GEOLOGY/GEOPHYSICS 101 Program 15
MINERALS: The Materials of Earth
Hello. Welcome to our second program on Minerals.
Now, more than 2,000 minerals have been identified, and many of these are very rare; in fact, some are found only in one isolation location somewhere on Earth. Luckily for us, the common minerals only number about 50, so as a geologist working in the field, you only have to be familiar with 50 or so different minerals. That sounds like a lot, but these 50 minerals are different enough that you can usually tell them apart pretty easily, but of those 50, most rocks are made of only about 20 different minerals, and of those 20, only about 10 are really important for our purposes for understanding crustal processes, so in the last program we learned about atoms and the physical properties of minerals, and this program will focus on the structures of minerals and the various types of minerals, including the silicates, which are the components of most rock forming minerals.
I'll remind you to
One of the characteristic features of minerals is the crystal structure, so in this program we want to take a look at the arrangement of the atoms and ions that make up crystals to see if we can relate that to the larger scale properties of the physical properties of minerals, so let's take a look at crystal growth briefly for a second.
There are basically two different ways in which crystals can form. One of these is from a molten liquid in the case of the crystallization of magmas or lavas. The other way is the deposition from a solution, much in the same way that salt water creates salt crystals as the water evaporates. In both cases the crystal grows ion by ion or atom by atom usually at the corners because, as we'll see from the crystal models later, it's the corners where the weaknesses occur that allow the ions to grow. When the ions arrange themselves, they produce different patterns which depend upon the size of the ions and their charges.
So, you see, each mineral has a unique composition and structure, and these are really two separate considerations. The composition, of course, is the type of atoms that are involved, and the structure is the way that those atoms are arranged or the pattern that they are arranged in.
Every crystal consists of atoms in a repeating three- dimensional arrangement. Now, keep in mind that the atoms are really quite small, and there are something like 4 million atoms per millimeter. A millimeter is very small. If you pull out your ruler and look at the metric scale, the millimeter is the smallest division on there, so try to imagine, if you can, 4 million atoms fitting in the size of one millimeter.
Within a crystal, the smallest repeating unit in this three- dimensional structure is called the "unit cell". You might want to take a look at Box 9.4 on page 207 to get a sense of what we mean by "unit cell". Crystalographers have observed over the years that unit cells can come is several different shapes, and the shapes depend upon the size and the charge of the ions involved, and basically it depends upon how many ions can fit around a particular other ion, and I'll show you some examples of this in the second part of the program.
It turns out that there are only about six basic crystal shapes. These shapes are all boxes of one kind or another. Now, the boxes can be different sizes and different shapes. The size of the boxes, for example, may be the same lengths. They may be different lengths, and the size of the boxes may be perpendicular to each other, or they may be at different angles to each other. The crystal actually grows by repeating this unit cell, and crystalographers talk about symmetry operations, which you can create the unit cells by repeating these basic unit cells either by sliding, rotating, and various other symmetry operations.
Okay, and by the way, the symmetry is reflected in the arrangement of the faces on a particular crystal. Okay, so with this in mind, we note here that different minerals may have the same structure but different composition; that is, the atoms or ions may be arranged in the same way, but different atoms occupy the different sites in the crystal. A good example of this are two minerals. I showed you examples of them in the last program: halite, which is sodium chloride, and galena, which is lead sulphide.
Now, both of these minerals have similar properties. I should say they have the same cleavage. They both have the cubic cleavage, but they have very different properties because the composition is different. Halite is a soft, clear solid. Galena is a metallic silvery, very dense material.
I have an example of the sodium chloride crystal model here that we can use to see how this arrangement works. You'll note on this model first of all that there are two different colors of balls. Now, since this model can represent several different types of minerals, the balls can represent different types of atoms. If this was galena, for example, we could say that the red balls represent lead, and the white balls represent sulphur, or if it's halite, we could say that the red balls represent sodium, and the white balls represent chlorine.
Now keep in mind that this is a model, and it doesn't show all of the features of the crystal, just like all models, and by nature they have to be incomplete; in fact, this one's expanded a little bit, so we can see how the atoms relate to each other. In a real crystal, the balls are different sizes, and they're packed together a little bit more tightly.
