Driving forces for the weathering and alteration of silica in the regolith: Implications for studies of prehistoric flint tools

Reflecting on siliceous rocks in central Australia: Using advanced remote sensing to map ancient “tool‐stone” resources

“…Silcrete forms when unconsolidated regolith (e.g., sediments, saprolite, soils) is cemented by secondarily deposited silica through a process known as silicification (Callen, ; Milnes & Thiry, ; Summerfield, ; Taylor & Eggleton, ; Wopfner, ). Petrological studies indicate the siliceous mineral cement is a combination of amorphous, cryptocrystalline, and microcrystalline forms of silica, ranging from granular to fibrous (chalcedonic) crystalline structures (Hughes et al, ; Milnes & Thiry, ; Taylor & Eggleton, ; Thiry & Milnes, ; Thiry, Fernandes, Milnes, & Raynal, ). These microscopic forms of silica are frequently associated with a family of hydrated silicate minerals that include opal, chalcedony, agate, and other types of microquartz (Flörke, Graetsch, Röller, & Wirth, ; Graetsch, Flörke, & Miehe, ; Smith, Bandfield, Cloutis, & Rice, ).…”

“…Central Australian silcretes are typically gray or brown in color, but other variations are possible (Hughes et al, ). The precise manner in which silcrete forms is somewhat enigmatic (see Taylor & Eggleton, ), but our review indicates that silcrete can form in a number of geological contexts and it fundamentally involves the natural percolation and/or lateral movement of silica‐rich solutions through weathered sediment (see Simon‐Coincon, Milnes, Thiry, & Wright, ; Taylor & Eggleton, ; Thiry & Milnes, ; Thiry & Milnes, ; Thiry et al, ; Thiry, Milnes, Rayot, & Simon‐Coincon, ; Wopfner, ). Silica is released into the geological environment through continental weathering, hydrothermal activity, and biogenic precipitation.…”

“…As the solution dehydrates, the silica precipitates and indurates the clastic regolith matrix (Figure a). It is not uncommon for silcrete duricrusts to form on the upper profile of ancient weathered sediments (e.g., “pedogenic silcrete”) (Alley, ; Thiry & Milnes, ; Thiry et al, ) (Figure b); however, it can also form via phreatic conditions near the water table (e.g., “groundwater silcrete”), so localized subsurface epigenetic silcrete formations occur as well (Alley, ; Thiry & Milnes, ; Thiry & Milnes, ; Ullyott & Nash, ). Because silcrete is a resilient material, weathered silcrete pebbles and cobbles are common on the gibber surface (Figure c).…”

“…The hydrated silicate minerals of interest to this study include opal, chalcedony, agate, and microquartz (Ehlmann et al, ; Milliken et al, ; Smith & Bandfield, ; Smith et al, ). Opal is weakly crystalline, comprised of amorphous silica with no crystalline organization (Graetsch et al, ; Thiry et al, ). Chalcedony and agate (a variety of chalcedony) are cryptocrystalline to microcrystalline forms of silica with microfibrous crystalline structures (Graetsch et al, ; Leudtke, ).…”

Preliminary characterization of flint raw material used on prehistoric sites in NW Belgium

“…However, sample preparation is destructive and thus not suitable for sensitive archaeological artifacts. As SEM can be performed both on thin sections as well as on integral pieces of flint, without the need for destructive sample preparation, this technique has been abundantly used to study microtexture and micromorphology of fractured surfaces and cortex (Fernandes et al, ; Madsen & Stemmerik, ), weathering features (Thiry, Fernandes, Milnes, & Raynal, ), use‐wear traces (Bofill, Procopiou, Vargiolu, & Zahouani, ; Borel, Ollé, Vergès, & Sala, ; Ollé & Vergès, ; Stemp, Watson, & Evans, ), and many more. However, for detailed chemical analysis, SEM‐EDX (energy‐dispersive X‐ray spectroscopy) examination again requires sample preparation, such as polishing.…”