Figure 2
From: Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales
Isolated concretion formation mechanism.
A spherical isolated concretion formed by the reaction between 
, a byproduct from the breakdown of fatty acid originating from the dead organism in the tusk-shell and pore-water Ca, which was mostly derived from the surrounding matrix outside the concretion. Precipitation of these species as CaCO3 occurred very rapidly and formed a hard concretion. The concretions continued to grow until the carbon of the organs was all consumed. Surrounding sedimentary layers have been bent by compaction after concretion formation was completed. The relationship between L (width of reaction front), D (diffusion coefficient of fatty acid in clayey matrix) and V (linear growth rate of reaction front) can be described as L = D/V.
