Sandstone is a common sedimentary rock composed primarily of sand-sized grains. Sedimentary rocks are broadly divided into two main categories: detrital and chemical. Understanding sandstone’s classification requires looking at the origins of its main components. The distinction between detrital and chemical formations is based on the process that created the sediment. This article defines these two types to clarify why sandstone is classified as it is.
Understanding Detrital Sedimentary Rocks
Detrital sedimentary rocks (clastic rocks) form from the physical fragments (clasts) of pre-existing rocks. These fragments result from physical weathering, such as the grinding action of ice, water, or wind. The pieces are transported away from their source by agents like rivers and wind. The resulting rock is classified by the size of the deposited grains.
Sediments are deposited, often in layers, where they undergo lithification. This final process involves compaction and cementation, turning the loose sediment into solid rock. Examples of detrital rocks include shale, made of fine, clay-sized particles, and conglomerate, formed from large, rounded gravel and pebbles.
Understanding Chemical Sedimentary Rocks
Chemical sedimentary rocks form through a non-clastic process involving the precipitation of minerals from a water solution. The material is carried in a dissolved state as ions. When the water becomes supersaturated or evaporates, the dissolved minerals crystallize and settle out of the solution.
Precipitation can occur inorganically, such as when seawater evaporates, leaving behind evaporite minerals. Rock salt (halite) and gypsum are common examples. Chemical rocks also form through biochemical processes where marine organisms extract dissolved ions, like calcium carbonate, to build shells and skeletons. The accumulation and lithification of these organic remains form most types of limestone.
Sandstone: A Predominantly Detrital Rock
Sandstone is classified as a detrital (clastic) sedimentary rock because of the origin of its primary components: the sand grains. These grains are composed of physically weathered minerals, most commonly quartz and feldspar, eroded from older rocks. The particle size (1/16 to 2 millimeters in diameter) defines the rock as sandstone and aligns it with the detrital classification.
The grains are transported by water or wind and deposited in environments like beaches or riverbeds. Once buried, the weight of overlying sediment causes compaction, reducing the spaces between the grains. This transforms the sand into a cohesive rock. The detrital classification prevails because the physical origin of the sand grains constitutes the bulk of the rock’s volume.
The Role of Chemical Cementation
The ambiguity in classifying sandstone arises because the final step of lithification—cementation—is a chemical process. While the rock’s framework is built from detrital grains, the binding material is often a chemical precipitate. Groundwater circulating through the pore spaces carries dissolved ions which precipitate to form a crystalline cement.
The most common cementing agents are silica, calcite, and iron oxides. Silica cement, often forming quartz overgrowths, creates a strong and durable sandstone. Calcite cement is highly soluble in water and can result in a more porous rock. Although this chemical binding material fills the voids, the detrital nature of the sand grains prevails because classification is based on the origin of the majority of the rock mass.