Natural Cement and the Lithification of Sediments
Clastic Sedimentary rocks are formed by the processes of Compaction and Cementation.
The illustration above shows the process of Cementation.
In a water environment, if minerals are present in solution, they will flow
in and around loose sediments then crystallize, "gluing" fragments
together. This process, along with compaction due to the pressure of overlying
layers of sediment, transforms loose material at depth into solid rock. Quartz,
Calcite and Hematite are common minerals that form the natural cement that binds
clastic rocks.
In hand samples it is often possible to tell the difference between the cementing
medium holding larger clasts together in a rock. Observation of the matrix between
the actual fragments may reveal a reddish color, which is the iron oxide in
Hematite. Or, application of dilute HCl may cause portions of the rock to effervesce.
If this happens and the rock is clearly not a limestone to begin with then the
cement is most likely Calcite. Silica is more difficult to discern. One can
detect it by process of elimination concerning the other two cements and it
also helps to keep in mind that Quartz cement is hard and durable and that clastic
rocks cemented with Quartz would most likely not crumble or break apart easily.
