|Clastic Sedimentary Rocks|
|Grain Size||Original Sediment||Grain Roundness||Rock Name|
|coarse (> 2 mm)||gravel||angular||breccia|
|coarse (> 2 mm)||gravel||rounded||conglomerate|
|medium (0.06 - 2 mm)||sand||variable||sandstone*|
|fine (0.004 - 0.06 mm)||silt||not visible||siltstone|
|extra fine (< 0.004 mm)||clay||not visible||shale|
|Chemical Sedimentary Rocks|
|Composition||Common Depositional Environment||Features||Rock Name|
|calcite||warm shallow ocean||gray, softer than glass||limestone|
|calcite||warm shallow ocean||white, powdery||chalk|
|microquartz||deep ocean floor||conchoidal fracture||chert|
|microquartz||lakes in volcanic environments||white, powdery||diatomite|
|carbon||heavily vegetated swamp||black, soft||coal|
|halite||evaporating water body||tastes like salt||rock salt|
|gypsum||evaporating water body||softer than fingernail||gypsum rock|
As rocks undergo weathering and erosion on the surface of the earth, they turn into grains of sediment. Clastic sediment is the solid bits and pieces of rocks and minerals that have broken down from the source rock by a combination of physical and chemical weathering. Clastic sediment is sometimes called detrital sediment. Grains of clastic sediment range in size from microscopic (clay particles) to house size (large boulders), with sand and silt being two of the common sizes of clastic sediment.
As rocks on the surface of the earth undergo chemical and physical weathering, some of their chemical elements or ions become dissolved in water. Subsequently, the dissolved atoms or ions come back out of solution in water and crystallize into solid minerals, a process known as precipitation. Minerals that precipitate from solution comprise the chemical sediments. Common chemical sediments include the precipitated minerals halite (rock salt), gypsum, calcite, and various forms of ultra-microscopic quartz grains (cryptocrystalline quartz, listed in the table above as "microquartz").
Sediment turns into sedimentary rock by some combination of burial beneath more sediment, compaction into a smaller volume (due to being compressed by the weight of more sediment piled on top), and cementation by new mineral growth precipitating between the sediment grains. This is commonly summarized as "burial, compaction, and cementation."
Sedimentary Rock Classification Table
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes