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摘要
Glass stability (GS) indicates the glass reluctance or ability to crystallise upon heating and it can be characterised by several methods and parameters. GS is frequently used to retrieve glass-forming ability (GFA) of corresponding liquids: the likelihood of obtaining a crystal-free glass through melt-quenching. In the present study, GS has been determined for the first time on six sub-alkaline glasses having complex (natural) compositions, the most widespread and abundant on Earth. The two end-members are a basalt and a rhyolite, B-100 and R-100, plus intermediate compounds B80R20, B60R40, B40R60, B20R80. Each glass was heated in a differential scanning calorimetry (DSC) at a rate of 10 degrees C/min (600 degrees C/h) to measure in-situ T-g (glass transition), T-x (onset of crystallization) and T-m (melting) temperatures, from ambient to their liquidus temperatures. The ex situ run-products quenched at T-m have been characterised by scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA) techniques, in order to quantify textures and compositions of phases, respectively. R-100 and B20R80 do not shown any DSC peaks, whereas B40R60, E60R40, Esollao and B-100 thermograms display progressively more resolvable peaks. As SiO2 (wt%) in the melt increases from B-100 to B40R60, T-x linearly increases, Tm first decreases and then levels off, whereas Tg weakly changes. R-100 and B20R80 run-products are completely glassy, while from B-100 to B40R60 the amount of glass (gl) increases from 48.5 to 97 area%, counterbalanced by a decrease of clinopyroxene (cpx) content from 47.7 to 16 area%. The spinel (sp) content is constrained within a narrow range of 0.9-3.8 area%. Conversely, plagioclase (plg) crystallizes heterogeneously on the Al2O3 holders only in B-100 and B80R20 and at distance < 100 mu m from it R-100, B20R80, B40R60, and B60R40, ex-situ glasses exhibit chemistries very close to their starting compositions, according to the absence (or scarcity) of crystals formed during heating. Instead, B-100 and B80R20 glasses are enriched in Si, Al, and Na but depleted in Fe, Mg, and Ca due to internal crystallization of sp and mostly cpx. Specifically, the composition of cpx from B-100 is enriched in Ca-M2, Mg-M1, Fe-M2.M1, and Al-M1.T. The values of K-T, K-H, K-W, K-LL, and w(2) (as GS parameters) increase linearly and monotonically as a function of SiO2, showing high correlation coefficients (R-2 = 0.93-0.95). Moreover, T-x values and GS parameters highly correlate with GFA via R-c (critical cooling rate), as previously determined by ex-situ cooling-induced experiments. This leads to the conclusion that GS scales with GFA for natural silicate compositions. In addition, the in situ Re value of B-100 measured with DSC is > 45 degrees C/min ( > 2700 degrees C/h), corroborating the value of R-c of similar to 150 C/min (9000 degrees C/h) determined by ex-situ experiments. In turn, relevant solidification parameters on heating or cooling obtained by DSC investigations, also for chemically complex (natural) systems, extend the results from previous observations conducted on simple silicate systems. These outcomes are relevant for lavas or magmas that re-heat glass-bearing volcanic rocks, as well as for fabricating glass-ceramic materials with desirable texture and composition of phases starting from abundant and very cheap raw volcanic rocks.
- 出版日期2017-12-1
- 单位Perugia
