Wt % Oxides

Our students in Introductory Geochemistry had their first exam today. The class so far has covered introductory chemistry, cosmochemistry and the formation of the elements, the distribution of elements in the solar system and within the Earth over time, chemical analysis and how to work with data, and radioactivity and geochronology. They've also learned about the use of weight percent of oxides (wt % oxides) as a common way to express the amounts of certain rock-forming elements in a sample.

Anyone who knows something about geochemistry or analytical geology has probably heard of wt. % oxides as a means of depicting some geological data. In the early days of geochemistry, gravimetric techniques were utilized to measure the elemental makeup of certain minerals and rocks. One means of that was to burn everything in the presence of oxygen, thus removing any volatiles and oxidizing all the cations present in a sample to form oxides. For instance, in intro chemistry labs at my undergrad college the students would burn magnesium strips in crucibles in an aerobic environment and then would utilize measurements of mass before and after burning to determine the stoichiometry of the oxide of magnesium (just MgO in this case, so the moles of oxygen to magnesium after burning are 1:1). 

This use of wt. % oxides is how data has been presented for a long time in geochemistry. Now the convention continues, even though our methodologies and instrumentation for collecting the data have gotten much better. In fact, the wt. % oxides method of presenting data is just outdated and ridiculous to continue. 

We now know that the cations in minerals are not all in their oxidized form. Rocks aren't usually composed of mixtures of metal oxides. In fact, to even get wt. % oxides data these days, we get good abundance data or ratio data for an element and then create wt. % oxides data from those chemical analyses.  It makes no sense. None of our instruments are built to measure wt. % oxides in a material (since most of the rock-forming elements are not bound up as oxides). Instead, we now make out instruments take straightforward measurements of cations in a sample and then convert those data to wt. % oxides.

I've hopefully imparted upon my students the knowledge that it's good to know that these measurements are common so we must know how to work with them, but they are so archaic and pointless that we need to get geochemists to stop using them.

This quote from Joe Smyth, professor of mineralogy at CU, sums up my feelings on the issue:

“This is an unfortunate relic of wet chemical analysis, but is so firmly entrenched in the science that it is important that you be able to manipulate these and convert them to atom ratios.”