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Rocks and Minerals

Earth structure and Rocks and Minerals
by

Christopher Ervin

on 28 November 2016

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Transcript of Rocks and Minerals

photo credit Nasa / Goddard Space Flight Center / Reto Stöckli
Mr. Ervin
Earth Science

Rocks and Minerals
Solids
What do ice crystals, table-salt, and diamonds all have in common? All of them are solid structures, and all form from crystals.
Solids are substances with densely packed particles, which may be ions, atoms, or molecules, depending upon the substance.
States of Matter
An iceberg floating in the ocean beneath a blue sky not only captures the beauty of nature, but also illustrates three states of matter found on Earth.
Matter can be solid like the iceberg, liquid like the ocean, or gaseous, like the water vapor in the air. At normal room temperature and atmospheric pressure these are the three states of matter that normally exist. There is one other; plasma. We will talk more about that one later.
Solids
Some solid materials have no regular internal patterns. Glass is an example of this. Glasses are solids that consist of tightly packed atoms arranged at random. Glasses form when molten material is cooled so rapidly that the atoms don't have enough time to arrange themselves into a regular pattern.
These solids do not form crystals, or their crystals are so small that they cannot be seen. Window glass has a chemical formula of SiO2.
Liquids
Any temperature above absolute zero (-273 degree C), the atoms in solids vibrate. The vibrating increases as temperature increases, these are called thermal vibrations. At the melting point of material, these vibrations become violent enough to break the forces holding them together. These particles now have the energy to slide past one another, and the substance becomes a liquid.
Liquids do not have their own shape, liquids take the shape of the container they are in, but they do have a definite volume.
Gases
As the vibrating increases individual particles may gain enough energy to escape the liquid. This process of change from a liquid to a gas is called evaporation, or vaporization. The process of changing from a solid straight into a gaseous state is called sublimation. An example of this would be snow disappearing without melting on high mountain peaks.
Plasma
When matter is heated to temperature higher than 5000 degrees C, the collisions between particles are so violent that electrons are knocked away from atoms. Really extreme temperatures like this exist in stars. These hot, ionized, electrically conducting gases are called plasmas.
The particles of solids are arranged in a definite pattern and so a solid has a definite shape and a definite volume.
Most solids have a crystalline structure, in which the particles are arranged in regular geometric shapes.
Table salt, crystalline structure
This picture of the sun shows the gases in its corona that are plasma.
Conservation of Matter and Energy
The identity of matter can be changed through chemical reactions and nuclear processes, and its state can be changed under different thermal conditions.
BUT, matter can neither be created nor destroyed!!
This fact is called the Law of conservation of matter. This is also known as the first law of thermodynamics.
Minerals
What is a mineral?
Earth's crust is composed of about 3000 minerals. Minerals play important roles in forming rocks and in shaping Earth's surface, and a select few have played, and continue to play, a role in shaping civilization.
Mineral Characteristics
Take a look around the class. The metal in the chairs you're sitting on the glass in the window, and the graphite in your pencil are just a few examples of how we use products formed from minerals.
Minerals
A mineral is a naturally occurring, inorganic solid with a specific chemical composition and a definite crystalline structure.
Naturally Occurring
To say that minerals are naturally occurring simply means that they are found in nature, and are formed in natural processes. Secondly all minerals are inorganic, that is, they are not alive. Based on this criteria, salt is a mineral, but sugar that is harvested from a plant is not. So what about coal?
Solids with Specific Compositions
The third characteristic of minerals is that they all are solids. Solids have definite shape and volumes. Liquids and gases do not! So, no gas or liquid can be considered a mineral. Each mineral has a unique chemical composition unique to that particular mineral. A few minerals, such as, copper, silver, and sulfur are composed of single elements.
The mineral quartz, for instance, is a combination of two atoms of oxygen and one atom of silicon. Although other minerals contain oxygen and silicon, the arrangement and proportion of these elements in quartz are unique to quartz.
Minerals
Pyrite
Pyromorphite
Topaz
Feldspar
Definite Crystalline Structure
The last part of the definition of minerals relates to crystalline structures. The atoms in minerals are arranged in regular geometric patterns that are repeated again and again.
