Fine-Grained Siliclastic Rocks 1XX 2XX 3XX 4XX
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.
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.
If the secondary material is sand, the rock is a 3XX.
If the secondary material is coal, the rock is a 1X3 or 3X4.3.
Step 2: Determing the mineral color (second digit)
Fine grained siliclastic rocks are next classified by the color of the layer or silty material.