Small organisms such as foraminifers, small gastropods, pelecypods and brachiopods persists as whole shells in the sediment. Larger organisms, such as trilobite and crinoids, disarticulate into small pieces at death. Other organisms are disarticulated by scavengers, decomposers and burrowing organisms. We consider clastic rocks that are predominantely made up of carbonate in the next lecture. The cement in a sedimentary rocks may constitute as much as one-third of its volume.
Calcium carbonate is the most common kind of carbonate cement.
The third most abundant mineral cement is hematite, and it is far less abundant than siliceous or carbonate. Clay minerals can also act as cementing agents. The most popular classification system for clastic rocks falls into the latter category. It was designed for sandstones, but has been adapted to both coarser and finer grained rocks. The axes between the corners measure the ration between constituents in opposing corners.