The sediment in a rock may be assessed by measuring a few hundred randomly selected grains in a set of thin sections. Assuming that less than 5 percent of the rock is larger than sand (gravel), only three grades are used: 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. Rocks with less than 5 percent gravel may be classified according to grain size (on the Wentworth scale) using this diagram. In practice, just a few large grains is all it takes. And notice that conglomerate has a 30 percent threshold. It only takes very little gravel to make a rock 'conglomeratic.' If you pick up a rock and see any gravel clast at all, that's enough to call it conglomeratic. You can calculate your rock's position that way, too. Notice that the lines that fan downward from the gravel vertex are based on values, expressed as a percentage, of the expression mud/ sand and mud, meaning that each point on the line, regardless of the gravel content, has the same proportions of sand to mud.Naturally, the number used for mud will also be there. Read its name from the field in the diagram. The point where the lines for gravel and sand meet is your rock.That will be a line parallel to the left side. Do the same for sand (left to right along the bottom).Measure along one of the sides, then draw a horizontal line at that point. Draw a horizontal line on the ternary diagram to mark the value for gravel, zero at the bottom and 100 at the top.Once the percentages are determined, using the diagram is straightforward: For example, if the gravel/sand/mud/matrix numbers are 20/60/10/10, gravel/sand/mud normalizes to 22/67/11. In that case, the cement fraction is subtracted from the total and the three sediment fractions are recalculated so that they add up to 100 - that is, they are normalized. If the cement cannot be removed, the rock is examined under the microscope in thin sections and the fractions are estimated by area instead of weight. The sediment is then sifted through a graduated set of sieves to sort out the different sizes, and the various fractions are weighed. DMSO, ultrasound, and other methods are also used. Gravel is anything larger than sand and includes granules, pebbles, cobbles, and boulders on the Wentworth scale.įirst, the rock is disaggregated, typically using acid to dissolve the cement holding the grains together.Mud is anything smaller than sand and includes the silt and clay size grades of the Wentworth scale.Sand is between 1/16 millimeter and 2 mm.This diagram is used to classify sedimentary rocks according to the mixture of grain sizes in them. Sedimentary Rock Classification Diagrams.

Ternary diagram feldspar