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Povzetek: Traditionally, glass-ceramics are inorganic non-metallic materials obtained by the controlled crystallization of a glass. A modern definition has widened this class of materials to solid materials containing at least one glassy and one crystalline phase. The glass is usually obtained by quenching a melt. Re-heating it to a temperature slightly above the glass transition temperature allows nucleation while an often applied second annealing step at a higher temperature causes most of the crystal growth. As in most materials, the composition and the microstructure of glass-ceramics widely governs their properties. The morphology, i.e., size, and aspect ratio of the crystal phases is of special significance and depends on the crystal structure and the occurring growth mechanism. The morphology is also affected by the chemical composition and the temperature/time schedule of the crystallization process, here components of minor concentrations can have a great effect. This review addresses the effects of nucleating agents, phase separation, crystal orientation alignment and stress introduction as tools to tailor the properties of glass-ceramic materials. Future developments in the field of glass-ceramics are discussed.
Ključne besede: glass ceramics, controlled crystallization, microstructure, properties
Objavljeno v DiRROS: 12.02.2025; Ogledov: 761; Prenosov: 406
Celotno besedilo (28,08 MB)
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Povzetek: We present undercooling (∆T) experiments aimed at investigating the effect of growth kinetics on the textural and compositional evolution of clinopyroxene crystals growing from a high-K basalt erupted during the 2003 paroxysm of Stromboli volcano (Italy). The experiments were performed at P = 350 MPa, T = 1050–1210 °C, H2Omelt = 0–3 wt%, and fO2 = Ni-NiO + 1.5 buffer. An initial stage of supersaturation was imposed to the melt under nominally anhydrous (∆Tanh = 10–150 °C) and hydrous (∆Thyd = 25–125 °C) conditions. Afterwards, this supersaturation state was mitigated by melt relaxation phenomena over an annealing time of 24 h. Results show that plagioclase is the liquidus mineral phase of the high-K basalt at ∆Tanh = 10 °C and dominates the phase assemblage as the degree of undercooling increases. Conversely, clinopyroxene and spinel co-saturate the melt at ∆Thyd = 25 °C, followed by the subordinate formation of plagioclase. At ∆Tanh/hyd ≤ 50 °C, the textural maturation of clinopyroxene produces polyhedral crystals with {−111} (hourglass) and {hk0} (prism) sectors typical of a layer-by-layer growth mechanism governed by an interface-controlled crystallization regime. At ∆Tanh/hyd ≥ 75 °C, the attainment of dendritic and skeletal morphologies testifies to the establishment of diffusion-limited reactions at the crystal-melt interface. 3D reconstructions of synchrotron radiation X-ray microtomographic data reveal a composite growth history for clinopyroxene crystals obtained at ∆Tanh/hyd ≥ 95 °C. The early stage of melt supersaturation produces rosette-like structures composed of dendritic branches of clinopyroxene radiating from a common spinel grain, which acts as surface for heterogeneous nucleation. As diffusive relaxation phenomena progress over the annealing time, the elongate dendrites that constitute the inner crystal domain are partially infilled by the melt and develop skeletal overgrowths in the outer domain. With the increasing degree of undercooling, TAl and M1Ti cations are progressively incorporated in the lattice site of clinopyroxene at the expense of TSi and M1Mg cations. Because of the effect of H2Omelt on the liquidus depression and melt depolymerization, crystals obtained at ∆Thyd are also more enriched in TAl and M1Ti and depleted in TSi and M1Mg than those growing at ∆Tanh. The emerging picture is that the morphological and geochemical evolution of clinopyroxene is mutually controlled by the combined effects of melt supersaturation and relaxation phenomena. A new empirical relationship based on the cation exchange reactions in the lattice site of clinopyroxene is finally proposed to estimate the degree of undercooling governing the crystallization of augitic phenocrysts erupted during normal and violent explosions at Stromboli.
Ključne besede: clinopyroxene, undercooling, hourglass, crystallization, microtomography
Objavljeno v DiRROS: 27.10.2023; Ogledov: 1707; Prenosov: 1044
Celotno besedilo (6,46 MB)
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