of Congo. Pale-pink euhedral cobaltoan dolomite was associated with kolwezite [(Cu1.33Co0.67)(CO3)(OH)2] and cobaltoan malachite [(Cu,Co)2(CO3)(OH)2]. A crystal with a Co:Mg ratio of 1:5.6 (SEM/EDAX measurement), twinned on (11 -2 0) was used for crystal structural refinement. The refinement of the structural model of Reeder & Wenk [Am. Mineral. (1983), 68, 769-776; Ca at site 3a with site symmetry -3; Mg site at site 3b with site symmetry -3; C at site 6c with site symmetry 3; O at site 18f with site symmetry 1] showed that Co is totally incorporated in the Mg site, with refined occupancy Mg0.83Co0.17, which compares with Mg0.85Co0.15 from chemical data. The Co substitution reflects in the expansion of the cell volume, with a pronounced increasing of the c cell parameter
A crystal structural study of gabrielsonite from Långban, Sweden
No full textLångban is a worldwide famous mineralogical locality, almost unique for its mineral diversity and the peculiarities
of the occurring minerals.
The largest number of rare minerals occur in late-stage Pb–Mn–As–Sb-bearing fissures, typically oxychlorides,
arsenates and arsenites. The great species diversity is due to the unusual, almost unique co-presence of Mn2+, Mn3+, Sb5+,
As3+, As5+, Pb2+, Ba2+ and Be2+. Gabrielsonite was here established as a new species by Moore (1967), with chemical
formula PbFe(AsO4)(OH), space group P21ma, a = 7.86, b = 5.98, c = 8.62 Å. During the SYNTHESYS SE-TAF-5983
research visit to the Naturhistoriska riksmuseet, gabrielsonite crystals were extracted from a museum specimen
(NRM#19250353), with the aim of investigating its crystal structure. Single crystal intensity data were collected at
Elettra synchrotron facility, XRD1 beamline, with wavelength 0.59043 Å, resulting in 2490 unique reflections, with Rint
= 0.0796. The gabrielsonite structure was solved in space group Pmc21, with a = 6.006(1), b = 8.661(2), c = 7.909(2) Å,
and refined up to R1 = 0.0344, wR2 = 0.1140, S = 1.042. Two independent Pb cation sites, with half occupancy, are
found in gabrielsonite structure, showing distorted four coordination, with bond lengths ranging from 2.203(5) to
2.606(7) Å. The two independent As sites have both a half partial occupancy, and present the typical trigonal pyramidal
coordination, with bond lengths ranging from 1.785(6) to 1.814(5) Å. Fe shows octahedral coordination, with bond
lengths ranging from 1.936(5) to 2.118(6) Å. About the anionic part, two of the four independent oxygen sites show half
partial occupancy, while the two remaining anionic sites are fully occupied; a bond valence analyses allowed to identify
one of these sites as a hydroxyl. The crystal structure of gabrielsonite is made up by chains of edge sharing Fe octahedra
(FOC) running along a, whereas Pb1 and As1 polyhedra form through corner sharing “layers” (PAL) parallel to (010).
As2 and Pb2 polyhedra link by corner sharing to form chains (PAC) running along a, with Pb and As polyhedra
regularly alternating along the chain.
The structure of gabrielsonite can be described through a regular alternation along b of slabs hosting PAC, FOC, and
PAL structural elements. Alternatively, by analogy with the minerals of the adelite-descloizite group, it can be
described through a “framework” made up by FOC linked by corner sharing to AsO3 groups, with Pb atoms hosted in
the framework channels running along a. Present structural analyses indicate for gabrielsonite the new chemical
formula PbFeAsO3(OH), to be confirmed through appropriate EPMA, EXAFS, FTIR and Raman studies
On the Crystal-Chemistry of Rosasite and Parádsasvárite
No full textWe report the results of mineralogical and structural studies of para ́dsasva ́rite from Rudaba ́nya, Hungary, and rosasite from
Hayden, Arizona (USA). A preliminary investigation of the two minerals, which belong to the rosasite–malachite group, was
conducted using Raman spectroscopy, X-ray diffraction, and EPMA. Para ́dsasva ́rite has the chemical formula
(Zn1.91Cu0.06Mg0.02)R1.99(CO3)(OH)2, ideally Zn2(CO3)(OH)2, and rosasite has the formula (Cu1.14Zn0.84Mg0.02)R2(CO)3(OH)2,
matching the general formula CuZn(CO3)(OH)2.
