Petrographical and geochemical characterisation of hypersiliceous rocks in Belgium used in prehistorical and historical times

Over time archaeologists and historians have discussed the geographical origin of high siliceous stone materials from our cultural heritage composed, for Belgium, mainly by flint, sedimentary quartzite and phtanite. A new term in this study is introduced to include all these high siliceous rocks wich contain more than 90% silica, having similar mechanical and physico-chemical properties. This new introduced term “hypersiliceous rocks” refers to siliceous rocks such as flint and radiolarites, but also to detrital silicified quartz sandstones, better known as sedimentary “quartzites”, to phtanites, a collective term for all very-fine grained, high-siliceous, black rocks, the latter due to its carbonaceous content, including also pure metamorphic quartzites.

The aim of this research is to determine a unique fingerprint for each hypersiliceous rock type linked to a specific outcrop area. This study uses classical petrography, cathodoluminescence microscopy, energy-dispersive X-ray spectroscopy (EDS) on thin sections and backscattered electron imaging (BSE) and EDS on rock samples. Some techniques are only applied on specific types of hypersiliceous rocks according to their properties. For example, Raman-spectroscopy is being applied on phtanites to obtain Raman-spectra of the carbonaceous material within, based on the maturation grade assuming a different burial history for the different phtanite locations.

The final aim is a reference collection of hand specimens and thin slides of hypersiliceous rocks limited to Belgium with their geographical occurences. Complementary, explanative notes with macroscopic and microscopic descriptions obtained with the classical polarizing microscope, cathodoluminescence microscopy, backscattered electron imaging (scanning electron microscope-SEM) and chemistry by X-ray diffraction, Energy dispersive X-ray spectroscopy and Laser-ablation - ICP-mass spectrometry.

Objectives

• Literature study,
• Field trips – collecting raw stone materials,
• Raw stone material from the existing collection of Building Stones (GSB),
• Use of Geographical Information System (GIS) – mapping occurrences of hypersiliceous rocks in Belgium,
• Use of petrographical techniques, such as classical polarizing microscope,
• Use of spectral analyses, such as Energy Dispersive X-ray spectroscopy, and geochemical analyses by LA-ICP-MS,
• Use of other techniques such as hot cathodoluminescence microscopy, backscattered electron imaging (BSE-SEM) and X-ray diffraction,
• Making of explanatory notes,
• Establishing a website available for scientists.