**A crystal is a solid in which the atoms are arranged in repeating patterns.
Minerals From Magma
Minerals from a cooling of magma. **Magma is molten material found below Earth's surface. Density differences force magma upward into cooler layers of Earth's interior, where the magma cools. As the magma cools the material in the magma starts to interact chemically and starts to form minerals. The type and amount of elements present in a magma help determine which minerals will form, and the rate at which the magma cools will determine the size of the mineral crystals.
Minerals From Solution
A given volume of water in a solution can dissolve only so much of a solid before the water becomes saturated. At that point the water can no longer dissolve any more of the solid. When this happens in nature mineral crystals start to precipitate, or drop out of the solution. Minerals can also form from the evaporation of water, gypsum is a perfect example of this process.
Gypsum
Mineral Groups
Earlier, I said that there were about 3,000 minerals found in Earth's crust. However, only about 30 of these minerals are common. The most common are called rock forming minerals because they make up most of the rocks found in Earth's crust. Elements are also found in Earth's crust. About 90 known elements occur naturally in the crust.
Silicates
Oxygen (O) is the most abundant element in Earth's crust, followed by silicon (Si). Minerals that form containing oxygen and silicon are known as silicates.
Silicates make up approximately 96% of the minerals found in Earth's crust. The most common of these are, feldspar, and quartz. The figure below shows how 1 atom of silicon attaches to 4 atoms of oxygen to form a silica tetrahedron.
This basic silica tetrahedron has the ability to share oxygen atoms with other tetrahedron molecules. This unique structure allows molecules to combine chemically and structurally in a vast number of ways, which accounts for the diversity of silicates.
Minerals Groups
Group
Example
Native Elements
Copper metal (Cu)
Oxides and
Hydroxides
Hemitite (Fe2O3)
Brucite (Mg{OH}2
Halides
Halite (NaCl)
Carbonates
Calcite (CaCO3)
Sulfates
Anhydrite (CaSO4)
Silicates
Olivine (Mg2SiO4)
Sulfides
Pyrite (FeS2)
Carbonates
Oxygen easily combines with many other elements and thus forms other mineral groups, such as the carbonates and the oxides. Carbonates are minerals composed of one or more metallic elements with the carbonate compound CO3. Examples of carbonates are calcite, dolomite, and rhodochrosite. Carbonates are the primary minerals found in rocks such as limestone, coquina, and marble. Some carbonates have distinctive colorization such as the greenish hue of malachite and the blue of azurite.
Oxides
Oxides are compounds of oxygen and a metal. Hemitite (Fe2O4) are common iron oxides and good sources of iron. The mineral uraninite is valuable because it is the major source of uranium, which is used to generate nuclear power.
Other major mineral groups are sulfide, sulfates, halides, and native elements. Sulfides such as pyrite (FeS2) are compound sulfur and one or more elements. Sulfates such as anhydrite (CaSO4) are composed of elements with the sulfate compound (SO4). Halides such as halite (NaCl) are made of chloride or fluoride along with calcium, sodium, or potassium.
Geologists rely on several relatively simple tests to identify minerals. These tests are based on a mineral's physical and chemical properties. It is usually best to use more than one test to identify a mineral.
Identifying Minerals
Color
One of the most noticeable characteristics of a mineral. Color is sometimes caused by the presence of trace elements or compounds in a mineral. For example quartz comes in a variety of colors.
The way that a mineral reflects light from its surface is called luster. Luster is described as being either metallic or nonmetallic. Silver, gold, copper, and galena have shiny surfaces that reflect light, they are said to have a metallic luster. Nonmetallic minerals like calcite, gypsum, and quartz, do not shine like metals, and their lusters are said to be nonmetallic.
Luster
Texture
Texture describes how a mineral feels to the touch. Like luster and color, texture is often used in combination with other tests to identify a mineral. The texture of a mineral may be described as smooth, rough, ragged, greasy, soapy, or glassy.Fluorite has a smooth texture and talc feels greasy.
Streak
A mineral rubbed across an unglazed porcelain plate will sometimes leave a color powdered streak on the surface of the plate.
Streak is the color of a mineral when it is broken up and powdered.
Sometimes a mineral's streak does not match its external color.Pyrite is a great example; pyrite is gold in color on its surface, but leaves a greenish black streak. Gold on the other hand leaves a yellow streak.
A mineral's streak rarely changes, even if it is weathered or its external color varies slightly.
