Minerals and Rocks
Based on Earth an Introduction to Physical Geology
Minerals
Crystal shape helps a geologist identify a mineral.
Minerals are an important part of the makeup of rocks. They can make up minute parts of rocks or a rock can be composed entirely of one mineral. Minerals, however, are different than rocks. Minerals must have certain characteristics in order to be identified as a mineral.
Geologists have found that minerals exibit similar characteristics. Characteristics are:
1) Minerals are naturally occurring. Minerals are not manufactured by people. Natural processes form minerals.
2) Minerals have crystalline structures. This refers to the atoms within a mineral. Atoms within a mineral form an orderly pattern, which repeats.
3) Minerals have a defined chemical signature. Chemical compounds that make up a mineral will be within specified limits. This is because similarly sized atoms can replace each other in nature. When one atom of the same size is substituted for another, it doesn’t change the internal chemical structure.
4) Minerals are inorganic. They do not come from animals or plants.
5) A mineral is a solid substance at the Earth’s surface temperatures.
Geologists use a lot of properties to help them identify mineral varieties. Some common physical properties they use for identification are:
1) Luster. Luster is the light reflected off a material. Common terms for describing luster include dull, metallic, glassy, and nonmetallic.
2) Shape of crystal. The exterior shape of the crystal is very important in helping determine the type of mineral.
Crystals can be geometric in shape. If minerals don't have a geometric shape, there are certain characteristics found in the shape it takes.
3) Ability to transmit light. This looks at whether or not light is able to pass through the mineral. Opaque, translucent, and transparent are common terms used to describe this trait.
4) Color and Streak. The color of the mineral is what you see when you look at the mineral. It is an easily seen defining characteristic. The streak is the true color of the mineral. It is found by scratching the mineral across an unglazed piece of porcelain to produce grains of the mineral. The color of these grains may be the same as what one sees when looking at a large piece of the mineral, or they may be different. Minerals that are harder than porcelain do not have streaks.
5) How a mineral breaks or fractures. Whether the mineral breaks along planes or fractures shows the types of chemical bonds the mineral has.
Geologists have found that minerals exibit similar characteristics. Characteristics are:
1) Minerals are naturally occurring. Minerals are not manufactured by people. Natural processes form minerals.
2) Minerals have crystalline structures. This refers to the atoms within a mineral. Atoms within a mineral form an orderly pattern, which repeats.
3) Minerals have a defined chemical signature. Chemical compounds that make up a mineral will be within specified limits. This is because similarly sized atoms can replace each other in nature. When one atom of the same size is substituted for another, it doesn’t change the internal chemical structure.
4) Minerals are inorganic. They do not come from animals or plants.
5) A mineral is a solid substance at the Earth’s surface temperatures.
Geologists use a lot of properties to help them identify mineral varieties. Some common physical properties they use for identification are:
1) Luster. Luster is the light reflected off a material. Common terms for describing luster include dull, metallic, glassy, and nonmetallic.
2) Shape of crystal. The exterior shape of the crystal is very important in helping determine the type of mineral.
Crystals can be geometric in shape. If minerals don't have a geometric shape, there are certain characteristics found in the shape it takes.
3) Ability to transmit light. This looks at whether or not light is able to pass through the mineral. Opaque, translucent, and transparent are common terms used to describe this trait.
4) Color and Streak. The color of the mineral is what you see when you look at the mineral. It is an easily seen defining characteristic. The streak is the true color of the mineral. It is found by scratching the mineral across an unglazed piece of porcelain to produce grains of the mineral. The color of these grains may be the same as what one sees when looking at a large piece of the mineral, or they may be different. Minerals that are harder than porcelain do not have streaks.
5) How a mineral breaks or fractures. Whether the mineral breaks along planes or fractures shows the types of chemical bonds the mineral has.
One mineral found here in Utah is halite or sodium chloride. It is known to most people as table salt, but geologists call this mineral halite. It is very important when talking to a geologist to call it by the correct name. To get a closer look at halite in its natural environment, I went out near the Great Salt Lake. I visited an area where the water had evaporated, leaving the ground exposed. When the water evaporates it leaves a layer of halite over the ground. It was surprising to see how thick the layer can be. It is very hard and crunchy to walk on, but it can be deceptive because sometimes the ground underneath it is moist and soft. The halite I picked up from the lake bed was about four inches wide, six inches long, and one inch thick. It had a pink tint to it. I also found a ball of halite that was about three inches in diameter along the side of the road. Some halite crystals which have had longer to develop are much larger than the crystals I found. Whether large or small the crystals have the same shape characteristic of halite.
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While in Ophir I found large veins of a white mineral in some of the rocks. I could tell this was a mineral because of the individual crystals that had formed. I brought some home with me to try and figure out what it was.
