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I am a student at Oxon Hill High School in Maryland and I am doing a project on the rock cycle. The metamorphic rock we selected was marble, which is formed from limestone and contains dolomite and calcite as the two main sediments. Do you know what igneous rock would give you these sediments? My teacher has been helping and we can't find any occurrences. We know there must be one.

Great question! Unfortunately, I can't give an easy answer to this one. Before we continue, let's agree on some basics: calcium is an element, calcite and dolomite are minerals, and limestone is a sedimentary rock which can turn into the metamorphic rock called marble if the right conditions occur. Igneous rocks cool from molten magma and form the majority of the crust. Most igneous rocks are made up of minerals which are part of the silicate group. This means that they are primarily based on silicon (and oxygen). Calcite, dolomite, limestone, and marble are part of the carbonate group, which is based on calcium (and oxygen). The problem is how to start with a bunch of igneous minerals which have silicon and end up with a sedimentary rock assemblage based on calcium.

The minerals which make up the igneous rocks can't make calcite or dolomite directly, but they are a source of calcium (which is the main ingredient of both). Much of the calcium occurs in the common igneous rock-forming mineral called "plagioclase feldspar." The feldspar mineral family is VERY important to the earth (probably because it forms about 60% of the crust). Not all feldspars contain calcium, but many of them do, and as they weather at the surface the calcium is released (dissolved) into surface waters where it is washed down the rivers to the ocean.

At this point it gets a bit more complicated. We have to somehow get this dissolved calcium out of the sea water and make a solid, carbonate mineral out of it so that it can sink to the seafloor and pile us as sediments to make limestone. As fortune would have it, the earth is really good at this, and its really not that big of a deal. Huge amounts of calcium are taken directly from the sea water by tiny, one-celled organisms which use it to build their shells. When these critters die (or reproduce), their microscopic shells sink to the bottom of the ocean where they pile up in thick layers of calcium-rich ooze. Over the course of geologic time, the layers of debris pile up, and the carbonate ooze on the bottom compacts into limestone, a sedimentary rock. Dolomite is similar to limestone, but has a bit of magnesium substituting for some of the calcium. If heat and pressure continue to rise, the limestone can undergo additional changes and end up as the metamorphic rock we call marble.


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