![]() ![]() If no silt, then the piece of rock will be ground up and is not gritty. If there is any silt, you will feel some grittyness between your teeth. Take a clean piece of shale and put a tiny piece of it between your front teeth and grind it. It may sound silly, but all geologists who have ever worked in the field doing mapping have used this test. To tell them apart, you have to use your teeth. Some shale contains no silt (fine-grained quartz) and some does. When I was a kid, I always liked to skip some of these thin flat rocks on a pond or creek, if I could find some near the water. When you see shale on an outcrop that has been weathered, you may see a lot of platy pieces of rock below the outcrop. You may see a few sparkly grains of mica on the broad flat surface of a sample, but mica is really not too common. So a sample of shale may exhibit one or more colors, however, it is commonly black or gray, it splits into sheets (is fissile), is usually soft enough to scratch with your fingernail, but sometimes it takes a nail if the rock has really gotten hard, and is very fine-grained. A shale having a red color is evidence that the clay underwent some oxygen-rich process, like weathering, before it was consolidated into a rock. Shale may be black, gray, red, green, or brownish, depending on how much pyrite, iron oxide, or carbonaceous (organic) material was deposited with it or formed after it was deposited. Shale rarely contains any carbonate minerals, like calcite or dolomite, so it will not fizz if you put a drop of vinegar or weak hydrochloric acid on it. As this happens over a period of time, the resulting rock - shale - develops the property of fissility.įissile rocks can be split or weather into flat pieces, kind of sheetlike in form. Then as more sediment is deposited on top of the clay layer or bed, the water between the clay particles begins to be squeezed out and the clay minerals begin to all lay flat or horizontal. When they are first deposited, they lay at all angles from horizontal to vertical (standing on edge). Clay consists of very small mineral grains that are platelike in shape or form, in other words, they are generally flat and thin, kind of like a dinner plate or sheet of paper. Shale is initially deposited as clay, from either fresh or salt water. This property is due to the minerals that compose the rock. The partings are a property called fissility, that is shale is fissile. Shale is a sedimentary rock that may have a variety of colors, but because of its composition typically has partings that are parallel to the bedding of the layers of rock above and below it. As important as what it looks like, is to understand what shale is composed of. Shale looks like flat rock that splits into thinner and thinner sheets. Michael Howard answers questions about Geology, Rock Types, and Earth ScienceĪ. ![]()
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