Most of the rocks in Ophir are sedimentary rocks. A common mineral found cementing sedimentary rocks together is calcite. So I decided to test and see if it was this first. Vinegar reacts with calcite by bubbling. I put some vinegar on this mineral to test my theory and it bubbled, although it was hard to see. I concluded that this mineral is calcite. |
As I was getting in the car in Ophir I found this rock on the side of the road. This rock caught my eye because of its layers and also because one layer is green. I picked it up and found that there were gold chunks in the green layer.
This rock was really interesting to me. I really wanted to know why it was the way it was. I thought that the green layer might get its color from olivine because we learned about this mineral in class but olivine isn't something I had come across before. I decided to take this rock to a geologist, Ted LaCross. He was able to tell me more about this rock than I could have figured out on my own. It was very interesting to know how geologists determine how rocks are made up and which minerals are which. |
I found out that the green layer in this rock does get its color from olivine. The gold chunks found in this rock are pyrrhotite. This is the mineral iron sulfide. A magnet is attracted to these particular chunks which is how the conclusion was drawn that they were pyrrhotite and not pyrite like I thought.
Igneous Rock
Igneous rock is formed from molten rock such as the lava seen coming out of this volcano.
Igneous rocks are formed from molten rock. Magma forms deep within the earth’s crust and upper mantle when rock heats up to its melting point. Rocks that form from magma that does not reach the earth’s crust are called intrusive igneous rocks. Those that form from magma that cools at the surface are extrusive igneous rocks. Some igneous rocks are basalt, granite, rhyolite, pumice, and obsidian.
As magma cools different minerals crystallize. Rocks forming from magma have different compositions based on the temperature of the magma it formed from. The rocks composed primarily of minerals that have the highest melting point are called ultramafic. Those formed primarily from minerals with the lowest melting point are called felsic. In between these are mafic rocks which are composed of minerals that have melting points below the ultramafic but above rocks composed of the next group of minerals, which have a composition called intermediate. Darker minerals tend to melt at higher temperatures than lighter minerals. So rocks with ultramafic composition are darker than rocks with felsic composition. Felsic rocks tend to be higher in Muscovite, quartz, Plagioclase Feldspar, Biotite, Amphibole and Potassium Feldspar. Ultra mafic rocks tend to be higher in Olivine and Pyroxene. The rocks in between are composed of a combination of minerals with higher and lower melting points.
The texture of igneous rocks tells a lot about the way it formed. Rocks that form from slowly cooling magma develop large crystals that are equal in size. They are said to have a phaneritic texture. If large crystals form that are bigger than a centimeter in diameter then the rocks have a pegmatic texture. Rocks that form by cooling too quickly to form crystals have a glassy texture. Rocks that cool quickly, but slowly enough to form small crystals have an aphanitic texture. Rocks that form in different environments and are able to form both large and small crystals have a porphyritic texture. Sometimes individual rock and mineral fragments are ejected during an eruption and consolidate to form larger rocks; this type of rock is said to have a pyroclastic texture. |
I went out near Vernon to look at an igneous rock that is very beautiful. The pictures at the right are of Vernon Hills Wonderstone. This rock is welded tuff made up of volcanic glass particles which stuck together while they were still hot and then were compacted by weight. The colorful banding pattern is caused by iron rich water that circulated through the rock. (Wonderstone in Vernon Hills, Tooele County)
Another thing I went out to do was to go look at some volcanic ash that is near Faust. Volcanic ash is similar in size and texture to the particles that make up the Vernon Hills Wonderstone. The difference is that the volcanic ash in Faust didn't stick together while it was still hot and it hasn't been under any pressure. Volcanic ash is very fine in texture. Although there are large clumps of volcanic ash, it was surprising at how easily chunks of it crumbled when handled. |
Little Cottonwood Canyon is home to quartz mezonite. It is better known as granite. Quartz mezonite is an intrusive igneous rock. It is found in abundance around Temple Quarry Nature Trail (Case). This particular rock can also be seen in downtown Salt Lake City. The Church of Jesus Christ of Latter Day Saints used it to build their temple. They later used it again to build the Conference Center which is located directly across North Temple from their temple.
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Sedimentary Rock
Cliffs of sedimentary rock in Ophir, Utah.
There are three categories of sedimentary rock. Detrial sedimentary rock is formed when material that originates as solid particles accumulates. Chemical sedimentary rocks form when ions in a solution precipitate out by inorganic or biological processes. Organic sedimentary rocks are organic carbon based. They form when layers of plants and organic matter accumulate in an oxygen poor swampy area.
Once sediments are deposited, they go through a process called diagenesis. Diagenesis includes chemical, physical, and biological changes. Diagenesis includes recrystallization from less stable minerals to more stable ones, conversion of plant matter to solid carbon, and lithification. Lithification includes the processes of compaction and cementation. Compaction happens when material is compressed under the weight of overlying material. Cementation occurs when water percolates through material and new minerals crystallize within the pore spaces between the sediments gluing the sediments together.
