Classification of Sedimentary Rocks

Geologists use a number of parameters to identify and classify sedimentary rocks. These parameters differ depending on the type of rock you are classifying (clastic, chemical, or biochemical). So the first step is to determine which of these three types you have. The next step will be to determine what types of minerals are present in your rock and the size and shapes of this minerals.


	Clastic Sedimentary Rocks
If you have determined that your rock is made of bits and pieces of other rocks, your rock is considered a clastic sedimentary rock. The next step is to determine the size of the clasts. Gravel sized clasts - If the clasts are large (gravel size) then the mineralogy will not matter. If the clasts are rounded the rock is a conglomerate, if the clasts are angular the rock is a breccia. Sand size clasts - If the clasts are sand-sized then you have a sandstone. Sometime that is all you need to determine, but it is useful to sub-classify the sample based on the mineralogy of the sand-sized clasts. Use a hand-lens to get a close up look at the sand grains. If the rock is made primarily of quartz then it is a quartz sandstone. Quartz usually looks like small rounded glass beads. If the rock is made of both feldspar and quartz then it is an arkose. Feldspar has cleavage and will tend to have small flat surfaces that will reflect light like little mirrors. While using your hand-lens, twist your sample around and see if it has sparkly little surfaces. If your rock is a mixture of sand, silt, and clay then it is a graywacke. Silt and clay size clasts - If the clasts are so small you cannot see them then your rock is a mudstone, made of silt or clay sized particles. Silt tends to feel gritty and clay feels smooth. Eating rocks is generally not a recommended practice, but sometime it can help you distinguish between silt and clay. Grind a small sample of your rock between your front teeth. If it feels gritty then it's probably a siltstone, if it feels smooth and slimy then it's probably a shale or claystone.


	Chemical/Biochemical Sedimentary Rocks
Chemical and Biochemical Sedimentary rocks are sometimes grouped together and referred to as Non-Clastic Sedimentary Rocks. Non-clastic sedimentary rocks are those consisting of minerals that actually form within the basin of deposition (usually the oceans) as minerals that crystallize from ions in solution. This process can occur in two ways: (1) Biochemically - There are two groups of biochemical rocks; 1) those that form on the land from plant materials and 2) those that form in the sea as limestones. Limestones are a class of sedimentary rocks that are composed of CaCO₃. Organisms remove ions from sea water, combine them together and secrete them as hard parts such as shells. After death of the organisms, these hard parts may disaggregate and accumulate on the ocean floor. These rocks are easy to test for in that they will react with dilute HCl. (2) Inorganically - Some minerals crystallize out of water and precipitate on the ocean or lake floor from process such as evaporation. Examples are salt crystals forming at the bottom of a pan of evaporating water.