Now that we know something about the general types of sedimentary rocks, lets further discuss their
mineralogy, starting with terrigenous, or clastic rocks. Since these rocks are made of pieces
of preexisting rock that have been weathered, eroded, and transported, we must consider how the
common terrigenous minerals change during these processes. Any rock or sediment that is exposed
at the surface will be under constant attack from acidic precipitation and groundwater; therefore,
the minerals that survive and make it to the depositional area must be relatively unreactive.
The reactivity of the silicate minerals at the surface tends to follow Bowen’s Reaction Series,
with the mafic minerals at the top being highly reactive, and the felsic minerals at the bottom
being least reactive, the most stable of them being quartz and the clay minerals. Incidentally,
these are the minerals most commonly found in clastic sedimentary rocks. A sediment’s
maturity refers to the amount of time that it spent exposed at the surface and can be gauged
based on the relative amounts of quartz plus clay to feldspar, which breaks down to clay over time.
So the longer a pile of sediment sits near the surface, the more feldspar that becomes clay.
Now think about which minerals you would least expect to find in a terrigenous sedimentary rock.
What sort of chemical properties might they have? Not surprisingly, the most soluble minerals, the
salts, are rarely ever found as clastic sediments; this includes all the evaporite minerals.
In addition to being chemically altered, or as geologists say, chemically weathered, during
transport, grains of sediment are also abraded, as they crash into each other during turbulent
flow. This abrasion makes the grains more rounded, and the longer a grain of sediment is transported,
the more rounded it becomes. In addition, other processes that operate during transport act to
sort the grains based on their diameter. Consider a sandy beach; the constant breaking of the waves
on the shoreline continuously rinses out the silt and mud particles since they are small
enough to be suspended in the ocean water, which get carried offshore until the water is calmer.
The silt will drop out first, creating a silt deposit adjacent to the shore,
and the mud will get carried farther offshore; this is the sorting process in a nutshell.
Let’s now summarize what we’ve learned about clastic rocks. Sediments that have been
transported far from their source over a long period of time will be rich in quartz and clay,
well-sorted, and well-rounded. Sandstones that were formed in beach environments are a great
example of this, where it is not uncommon to see nearly pure quartz sand. Conversely,
sediments that have been deposited adjacent to their source will be composed of various
silicate minerals, especially feldspar, and will be relatively angular and poorly sorted. Breccia,
which is a type of rock that commonly forms from sediment that is deposited in landslides
and lahars, displays these characteristics. We now know about sediment, where it comes from,
how its transported, and how it changes during transport. All that is left to explore is how
sediment changes after it is deposited. The chemical and physical changes that occur during
and after the burial of a body of sediment is called diagenesis. Sediment is deposited in
basins, or low-lying areas that are typically near sea level. As successive layers of sediment stack
up, their weight acts to compact the underlying sediment. In a typical mud, there can be more
than 60% pore space, that is to say, 60% of the volume of this mud is empty space. In sediment
that is buried in a basin, the pore space will almost exclusively be filled with groundwater,
which becomes important later. The deeper a sediment is buried, the more it will compact,
with a 50% reduction in porosity occurring by a depth of about 2000 meters. The chemical changes
that occur during diagenesis generally involve either precipitating or dissolving minerals with
groundwater. The most important chemical change during diagenesis is cementation,
which involves the precipitation of minerals into the pore space of a body of sediment,
transforming it into a rock. The most common cement-forming minerals are quartz and calcite.
Some sediments, especially those made of carbonate minerals, will recrystallize during diagenesis,
turning many small, irregular crystals into larger, more equant ones. Being one of the more
soluble mineral groups, the carbonates can also dissolve during diagenesis, forming large voids.
Whether cementation or dissolution occurs depends on the chemistry of the sediment’s pore water. For
example, calcite tends to dissolve under acidic conditions and precipitate when the pH is higher.
That covers the mineralogy of sedimentary rocks,
so let’s move forward and get some information on their classification next.