Monday, January 2, 2012

What is a mineral? (Part 1)

What is a mineral?  Ever play the game 20 Questions?  In that guessing game, the first thing established is whether or not the mystery object is an animal, vegetable, or mineral.  In this case, the term mineral refers to anything inorganic (a coin in my pocket, for example, would be a "mineral").  This is not, however, how a geologist would define a mineral.

Every morning I take a multivitamin with minerals.  The "minerals" listed are things like iron (Fe), copper (Cu), magnesium (Mg), selenium (Se), zinc (Zn), and others.  Geologists would call these things elements, since they're found on the Periodic Table of the Elements, not minerals.

If you've ever taken a geology course, you should already be aware that geologists have a very specific definition for the term "mineral".  Let's look at that definition.

From Tarbuck & Lutgen's Earth Science (2012, 13th ed., Prentice-Hall), a standard college-level earth science text for non-science majors, a mineral is defined as:

...any naturally occurring inorganic solid that possesses an orderly crystalline structure and a well-defined chemical composition.

Let's look at each part of this definition in a bit more detail.

Minerals are naturally-occurring.  This means that anthropogenic ("man-made") materials, like synthetic gemstones for example, are not minerals.  Diamonds (C) are naturally-occuring minerals while cubic zirconia (ZrO2) are made in the laboratory and not considered to be true minerals.  Can you tell which is which in the image below?

Some definitions, however, limit this a bit more.  The International Mineralogical Association, for example, defines a mineral as: element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.

In this definition, biogenic compounds - those created by living things - are not minerals.  Examples include things like the kidney stones (calcium oxalate - CaC2O4) shown at left and bivalve shells (aragonite - CaCO3).

The textbook definition, however, would consider these thing to be minerals.  Keep in mind that terms like "minerals" are defined by people (kind of like the term "planet" in astronomy) and people disagree about the definitions at times!

Minerals are inorganic.  Here it gets all complicated too.  Turns out there really isn't any formal chemical definition of the terms organic and inorganic.  Generally speaking, however, inorganic compounds aren't organic compounds.  OK, so what are organic compounds?  Organic compounds are basically carbon atoms bonded together with attached hydrogen, oxygen, or nitrogen atoms.

As an example, both salt (left) and sugar (right) look crystalline under magnification.  The difference is that salt is NaCl which is an inorganic molecule whereas table sugar (sucrose) is C12H22O11 - an organic molecule.

Minerals are solid.  The state of a material (solid, liquid, or gas) obviously depends on temperature and pressure conditions so, once again, we have to clarify things a bit more.  Generally, we specify that a mineral must be solid at the range of temperatures and pressures found on the Earth's surface.  We can even be more precise and specify 1 atmosphere of pressure at 0° C (STP) or 1 atm at 20° C for room temperature.

Minerals also have an orderly crystalline structure and a well-defined chemical composition.  I'll come back to these tomorrow...

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