Okay, one of the things you'll notice about this particular model. I can rotate it a little bit here, so you can get a sense of the three-dimensional nature of it. One of the things you'll notice about this particular model is that if you sort of peer down into the center of the crystal, and I may get the pencil to point here, you'll notice that a white ball down here is completely surrounded by six red balls, and by the same line of reasoning if you look at a red ball, you'll see that it's completely surrounded by six white balls, so the nature of this crystal is that basically the sodium and the chlorine are both completely surrounded by each other in the most efficient way possible.
Okay, I have a couple of other models here just to note here. I'll come back and look at these models in a little more detail in a second. Whoops! Live TV again! This is a model of calcite. Now calcite has the chemical formula calcium carbonate, which is "Ca", that's 1 calcium, "CO3", that's 1 carbon and 3 oxygen, and you can see represented in the model, here's a calcium ion. Here's a carbon atom attached to 3 oxygen atoms. Now, in this particular model, you'll note that the oxygen is attached to the carbon, and each carbon is attached to the calcium. This is kind of significant because the mineral calcite, as we'll see in later programs, is a very commonly occurring mineral on the Earth's surface, but when calcite dissolves in water, for example, the carbon and the oxygen stay attached to each other, and the bond that's broken is the bond between the calcium and the carbon atom. There, by the way, are not too many other examples of this particular structure although the mineral "dolomite" is related to calcite except that some of the calcium ions are replaced by magnesium ions instead.
Okay, we also note here that different minerals may have the same composition but different structures. These are called "polymorphs", and in this case, the arrangement depends upon the composition; that is, which atoms are involved and also the temperature and pressure at the time of formation of a particular mineral.
For example, there are several different varieties of silica. Silica is silicon dioxide. The most common is quartz, but there are other varieties that are formed at high pressures, and, in fact, some of these, the mineral called coesite, for example, has been identified in impact craters where meteorites smashed into the Earth, and this quartz in the rocks was reformed into a different crystal structure to form the new mineral called "coesite". Even ice, which we're all familiar with, has many different structures. The most common, of course, is the one that you find in your freezer, but under different conditions of temperature and pressure, ice can actually form several different crystal structures.
I also have some models here of graphite and diamond. I noted in the last program that graphite and diamond are both examples of carbon. This particular model is the graphite structure, and, you see there's something very distinctive about graphite. On one hand, if we turn this to the top, you see that the graphite consists of these rings of six carbon atoms. Okay, 1, 2, 3, 4, 5, 6, and the rings are all connected together, but on the other hand, these rings are in layers. I'll come back to look at this model again in a minute, but let me pull the diamond model out here. Here you see again the black balls represent carbon atoms but notice this time the structures is packed differently. The atoms are arranged a little bit differently. They're still six-sided rings if you still count them, 1, 2, 3, 4, 5, 6, but they're in a little bit different shape and a little bit different configuration than they were before, so with these crystal models, which, again, are models. They don't really represent reality completely, but they represent the models well enough that we can get a sense of how these things work.
I want to go back to the halite models, and I'll come back to these in a minute as well and see if we can relate this to some of the larger scale properties. Okay, I'm going to put a crystal of halite here on the table and bring out the crystal model again. You may remember in the last program I cleaved a piece of halite, and we saw that it has this nice what we called "cubic cleavage"; that is, it has three directions of cleavage at right angles to each other. If we go back to the crystal model, I think we can see how this cleavage comes about.
Okay, this is the cubic model that represents a unit cell of halite, and you can see that first of all, the shape of the model itself is a cube. All right. The sides are nearly perpendicular to each other. We also note that when you want to break a crystal of halite, what you're actually doing is breaking the bonds between adjacent sodium and chlorine ions. So which way do you think it would be easiest to break this? If your job was to cut this thing apart into pieces, how would it be easiest to break it? Well, you can see breaking it along here along this plane would be easier than breaking it diagonally because there are fewer bonds to cut. Not only that but we can also see that breaking it along this direction would be easy, and breaking it along this direction would be easy, so that when I cleave this, when I used the knife to cleave it yesterday and put the knife like this, what I'm doing is splitting apart those layers of atoms along these particular planes of weakness.
