term. These can be broken down into
five broad categories of chemical reactions: equilibrium, radioactive decay, microbial, precipitation,
and authegenic processes. We will consider each in its own turn.
File last modified 1 December 1996
18.3 Chemical Diagenetic Processes
In this section we will discuss all the stuff that goes into the term. These can be broken down into
five broad categories of chemical reactions: equilibrium, radioactive decay, microbial, precipitation,
and authegenic processes. We will consider each in its own turn.
If we assume that we have a simple chemical reaction of the form:
then we can write the diagenetic equation for A as:
and similarly for B. In these equations RAsol refers to the rate of dissolution/precipitation of AB and R'A refers to all other, slow, non-equilibrium reactions affecting A and B. Since
and we can write the thermodynamic equilibrium constant as:
ignoring, for the time being, the activity coefficients. We now can combine the diagenetic equations for A and B arriving at:
and if you think this looks horrific, it can get a lot worse.
Like chemical reactions in the sediment, radioactive decay can also change the components within the sediment column. Consider the simple first order decay equation, written as:
where the concentrations are represented as 
because the diagenetic equation in these cases is
typically applied to the solid phase of sediments. It looks like:
where 
represents the burial rate of the solids. It is perhaps helpful to remind you of the following
relationships solids, pore fluids and whole sediment.
Typically, in sediments, we will be looking at catabolic processes, i.e. processes that are involved in the decomposition of organic matter, as opposed to anabolic (assimilation) processes. Because of the incredibly complex biological activities taking place, we generally simplify by making an analogy to the laboratory enzyme reaction studies we are familiar with. One of the most basic of such reaction equations is the Michaelis-Menton kinetics equation:
where C is the concentration of the metabolite, Rmax is the maximum rate of reaction, and KM is known as the Michaelis constant. Then by analogy we can write the fundamental equation that describes Robert Berner's well known multi-G model:
where Gi is the concentration of complex organic matter of type i and 
represents the total
decomposable organic matter in the sediment. Of course, this can be substituted into one of those
terms. In section 18.4 I'll make a very simple demonstration of this and apply it to the anoxic
sediments from the Long Island Sound FOAM (Friends Of Anoxic Mud) site.
The mathematics that lies behind precipitation diagenetics is based largely on what we've learned about the energetics of crystal formation, from aqueous solutions. Take, for example, the free energy of formation of a crystal:
where n refers to the number of atoms or ions precipitated. The terms of (18.3.11) can be written as:
where 
is the ratio of initial ion activity product of supersaturation to equilibrium, kB is the
Boltzmann constant, and T is the absolute temperature. The second term of (18.3.11) can be written
as:
where 
is the specific interfacial free energy between crystal and solution and A is the surface area
of the crystal.
There are several processes to consider when considering precipitation, but there are two (at least) that are fundamentally important: nucleation and growth rate. Nucleation requires an increase in free energy before the nucleation process can happen. When considering growth rate it is important to identify whether the growth is controlled by transport or surface reaction rates. In transport controlled growth, the rate is determined by how quickly ions can get to the active surface. In an idealized model one might approximate the surface as part of spherical grains and then use the spherical coordinate frame form of Fick's 2nd Law:
Surface reaction controlled growth is when the rate limiting step is the formation of a flat 2-D crystal one ion thick on a smooth crystal surface.
Dissolution can, in many ways, be considered the reverse of precipitation.
An authegenic mineral is one that formed in the sediment after burial, i.e. via diagenesis. There are a number of authegenic processes, for example, cementation is a process where the mineral forms or crystalizes in the pore space of the sediments. Another process is replacement where the mineral replaces another that has dissolved. Finally, if the mineral dissolves in one part of the sediment and migrates elsewhere to reprecipitate then this is known as diagenetic redistribution.
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