Why don't organisms use dolomite to build skeletons? A recent paper in Geology (open access) links crustal evolution and climate change as controls on skeletal mineralogy. It doesn't address my question on dolomite, but its quite interesting and informative work:
Carbonate mineralogies have oscillated between aragonite and calcite seas through geological time, proposed to be due mainly to secular variation in the magnesium/calcium ratio driven by changing rates of ocean crust production. A quantitative compilation of inorganic and biominerals from the onset of biomineralization (late Ediacaran–Middle Ordovician) reveals a correspondence between seawater chemistry and the first adopted mineralogy of skeletal clades.
The term calcite and aragonite seas implies that low-mg calcite is thermodynamically stable in calcite seas, while aragonite is stable under a higher mMg/Ca ratio of sea-water. Late Proterozoic (Ediacaran) to early mid Cambrian was a time of aragonite seas. Mid Cambrian to late Ordovician was a time of calcite seas. The figure below shows the oscillating sea water chemistry through geologic history.
Organisms use a variety of minerals to build skeletons around their soft tissue. Calcium carbonate in the form of three minerals,low Mg calcite (< 4 mole% Mg), high Mg calcite (> 4 mole% Mg) and aragonite are the most widely used materials for building skeletons. Silica and calcium phosphate is also used by some groups.
What this research shows is that oscillating sea-water chemistry from late Proterozoic to Ordovician, either changes in Mg/Ca ratio of seawater or changes in pCO2 driven by climatic changes from icehouse conditions to greenhouse conditions has influenced the adoption of specific carbonate phases as skeletal material. Here's the distribution of the mineralogy of major skeletal taxa and carbonate inorganic precipitates (ooids and synsedimentary marine cements) from upper Ediacaran to Middle Ordovician.
Organisms are pretty conservative about which clothes they wear. Once a skeletal mineralogy has been selected, organisms are loathe to change it. Corals are a famous exception. Paleozoic corals (Tabulates and Rugose) wore calcite armor. Mesozoic and Cenozoic corals have switched to aragonite.
Besides the changes from aragonite / high Mg calcite to low Mg calcite biota at various times as sea water chemistry changed, the figure also reveals an interesting pattern of metazoan evolution. Notice the greater diversity of metazoans using aragonite and high-Mg calcite. Actually the diversity is more than the figure shows, since a lot of the phosphate skeletal minerals are secondary, formed by early alteration of aragonite. And most of this diversity appears by early mid Cambrian before the switch to low-mg calcite seas.
That shows that by the end of Tommotian times the metazoan biosphere had greatly diversified with many clades acquiring an ability to secrete skeletons. Fewer new skeletal groups arose after the middle Cambrian during the time of low Mg calcite seas.
So, back to my original question. Why was dolomite left out of the organic wardrobe?
Dolomite is a Ca Mg carbonate. Mg content varies from 48 to 52 percent, sometimes more. It is not just high Mg calcite with more Mg. In mineralogy jargon, high Mg calcite and dolomite don't form a solid solution series. You can't make dolomite by simple adding Mg ions to the high Mg calcite lattice. In high Mg calcite the Mg ions substitute for Ca ions randomly throughout the unit cell. In dolomite there are MgCO3 layers alternating with a CaCO3 layers. There is almost no substitution of Mg for Ca and vice versa within layers.
There is usually plenty of Mg in seawater for dolomite to precipitate. The figure below shows the distribution of dolomite through the Phanerozoic. Most of this dolomite is diagenetic in origin formed by the replacement of earlier calcite phases.
You can see that dolomite peaks actually coincide with calcite seas i.e. sea water having lower Mg/Ca ratio.
So its not the lack of Mg that is the problem. The problems are kinetic.
Sulphate and phosphate ions in sea water inhibit or slow down dolomite precipitation. The environments where dolomite readily precipitates directly from sea-water and in fact forms a sort of a skeletal coat on microbial colonies is in supratidal flats and hypersaline settings. This is where sulphate reducing bacteria remove sulphate ions from solutions triggering rapid precipitation of dolomite.
But for the vast majority of organisms under normal marine conditions, dolomite is out of reach. Post Ediacaran times as metazoan ecosystems increased in complexity, skeletonization evolved most likely as a response to predation.
The hurly burly of predator prey interaction would have favored minerals that precipitate rapidly as a choice for building skeletons. Dolomite the slow poke of carbonate phases lost out.
I like these big themes. There may be a post series here, exploring these kind of complex causal chains linking crustal and climate processes to patterns of biological evolution. I might post a few on this topic in the future.