12.2: Obtaining a Mineral Analysis

X-ray diffraction has historically been, and still is, a very important technology used by mineralogists. It also has important applications in solid-state physics, biophysics, medical physics, chemistry, and biochemistry. It allowed Watson and Crick to discover the structure of DNA in 1953. Yet, diffraction rarely tells us what elements a mineral contains, instead telling us how atoms are arranged. So, petrologists and mineralogists use other approaches to obtain chemical analyses.

Until about 1960, geologists obtained mineral and rock analyses primarily using an approach called “wet chemistry.” To obtain a wet chemical analysis, we can dissolve samples in acid and then analyze them by reactions involving precipitation, titration, or colorimetry. Gravimetric analysis involves reacting the acid solution with reagents to produce a precipitate. We then weigh the precipitate, and the weight tells how much of an element of interest was present in the original sample. Volumetric analysis involves titrating the sample until a specific reaction occurs. The volume of reagent necessary to make the reaction occur is proportional to the amount of the element of interest that is present. Colorimetric analysis involves reaction with a reagent that changes the color of the solution. The color intensity is proportional to the amount of the element of interest present, which we quantify by comparison to standards. The advantage of wet chemical analysis is that we can analyze just about any element. However, different elements require different approaches, the technique requires large samples that are destroyed during analysis, and analyses are difficult and time-consuming to do with accuracy. For these reasons and because other, simpler techniques are now available for most purposes, wet chemical analysis is rarely done today.

In the 1960s, researchers developed several new kinds of instruments that made mineral and rock characterization and analysis easier, while also providing new and different kinds of information. Today, we use many different kinds of analytical techniques and devices; for a good summary, go to https://serc.carleton.edu/research_education/geochemsheets/browse.html. The development of electron microscopes and electron microprobes was of particular importance – we discuss these instruments below.