Slate is a type of metamorphic rock. It comes in many different colors including black, white, red and gray. Other colors, such as green or blue, often highlight pieces. Slate's color is determined by the materials that make up the rocks. Gray slate is made up of mud, clay, volcanic ash and various minerals. The ash is what gives it the gray color. Does this Spark an idea?
Metamorphic Rocks
Metamorphic rocks are created when igneous or sedimentary rocks are subjected to extreme levels of pressure and high temperatures. Eventually, the rocks change their structure. The pressure is created by the weight of multiple layers of rock and sediment creating downward force. More pressure is generated from magma underneath the Earth's surface. The magma releases gaseous pressure and heat. The heat helps to soften the rock while the pressure reshapes it; temperatures can reach in excess of 392 degrees Fahrenheit.
Mud and Clay
Mud, along with clay, comprise the majority of slate. Mud is simply highly saturated soil. It provides the medium in which the volcanic ash and minerals are suspended. Clay is more firm than mud, and provides the mud with structure while the slate forms. When wet, clay does become more pliable, but as it dries it returns to its more rigid state. When mud is introduced to clay, the moisture in the mud causes the clay to become wet and the two substances mix.
Volcanic Ash
Volcanic ash is what gives gray slate its color. The more ash in the slate, the darker the slate will be. Volcanic ash is made up of rock and glass. The particles are very small and jagged. While ash is often thought of as soft and flaky, volcanic ash is hard and insoluble, gaining electrical conductivity when wet. It also has corrosive properties and is abrasive to the touch. Like any other form of ash, volcanic ash is easily spread over long distances by wind.
Minerals
There are many minerals in slate. Some add a gray color to slate, even helping add a gray shade or tint other colors of slate. Graphite is a dark mineral that is found in slate and helps the volcanic ash create a deep gray color. Graphite is commonly used in pencils and as a lubricant. Muscovite is a flaky, crystalline mineral that comes in many colors and is often transparent. Muscovite is used in electrical insulators, to give shine to wallpaper and as glitter in makeup. Chlorite is another mineral found in slate. It has very few uses, being found mostly as filler.
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