Mineral Deposits and the Rock Cycle

In elementary school, students learn about the rock cycle, that rocks change over geologic time and can exist if five major states:

Diagram from the BBC's Essential Guide to Rocks

When studying formation of mineral resources, the situation becomes more complex, because not only can they be deposited as part of the rock formation process, igneous, sedimentary or metamorphic, but they can be deposited after the rock is formed - generally precipitating out of solution of water which is forced through the rock. This site from the University of Texas presents information on the formation of minerals. The page on the role of hydrothermal and aqueous solutions in the formation of minerals is particularly interesting.

The Rock Cycle: Caught in the Act

The existence of the rock cycle means that rocks are constantly in transition. Even diamonds aren't forever. While volcanic eruptions are the most spectacular examples of the rock cycle in action it is possible to see it in action even in the sedimentary rocks of middle Tennessee.

Chert (Chalcedony) replacing limestone: (Parsons, Decatur Co., TN) The white patches on this brachiopod are sites where chert crystals are growing, replacing the limestone. This is a secondary replacement process.
	Limonite replacing chert: (Cumberland Furnace, Dickson Co., TN) In another secondary replacement process, iron rich water seeps into fractured chert, at first cementing the chert into breccia, and finally replacing the chert. In this specimen, the white rock is chert, the dark brown rock is limonite, and the tan rock is where the replacement is taking place
Sulfides Oxidizing: The Chattanooga Shale contains a great deal of pyrite (FeS2). This is an example of pyrite forming by sedimentary processes in anoxic conditions. When the Chattanooga Shale is newly exposed it is pure black, Over time, oxygen breaks down the pyrite, leaving a powdery residue on the rock's surface. The brownish powder is iron oxide. The yellow powder is sulfur.

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