 Welcome to OpenGeology. This is an effort to share geology with everybody. This is created by Salt Lake Community College instructors. This goes with a textbook at opengeology.org slash textbook. Today we'll be talking about sedimentary rocks, specifically chemical and biochemical sedimentary rocks. Chemical sedimentary rocks are, unlike plastic sedimentary rocks, rocks that precipitate from solution. This is, and by precipitate I mean crystallize or form out of water, which is the opposite of dissolve. I have included a video of crystallization and I'll put the link to that video in the description of this one. Precipitation can occur either by inorganic processes like evaporation or from groundwater or organic processes such as critters pulling chemicals out of the water to build their skeletons in homes or large piles of shells from those critters. We're going to talk about several different types of chemical sedimentary rocks. So let's start with limestone, which is one of the most abundant ones and one of my favorites. Limestone is made mostly of the mineral calcite, which is calcium carbonate and the depositional environments for limestone can include both inorganic and biochemical origins. Here's a picture of some limestone in beautiful Logan Canyon. Some biogenic biochemical limestone is generally made mostly by marine organisms and a good guess for a depositional environment for this type of limestone is coral reefs. Coral reefs often produce fossiliferous limestone, which is a shell and fossil hash with cement and calcium carbonate mud in between the fossil shells. I'll have a picture of that in an upcoming slide. And then another type of this biochemical limestone is chalk, which is made up of these microscopic shells. Here's a picture. Some of the microscopic calcium carbonate shells. I think they're called coccolithophores, but I'm not positive on that. Here's a picture of that depositional environment we were just talking about. And here is a picture of some fossiliferous limestone. You can see in the profile view where this limestone has been cut, all these little shells that kind of look like cornflakes mashed together with little fine gray mud in between each shell. The fossils in this particular picture are a little bit darker, just so you can differentiate. Uletic limestone is another type of limestone and its depositional environment is warm shallow ocean water. To make uids you need three things. Water that is super saturated or has a very high amount of calcium and carbonate ions. You need something for the crystals to nucleate on to grow on. And a very common thing for that is fish poop. And then finally you need a gentle rolling motion in shallower and water to kind of roll these nucleating crystals around. And when they get glued together you get beautiful limestone like uletic limestone. Next we have inorganic limestones like travertine and tufa. These form from the precipitation of calcium carbonate or calcite as the water that is carrying that calcite the ions for that calcite degasses. So here is a picture of some tufa forming at mammoth hot springs in Yellowstone National Park, the north end of Yellowstone. And as the water leaves the groundwater system and emerges to the surface it degasses and releases its calcite in these beautiful tufa terraces. And then in a fairly similar process here's water moving through a cave system and as it moves to the cave system it precipitates its calcium carbonate as travertine. A similar or close relative of limestone is dolastone. It's made of dolomite which is very much like calcite except it's magnesium calcium carbonate. And there's still some debate as far as I understand of the origin of dolastone but some potential depositional environments include flat tidal lagoon areas where you have super concentration of magnesium and calcium ions. And then a lot of people think that a lot of dolastone could form as a secondary process after the rock becomes lithified. You can have groundwater moving through that's magnesium rich that changes limestone into dolastone. Dolastone is generally harder than limestone and it doesn't fizz as readily when you put hydrochloric acid on it. Another cool type of chemical sedimentary rock is chert and you can have both biogenic and non-biogenic chert. Biogenic chert forms like biogenic limestone from like biogenic limestone can at least from these tiny microscopic shells. So this is kind of the equivalent to chalk for biogenic limestone except these shells are made out of silica right. I think these are diatoms in this picture. And these will accumulate at the ocean bottom beyond the zone where calcium carbonate generally can precipitate. So at a fairly great depth you get these salacious oozes that will pile up and then later lithify. So a good depositional environment for this is an ocean floor. For the non-biogenic chert this can commonly form as salacious center which can happen at hot springs or a lot of times you can be depositing quartz in groundwater systems. Special groundwater silica rich groundwater systems. Another type of chemical sedimentary rock are evaporites and a great example of a depositional environment for these is the Bonneville salt flats. And here you see an immense amount of evaporites which is the white rock in this picture. These are created by the process of evaporation. As water evaporates from apply a lake or something similar it concentrates the ions in that water and causes precipitation of halite and gypsum creating rock salt and rock gypsum. Coal is a very important chemical sedimentary rock and this forms from the accumulation of plant matter. So a really common depositional environment for this is swamps and peat bogs. And here is an example of what that depositional environment might look like. This will eventually create peat which can accumulate in large quantities and then they'll get compressed and over time through the process of diagenesis you'll create different grades of coal. And so that is it for chemical sedimentary rocks. Please do come back for some depositional environments and sedimentary structures.