Fluorite can be purple, yellow, green, or blue, but its streak is always white. The streak test can only be used on minerals softer than the porcelain plate.
One of the most useful tests for identifying minerals is hardness.
Hardness is a measure of how easily a minerals can be scratched.
German geologist Friedrich Mohs developed a scale in which an unknown mineral's hardness can be compared to the known hardness of ten minerals. The minerals in Mohs scale of mineral hardness were selected because they are easily recognizable and, with the exception of diamond, readily found in nature. Talc is one of the softest minerals, and can be scratched with a fingernail, thus it represents 1 on the Mohs scale of hardness. On the other end of the spectrum is diamond, it is so hard that it can be used as a sharpener or cutting tool, it represents 10 on the Mohs scale.
Hardness
Atomic arrangement also determines how a mineral will break. Minerals break along planes where atomic bonding is weak. A mineral that splits relatively easy and evenly along one or more flat planes is said to have cleavage.
To identify a mineral by cleavage, geologists count the number of cleaved planes and study the angle or angles between them.
Cleavage and Fracture
Density and Specific Gravity
Sometimes, two minerals of the same size may feel quite different when they are held. one is heavier than the other. Differences in weight are the result of different densities, which is defined as mass per unit volume.
D=M/V.
Specific gravity is the ratio of the weight of a substance to the weight of an equal volume of water.
Minerals have several special properties that can be used for identification.
Refraction is one of those properties. Refraction is how a mineral bends the light that pass through the mineral.
Special Properties
Another special property of minerals, like magnetite, is that they can be magnetic. Another characteristic is smell, minerals like sulfur produce an odor like rotten eggs.
There is a good chance that while your reading this you are sitting on minerals, wearing minerals, and perhaps even eating minerals. Minerals are virtually everywhere. They are used to make computers, cars, televisions, desks, roads, buildings, jewelry, beds, paints, sports equipment, and medicines.
Mineral Uses
Ores
Many of the items mentioned above are made from ores. A mineral is an ore if it contains a useful substance that can be mined at a profit. Hematite, for example, is a useful ore that contains the element iron. Look around you. Are any items made from iron? Is there anything in here made from aluminum? Aluminum is found in bauxite, another mineral.
Rutile, an important ore containing titanium
Bauxite, aluminum bearing ore
Hematite, iron bearing mineral
Ores that are located deep within Earth's crust are removed by underground mining. Ores that are near Earth's surface are obtained from, large open-pit mines, such as the one seen here.
Mines
Open-pit mine
The classification of a mineral as an ore may change if the supply or demand for that mineral changes. Consider a mineral used to make computers. Engineers might develop a more efficient design or a less costly alternative material. In either case, the mineral would be no longer used in the production of computers.
What makes a ruby more valuable than mica?
Gems
Rubies are much rarer and more visually pleasing than mica. Rubies are thus considered gems.
Gems
are valuable minerals that are prized for their rareity and beauty.
Raw emeralds
Because of their rareness, rubies and emeralds are actually more valuable than diamonds.
In some cases the presence of trace elements can make one variation of a mineral more colorful and thus more prized.
Amethyst, for instance, is the gem form of quartz. Amethyst contains trace minerals, which gives the gem a lovely purple color. The mineral corundum, which is often used as an abrasive, also can be found as rubies and sapphires. Rubies contain trace amounts of chromium; sapphires have trace amounts of cobalt or titanium.
Rock are defined as a naturally formed aggregate of mineral matter constituting a significant part of the earth's crust.
There are three types of rock; Igneous, sedimentary, and metamorphic.
Igneous rocks are formed from the crystallization of magma or lava. The term igneous comes from the Latin word "ignis" which means "fire" because early geologists often associated igneous rocks with fiery lava flows.
Rock Flavors
Types of Igneous Rocks
There are two types of igneous rocks. Fine grained igneous rocks that cool quickly on Earth's surface are called
extrusive
igneous rocks. Coarse grained igneous rocks that cool slowly beneath Earth's surface are called
intrusive
igneous rocks.
Granite is the most commontype of intrusive igneous rock.
The cross-cutting relationships are evidence that granite was intruded, or forced into, existing rocks. The figure below shows the conditions under which both types of igneous rocks are formed.