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Sedimentary rocks are classified by their textures. Clastic sedimentary rocks consist of individual pieces that go through cementation or compaction. Nonclastic sedimentary rocks have a crystalline texture. Some nonclastic sedimentary rocks look like igneous rock but their mineral composition is very different. Crystals in sedimentary rock are intergrown and can be large or small in size.
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To get a better idea of how sedimentary rock forms, I took a trip out to Stansbury Island. There I found large clumps of sediment and rocks stuck together. If pushed, these large chunks would move as one piece but would probably break easily as they moved. There are large obvious voids between some pieces. The bond between the pieces of sediment is easily broken. These clumps have not completed the process to become a true sedimentary rock but they are a good example of how individual pieces of sediment begin to form a much larger rock.
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While out on Stansbury Island we took some time to look at the oolitic sand. Oolitic sand is similar in size to sugar and is formed by bacteria. This sand may be part of what composes the large sandstone boulders that can be found on the mountains of Stansbury Island. When touching these boulders it feels like you are running your hand over sand paper.
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One day I went on a drive out to Ophir, Utah. Along the south side of the road are some beautiful cliffs. The various layers of sedimentary rock can be seen when looking at them. Since the layers are different colors that means different materials were deposited to make up the sediment beds that formed these cliffs.
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Metamorphic Rock
This slate is found up Big Cottonwood Canyon.
Metamorphic rocks are rocks that have undergone a change. Their parent rock, which means the rock they formed from, is usually igneous or sedimentary rock although sometimes metamorphic rock undergoes further change and becomes a different rock. Change in rock is caused by heat, pressure, or chemically active fluids. Each of these can cause changes simultaneously but how much one will influence metamorphism varies.
Heat comes from within the Earth. It is caused by radioactive decay and stored thermal energy from when the earth was formed. The greater the depth, the higher the temperature will be. Changes within rocks that are caused by heat are recrystallization of mineral grains and chemical reactions to make more stable minerals.
Pressure and stress compress or deform rock. Confining pressure is applied equally in all directions. It causes pore space between mineral grains to become smaller. When pressure gets high enough, it causes minerals to recrystallize. Differential stress is when forces of pressure from different directions are unequal. Rocks that go through differential stress are folded or flattened. As a result, the rock is shortened where it is under the greatest stress or lengthened perpendicular to the stress.
Chemically active fluids escape from magma bodies and invade surrounding rocks. As rocks are buried deeper under overlying layers of sediment, they heat up and chemically active fluids are forced out of particles due to dehydration. This promotes recrystallization or metasomatism. Metasomatism is when the chemical composition of the rock changes during metamorphisis. Minerals can be transported long distances by hot fluids moving through cracks and fissures in rocks.
Texture within metamorphic rock is determined by the orientation of the mineral crystals within it. Rocks
with random orientation will look the same no matter how you look at them. Preferred orientation causes the mineral grains to line up in a parallel or sub-parallel alignment. This will make the rock look different from different angles. Foliation is caused by compressional stress and results in preferred orientation.
Heat comes from within the Earth. It is caused by radioactive decay and stored thermal energy from when the earth was formed. The greater the depth, the higher the temperature will be. Changes within rocks that are caused by heat are recrystallization of mineral grains and chemical reactions to make more stable minerals.
Pressure and stress compress or deform rock. Confining pressure is applied equally in all directions. It causes pore space between mineral grains to become smaller. When pressure gets high enough, it causes minerals to recrystallize. Differential stress is when forces of pressure from different directions are unequal. Rocks that go through differential stress are folded or flattened. As a result, the rock is shortened where it is under the greatest stress or lengthened perpendicular to the stress.
Chemically active fluids escape from magma bodies and invade surrounding rocks. As rocks are buried deeper under overlying layers of sediment, they heat up and chemically active fluids are forced out of particles due to dehydration. This promotes recrystallization or metasomatism. Metasomatism is when the chemical composition of the rock changes during metamorphisis. Minerals can be transported long distances by hot fluids moving through cracks and fissures in rocks.
Texture within metamorphic rock is determined by the orientation of the mineral crystals within it. Rocks
with random orientation will look the same no matter how you look at them. Preferred orientation causes the mineral grains to line up in a parallel or sub-parallel alignment. This will make the rock look different from different angles. Foliation is caused by compressional stress and results in preferred orientation.
Textures of metamorphic rock include foliated textures such as rock cleavage, schistosity, and gnessic textures as well as nonfoliated and porphyroblastic textures. Rock cleavage refers to the flat surfaces that a rock breaks along. Schistosity refers to minerals that are big enough to be seen without a microscope and exhibit a layered structure. Gnessic texture refers to the banded appearance that results when minerals separate and usually doesn’t split along planes. Nonfoliated texture is seen when rocks have equidimensional crystals. Rocks with porphoryblastic texture have unusually large crystals, called porphyroblasts, surrounded by fine grained minerals.
