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Ternary Diagram for Sandstone and Mudstones

Sedimentary Rock Classification Diagrams


Rocks with less than 5 percent gravel may be classified according to grain size (on the Wentworth scale) using this diagram. (more below)
Sand, silt and clay
Diagram (c) 2009 Andrew Alden, licensed to About.com (fair use policy)
This diagram, based on the Folk classification of sediment, is used to classify sandstones and mudstones according to the mixture of grain sizes making them up. (Click the image to see a larger version.) Assuming that less than 5 percent of the rock is larger than sand (gravel), only three grades are used:
  1. Sand is between 1/16 millimeter and 2 mm.
  2. Silt is between 1/16 and 1/256 mm.
  3. Clay is smaller than 1/256 mm.

The sediment in a rock may be assessed by measuring a few hundred randomly selected grains in a set of thin sections. If the rock is suitable—for instance if it is cemented with easily soluble calcite—the rock can be disaggregated into sediment, using acid to dissolve the cement holding the grains together (although DMSO and ultrasound are also used). The sand is sifted out using a standard seive. The silt and clay fractions are determined by their settling speed in water. At home, a simple test using a quart jar will give the proportions of the three fractions.

Once the percentages of sand, silt and clay are determined, using the diagram is straightforward:

  1. Draw a line on the ternary diagram to mark the value for sand, zero at the bottom and 100 at the top. Measure along one of the sides, then draw a horizontal line at that point.
  2. Do the same for silt. That will be a line parallel to the left side.
  3. The point where the lines for sand and silt meet is your rock. Read its name from the field in the diagram. (Naturally, the number for clay will also be there.)
  4. Notice that the lines that fan downward from the sand vertex are based on values, expressed as percentage, of the expression clay/(silt + clay), meaning that each point on the line, regardless of the gravel content, has the same proportions of silt to clay. You can calculate your rock's position that way too.

This graph is related to the previous graph for gravel/sand/mud: the center line of this graph, going from sandstone through muddy sandstone to sandy mudstone to mudstone, is the same as the bottom line of the gravel/sand/mud graph. Imagine taking that bottom line and fanning it out into this triangle to split the mud fraction into silt and clay.

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