Editing Chert (section)

===Nodular chert===
[[File:Çört yumrusu, Chert.2.jpg|thumb|Chert nodule within soft limestone at [[Akçakoca]], Turkey]]
Nodular chert is most common in limestone but may also be found in [[shale]]s{{sfn|Boggs|2006|p=216}} and sandstones.{{sfn|Blatt|Tracy|1996|p=335}} It is less common in [[dolomite (rock)|dolomite]].<ref name="Knauth1979"/> Nodular chert in [[carbonate rock]]s is found as oval to irregular [[nodule (geology)|nodules]]. These vary in size from powdery quartz particles to nodules several meters in size. The nodules are most typically along bedding planes or [[stylolite]] (dissolution) surfaces, where fossil organisms tended to accumulate and provided a source of dissolved silica, but they are sometimes found cutting across bedding surfaces, where the chert fills [[fossil burrow]]s, [[fluid escape structures]], or fractures. Nodules under a few centimeters in size tend to be egg-shaped, while larger nodules form irregular bodies with knobby surfaces. The outer few centimeters of large nodules may show [[desiccation]] cracks with secondary chert, which likely formed at the same time as the nodule. Calcareous fossils are occasionally present that have been completely silicified.{{sfn|Blatt|Tracy|1996|p=335}} Where chert occurs in chalk or [[marl]], it is usually called [[flint]].{{sfn|Boggs|2006|p=207}}

[[File:Flint with weathered crust.JPG|thumb|Flint with white weathered crust]]
Nodular chert is often dark in color.{{sfn|Blatt|Tracy|1996|p=335}} It can have a white [[weathering]] rind that is known in [[archaeology]] as ''[[Cortex (archaeology)|cortex]]''.

Most chert nodules have [[Texture (geology)|textures]] suggesting they were formed by diagenetic replacement, where silica was deposited in place of [[calcium carbonate]] or [[clay mineral]]s.{{sfn|Boggs|2006|p=207}} This may have taken place where [[meteoric water]] (water derived from snow or rain) mixed with saltwater in the sediment beds, where carbon dioxide was trapped, producing an environment supersaturated with silica and undersaturated with calcium carbonate.<ref name="Knauth1979"/> Nodular chert is particularly common in continental shelf environments.{{sfn|Boggs|2006|p=216}} In the [[Permian Basin (North America)]], chert nodules and chertified fossils are abundant in basin limestones, but there is little in the carbonate buildup zone itself. This may reflect dissolution of opal where carbonate is being actively deposited, a lack of siliceous organisms in these environments, or removal of siliceous skeletons by strong currents that redeposit the siliceous material in the deep basin.{{sfn|Blatt|Middleton|Murray|1980|p=576}}

The silica in nodular chert likely precipitates as opal-A, based on internal banding in nodules,{{sfn|Blatt|Middleton|Murray|1980|p=576}} and may recrystallize directly to microquartz without first recrystallizing to opal-CT.{{sfn|Boggs|2006|p=216}} Some nodular chert may precipitate directly as microquartz, due to low levels of supersaturation of silica.{{sfn|Blatt|Tracy|1996|p=335}}