Chemistry in Geology

Chemistry has been defined as the science that is concerned with the composition, properties, and structure of matter and with the ways in which substances can change from one form to another. But this definition is too broad to be useful. Chemistry isn't the only science that deals with the composition and transformations of matter. Matter is also composed of minerals, which transform by geologic processes like erosion, and of continents which transform by earthquakes and plate drift. These are normally considered the subject of Geology. Chemists are unique because they understand or explain everything, even the subjects studied by geologists, in terms of the properties of just over 100 kinds of atoms found in all matter, and the amazing variety of molecules that are created by forming and breaking bonds between atoms. So chemistry is defined by its approach, not its subject matter. Chemistry explains or understands any subject in terms of the properties of atoms and molecules.

Chemistry provides a unique perspective that is particularly important in Geology; as a matter of fact, "The Earth Scientist's Periodic Table"^[1] has been designed according to geological properties of elements, and can be viewed here. Another periodic table has links from the elements to pages showing their minerals and geologic properties^[2]. Both of these helps us understand the formation and properties of "fools gold", or iron pyrite.

 

Pyrite, "fool's gold".

Pyrite is FeS₂, and is made up of the Fe²⁺ ion and the S₂^2- ions. Chemists know that lower charged ions like Fe²⁺ bond with large, polarizable ions like S₂^2-, while higher charged ions like Fe³⁺ ions bond more strongly with smaller ions like O^2-. Pyrite is the most common sulfide ore, but others have large, low charged ions like Hg²⁺ (HgS is cinnabar or vermillion) and PbS (galena). The Earth Science Periodic Chart shows relationships between these large, low charge metal ions. These are considered chemical properties, because they determine the tendency to form bonds between atoms.

Chemical reactions, resulting in bond formation or breaking, distinguish fools gold (Pyrite, FeS₂) from real gold, Au ^[3]. The physical properties (which are not related to tendencies to form or break bonds) are similar; both are minerals with a gold lustre, characteristic crystal shapes, and geologic locations. A chemical property of gold is that it is inert, and reacts with (forms bond with) almost nothing. In the presence of Acidithiobacillus bacteria, FeS₂ reacts with oxygen in the atmosphere and water during mining, and with forms sulfuric acid, the major type of acid mine drainage. Acidithiobacilli generate their energy by oxidizing ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) using oxygen.

So is pyrite a "chemical"? When it's understood in terms of the properties of its constituent atoms, for instance, when it's used in the synthesis of sulfuric acid, it's usually called a "chemical". Chemists often say "everything is a chemical" because they understand things that way, but it makes sense to consider things "non chemical" when they are not understood in chemical terms (a glass of water, a diamond), or when they are mixtures and thus can't be described by a single chemical formula like FeS₂.

Important Terms: Chemistry

Chemical

Chemical Property

Chemical Change, Reaction

From ChemPRIME: 1.0: Prelude to Chemistry

References

1. ↑ www.gly.uga.edu/railsback/PTPopups2.html
2. ↑ http://webmineral.com/chemical.shtml
3. ↑ en.Wikipedia.org/wiki/Pyrite

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Contributors and Attributions

• Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.