Weathering

Weathering is when rocks break down in place, that is, without moving the rock. This is usually done by water, but there are plenty of other physical and chemical processes that break down rocks without moving them. Physical weathering occurs when a tree root grows into a rock and breaks through, when a river cuts through a canyon, when particles carried by the wind abrade the rock, or during the process of frost wedging, which is when water fills a crack in a rock and freezes, then the ice expands and makes the crack deeper. Chemical weathering can be caused by acid rain or even regular rain, as minerals in the water weaken the rocks and make it easier for them to be eroded or broken later. Minerals can even react with chemicals in the air (such as iron and oxygen reacting to form rust, also known as iron oxide) or with other minerals nearby. Minerals are made of chemicals, after all, and there is nothing stopping them from reacting with one another.

There are a lot of interesting ways that minerals can change due to weathering, both physical and chemical. For example:

  • Limestone dissolves
  • Calcite dissolves
  • Gold may dissolve if manganese is present
  • Silver minerals can change to horn silver (cerargyrite) or dissolve
  • Feldspar changes to clay
  • Olivine and hornblende change to serpentine or chlorite
  • Pyrite changes to limonite and hematite
  • Rhodochrosite and rhodonite change to psilomelane or pyrolusite (manganese) minerals
  • Copper sulfide minerals change to malachite, azurite, cuprite, or metallic copper, or may dissolve entirely
  • Some copper minerals become partly limonite

Adapted from an article in Cycad, Flint Chips, Osage Hills Gems 11/1992

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