Skeletal Carbonate Mineralogy of Brachiopods: Smith, Freeman, Dixon-Anderson and Lee

Measurements of 1711 specimens in 147 species included 253 fossil specimens: living species range from 79°N to 74°S and from intertidal to almost 4000 m deep. Calcareous brachiopods create strong and resilient valves, formed of very low-Mg calcite (χ ̅ =1.3 wt% MgCO3 in the phylum).

The Craniida (χ ̅ = 8.9 wt% MgCO3) and Thecideida (χ ̅ = 6.5 wt% MgCO3) are unusual in precipitating calcite with higher Mg content; these taxa are unusual in that they cement themselves to the substrate. This is the first study to find bimineralic brachiopods; a few species show a combination of low-Mg (χ ̅ = 0.8 wt% MgCO3) and intermediate-Mg calcite (χ ̅ = 7.2 wt% MgCO3). While Mg in calcite varies systematically among valve layers and sometimes along the growth axis, we found no consistent difference between valves of the same individual.