Okay, I want to put the calcite model up here. I'll replace this crystal halite with the calcite crystal and remember from last time that the calcite crystal has this distinctive rhombehedral shape. The rhombehedral is sort of like a squashed cube. Go back to the calcite structure for a second. Here's the calcite crystal structure. You'll note here that it has distinctly a rhombehedral shape. I also want to note here that strictly speaking the mineral calcite is in what's called a "hexagonal crystal system", and if you look at it from the end like this it is actually a six-sided figure, a six-sided shape, but when you turn the six-sided figure in this direction, it turns out to be a rombehedron, and again you`ll see that the cleavage can be explained very easily by the fact that when you cleave the calcite, what you're doing is cleaving it along this plane, again breaking the bonds between the calcium and the carbon, or the same thing in this direction, or the same thing in this direction.
Now, in this particular model, the bonds between the carbon and the oxygen are covalent bonds and they're quite strong. The bonds between the calcium and the carbon are ionic bonds, and they're quite weak, so when the mineral is cleaved, the carbon and oxygen tend to stay stuck together with the strong covalent bonds, and, in fact, these bonds between the carbon and the oxygen atoms are so strong that they tend to stay together as a unit even through the weathering process and even through various transformations of the mineral calcite.
Okay, let's go back and look at the graphite model for a second. I noted in the last program that graphite is very soft. You can write on a piece of paper with it. I think we can see why from the crystal model. Now, in this particular model, again turning it end on like this, these rings of carbon atoms that form the individual sheets are covalently bonded, so these bonds between the adjacent carbon atoms are quite strong, and it tends to resist breaking, so if I try to break this by cutting it this way, I'm dealing with breaking these covalent bonds, which are quite strong. On the other hand, the bonds between the various layers. Here's a layer. Here's a layer. Here's a layer. These bonds between the various layers are what are called "Vander Waals bonds". I mentioned them briefly in yesterday's program, and these bonds are extremely weak, so if I pick up a piece of graphite and rub it between my fingers, what I'm doing is rubbing off these individual layers and breaking these very weak Vander Waals bonds, and the same thing happens, of course, when you write on a piece of paper with graphite.
If I bear down on the piece of graphite, basically I scrape these layers off, so graphite because of these weak Vander Waals bonds between the layers is very soft and can be used to write on a piece of paper.
Let's compare this with the diamond model. Now, we know that diamond is the hardest known substance. First of all, look at this model. Look how dense and compact it is. In fact, if you examine this a little bit closer, you see that there are these six-sided rings of carbon atoms, and no matter how you look at it, these six-sided rings are joined together in an interlocking three- dimensional framework. Oh, and by the way, the white bars in here represent the unit cell for the diamond atom. You can see that the unit cell is actually a cube. Now, diamond is very hard because this particular type of bonding is what some people have described as the "perfect covalent bond". They say it's the perfect covalent bond because the bond is between atoms of the same type, so neither one of the atoms has control over the electrons, and the electrons are equally shared between all of the atoms. The diamond is very hard, and it's very difficult to break these bonds, but as I pointed out in the last program, diamond does actually have cleavage, and, I think, you can see this if I turn this around this way, you can see that there are gaps in here. Here, for example, along this line. I'll put the pencil in front. You see that there's a gap between this set of carbon atoms and this set of carbon atoms; in fact, that represents a cleavage direction for a diamond. You may notice that the cleavage direction is oriented at an angle to the unit cell, so when a diamond cutter wants to cut a diamond, he moves the diamond around and analyzes under polarized light until he finds that particular cleavage direction in the actual mineral, and then he can cleave it along that direction, and we see that the diamond breaks very nicely along those lines.
I think that gives a fairly good background, at least for watching the video. The video shows some of these principles, and it talks about some of the other aspects of minerals as well, so we'll come back after the video and specifically talk about the types of minerals and the structure of the silicate minerals, so in the meantime, let's watch the video.
Funding for this program was provided by the Annenberg C.P.B. Project.
At first glance, there's nothing particularly remarkable about this scene. These are objects that you might find at any typical campsite; however, there's a connection between them that goes beyond their obvious function. Most of these items, as well as those that fill our everyday lives, are made at least in part of minerals, the natural minerals of which the Earth is composed. Geologists define minerals as "solid substances that are naturally occurring and inorganic". Minerals also have a definite chemical composition in which the atoms are arranged in an orderly pattern called the "crystalline structure". Thousands of different chemical compositions in crystalline structures occur in nature, and combinations of these result in thousands of different mineral varieties. If we were to take away the objects from this campsite around me that require minerals in their manufacture, there'd be very little left to look at. Or sit on.
