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Garnet
Garnet is the birthstone of January. Although many people think of a red gem when they think of garnet, it is actually available in almost every color – green (tsavorite and demantoid), orange (spessartine and hessionite), purple (rhodolite), yellow (mali and topazolite), pink (malaya), red (almandine and pyrope), black (melanite). These are just some of garnet’s…
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Birthstones
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Siltstone Uses and Economics
Siltstone has very few uses. It is rarely the target of mining for use as a construction material or manufacturing feedstock. The intergranular pore spaces in siltstone are too small for it to serve as a good aquifer. It is rarely porous enough or extensive enough to serve as an oil or gas reservoir. Its…
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Field Identification
Siltstone can be difficult to identify in the field without close examination. Weathered surfaces often appear to show sedimentary structures where none are present. Different layers weather at different rates. Siltstone is often interbedded with other lithologies. Identification requires breaking off a small piece and observing the grain size. Scraping the surface with a nail…
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What Color is Siltstone?
Siltstone occurs in a wide range of colors. It is usually gray, brown, or reddish brown. White, yellow, green, red, purple, orange, black, and other colors occur. The color is caused by the composition of the grains, the composition of the cement that binds them together, and stains produced by contact with subsurface waters.
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What Is Silt?
The word “silt” does not refer to a specific substance. Instead, it is a word used for loose granular particles in a specific size range. Silt-sized particles range between 0.00015 and 0.0025 inches in diameter, or between 0.0039 and 0.063 millimeters in diameter. They are intermediate in size between coarse clay on the small side…
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What is Siltstone?
Siltstone is a sedimentary rock composed mainly of silt-sized particles. It forms where water, wind, or ice deposit silt, and the silt is then compacted and cemented into a rock. Silt accumulates in sedimentary basins throughout the world. It represents a level of current, wave, or wind energy between where sand and mud accumulate. These include fluvial, aeolian, tidal,…
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Environments of Shale Deposition
An accumulation of mud begins with the chemical weathering of rocks. This weathering breaks the rocks down into clay minerals and other small particles which often become part of the local soil. A rainstorm might wash tiny particles of soil from the land and into streams, giving the streams a “muddy” appearance. When the stream…
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Slope Stability
Shale is the rock most often associated with landslides. Weathering transforms the shale into a clay-rich soil which normally has a very low shear strength – especially when wet. When these low-strength materials are wet and on a steep hillside, they can slowly or rapidly move down slope. Overloading or excavation by humans will often trigger…
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Expansive Soils
The clay minerals in some shale-derived soils have the ability to absorb and release large amounts of water. This change in moisture content is usually accompanied by a change in volume which can be as much as several percent. These materials are called “expansive soils.” When these soils become wet they swell, and when they…