How to ID

Identification of Common Sedimentary Rock

Learning to identify common sedimentary rocks by name requires a knowledge of features that one can recognize by observing an unknown sample. In this section, an arbitrary generalized procedure is presented for the introductory student to make relevant observations used to assign a name in the simplified classification of sedimentary rock to an unknown sample.

A process of identification

A process of identification is usually a multi-step procedure.

  • make visual observations
  • perform simple tests
  • make inferences, if any, based on given and observable data
  • compare observations and test results to properties of known rock types in a classification
  • assign a name to the unknown sample based on the closest match between the unknown and a known rock the classification

Click to enlarge image.

sample size - about 4 cm width

sample does not react to cold, dilute HCl

Unknown Sedimentary Rock

Example of Identification Process

make observations

  • The caption says that the rock is 4 cm (40 mm) wide.
  • The caption says that the sample does not react to acid.
  • I see that the rock appears to have a bunch of fragments of various colors embedded in a fine-grained matrix
  • I see that the fragments appear to be relatively rounded in shape (not many sharp corners or edges)
  • I can estimate that the rounded fragments range in size from less than 1 cm to about half the width of the rock, about 2 cm

perform simple tests

  • There aren't any links to videos of simple tests so I can't get any additional information to supplement my visual observations from the image.

make inferences, if any

  • My eyeball estimate based on the size range of the rounded fragments allows me to identify gravel (larger than 2 mm, or 0.2 cm)
  • The caption does state that the rock does NOT react to acid. I can infer that the rock is not made of CaCO3 (calcite/aragonite), otherwise it would fizz (react). This information eliminates any of the CaCO3 rocks, the limestones. I can focus on rocks in the other categories in the classification.

compare observations to classification

  • I have to consult the simplified classification chart (and below) to find named categories with the features I observed in the unknown sample.
  • A major group of sedimentary rocks is made of CaCO3. These are the various types of limestone. I know that CaCO3 reacts strongly with cold, dilute HCl. Because I'm told that the unknown DOESN'T react to acid, I can exclude it from being one of the limestones.
  • The prominent feature I see in the unknown are the larger, rounded fragments. The only place in the classification where larger, rounded fragments are used in the naming of a category is conglomerate.

name the unknown

  • I'll use the prominent feature in the unknown of the larger, rounded fragments to make a match with the category conglomerate. So that's what I'll name this sample - conglomerate.

Click image to enlarge.

Strategies for Identification of Unknowns

Do the Easy Things 1st!

What are the easy things?

  • Observe results of an acid test. Include or exclude limestone. If the rock fizzes strongly, the sample may be a limestone. Now you've eliminated all but 6 rocks in the simplified classification. If the rock doesn't fizz, we may eliminate the 6 limestones
  • Is the sample all black, and does it feel a bit lighter in weight for its size (density), compare to other samples? If so, it might be one of the 3 types of coal.
  • Does the sample exhibit gravel-sized fragments? If fragments are overall rounded, it might be a conglomerate. If fragments are overall angular, the sample might be breccia.
  • Is the sample overall fine-grained and friable? Test with acid - a positive result means the sample might be micrite (a limestone). If negative result, the sample might be diatomite.