It is found mostly in dark colored igneous rocks.Ĭalcite and Dolomite: The color is usually white, but can be other colors when impure. If there is much of it, it is almost sugary. Olivine*: Olivine in rocks is an olive green to greenish yellow. Actinolite is usually found in schists or gneisses. The crystals may be parallel to each other, or spread from a point. Actinolite is dark green tremolite is white to gray. Hornblende is usually found in dark colored metamorphic rocks sometimes in igneous rocks.Īctinolite and Tremolite: Actinolite and tremolite are usually in long thin blades or needle like crystals. It shows nearly flat, shiny faces in almost rectangular or long thin needle like crystals in rock. Hornblende: Hornblende is dark green to black. The color is medium to dark green, sometimes almost black but with a greenish tint. Phlogopite may be found in marble.Ĭhlorite*: Like mica, but the flakes are usually not as thin and do not peel apart as easily. Muscovite is silvery to brown biotite is black phlogopite is a reddish brown. In rocks they are usually flakes or layers of flakes. Micas*: (muscovite, biotite, phlogopite) Micas have very thin layers that peel off (or cleave) very easily. Plagioclase Feldspars*: (albite, labradorite) Look like the potassic feldspars, except they are white to dark gray, sometimes black. The crystal grains are usually blocky and nearly rectangular. They show flat, shiny faces in igneous rocks. Potassic Feldspars*: (microcline, orthoclase) Potassic feldspars are pink or tan, sometimes white. It has a glassy, or sometimes waxy, look to it. It is usually gray in igneous rocks gray, white, yellow, or red in sedimentary rocks and gray or white in metamorphic rocks. Quartz: Quartz is the last mineral to crystallize, so in igneous rocks it never has any definite shape. Descriptions of some of the minerals, as they look in rocks, follow: With a little practice you will recognize most of them when you see them. The list of minerals that commonly form rocks is short. This whole process is called the Rock Cycle. If the magma moves upward toward the surface it cools and crystallizes to form igneous rocks. When the rock material is molten, it is called a magma. If rocks are buried deep enough, they melt. If a great amount of pressure is exerted on the sedimentary rock, or it is heated, it may turn into a metamorphic rock. As the sediment is buried it is compressed and material dissolved in water cements it together to make it into sedimentary rock. The weather, running water, and ice wear them down. Rocks, like mountains, do not last forever. Hot liquids or gases from the magma also can cause chemical changes in the rock around the magma. It is very hot and bakes the rock through which it moves. Another way that high temperatures occur is when magma rises through the earth's upper crust. The deeper below the surface of the earth, the higher the temperature, so deep burial also means high temperatures. The pressure can come from being buried very deep in the earth's crust, or from the huge plates of the earth's crust pushing against each other. Metamorphic Rock is formed by great heat, or pressure, or both. Often, both cementing and compaction take place together. Another way it becomes rock is from being cemented together by material that has been dissolved in water. The sediment gets turned into rock by being buried and compacted by pressure from the weight above it. If there are a lot of pebbles mixed with the sand, it is called gravel. Sand has the largest particles while clay has the smallest. Sedimentary Rock forms from particles, called sediment, that are worn off other rocks. Sometimes, when the magma cools very quickly, it forms a kind of black glass that you cannot see through. When it cools quickly on the surface, the crystals are very small and you would need a magnifier or a microscope to see them. When magma cools slowly underground the crystals are large enough to see. Magma that erupts onto the surface is called lava. Igneous Rock is formed when a magma cools underground and crystallizes or when it erupts unto the surface of the ground, cools and crystallizes. A rock is like the car, a mineral is like the steel, or glass, or plastic. A car is made of steel, glass, and plastic. Minerals are not rocks, rocks are made of minerals. Most rocks are made from more than one mineral, but there are quite a few kinds that are made from only one mineral. Rocks are made of minerals, like quartz, calcite, feldspars, and micas. They are everywhere on earth, but often buried under soil. They are the mountains and the bottom of the ocean. Rocks are what the crust of the earth is made of. Gneiss, Marble, Quartzite, Schist, Serpentinite, Slate The Rock Identification Key - by Don Peck Rock Key Table of Contents The Rock Keyīasalt, Diabase, Diorite, Gabbro, Granite, Obsidian, Pumice, Rhyolite, Scoriaīreccia, Conglomerate, Limestone, Sandstone, Shale
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