Types of Igneous Rocks
Lava cools on the surface & forms fine grained rocks like Rhyolite,
Magma cools slowly beneath the Earth's surface to form coarse grained rocks like granite
Bowen, a Canadian geologist, demonstrated that minerals form in predictable patterns as a magma cools. Bowen's reaction series illustrates this relationship between cooling and mineral formation.
Bowen's Reaction Series
Igneous rocks are classified as intrusive or extrusive. Geologists break the classifications down a little further too. These classifications are done according to things like grain size and texture.
Classifying Igneous Rocks
Mineral Composition
As shown in the table below, the three main groups of igneous rocks are, felsic, mafic and intermediate.
Felsic rocks such as granite are light in color and high in silica content, and contain quartz, and the feldspars, orthoclase, and plagioclase.
Mafic rocks such as gabbro are dark colored and have lower silica content, but are rich in iron and magnesium.
Ultramafic rocks like peridotite and dunite have very low silica content and very high contents of iron and magnesium.
Mineral Composition
Granite, light colored, high in silica - Felsic
Peridotite, dark colored, very high in iron and magnesium content
Gabbro, dark in color high iron and magnesium content, Gabbro
Along with the different mineral content of igneous rocks their grain sizes also varies. Obsidian is an extrusive igneous rock that cools rapidly and gabbro is an intrusive igneous rock that cools very slowly, the grain size in these rocks is very evident as seen below.
Grain Size
Gabbro, large grain size, cools slowly
Obsidian, cools rapidly, very small grain size.
Porphyritic texture is texture of a rock with varying mineral sizes. Very large crystals of one or more minerals combined with smaller crystals of the same or different minerals.
Porphyritic textures indicate a complex cooling history. Probably when a slow cooling magma starts to cool more rapidly. This change in cooling results in the different sizes of the mineral grains.
Texture
Igneous rocks have many properties that make them really good for building materials. The interlocking grains help give them good strength. They are also usually resistant to many types of weathering. If you have ever eaten at Panda Express, you have eaten on a granite table top.
Igneous Rocks as Resources
Ore Deposits
Ore deposits come in many shapes and forms. They include, veins, kimberlites, and pegmatites.
Sediments are pieces of solid material that have been deposited on Earth's surface by wind, water, ice, gravity, or chemical precipitation. When sediments become cemented together, they form sedimentary rocks. The formation of sedimentary rocks begins with weathering and erosion produce sediments. Sedimentary rocks have to have a certain process they go through to become actual rocks. Erosion, deposition, compaction and cementation, for the rocks to lithify.
Lithification is the physical and chemical processes that transform sediments into sedimentary rocks.
Formation of Sedimentary Rocks
Types of Sedimentary Rocks
The most common type of sedimentary rocks, clastic sedimentary rocks, are formed from the abundant deposits of loose sediments found on Earth's surface.
Sedimentary rocks consisting of gravel-sized fragments are classified as coarse-grained clastics.
Coarse grained Clastics
Breccia
Conglomerate
Medium-grained Clastics
Sedimentary rocks that contain sand sized rock and mineral fragments are classified as medium-grained clastic rocks. The most common medium-grained clastic sedimentary rock is sandstone. Sandstone has many features of interest to geologists. One important feature of sandstone that is very much of interest to geologists is porosity. Porosity is the percentage of open spaces between the grains in a rock.
When pore spaces are connected to one another, fluids can move through them. This features alone, makes sandstone layers valuable as underground reservoirs of oil, gas, and groundwater.
Medium-grained Clastics
Fine-grained Clastics
Sedimentary rocks consisting of silt and mud are called siltstone and mud stone. These types of rocks have very poor porosity. Because of its low porosity, it often forms barriers that make it difficult for the movement of groundwater and oil.
Siltstone
Mudstone
What happens to a glass of saltwater when you allow it to sit and evaporate? The water evaporates and a layer of salt is left behind. A process similar to this occurs in nature as well.
During chemical weathering, minerals can be dissolved and carried into lakes and oceans. As water evaporates from the lakes and oceans the dissolved minerals are left behind. The Great Salt Lake is a great example of this process.
Types of Sedimentary Rocks
Great Salt Lake, Utah
So, there are 2 types of sedimentary rocks, chemical sedimentary rocks and clastic sedimentary rocks.
Chemical sedimentary rocks are formed in 2 ways; from evaporation and from organics. Organic sedimentary rocks are formed from the remains of once living things.