Human society depends on the products that it invents and manufactures, and minerals are an important raw material. The minerals we use in the manufacture of consumer goods and that are a part of virtually any man-made object you can name are also found here in the rocks that make up the Earth's crust.
In this open pit mine, for example, iron ore is extracted from the Earth. It is smelted and combined with other mineral products to form steel which is molded to make automobiles, ships, and skycrapers.
From these sand dunes, quartz grains are separated, then melted and molded to form glass, which is used to fill the windows of the world. Rocks are simply aggregates of mineral grains. Many granitic rocks, for example, contain mostly orthoclase, quartz, and plagoclase. While basalt typically contains plagoclase, pyroxene and olivine, and so apart from their value as components of everyday objects, minerals are also useful tools for classifying rocks.
The minerals contained in rocks provide hidden clues about the conditions under which the rocks formed. A mineral is like a little fossil. It's a story of a past time. Fossils to us tell us about past living conditions and where that fossil grew and lived at a very different age, and minerals do the same thing. Like fossils, minerals in a given rock are millions, if not billions, years old, but they trap within themselves, within their own internal compositions, their own history.
Different geologists use minerals in different ways. A chronologist uses minerals to determine the age of a rock, whether it be in millions or billions of years. He uses the radioactive elements that are in each mineral. The sedimentary petrologist and stratigrapher use minerals in a sediment to determine how that sediment was formed into a sedimentary rock, and some of those minerals in the sediment tell that geologist about mountains that were once there eroded to deformed sedimentary rock. The igneous petrologist and the metamorphic petrologist used minerals to determine the pressure and temperature recorded during a rock's crystallization from a molten magma or deformation during metamorphism.
In plate tectonics, a structural geologist uses minerals as well. Many minerals record magnetic direction, and as the plates have migrated, the magnetic directions are shifted, and so minerals have recorded plate motion, so we have learned about where the plates once were relative to today from the minerals. Different geologists have learned different things, but the minerals have recorded that information despite their great antiquity of age.
Over 2,000 varieties of minerals on Earth have been identified, and new ones are still being discovered, but most are rare, including some that have only been found at a single location on the planet. In fact, the common types of minerals number only about 200. Examples include quartz, olivine, orthoclase and plagoclase. These common mineral varieties are called rock forming minerals because they comprise most of the rocks on earth and also serve as the basis for classifying them.
Of course, before a rock can be classified, its minerals must be identified. This is one of the most fundamental tasks in all of geology. The differences between mineral varieties are related to their atomic structure. The atoms that make up a mineral are perfectly and symmetrically arranged in an almost infinite three- dimensional crystal lattice work. This structure is held together by a variety of chemical bonds. Individual atoms often occur as electrically charged particles called "ions". One important bond is formed when these ions combine to neutralize their charges. This results in the more stable configuration of a crystal structure. A somewhat analogous situation might be the relationship between loose cinder blocks and a cinder block wall that has been carefully constructed and mortered together. Both are composed of the same raw material, but the wall is strong and stable because of the way the individual blocks are mortered together.
On an atomic level, each type of mineral has its own unique crystal framework based on an orderly arrangement of bonded atoms. Crystal growth occurs atom by atom, layer by layer, in exactly the same pattern repeated over and over again. This regular internal structure has a great deal to do with the shape and physical properties of the resulting mineral.
As it turns out, a mineral's physical properties are usually quite different from those of the elements that compose it. A good example is halite. Halite is a mineral that forms when sodium and chlorine atoms join during the evaporation of a lake. By themselves, each of these elements is extremely dangerous. Sodium, being an explosive metal; and chlorine, a poisonous gas. Yet when they are joined together, sodium and chlorine combine to form something that most of us use all the time: ordinary table salt.
Another mineral with physical properties that are different from those of its chemical proponent is quartz. And if you look at that mineral quartz, it's composed of silicon, which in its pure state is a silvery solid substance, and oxygen, which isn't a solid at all, but an important atmospheric gas that also behaves flammably. Silicon and oxygen are very different individually. They don't combine to form quartz under ordinary surface conditions, but inside the Earth quartz forming reactions are common. Quartz is harder than steel due to the three-dimensional bonding of its individual silicon and oxygen ions.
