Sedimentary rocks form through two major processes 1

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Sedimentary rocks form through two major processes: 1. Detrital and clastic sedimentary rocks are derived from the weathering of pre-existing rock, where the sediment is transported, deposited, buried, and lithified (solidified into stone). Examples of such rocks are: conglomerate, sandstone, and shale. 2. Biological and chemical sedimentary rocks are a result of the settling and lithification of precipitates (such as mineral grains) and organisms (such as plankton). Examples of such rocks are: limestone, halite, chert, and diatomite. Origin of Sedimentary Rock There are 4 main steps in the formation of most sedimentary rocks: I. Weathering of pre-existing rocks II. Erosion and transportation of sediments or ions in solution III. Deposition IV. Burial and lithification I.Weathering Weathering of pre-existing rocks produces sediment (Fig. 1). There are two main ways that rocks can weather: Mechanical (physical) weathering is the process of breaking rock material into smaller particles. If you hit a rock with a hammer and it breaks into various-sized pieces, you have physically weathered the rock by decreasing the particle size. This increases the surface area without changing the chemical composition of the minerals. Chemical weathering occurs when minerals interact with the environment and are chemically changed. Some minerals dissolve in water (e.g. halite), or oxidize (e.g. iron rusts) to form ions in solution or new minerals. Feldspar reacts with water to form clay, which is the constituent of shale, the most abundant sedimentary rock. The main products of mechanical and chemical weathering are: Lithic (rock) fragments: broken pieces of parent (pre-existing) rock. Resistant mineral grains: some minerals are relatively stable at the Earth’s surface and are resistant to alteration. Quartz is the most common resistant mineral. Clay : clay is formed by the chemical weathering (specifically hydrolysis , the chemical breakdown of a compound due to reaction with water) of feldspar minerals. Besides being a mineral, the term clay also refers to sediment that is smaller than 1/256 mm. Ions in solution: chemical weathering of non-resistant minerals releases ions such as Si, Ca, Na, Fe, Mg. These ions are present in lakes, rivers, groundwater, and the ocean where chemical sedimentary rocks form by precipitation or evaporation. Ions dissolved in groundwater can precipitate and cement clastic particles together. Ion precipitation can also form beautiful geodes, agates, and thundereggs by filling rock cavities (holes).
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Lab #3: Sedimentary Rocks and Depositional Environments 49 Weathering products are dependent on the composition, grain-size, and the type and degree of weathering. For example, weathering of a coarse-grained granite will produce sand-sized particles, clay minerals, and dissolved ions. However, since basalt is fine-grained and does not contain any resistant minerals, basalt will eventually break down completely into clay minerals and dissolved ions.
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