File talk:Glauconite-281086.jpg

File description

The description “Glauconite (Dimensions: 2 cm in diameter)” was somewhat misleading because it suggested that the whole structure is made up of glauconite or that glauconite is its main component. This, however, is not the case. Instead it is more than probable that the grains visible in the image do not consist of glauconite but of quartz, as it is common in sands and sandstones throughout the global geologic record. Glauconite is an authigenic mineral that forms under certain circumstances in larger quantities in marine sediments after their deposition, giving these deposits a greenish colour. It belongs to the so-called clay minerals because it forms very fine (i.e. clay-sized) grains which are not individually visible to the naked eye. If the sediment in which the glauconite formed is a sandstone, it is often referred to as “glauconite sand”. Such a denomination, however, should be avoided because a sand should be named after the mineral that makes up a considerable fraction of the sand grains proper, as it is the case in the terms “quartz sand”, “monzonite sand”, “olivine sand” and “carbonate sand”. Hence, the nodules overleaf are better referred to as “glauconitic” and/or “glauconitic quarz sand” than to as “glauconite” and/or “glauconite sand”.
The same applies to File:Glauconite-281085.jpg
Regarding the geological age: Since the whole surficial bedrock (i.e. the pre-plesitocene) geology of Green County, Wisconsin, comprises sedimentary rocks of Ordovician age (see e.g. Revised Bedrock Geology Map of Wisconsin, Wisconsin Geological and Natural History Survey 2005) it is highly likely that these nodules were recovered from Ordovician sand(stone)s, possibly from the St. Peter Sandstone. --Gretarsson (talk) 16:59, 16 November 2017 (UTC)