Chemical Sedimentary Rocks

Minerals precipitate as the shallow water that has flooded the bottom of Death Valley evaporates. Click on image for full size (66K JPG) Courtesy of Martin Miller, University of Oregon

Like other types of sedimentary rocks, chemical rocks form at the Earth’s surface, are usually found in horizontal layers, and do not form from molten rock. However, unlike most other sedimentary rocks, they are not made of clasts of sediment. Instead they are made of mineral crystals that formed from chemicals dissolved in water.

The water in the oceans, lakes, and underground is often full of dissolved minerals and other elements. All sorts of things can dissolve into water. If you put a spoonful of salt into water, the salt will eventually dissolve into the water. Seawater tastes salty mainly because there are salts such as halite (NaCl) dissolved in it. There are other chemicals dissolved in water as well, including Calcium (Ca), Carbon ( C), and Oxygen (O).

When water is so full of elements that not all will fit, some are not able to remain dissolved. They come out of solution, or precipitate, forming solid mineral crystals. Precipitation is the opposite of dissolving. Minerals precipitate when some water has been evaporated or when a chemical reaction occurs. Evaporation takes only water molecules into the atmosphere so the elements in the remaining liquid water become very concentrated. Eventually, they do not all fit and some precipitate out as mineral. In the picture at the left, minerals, mainly halite and gypsum, are precipitating out of shallow water that has flooded the bottom of Death Valley in California. The valley is so hot and dry that water evaporates very quickly, leaving behind the minerals that were dissolved within it.

Mineral crystals are made as the shallow water that has flooded the bottom of Death Valley evaporates. Click on image for full size (66K JPG) Courtesy of Martin Miller, University of Oregon

Unlike most other sedimentary rocks, chemical rocks are not made of pieces of sediment. Instead, they have mineral crystals made from elements that are dissolved in water.

The water in the oceans, lakes, and ground is often full of dissolved elements. All sorts of things can dissolve into water. If you put a spoonful of salt into water, the salt will eventually dissolve. Seawater tastes salty mainly because there are salty minerals such as halite dissolved in it.

Sometimes water becomes so full of dissolved elements that they will not all fit. Some are not able to remain dissolved and form solid mineral crystals. This usually happens when some of the water has evaporated away, leaving less room for the dissolved elements. If enough water evaporates, they do not all fit and some form crystals of minerals such as halite, gypsum, and calcite. In the picture to the left, minerals are forming out of shallow water that has flooded the bottom of Death Valley in California. The valley is so hot and dry that water evaporates very quickly, leaving behind the minerals that were once dissolved in it.

Crystals are forming out of shallow water that has flooded the bottom of Death Valley. It is so hot and dry that the water evaporates and mineral crystals are left behind! Click on image for full size (66K JPG) Courtesy of Martin Miller, University of Oregon

Some sedimentary rocks are made of mineral crystals that come from oceans, lakes, and groundwater. Water can have all the ingredients that are needed to make mineral crystals!

How does water hold the ingredients to make minerals? See for yourself by putting a spoonful of salt into a glass of water. The salt will dissolve into the water. You can’t see it anymore, but it is still there and if you take a sip of the water you will know it! Seawater tastes salty because there are salty minerals such as halite dissolved in it.

Sometimes water becomes so full of dissolved minerals that they will not all fit. When some of the water evaporates away, there is less room for the dissolved minerals. If they do not all fit, crystals of halite, gypsum, and calcite may form.