Dolomites

Geologists speak of the “dolomite problem” because we see thick layers of dolomite in the rock archive, whereas today it is formed directly under natural conditions almost nowhere. Dolomite (CaMg(CO3)2) is thermodynamically stable, but at low temperatures crystal growth is extremely slow: magnesium ions are strongly enveloped by water molecules and have difficulty entering the crystal lattice. Therefore, in modern seas, mainly limestone (calcite, aragonite) precipitates, and we find primary dolomite only in exceptional environments such as very saline, shallow lagoons and some microbiologically dominated environments.

Therefore, we suspect that large-scale dolomite formation was widespread in the past under chemical and biological conditions that rarely occur today, for example, highly evaporating, magnesium-rich tropical seas with extensive microbial mats. The Late Paleozoic and Mesozoic in particular (especially the Permian and Triassic) show thick dolomite packages, often formed by diagenetic alteration of limestone in such specialized marine basins. The dolomite problem is thus the tension between “much dolomite in the past” and “almost no dolomite formation now”—precisely what makes this stop so interesting.

Pascal Brackman