Sedimentary Rocks
The following keys shows how sedimentary rocks are classified on the basis of texture and composition. The descriptions of the rocks in the main body of the text provide additional information, such as common rock colors.
Recommended equipment for identification of sedimentary rocks includes a bottle of dilute hydrochloric acid, a hand lens or magnifying glass, a millimeter scale, a glass plate for hardness tests, and a pocketknife or rock hammer.
Begin by testing the rock for carbonate minerals by applying a small amount of dilute hydrochloric acid (0.1 molar HCI) to the surface of the rock (CAUTION-dilute HCl can burn eyes and clothing; use with supervision).
1. The rock does not effervesce (fizz) in acid, or effervesces weakly, but when powdered by a knife or hammer, the powder effervesces strongly. If so, the rock is dolomite.
2. The rock does not effervesce at all, even when powdered, or effervesces only in some places, such as the cement between grains. Go to part I of this key.
3. The rock effervesces strongly. The rock is limestone. Go to part II of this key to determine limestone type.
4. With a hand lens or magnifying glass, determine if the rock has a clastic texture (grains cemented together) or a crystalline texture (visible, interlocking crystals).
PART I.
CLASTIC SEDIMENTARY ROCKS
A. If clastic texture and : 1. Most grains are more than 2 mm in diameter. a. Angular grains - sedimentary breccia. b. Rounded grains - conglomerate.
2. Most grains are between 1/ 16 and 2 mm in diameter. Rock feels gritty to the fingers. Rock is sandstone. a. More than 90% of the grains are quartz - quartz sandstone. b. More than 25% of the grains are feldspar - arkose. c. More than 25% of the grains are fine grained rock fragments, such as shale, slate, and basalt - lithic sandstone. d. More than 15% of the rock is fine grained matrix - graywacke.
3. Rock is fine-grained (grains less than 1/16 mm in diameter). Feels smooth to fingers. a. Grains visible with a hand lens - siltstone. b. Grains too small to see, even with a hand lens. and 1. Rock is laminated, fissile - shale. 2. Rock is unlayered, blocky - mudstone. PART II.
CHEMICAL SEDIMENTARY ROCKS – often crystalline texture, but can be clastic or bioclastic
A. If crystalline: 1. Crystals fine to coarse, hardness of 2 - rock gypsum 2. Coarse crystals that dissolve in water - rock salt.
B. Hard to determine if clastic or crystalline: 1. Very fine-grained, smooth to touch, conchoidal fracture, hardness of 6 (scratches glass), nonporous - chert (flint if dark) 2. Very fine-grained, smooth to touch, breaks into flat chips - shale. 3. Black or dark brown, readily broken, soils fingers - coal.
Limestone may be clastic or crystalline, fine- or coarse-grained, and may or may not contain visible fossils. Usually gray, tan, buff, or white. A. Limestone – clastic texture, no fossil shells B. Fossiliferous limestone – bioclastic, clastic texture, grains are whole or broken fossils in limestone mud. C. Two relatively rare limestone varieties are: 1. Coquina-very coarse, recognizable shells, much open pore space. 2. Chalk-very fine-grained, white or tan, soft and powdery. B. Oolitic limestone-grains are small spheres (less than 2 mm in diameter), all about the same size. C. Travertine-coarsely crystalline, no pore space, often contains different-colored layers (bands).