It's usually transparent, forms beautiful crystals, very different from its pure separated elements. With a few simple tools: a steel knife, hydrochloric acid, a rock hammer, geologists in the field can perform tests to identify minerals.
Each mineral has a distinctive set of physical properties based on its own unique combination of chemical composition and crystalline structure. Physical properties include the color of the mineral, the way it reflects light, the way in which the mineral breaks, and some simple chemical reactions. These are used to help identify the mineral. It's easy to see that this rock is made of different minerals because there are four different colors of mineral crystals. Color is a fundamental physical property of minerals. Look at this silver mineral called "muscovite". It looks almost like a stack of paper with the individual sheets flaking apart quite easily. The tendency of minerals to break along flat planes is called "cleavage", and cleavage is a property that's determined by the crystalline structure of the mineral.
This pink mineral is "feldspar". Unlike muscovite, it has cleavage, but there are two directions of cleavage about 90 degrees to one another. The hardness of minerals is another identifying characteristic. Quartz is quite hard. It can't even be scratched by this steel hammer. Calcite looks similar to quartz but is much softer and scratches easily. Like cleavage, hardness is a physical property that's determined by the crystalline structure of the mineral and is a good way of differentiating between these two minerals.
Another physical property of calcite is that it dissolves in dilute acid. Calcite is a carbonate mineral, and the acid releases the carbon as carbon dioxide gas. Quartz is a silicate mineral. It doesn't dissolve in acid, and so there's no obvious chemical reaction. The way in which of the minerals reflect light is the physical property called "luster".
Feldspar has a dull luster. It doesn't shine at all. But compare that to muscovite, which has a glassy luster. Metallic minerals likes galena reflect light like a polished metal surface. Pyrite also has a metallic luster but is a different color than galena. One useful way to distinguish between some metallic minerals is a physical property called "streak". When we rub a mineral against a porcelain plate, we powder the mineral, and by comparing the color of the mineral in its powdered form to the coarse crystalline form, we can distinguish some types of minerals. Hematite is reddish brown in its powdered form and grey metallic in its coarse crystalline form. Compare this to galena, which is grey, both the powdered form and the coarse crystalline form.
Geologists in the field use simple tests like these to help identify minerals in rocks, but this is only the first step. Some minerals are only present as microscopic crystals in rocks. Others, in only extremely small quantities, and some materials can't be identified by physical properties alone.
Petrologists, the geologists that study the composition and origin of different types of rocks need to know much more about a rock sample like this and the minerals it contains. Once a sample is collected and identified in the field, it's taken back to the laboratory for a much more thorough analysis of the minerals. Petrologist Lawford Anderson is analyzing a piece of granite from the Whipple Mountains, which lie along the Colorado River in Southeastern California.
The purpose of the investigation is to determine the age of the granite, as well as to figure out exactly where in the Earth's crust it originated. That rock comes back to the laboratory. We're going to learn to read that part of Earth's history. We've got to open that rock up like opening up a book and start to read what kind of secrets are pent up in its minerologic or elemental composition. One of the thing that happens is that we saw that rock, and from that slab of rock that's removed, we have a piece of the rock here, and from the slab, we break it down to a smaller piece from which a very thin slice is made.
That is a layer of rock that is sliced so thin that we can pass light through it in a microscope to look down and see how the different minerals are arranged, be they sedimentary, igneous, or metamorphic minerals, the nature of the way they're intergrown, their composition tells us about the conditions of that rock's history, that part of Earth's history.
In addition to microscope work, Anderson also uses X-ray analysis to provide important imformation about the composition of minerals and rocks. We had that low one last week. Has that been corrected? To prepare the sample for analysis, the rock is literally broken down. This is done by first crushing it into smaller and smaller pieces. Ultimately, the rock is pulverized the consistency of a powder. The powder is carefully measured out. Then melted and pressed into the shape of a disk.
Finally, the disk is subjected to X-ray bombardment that yields the precise composition of the rock element by element. Another important analysis involves shooting beams of electrons at thin sections of rock to determine the individual mineral compositions within the rock.
