Detailed FT near-infrared study of the behaviour of water and hydroxyl in sedimentary length-fast chalcedony, SiO2, upon heat treatment

Chalcedony is a porous spatial arrangement of hydroxylated nanometre sized α-quartz (SiO2) crystallites. Due to micro-structural transformations upon heat treatment, the optical and mechanical properties of the rock are modified. We investigated these transformations in sedimentary length-fast chalcedony through Fourier Transform near- and mid-infrared spectroscopy using direct transmission and the reflectivity. Chemical adsorption potential and absorption of H2O by pores was studied after heat treatment. We found that water held in open porosity is reduced upon heat treatment to temperatures above 150 °C. Silanole is noticeably lost from 250 to 300 °C upwards and new bridging Si–O–Si further reduces the surface of open pores, creating a less porous material. Molecular water, resulting from the reaction Si–OH HO–Si → Si–O–Si + H2O creates new isolated pores within the material. At temperatures above 500 °C, the samples start internal fracturing, permitting water held in isolated pores to be evacuated. These results shed light on thermal transformations in chalcedony and allow for a better understanding of mechanical transformations after heat treatment.

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• We found that open porosity is reduced upon heat treatment to temperatures above 150 °C.
• Silanole is lost from 250 °C upwards.
• Si–OH HO–Si → Si–O–Si + H2O creates new water in isolated pores.
• At temperatures above 500 °C, the samples start internal fracturing.