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Soil Genesis and Development, Lesson 1 - Rocks, Minerals, and Soils

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1.2 - Minerals and Rocks

It is important to understand from the beginning what is the difference between minerals and rocks.  Simple, but useful definitions, are:

  • mineral – a naturally-occurring solid material with a limited range of chemical composition and a specific structure;
  • rock – a naturally-occurring solid material composed of one or more minerals.

Mineral particles may be very small (not visible without magnification), but still have the same limitations on composition and specific structure as a hand-sized specimen. Minerals in soil are important for several reasons:

  • They provide volume and mass to the soil;
  • As they weather, they supply elements that are required to grow plants;
  • As they weather, they provide the materials to form other minerals.

It is beyond our ability to describe or learn about the approximately 3000 minerals in nature. Luckily, only a few of those minerals are important in soil and those are the ones we will describe here. First, we need to make a few more definitions:

  • primary mineral – a mineral which forms under conditions different from those at the surface of the earth, often by solidification from magma.
  • secondary mineral – a mineral which forms at or near the surface of the earth, often from weathering products of primary minerals.

The table below provides brief descriptions of minerals found in soils. You can link back  to the table below from the rock and mineral sections that follow.

Common Mineral Groups: Elemental Composition and Relative Stability
Mineral Group Specific Example(s) Primary or Secondary Principal Elemental Composition Stability at Earth’s Surface
 feldspars  orthoclase, plagioclase  primary  Na, Ca, K, Si, Al, O  stable
 nesosilicates   forsterite (olivine)   primary  Mg, Fe, Al, Si, O  unstable
 chain silicates  diopside (pyroxene)
hornblende (amphibole)
 primary  Mg, Fe, Si, O  unstable
 network silicates  quartz     primary  Si, O  stable
layer silicates (clay minerals) montmorillonite, biotite, muscovite primary or secondary Si, Al, O, K, Mg, Fe stable
carbonates calcite secondary Ca, Mg, C, O unstable

Note that this classification will be useful for us to complete this lesson, but more exact categories are necessary if you want to learn more about minerals and the rocks they form. Also, the stability of a mineral is relative to the other minerals in the list and to a particular environment. Forsterite (an olivine), for example, is described as unstable because it will not persist in a surface soil as long as, for example, orthoclase (a feldspar). Forsterite may still persist in a soil for thousands of years, though.
 

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