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Understanding Our Planet Through Chemistry

I. Introduction

Questions about geology -the science of the Earth- can be difficult to answer because many times we can't safely get close enough to the event. Even if we can, our senses are not sharp enough to detect everything that is happening. The Earth is relentless in its course of change, but the transformation occurs over a vast amount of time. Some geologic processes can take a million years or more to complete. We know that today's events have also occurred repeatedly throughout geologic time. To understand our planet Earth, we need to read and interpret the permanent records in the Earth's crust and interior. These records are the key to the future, and many of these clues are preserved in the chemistry of geologic samples.

Everything we touch in our daily lives is made up of elements. There are 92 elements that occur naturally, and in most cases, the human senses cannot recognize these elements when they are present in a compound. If, for example, we could always recognize what something is made of, there would be no such thing as "fool's gold" (a natural combination or iron and sulfur called pyrite). Because we have difficulty identifying these relatively pure compounds, it's not surprising that when rock or soil contains only a very small amount of an element we are incapable of recognizing the element's presence.

Photo of the mineral pyrite. Pyrite.[36k] [130k]

Photo of gold. Gold. Using only our vision, pyrite is easily confused with gold, so much so that the common name for pyrite is "fools gold." [39k] [69k]

Using analytical chemistry, we can even determine trace elements (elements present at very low levels) at the parts per million (ppm) or parts per billion (ppb) level. It's difficult to comprehend the concentration of a substance at this low a level. To get a mental picture, imagine an average 3-bedroom home. It would take about 1 million marbles to cover the floors of the home. One part per million would be represented by just one marble among all the other marbles. For that same marble to represent one part per billion, however, it would take 20 football fields covered with marbles.

Different elements have different physical properties. These properties determine what methods can be used to analyze each element (or group of elements). The methods described in this WWW document can be applied to many different geological problems, but no one method can solve every problem. The analytical methods described here are only a few that were selected to show the role of chemistry in geology.

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