Sedimentary minerals most sedimentary rocks are

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Sedimentary Minerals Most sedimentary rocks are transported and deposited in water, and those rocks that are not formed in water often have groundwater moving through them. The dissolved ions in the groundwater can form sedimentary minerals, either in layers or filling cracks in rocks. Below is a chart listing the key properties of a few important sedimentary minerals (Fig. 4). Figure 3. Sorting of grains. Figure 4. Common sedimentary minerals and their properties. Important Sedimentary Minerals Mineral Diagnostic Mineral Properties Quartz Many colors (often dull, sometimes translucent), hard (scratches glass), conchoidal fracture (can break in smooth curves), no cleavage Gypsum White (can be almost clear), soft (can be easily scratched with a fingernail), good cleavage in 2 directions (but not at 90 o ) Calcite White, crystals can be rhombic, reacts (fizzes) with dilutes acid (HCl), soft (scratched with a glass but not with a fingernail) Halite White or translucent, soft, 3 cleavage planes (cubic crystals), salty taste (taste at your own risk!) Limonite Yellow-orange, soft, amorphous (no constant or regular shape) Feldspar White/pink/tan, hard (scratches glass), good cleavage in 2 directions
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Lab #3: Sedimentary Rocks and Depositional Environments 51 Figure 6. a. An illustration of cross-bedded layers. Cross-beds are common indicators of a dune environment. b. Ripple marks are common indicators of tidal flats, and can be used to tell current direction. c. Mud cracks help geologists determine the up-right direction during deposition. Depositional Environments Sediments accumulate in depositional environments such as alluvial fans, river channels, flood plains, deltas, lakes, desert valleys, beaches, shallow marine, and the deep sea floor. An important task of a geologist who studies sedimentary rocks is to interpret the ancient environment in which the rock formed (Fig. 5-9). By making detailed observations, a geologist can read the many clues that tell the depositional story of a rock sequence. Property Observation Interpretation Color Red, orange, and yellow colors occur where Fe- and other oxides form Black Oxidizing environment on continents Suggests carbon that was preserved in a reduc- ing environment (i.e. swamps or deep marine) Texture Grain size Rounding Sorting Energy or distance from source Abrasion history Constancy of energy Composition Transported minerals or fragments Minerals that form in sedimentary environments Indicates the type of source area Conditions in the environment must be just right to form rocks made of calcite, halite, gypsum, quartz, or iron-oxides Sedimentary Structures Bedding, cross-bedding, graded bedding, ripple marks, mud cracks, etc. Indicate mechanism of deposition, such as wind or water currents, wind moving over shallow water, underwater density currents, dessication of mud, etc.
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