Metamorphic rocks are created when the pressure or temperature increases on either sedimentary rocks or igneous rocks, or metamorphic rocks.
Metamorphic Rocks
Causes of Metamorphism
Pressure and temperature increase with depth into the Earth. When pressure and temperature become high enough, rocks begin to melt and form magma. But when this process occurs and the pressures and temperatures do not get quite high enough for the rocks to melt completely, the pressure and temperature combine to change the mineral structure of the existing rock. The word Metamorphism comes from the Greek words, meta, meaning "change", and the word morph, meaning "form". so, during metamorphism the rocks changes form while remaining a solid. The high temperatures required for metamorphism come from Earth interior heat, the pressure comes from 2 different things; vertical pressure, caused by the overlying weight and from compressive forces generated during mountain building.
Different combinations of temperature and pressure result in different types of metamorphism. As seen below, each combination produces a different group of metamorphic minerals and textures.
Types of Metamorphism
When high temperatures affect large regions of Earth's crust, they produce large belts of regional metamorphism. Regional metamorphism can be low grade, intermediate grade, and high grade. The grade reflects the relative temperature and pressure of the surrounding area.
When igneous rocks, such as those in an intrusion, come in contact with solid rock, a local effect called contact metamorphism occurs. High temperatures and moderate pressure form the mineral assemblages.
Contact Metamorphism
Types of Metamorphism
When very hot water reacts with rock and alters the chemistry and mineralogy, hydrothermal metamorphism occurs. The word "hydrothermal" is derived from the Greek words "hydro", meaning "water", and "thermal", meaning "heat". Hydrothermal fluids can dissolve minerals, break down others, and deposit new ones. Hydrothermal metamorphism is common around igneous intrusions and near active volcanoes.
Metamorphic rocks are classified into two textural groups; foliated and nonfoliated. Wavy layers and bands of minerals characterize foliated metamorphic rocks.
Metamorphic Textures
Gneiss
Unlike foliated rocks, nonfoliated rocks lack mineral grains with long axes in one direction. Nonfoliated rocks are mainly composed of minerals that form with blocky crystal shapes.
Marble
Porphyroblasts
Metamorphic Textures
Under certain conditions, new metamorphic minerals can grow quite large while the surrounding minerals remain small. The large crystals, which can range in size from a few millimeters to a few centimeters, are called porphyroblasts. Porphyroblasts are found in areas of both contact and regional metamorphism.
Mineral Changes
Minerals are stable at certain temperatures and pressures and crystallize from magma at different temperatures, Scientists have discovered that these stability ranges also apply to minerals in solid rock.
During metamorphism, the minerals in a rock change into new minerals that are stable under the new conditions. Minerals that change this way are said to undergo solid-state alterations. Because scientists have an understanding of how and when these minerals form, when they find them in a rock, they can interpret the conditions under which they formed.
Compositional Changes
Most metamorphic rocks reflect the original chemical composition of the parent rock. Gneiss, for example, has the same general chemical composition as granite.
In some instances though, the chemistry of a rock can be altered along with its minerals and textures. This occurs because hot fluids migrate in and out of the rock during metamorphism, which can change the original composition of the rock. Chemical changes are especially common during contact metamorphism near igneous intrusions. Hydrothermal fluids invade the surrounding rocks and change their mineralogy, texture, and chemistry. Valuable ore deposits of gold, copper, zinc, tungsten, and lead are formed in this way.
Gold
Copper
Zinc
Tungsten
Lead
As we have discussed, metamorphic rocks are formed by changing other rocks. The three types of rocks are igneous, metamorphic, and sedimentary.
The rock cycle is summarized in the figure above. The continuous changing remaking of rocks is called the rock cycle.
Essentially, rocks are recycled into different rock types much like glass is recycled into different types of jars and bottles.
Rock Cycle
There is obviously more than one path in the rock cycle. Sandstone might just as easily become uplifted and weathered back into sediments, thereby bypassing the metamorphic and igneous stages. Another possibility is that sandstone could be intruded by a magma and melted, thereby directly becoming an igneous rock bypassing the metamorphic stage.
Rocks of Earth's crust are constantly being recycled from one type to another. At any given time a magma is crystallizing, sediments are being cemented, and deeply buried rocks are metamorphosing. And finally, the processes that help shape Earth's landscapes are also part of the rock cycle.
Other Possible Paths
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