Saturday, May 4, 2013

CHERT / LIMESTONE: The Oil and Vinegar of the Rock World

Just like the dressing on your salad, the silica of chert and carbonate of limestone don’t mix.
Within the oceans, plankton either have shells of carbonate material (more commonly aragonite today but more commonly calcite in the Cretaceous) or silica (that is, the material quartz is made of). In rough approximations, generally there are just a few percent of siliceous-bodied plankton among the much higher population of carbonate-bodied plankton. When the plankton die and their remains slowly settle onto the bottom of the more shallow seas, they accumulate as fine-grained sands, layers upon layers of fine-grained sands, meters and tens of meters and hundreds of meters of fine-grained sands that eventually are sufficiently buried to a depth where they become rocks.

While they are becoming rocks, a process known as diagenesis, the unmixing magic occurs: aragonite’s structure changes to that of its more stable pseudomorph calcite. And all those miniscule calcite grains begin to merge to larger calcite grains, crystal lattices growing growing growing within the rock, become a proper limestone.

And then, there’s the silica grains mixed with the carbonates. In common ocean conditions, these were well mixed with the carbonates, but now that the carbonates are recrystallizing all over the place, what’s a poor silica molecule to do? It doesn’t bond with the calcite structure, so… it becomes an outcast, pushed away from the expanding calcite minerals, expelled to layer contacts, fractures and voids within the hardening sands and even into the dissolving remains of larger sea creatures.

When enough silica material is expunged from the carbonate host, it also joins together in forming rocks of its own, chert nodules consisting of horrendously fine-grained silica. These silica deposits can then be found in layers or, more easily recognized, as spherical concretions within the limestone. Flint is one variety of cryptocrystalline quartz formed in this way: when the silica crystallizes with water in its molecular framework, it can form opal.

Chert and flint nodules within limestone are among the most common kinds or rocks used by our stone age ancestors in making tools. They are so homogeneous in make-up that they don’t split up along fracture planes or break into unwanted awkward shapes. They are so fine grained as to have a glassy-kind of cleavage when broken (like in the broken glass of a pop bottle) so they can make durable sharp cutting edges. So, it’s not surprising that any rock formation of limestone that contains chert nodules is also a good place to start looking for the tools and chips made by our early ancestors.

Stone Age tool makers did not understand about the geology of diagenesis: but they did use its products to find the right rocks to make their axes and knives.

Photo from Vikos Gorge by Dina Ghikas; Neolithic tool added by me from my photo collection.

No comments: