Physical Geology lecture outline - Sedimentary rocks

Quartz - why is it so common?

It is abundant in source rocks, physically tough and chemically stable at the earth's surface. Very quartz rich sandstones are often considered as very mature sediments that have been through many cycles of weathering and transport and winnowing so that all that has survived in the locale of deposition is the quartz. The rest has been altered, ground down and carried away.

Clays - how are they formed?

The simple answer is by the chemical weathering of silicates. A better answer is by a type of chemical reaction known as silicate hydrolysis. A common example is given below.

K-Feldspar + water ->> K ions in solution and kaolinite and a weak acid.


2KAlSi3O8 + H+ + H2O ->> 2K+ + H4Al2Si2O9 + 4H4SiO4 (aq)

In general: silicate mineral + water -> ions in solution and clay


Feldspar - what does its presence tell you about the conditions of deposition?

As seen above feldspars weather to clays. So the conditions favorable to silicate hydrolysis are not favorable to preserving feldspar in sediments. Weathering is aided by three factors - time, water, and temperature. Taking the inverse, feldspars can be preserved in cold and/or arid environments when the time between exposure and deposition and burial is not great.

What are important factors in classification of sedimentary rocks?

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