Banded sandstone will have colored bands caused by oxidation or mineralization.
Sedimentary rocks are a type of rock formed by deposition and compaction of sediment. Identifying these rocks can be difficult if you do not know what to look for. Several characteristics can help in identifying sedimentary rocks, including composition, grain size, sorting, rounding and internal features. Recognizing a few of these attributes in a rock can help to make identification possible.
Composition and Grain Size
The minerals making up the rock are important to the identification of sandstones and limestones. Quartz is a common mineral in sedimentary rocks as it is resistant to weathering. It can be identified by its clear to milky white appearance and its hardness. Feldspar is another common mineral, but weathers much faster than quartz. It can be identified by its white to pink color and its hardness. Sandstone is aptly named as it is a rock made of sand-sized particles. The size of the grains making up a rock is one of the most important factors in identification. Grain sizes are broken down according to gravel-sized, sand-sized, silt-sized and clay-sized particles. Gravel is any grain greater than 1-mm, sand is between 1-mm and 1/16-mm, silt is less than 1/16-mm and is often too small to see, but has a gritty feel and clay cannot be seen nor felt. Rocks composed of only one grain size are called "massive."
Special Features
A rock is considered "immature" if it contains a significant amount of matrix. Matrix is the smaller-grained groundmass contained in some sedimentary rocks. Some sedimentary rocks have features such as cross-bedding, banding of color, and/or ripple marks, which can help to identify them. Marks such as these are caused by wind, water and mineralization. Some sedimentary rocks contain carbonate (CO32-) and this can be discovered through use of dilute Hydrochloric acid. If the fresh surface of a rock "fizzes" when in contact with HCl, then the rock contains carbonate. Color is the least reliable method of identification because sedimentary rocks can be various colors, but should be noted in the description of the rock.
Sorting and Rounding
Sorting of grains refers to similarity of grain sizes within a rock and can be used to help identify sedimentary rocks. A rock with all sand-sized particles is well-sorted, while a rock containing gravel-sized, sand-sized, and silt-sized particles is poorly-sorted. However, a caveat to this is that a rock containing only clay-sized particles is always poorly-sorted because of the nature of clay. Rounding is another way to test rocks. Rounding refers to the shape of the grains composing the rock. If the grains appear to be spherical, then they are rounded or well-rounded. If the grains have many sharp edges and are not spherical, then they are considered sub-angular to angular.
Types of Sedimentary Rocks
Limestone is a crystalline carbonate rock that is usually white to gray, but has varying types and appearances, but will always effervesce with HCl. Sandstone is a rock composed of sand-sized grains. Sandstone with 0 to 15 percent matrix is termed arenite, 15 to 75 percent is termed wacke and matrix amounts higher than that are mudstone. The composition is split into quartz, feldspar and rock fragments and will tell what type of arenite, wacke or mudstone it is (either quartz, feldspathic or lithic, respectively). Shale is a mudstone or claystone composed of clay- to silt-sized grains. Conglomerate is a rock composed of rounded gravel-sized grains embedded in a smaller-grained groundmass. Breccia is a rock composed of angular gravel-sized grains embedded in a smaller-grained groundmass.
Tags: rock composed, sedimentary rocks, smaller-grained groundmass, clay-sized particles, common mineral, embedded smaller-grained, embedded smaller-grained groundmass
