Summary: We’ve talked about limestone quite a few times, and we’ll talk about it a lot this month. I thought we should focus a bit on the rock itself. I imagine most people have a concept of limestone. It’s a rock, not a mineral. A mineral is a compound with a distinct chemical composition as well as a specific crystalline structure, and a rock is an aggregate of minerals. Sometimes, a rock might be just one mineral, and that’s often the case with limestone. It’s usually mostly the mineral calcite, CaCO3, calcium carbonate. Calcium, carbon, and oxygen are all pretty common in the earth’s crust, and carbon and oxygen are obviously also in the atmosphere. They all also get into the hydrosphere, the world’s oceans and other waters. Limestone quarry in Italy Photo by Michael J. Zirbes via Wikimedia Commons, under Creative Commons license. As a sedimentary rock, limestone is often mostly grains of calcite that are cemented together – often by more calcite. The grains can come from several different sources – they might be broken pieces of older limestone, or they might be broken pieces of the calcareous shells produced by a great many organisms including clams, snails, crinoids, bryozoans, and many more. Obviously you could not get broken shells until shelly animals had evolved, and that didn’t really happen in large volumes until the Cambrian Explosion that we talked about in February. But there are Precambrian limestones too. It’s also possible for calcium carbonate to precipitate directly from water that’s saturated with calcium that can react with carbon and oxygen – an inorganic process not related to life. That chemical process happens today, in places like caves. Stalagmites and stalactites are chemically precipitated calcium carbonate, usually resulting from pre-existing limestone being dissolved by water. Calcite is easily soluble in acids, and in fact geologists use the fizzy reaction between calcite and weak hydrochloric acid to test a rock or mineral for the presence of calcite. But there are acids in nature as well, including the acids produced by chemical weathering of rocks and by the reaction between rain and the carbon dioxide in the atmosphere. The latter is called carbonic acid, and it’s really a very weak acid, but a weak acid is enough to dissolve limestone when you’re talking about millions of years. The earth has had acid rain – slightly acid rain – pretty much forever. Modern acid rain that comes from human pollutants can be much more significant over shorter periods. So we can get limestone, calcite, deposited in vast layers through both chemical precipitation and as a result of the activity of marine organisms that secrete calcium carbonate to make their shells, which can become the main part of some limestones. Usually the rock is a combination of both factors. Something like 10% of all sedimentary rocks are limestones, but in some places it can seem that they are a lot more than that. In part that’s because in arid country, such as western North America, limestone does not dissolve as much as in areas where it rains a lot. It’s just that carbonic acid reaction again – lots of rain, more weak acid, more dissolution of limestone. In arid country, limestones often form prominent ridges and cliffs. Limestones can be pretty complex rocks, including grains that are really broken shells, as well as little grains that are chemically precipitated calcite forming tiny round balls maybe a half-millimeter across. Those things are called oolites – from the Greek word for “egg” because they are round or oval – and they often show concentric layers of calcite deposited on some nucleus such as a sand grain. As they get swirled by waves, they roll around but grow as thin layer after thin layer of calcite is deposited. While I said earlier that limestone is usually mostly calcite, that’s by no means the only thing that you can get in limestones. Sometimes you get calcium carbonate with a different crystal form –