The first single-crystal full structural study of rosasite from Hayden, based on synchrotron X-ray data, is reported here and
fully confirms the results obtained by Perchiazzi (2006) on the basis of X-ray powder data. The presence of structural disorder
in Cu-Zn distribution in these minerals is indicated by the streaking of reflections along c*. Rosasite is monoclinic, P21/a, a
12.2436(29) A, ̊ b 9.3555(19) A, ̊ c 3.1535(6) A, ̊ b1⁄498.69(3)8, and its crystal structure was refined to R11⁄412.4%, wR21⁄435.5%.
The presence of Cu2þ causes a Jahn-Teller distortion of coordination polyhedra, a major feature of the rosasite crystal structure,
particularly evident in the Me1 coordination polyhedron, which is fully occupied by Cu.
A Rietveld study of para ́dsasva ́rite, based on synchrotron radiation data, shows it is isostructural with rosasite, monoclinic
P21/a, a 12.253(4) A, ̊ b 9.348(3) A, ̊ c 3.167(1) A, ̊ b 1⁄4 97.700(4)8, and its crystal structure was refined to R1 1⁄4 1.45%, wR2 1⁄4
2.45%. The Me1 and Me2 coordination polyhedra in para ́dsasva ́rite, the former hosting Zn with a minor presence of Cu, the
latter fully occupied by Zn, are as expected both more regular than the corresponding polyhedra in rosasite.
Crystal-chemical considerations, based on literature chemical data for rosasite and Zn-rich malachite, indicate that a
minimum Cu content of 0.4–0.5 apfu is necessary to stabilize the malachite-type structure
Crystal structure determination and Rietveld refinement of rosasite and mcguinnessite
No full textThe crystal structure of rosasite, (Cu, Zn)(2)(CO3)(OH)(2), and mcguinnessite, (Mg,Cu)(2)(CO3)(OH)(2) have been determined from powder data. The two minerals are isostructural, with space group P2(1)/a and cell constants a=12.8976(3), b=9.3705(1), c=3.1623(1) angstrom, beta=110.262(3)degrees, V =358.54(2) angstrom(3), for rosasite and a=12.9181(4), b=9.3923(2), c=3.1622(1) angstrom, beta=111.233(3)degrees, V=357.63(2) angstrom(3) for mcguinnessite. The crystal structure refinements were lead up to R-p =7.51%, wR(p)=10.39% for rosasite and R-p =5.12%, wR(p)=6.22% for mcguinnessite. In both the two structures, the Cu coordination octahedron is distorted towards an elongated tetragonal bipyramid, whereas the Zn (in rosasite) and Mg (in mcguinnessite) coordination octahedra display an almost regular shape, their distortion being due to a partial occupancy of Cu. The carbonate group was refined as a rigid body, with a regular triangular geometry. Metal coordination octahedra polymerize through edge sharing to form octahedral "columns" and "ribbons", running along [001] and responsible for the acicular habit of these minerals. The structural relationships between rosasite and malachite are discussed
Attività della sezione di mineralogia del Museo di Storia Naturale e del Territorio dell'Università di Pisa.
No full text
Artificial Neural Networks (ANN) in the Assessment of Respiratory Mechanics
No full textThe aim of this thesis was to test the capability of Artificial Neural Networks (ANN) to estimate respiratory mechanics during mechanical ventilation (MV). ANNs are universal function approximators and can extract information from complex signals. We evaluated, in an animal model of acute lung injury, whether ANN can assess respiratory system resistance (RRS) and compliance (CRS) using the tracings of pressure at airways opening (PAW), inspiratory flow (V’) and tidal volume, during an end-inspiratory hold maneuver (EIHM). We concluded that ANN can estimate CRS and RRS during an EIHM. We also concluded that the use of tracings obtained by non-biological models in the learning process has the potential of substituting biological recordings. We investigated whether ANN can extract CRS using tracings of PAW and V’, without any intervention of an inspiratory hold maneuver during continuous MV. We concluded that CRS can be estimated by ANN during volume control MV, without the need to stop inspiratory flow. We tested whether ANN, fed by inspiratory PAW and V’, are able to measure static total positive end-expiratory pressure (PEEPtot,stat) during ongoing MV. In an animal model we generated dynamic pulmonary hyperinflation by shortening expiratory time. Different levels of external PEEP (PEEPAPP) were applied. Results showed that ANN can estimate PEEPtot,stat reliably, without any influence from the level of PEEPAPP. We finally compared the robustness of ANN and multi-linear fitting (MLF) methods in extracting CRS when facing signals corrupted by perturbations. We observed that during the application of random noise, ANN and MLF maintain a stable performance, although in these conditions MLF may show better results. ANN have more stable performance and yield a more robust estimation of CRS than MLF in conditions of transient sensor disconnection. We consider ANN to be an interesting technique for the assessment of respiratory mechanics
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