1,721,017 research outputs found
I-1 - I2/c ferroelastic phase transition in the feldspar Ca0.2Pb0.8Al2Si2O8 as a function of temperature.
No full textTEM observations on the P -I phase transition in feldspars along the join CaAl2Si2O8-SrAl2Si2O8.
No full textLimiti ed utilità clinica delle risposte evocate visive nella patologia neurologica dell'adulto
No full textIncomplete blueschist re-crystallization in high grade metamorphics from the Sesia-Lanzo unit (Vasario-Sparone subunit, Western alps): a case history of metastability
No full textThe Vasario-Sparone (VS) subunit is part of the Sesia-Lanzo unit (Western Alps) and represents a thin and discontinuous slice of pre-Alpine deep continental crust. It is tectonically interposed between the "eclogitic mica schists" and the "gneiss minuti" subunits. Metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the VS subunit. The VS rocks record a pre-Alpine high grade of metamorphism (T=780±50°C, P≈6 kbar) which is comparable to the metamorphism dated around 500 Ma in the Ivrea Zone. During the Alpine evolution the VS rocks sustained an early blueschist stage (T=300-400°C, P≥8-10 kbar) of probable Cretaceous age and a later low-T, low-P stage of Tertiary age. The blueschist transformations are governed by slow reaction kinetics, by the heterogeneous distribution of pervasive deformations and by syn-metamorphic fluid phase. In particular, transient kyanite formed in the early stages of the blueschist re-crystallization in response to a process controlled by fluid-deficient conditions and by internal buffering of aH2O at low value
Low-pressure ferroelastic phase transition in rutile-type AX2 minerals: cassiterite (SnO2), pyrolusite (MnO2) and sellaite (MgF2)
No full textThe structural behaviour of cassiterite (SnO2), pyrolusite (MnO2) and sellaite (MgF2), i.e. AX2-minerals, has been investigated at room temperature by in situ high-pressure single-crystal diffraction, up to 14 GPa, using a diamond anvil cell. Such minerals undergo a ferroelastic phase transition, from rutile-like structure (SG: P42/mnm) to CaCl2-like structure (SG: Pnnm), at ≈ 10.25, 4.05 and 4.80 GPa, respectively. The structural evolution under pressure has been described by the trends of some structure parameters that are other than zero in the region of the low-symmetry phase’s stability. In particular, three tilting-angles (ω, ω′, ABS) and the metric distortion of the cation-centred octahedron (quantified via the difference between apical-anion and equatorial-anion distances Δ|Xax−Xeq|) are used to express the atoms’ readjustment, i.e. relaxation, taking place in the CaCl2-like structures under pressure. The crystallographic investigation presented is complemented with an analysis of the energy involved in the phase transition using the Landau formalism and adopting the following definition for the order parameter: Q = η11–η22, ηij being the spontaneous strain tensor. The dependence of ω, ω′, ABS and Δ|Xax−Xeq| on Q allows determination of a correlation between geometrical deformation parameter and energy. Lastly, the relaxation mechanisms that exploits ω, ω′, ABS and Δ|Xax−Xeq| may be related to the ionic degree of bonding, the latter modelled via quantum mechanics and Bader theory. Sellaite, the mineral exhibiting the highest degree of ionic bonding among those investigated, tends to accomplish relaxation through pure rotation of the octahedron, rather than a metric distortion (Δ|Xax−Xeq|), which would shorten inter-atomic distances thus increasing repulsion between anions
Thermodynamic behaviour of the high temperature phase transition P-1 – I-1 along the join An-SrF (CaAl2Si2O8 – SrAl2Si2O8)
No full text
THERMODYNAMIC BEHAVIOUR OF THE HIGH TEMPERATURE PHASE TRANSITION P-1 - I-1 ALONG THE JOIN AN-SRF (CAAL2SI2O8 - SRAL2SI2O8)
No full textThermodynamic behaviour of the high temperature P -I phase transition along the CaAl_2Si_2O_8-SrAl_2Si_2O_8 join
No full textIn situ high-temperature synchrotron radiation powder diffraction patterns were taken from room temperature to T = 740°C from synthetic feldspars along the join CaAl2Si2O8–SrAl2Si2O8 (An–SrF). Three samples of composition An95SrF5, An90SrF10 and An85SrF15 were investigated, and the evolution of cell parameters with T was determined by Rietveld analysis of powder X-ray diffraction patterns. The high-temperature P[`1]\text-I[`1]P1-I1 phase transition, previously observed with Tc = 241°C in anorthite, was found in An95SrF5, An90SrF10 and An85SrF15 feldspars at Tc = 233(5)°, 195(2)° and 174(2)°C respectively. The transition was revealed by the disappearance of critical reflections and variations in the rate of change of cell parameters with temperature. A significant, although small (between 0.0025 and 0.0012 at room temperature), spontaneous strain could be measured, allowing the thermodynamic behaviour of the transition to be modelled. A second-order trend for An90SrF10 and An85SrF15 [P[`1]P1 phase is An60SrF40
Vincenzo Neri (1880–1960) and his sign to detect leg weakness due to corticospinal tract injury
No full textThe Italian neurologist Vincenzo Neri (1880–1960), a pupil of Joseph Babiński (1857–1932), greatly contributed to refining the semiotics of neurological examination and was a pioneer in medical cinematography. In 1909, Neri proposed a sign to diagnose leg paresis due to a pyramidal tract lesion. According to Neri, if a patient standing with the legs apart and the arms crossed on the chest bends the trunk of the pelvis, when the trunk has almost reached the horizontal line, the leg on the paralyzed side flexes, whereas the unaffected leg remains extended. This sign reflects a spinal hyperfunctioning emerging after a pyramidal lesion, and should be interpreted as a part of a triple flexion reflex. Beyond the acute stage, it could reflect an unusual pattern of flexor spasticity involving the lower limb due to corticospinal tract injury. The sign described by Neri retains its validity in identifying this organic leg weakness due to pyramidal lesions, particularly when it is mild or in its early stages
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