Rocks and Minerals
There are many types of rocks and minerals, but do you know how to distinguish one from another? On this page you will learn about things like lustre and the Mohs hardness scale, to sedimentary and igneous rocks. I bet you don't know what the softest and hardest minerals are. The softest mineral is talc and is a 1 on the Mohs scale; but diamond is the hardest and is a 10 on the Mohs scale. A mineral is a chemical element or compound, and a rock is made up of one or more minerals. Now that you have learned the basics about rocks and minerals lets go into depth about the different types of rocks and the test geologists use to examine minerals. Read below to find out.
Minerals:
Geologists use the following tests to determine which mineral is which. Some tests give more insight to what the mineral/rock is then others, by the information given below can you guess which one of these tests would be better to find out which mineral is which?
Crystal Structure:
Crystal structure is the shape of the crystals on a certain rock. There are six major types of crystal systems they are called cubic, tetragonal, hexagonal, orthorhombic, monoclinic, and the last one triclinic. Sometimes the crystal structure is big enough to see with the naked eye but most of the time they are too small for us to see and we need a microscope.
Hardness:
Hardness means the rocks resistance to being scratched. It is a very important physical feature in distinguishing different types of rocks. Right now you are probably wondering, “Well how do we measure the hardness?” and the answer to that my friends is the Mohs Hardness Scale. A German scientist named Friedrich Mohs developed a scale that measures rocks hardness from a scale from 1-10. From talc being the softest at a 1, and a diamond being the hardest at a ten.
Lustre:
Lustre means well quite simply how shiny a rock is. It all depends on how the light is reflected from its surface. If a mineral shines like polished metal then it is metallic, but if it is dull and there is no shine then it is classified as non-metallic. There are 6 different types of non-metallic lustre: adamantine, glassy, greasy, waxy, pearly, and silky.
Color:
The color test is one of least important tests in classifying a rock. It may give a slight clue in what its identity is, but color alone cannot identify a mineral. Some minerals are the same color or when mixed with other minerals the can be several different colors depending on what mineral are mixed.
Streak:
The streak is the color of the powder within the mineral. If it is rubbed against an unglazed porcelain tile (which is a 7 on the Mohs scale) it leaves a streak. A mineral harder than 7 will not leave a streak.
Cleavage and Fracture:
If a mineral breaks apart and it breaks straight and flat then it is called cleavage. It would look like really thin sheets. If the mineral breaks and it had jagged or rough edges then it is called fracture. Quartz is an example of a fractured mineral. These are the different tests that geologists use to determine the different types of rocks. Remember my question from before? Well hardness, streak, and cleavage and fracture are good tests to use for identifying minerals.
Rocks:
Igneous Rocks:
An igneous rock is one of the three main types of rocks. They are usually formed by the cooling and solidification of magma and lava. The magma mainly melts where there is an increase of temperature, or a decrease in pressure. There are 700 types of igneous rocks some examples are, quartz, komatite, aplite, and granite. When igneous rocks are formed in the Earth they are called intrusive igneous rocks, they are cooled very slow and are usually pushed up to the surface by magma. When they are formed on the Earth's surface they are called extrusive igneous rocks. These rocks are cooled quite quickly and we see extrusive igneous rocks everday.
Sedimentary Rocks:
Sedimentary rocks are also one of the main three types of rocks. They are made from sediment or by chemical reaction. Sediment is loose material like bits of rock, minerals, plants, and animals. (Animals I know it sounds gross but just keep with me here) This is how sedimentary rocks are formed; a layer of sediment is laying on a river or lake floor. Over time more sediment from rocks, sand or other loose materials make more sediment and lay on top of it. Once lots of layers of sediment are overlying on the first layer then all the weight eventually compresses into a rock. This is called compaction. Also water flows through the minerals causing them to dissolve. Dissolved minerals form natural cement that sticks larger pieces together this is called cementation. Most sedimentary rocks are formed by cementation and compaction. Sedimentary rocks are rich in plant and animal fossils, these fossils give us a look into the climate and area where these animals and plants lived. Most of the fossils are of footprints, teeth, scales, bones, and shells. Normally there are no organs, muscles, or skin tissue because it is too soft to fossilize. Some examples of sedimentary rocks are salt, gypsum, and sandstone.
Metamorphic Rocks:
Metamorphic rock is the last of the three main types of rocks. It is made below the Earth's surface where pressure and heat are high. When metamorphic rocks have layers inside of them then this means they are foliated. If they don’t have layers this means they are non-foliated. Metamorphic means transformation, this means that the rocks can change physically and chemically over a long period of time. When heat, fluids, and the pressure changes within the rock then this rock can change into another rock. Depending on the how high the pressure and temperature change is the rock can change into as many as seven different rocks. Some examples of metamorphic rocks are gneiss, slate, marble, schist, and quartzite. Did you know that the rock that has been changed is now called the parent rock? Or that they make up a large part of the Earth's crust? Well now you do; scroll down to lean about minerals.
The Rock Cycle:
The rock cycle is what happens to rocks when they change into different types of rocks. When an igneous rock is changing into a sedimentary rock it has to become sediments first. It does that by weathering and eroison, once it has become sediments then from compaction and cementation the igneous rock can turn into a sedimentary rock. An igneous rock can also turn into a metamorphic rock. It can do this by crystalization. Sedimentary rocks can turn into metamorphic rocks to, they can do this by heat and pressure. Metamorphic rocks can turn into sedimentary rocks by weathering and eroison, they can also turn back into igneous rocks by heat, the heat melts the rock into magma and them the magma is hardened and is turned into an igneous rock.