Mineralogical Transformations in Granitoids during Heating at Fire-Related Temperatures

Abstract

Fire is a major decay agent of rocks and can generate immediate catastrophic effects as well as directional and anisotropic damage that affect long-term weathering processes. Temperature increase is the most relevant factor, among other components in a fire, generating mineral transformations and bulk mechanical damage. Mineralogical changes at high temperatures are key to understanding the overall mechanical behaviour. However, most studies to date were carried out after rock specimens were heated to a target temperature and cooled down to room temperature. Therefore, these studies are missing the observation of the actual mineral processes during heating. This paper aims to compare mineralogical changes in crystalline rocks during heating by means of XPS and different XRD techniques. Samples of four different granitoids were heated to several temperatures up to 1000 °C to evaluate their chemical and structural changes. Results show how standardised thermal expansion coefficients are not a suitable indicator of the bulk effect of high temperatures on rocks. Results also show how thermal expansion estimations from XRD lattice measurements may be an alternative to bulk dilatometric tests, as they can be performed with limited sampling, which may be necessary in some studies. Nevertheless, XRD and XPS results need to be interpreted carefully in relation to the bulk effects of temperature increase in the rocks, as the structural behaviour may seemingly contradict the macroscopic effect.

Thermal-Induced Microstructure Deterioration of Egyptian Granodiorite and Associated Physico-Mechanical Responses

“…At 200 °C and 400 °C, XRD spectra show the same mineralogy. However, the main biotite peak (corresponding to the (001) plane) moves towards higher angles, which reflects a decrease in the network spacing (contraction) [ 55 ]. In addition, biotite plays an important role in accommodating the stresses caused by the mineral thermal expansion mismatch [ 56 ], which explains why granodiorite withstands the thermal effect and the absence of surface cracks in the specimens.…”

“…At 800 °C, XRD spectra show the presence of quartz, biotite, albite, microcline, and hornblende and the disappearance of chlorite peaks. Quartz crystals, after transitioning from the alpha to beta phase (>600 °C), start to contract (quartz volume recovery), which explains the creation of intergranular cracks, which produce a loss of integrity of the rocks [ 55 ].…”

Reducción de la resistencia por aumento de la temperatura en una granodiorita de una zona arqueológica afectada por incendios

“…Esta formación catastrófica y penetrativa de fracturas se interpreta como el resultado de la transformación del cuarzo α en β a los 573ºC que supone un cambio de volumen de los cristales de cuarzo de un 2% (Sirdesai et al, 2019) que se suma a la expansión térmica de los granos minerales (Chaki et al, 2008;Vázquez et al, 2022.…”

“…Así, aunque existen diversos estudios recientes sobre los efectos del fuego en las rocas en un contexto patrimonial (p.ej. Sanjurjo-Sánchez et al, 2016;Vázquez et al, 2016;Vigroux et al, 2021;Vázquez et al, 2022), es importante seguir profundizando en la caracterización y comprensión de los procesos de degradación que ocurren en las rocas durante el aumento de temperatura producido durante un incendio.…”

The Use of Magnetic Susceptibility as a Technique to Measure the Impact of Wildfires on Archaeological Heritage

“…However, their use may be limited if the rock attained very high temperatures and the fracturing degree is large. In these cases, an evaluation of mineralogical changes may be used to assess the extent of damage generated in the rocks because of a fire [20].…”