1.2.1: Traditional Definition

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James D. Dana (1813-1895), who developed the first widely used mineral classification system (which forms the basis of the one used today), defined a mineral as “. . . a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties.”

Dana’s definition was arguably the first, but more recent definitions have been quite similar. Some sticking points today are whether minerals can be wholly or partly anthropogenic (created by human activity, such as crystalline compounds in coal ash), whether minerals may have biological origins, and whether some mineral-like, non-crystalline materials can be considered minerals.

The International Mineralogical Association (IMA) is the most widely recognized authority on mineral names and nomenclature. The IMA has developed specific criteria that must apply for a substance to be considered a mineral. Today, the IMA list of approved minerals contains 6,000-7,000 entries (http://cnmnc.main.jp/). Many of the names, however, were “grandfathered-in.” They were approved for historical reasons but do not meet the current IMA criteria. And other names, such as olivine and biotite, that are commonly used to refer to minerals, are not on the list because they do not have specific compositions. (Olivine is a general name given to minerals that are primarily solutions of fayalite and forsterite, and biotite refers to micas with compositions between annite and phlogopite.)

1.6 Ions in a fluorite crystal

The IMA definition requires that minerals have highly ordered structures, which means that minerals must be crystalline. This is a key thing that separates minerals from many other solid materials. The drawing in Figure 1.6 shows calcium and fluorine ions in a crystal of fluorite. The fluorine ions (small and yellow) are at the corners of cubes called unit cells; the calcium ions (larger and lighter colored) are in the centers of the unit cells.

The model crystal in Figure 1.6 contains a few hundred unit cells and the crystal faces have “steps” in them. A real fluorite crystal, if large enough to see, will contain more than 10²⁰ unit cells. If the crystal contains no defects, the unit cells stack together in an orderly and repetitive way – as in this model drawing – but cells are so small that crystal faces are smooth.

For more discussion about what constitutes a mineral, try this video:
▶️ Video 1-1: What is a mineral? (9 minutes)