Fine-grained rocks are typically darker in color than sandstones and do not contain easily visible grains except in the case of sandstone streaks or layers. Fine-grained rocks can be homogeneous or heterogeneous with respect to grain size or mineral composition.

Homogeneous fine-grained rocks may be either smooth or gritty to the touch. Smooth varieties are clay-rich and fall in the 1XX series. Gritty varieties have mixtures of clay, silt and sand, and fall in the 3XX series.

Examples of homogeneous, fine-grained rocks. These rocks contain no streaks, layers, or nodules of other constituents aside from ironstone, that is ubiquitous in fine-grained rocks.

Heterogeneous fine-grained rocks contain secondary streaks, layers, or masses of a sand, carbonate, or coal.

If the second component effervesces when HCl is applied, the rock is a 2XX or 4XX.

Example of shale with limestone nodules (234). The nodules effervesce when acid is applied, but the darker clay matrix does not.

If the secondary material is sand, the rock is a 3XX.

Examples of heterogeneous, sandy, fine-grained rocks.

If the secondary material is coal, the rock is a 1X3 or 3X4.3.

An example of a heterogeneous, fine-grained rock with coal streaks (113).

Step 2: Determing the mineral color (second digit)

Fine grained siliclastic rocks are next classified by the color of the layer or silty material.

 

 

Last Modified on 2017-06-30
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