Glass stability (GS) of chemically complex (natural) sub-alkaline glasses

Abstract

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, B100 and R100, plus intermediate compounds B80R20, B60R40, B40R60, B20R80. Each glass was heated in a differential scanning calorimetry (DSC) at a rate of 10 °C/min (600 °C/h) to measure in-situ Tg (glass transition), Tx (onset of crystallization) and Tm (melting) temperatures, from ambient to their liquidus temperatures. The ex-situ run-products quenched at Tm 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.

R100 and B20R80 do not shown any DSC peaks, whereas B40R60, B60R40, B80R20 and B100 thermograms display progressively more resolvable peaks. As SiO2 (wt%) in the melt increases from B100 to B40R60, Tx linearly increases, Tm first decreases and then levels off, whereas Tg weakly changes. R100 and B20R80 run-products are completely glassy, while from B100 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 B100 and B80R20 and at distance < 100 μm from it. R100, 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, B100 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 B100 is enriched in M2Ca, M1Mg, M2,M1Fe, and M1,TAl.

The values of KT, KH, KW, KLL, and w2 (as GS parameters) increase linearly and monotonically as a function of SiO2, showing high correlation coefficients (R2 = 0.93–0.95). Moreover, Tx values and GS parameters highly correlate with GFA via Rc (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 Rc value of B100 measured with DSC is > 45 °C/min (> 2700 °C/h), corroborating the value of Rc of ~ 150 °C/min (9000 °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.