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I found out that Big Cottonwood Canyon is a great canyon to go up to look at metamorphic rock. (Case) I drove up it in order to get a closer look at metamorphic rock. While up the canyon I looked at purple slate and white marble.
The purple slate was interesting simply because of its color. I had never seen rock such a deep shade of purple. The rock cleavage that slate has is known as slaty cleavage and can easily be seen in the picture above. Further up the canyon I stopped to look at a white marble outcrop. It can be seen on both sides of the road. This marble has some darker areas in it which are caused by an intrusion of diorite. Further up the canyon I stopped to look at a white marble outcrop. It can be seen on both sides of the road. This marble has some darker areas in it which are caused by an intrusion of diorite. It is very different to see marble in its natural environment than to see it when it is cut and used for things such as flooring material and bathroom sinks.
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My Trip to Ophir
Limestone outcrop found in Ophir, utah. Calcite within the limestone gives some of it a sparkly appearance.
My initial trip to Ophir brought me back two more times. That was because of dark rock outcrops such as this one. On my initial trip when I found this rock I thought this was metamorphic rock because the white calcite lines in them weren't planar. The top layer of the rock seemed to have beds and was dull and the other area was sparkly.
Then I went up Big Cottonwood Canyon and looked at slate. These dark rocks didn't have the rock cleavage of slate or phyllite. They didn't have the look I expected to see for schist. I could eliminate marble and quartzite as well so if these were metamorphic I couldn't figure out what these rocks were. So I went out a second time and a third time to look at these particular rocks to try and figure out what they were.
I decided to try and see if these might be shale or limestone. Limestone is suppposed to react with vinegar. I tried seeing if they would react with vinegar but as far as I could tell there was no reaction. As a result I didn't think they were limestone. These rocks didn't break the way shale should break so I didn't think they were shale. I finally decided I needed to get help identifying these particular rocks.
Ted LaCross was also able to help me identify these rocks as well. He had hydrochloric acid, which I don't have at home. Limestone reacts with hydrochloric acid. The rocks from this outcrop did react with the hydrochloric acid. Which lead him to conclude that these rocks were indeed limestone. He explained how calcite fills voids in sedimentary rocks and sometimes replaces fossils. The calcite replacing fossils is what gives some of this limestone its sparkles and the lack of calcite in other parts makes it appear dull.
After finding out that these rocks were limestone I decided to go back to my vinegar and see why I hadn't seen it fizz. I found that when I did squirt some vinegar on the rock it did have tiny, tiny bubbles bubbles coming up. These bubbles were only visible if I was close to the rock. I had been expecting the vinegar to react more like what I had seen the hydrochloric acid react in class.
This is another dark rock outcrop in Ophir that I collected samples from which Ted LaCross helped me identify. It was another rock that had not appeared to me to react with vinegar. This rock was a bit different in appearance than the limestone down the road. However once again it also didn't seem like any of the metamorphic rocks. When hydrochloric acid was put on it there was no fizzing except in one small spot. |
This rock is dolostone which some people might still call dolomite. Dolostone isn't supposed to fizz unless it is ground up to a powder. The conlcusion was drawn that this dolostone was originally going to be limestone. The limestone underwent a chemical change that wasn't quite complete throughout the rock. I didn't know that chemical sedimentary rocks could change like this.
Sources
Case, William F, Easily Accessible Examples of Igneous, Metamorphic, & Sedimentary Rocks in Wasatch Front Canyons. October 15, 2011, http://geology.utah.gov/teacher/tc/tc0198.htm.
Tarbuck, Edward J., Frederick K. Lutgens, Dennis Tasa. Earth An Introduction to Physical Geology, Salt Lake Community College Edition, Prentice Hall, Upper Saddle, New Jersey, 2011 pgs 74 - 250.
Wonderstone in the Vernon Hills, Tooele County.Utahgeology.gov. September 30, 2011, http://geology.utah.gov/utahgeo/rockmineral/collecting/hpwndrst.htm.
Case, William F, Easily Accessible Examples of Igneous, Metamorphic, & Sedimentary Rocks in Wasatch Front Canyons. October 15, 2011, http://geology.utah.gov/teacher/tc/tc0198.htm.
Tarbuck, Edward J., Frederick K. Lutgens, Dennis Tasa. Earth An Introduction to Physical Geology, Salt Lake Community College Edition, Prentice Hall, Upper Saddle, New Jersey, 2011 pgs 74 - 250.
Wonderstone in the Vernon Hills, Tooele County.Utahgeology.gov. September 30, 2011, http://geology.utah.gov/utahgeo/rockmineral/collecting/hpwndrst.htm.