What is petrographic analysis?

Petrographic analysis is the science and utilization of polarized-light microscopy to determine the mineralogy, micro-structures, and textural aspects of rocks, cement, and pottery. Thin sections of material provided by the client must be prepared before analysis is conducted. These are cut and ground to a thickness of 0.30 micrometers, which is thin enough for light to pass through some of the materials present in the samples.

For example, shown on the right is a high-resolution image from the Kem Kem Formation near Errachidia, Morocco. Deposition of this sandstone occurred during the Middle Cretaceous, during the formation of the Atlas Mountains as part of the Alleghany Orogeny. The regional uplift led to a inversion of sedimentary rocks. Although this grain is subrounded, which might in other cases suggest an immature sandstone, multiple episodes of erosion, deposition and lithificaiton are evident in this grain. The grain has a rounded edge with remnants of smaller grains of the previous rock cemented to it.

It is through this type of interpretive description that the history of a rock can be unraveled. By revealing these microscopic details, petrographic analysis leads to a greater understanding of the rocks, in this case of the larger sedimentary system.

A grain from a supermature Middle Cretaceous sandstone near Errachidia, Morocco. Although this grain is subangular, which in some cases may indicate an immature rock, the above grain has been through multiple cycles of uplift, erosion, deposition and lithification.
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