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

Abstract

Chalcedony is a porous spatial arrangement of hydroxylated nanometre sized α-quartz (SiO(2)) 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 H(2)O 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+H(2)O 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.

Heat treatment as a universal technical solution for silcrete use? A comparison between silcrete from the Western Cape (South Africa) and the Kalahari (Botswana)

“…However, no porosity increase upon heating has so far been identified in silcrete from the Cape coastal zone [7, 8]. Increasing pore space in silica rocks upon heating was previously assigned to internal fracturing [7, 8, 34, 44, 48, 49]; its dramatic increase in our Kalahari samples suggests intense fracturing upon heat treatment at relatively low temperatures. The mechanism behind such fracturing is the critical steam pressure of trapped H 2 O [46, 49] that is produced by reaction (1) and that cannot be evacuated through open pores [50].…”

First evidence of heat treatment during the early Neolithic in northeastern Italy

“…Fourier-Transform infrared (FT-IR) spectroscopy is particularly effective in this sort of study, as it is capable of revealing the occurrence in the flint material of those microstructural and crystallographic changes that are regarded to be the main reasons for the improved workability, by knapping, of heated flint materials (Schmidt et al, 2013a;Weiner et al, 2015). The changes brought about by flint heat treatment can be mainly ascribed to a reduction or complete elimination of open porosity and contemporary release of molecular water, forming SieOH bonds inside the quartz structure (Schmidt et al, 2011(Schmidt et al, , 2012. The main vibrational modes associated with the SieOH bonds have their maxima at 4545 cm À1 , 4469 cm À1 and 4345 cm À1 .…”

“…5B). Relevant dehydration of flint material is that induced by possible heat treatments during the manufacture of flint artifacts, typically at temperatures in excess of 200 C. For this reason, Schmidt et al (2011;2013a) have proposed the intensity ratio of the IR lines at 4545 cm À1 and 4469 cm À1 as a reliable indicator of the material heat treatment, with the additional advantage that the approach is fully non-destructive. FT-IR tests can be carried out on the archaeological flint objects directly, provided the thickness of the sampled region is in the comparatively broad range of 0.5 mm up to 4 mm.…”

Detecting and Quantifying Heat Treatment of Flint and Other Silica Rocks: A New Non‐destructive Method Applied to Heat‐treated Flint From the Neolithic Chassey Culture, Southern France

“…1985) that is subdivided into molecular water (H 2 O) and chemically bound hydroxyl (silanole, SiOH). Typically, flint contains 0.5–0.8 wt% of SiOH and 0.3–0.6 wt% of H 2 O, strongly retained in liquid inclusions, crystal defects and grain boundaries (Schmidt et al . 2011).…”

“…2011). LF‐chalcedony can also absorb 0.2–0.4 wt% of molecular water within a network of open pores (Schmidt et al . 2011).…”