Minerals

Just like chemistry, the minerals that comprise rocks have stories to tell. Often, geochemistry is only so useful without accompanying information about mineralogy. As such, I spent the early part of my Ph.D. developing Raman spectroscopic methods to distinguish between different, chemically identical iron sulfide minerals at the micron-scale, prior to grain-specific geochemical analyses like SIMS. This was important to do, for two reasons: 1) different minerals, such as pyrite and marcasite (see gray crystals in left panel, above), are expected to form under very different environmental conditions, and therefore may record information about different parts of the sedimentary environment, and 2) different minerals, such pyrite and marcasite, have different structures and behave differently when ionized during SIMS, resulting in mineral-specific instrumental mass bias effects.

Marcasite (coarser grains in the above secondary electron microprobe image) is pyrite’s (finer grains, above) lesser known sibling. Despite having the same chemical formula (FeS₂), marcasite is rarely found in sedimentary rocks, whereas pyrite is almost ubiquitous. Experiments and theory suggest that this dichotomy results from the propensity for pyrite to form preferentially at pH values > 5 (i.e., encompassing the pH of the entire modern ocean and marine sediments). However, the presence of marcasite in sedimentary rocks of certain ages, as highlighted by the work of Juergen Schieber, would appear to suggest that diagenesis sometimes creates very acidic conditions that result in the formation of marcasite. As a side project, I am investigating this phenomenon, trying to understand what diagenetic processes gave rise to the occasional instances of marcasite we find in the rock record, and whether these occurrences might provide useful information about ocean chemistry.