The data derived from these procedures is vital. It enables Anderson and his colleagues to ascertain the pressure and depth at which the granite formed.
What we found out about the rocks in the Whipple Mountains is that they originated from the middle crust of the Earth some 25 or perhaps even more that 30 kilometers down, those minerals were crystallizing fom a magma that was in place in that level deep in the Earth crust at their age of 89 million years, so today we brought the rocks back.
They're at the surface now, but they were once deep, and they record in their composition and in their minerology how their inner crust originates. The conditions under which a mineral is created may be clearly reflected in its atomic structure, and, therefore, in its physical properties. Diamonds and graphite are perfect illustrations of the relationship between the mineral's environment of formation, crystal structure, and physical characteristics.
Diamonds have long been coveted as perhaps the most beautiful and precious of all gems. Graphite, which is used in pencils is extremely commonplace and far less valuable; yet both minerals are made of the same substance: pure carbon. The great contrast between their physical properties can be attributed to the differing structural arrangements of their carbon atoms. Diamond is the hardest of all minerals. Why is it so hard? It's because it has a very special and unique covalent bond that holds the different carbon atoms so tightly that they cannot be scratched.
In contrast, graphite, also a carbon mineral, the same carbon atoms are held with a very different kind of bond, and it's a very soft bond, and that mineral becomes soft, and that's why we can use graphite in pencils. So hardness is one aspect, and it's directly related to the bonding that holds the structure together. The covalent bond gives a strongly interlocking atomic arrangement to the carbon atoms in diamond. The weak bonds of carbon in graphite, however, develop a layered crystal structure.
Graphite is formed under low pressure conditions near surface, while diamond is formed under tremendously high pressures, in fact, needs great depths in the Earth to form; depth that are well within the mantle. It is these depths and pressures that give a diamond its covalent bonding and dazzling beauty and make it the rare and sought after jewel it has been throughout history.
Another rare mineral with a long and illustrious past is "gold". Few other minerals have ever had its economic or political power; yet unlike copper and silver, which have various industrial uses, gold has only limited practical value. The considerable value that gold does possess is based on its historical function as kind of a universal currency in a world where countries have little faith in each other's paper money.
The power of gold is exemplified by the settlement of California. Until the middle of the Nineteenth Century, this was a wild, uncharted, sparsely populated region. Then came the discovery of gold at Sutter's Mill, and practically overnight, thousands of people from all walks of life pulled up stakes and converged on the area hoping to strike it rich. The same kind of frenzied activity was then repeated at the end of the Nineteenth Century.
Following a gold strike in the Klondike, 30,000 adventurers poured into what is now the Yukon Territory. Gold and other pure minerals are relatively uncommon. What makes them so rare is that unusual conditions are required for them to concentrate within the Earth's crust.
Metallic minerals, such as gold, and silver, and copper all form the same way; they're precipitated from very hot water solutions called "hydrothermal" solutions that percolate up through cracks, up through fissures in the Earth, and as they reach cooler regions, they begin to crystalize, and whatever metals and non- metals that are in, dissolved in that hot water, precipitate out.
In case of gold, or silver, or copper, it calls for very special waters, and that's why they're all very rare. These waters are generally derived directly from crystallizing magma or from hot ground water circulating through the rock overlying an igneous intrusion. Rocks containing economically viable concentrations of metallic minerals are called "ore deposits", and hydrothermcal ore deposits are among the more important sources of metal known.
Most of the elements that make up ores don't have a home in everyday minerals, don't fit into the structures of quartz, or feldspar or mica, and as a lava or a magma begins to crystallize the common minerals, the elements are bunched up together and concentrated being dislodged away from the growing crystals.
Water is also building, and at some late stage in the crystallization of almost all magmas, water begins to boil off, and as it boils and rises out of the magma system, it takes with it all those elements that didn't have a home, and these go off and fill up fractures up through the rocks above the magma chamber, and as they reach the cooler rocks, they begin to precipitate.
Crystallization and precipitation from a hot solution is only one of several ways that minerals commonly form. A number of minerals crystallize directly from water. This occurs under certain conditions that favor chemical reactions between elements already present in the water. A common mineral that forms this way is hematite, sometimes called "bloodstone" by jewelers.
Hematite usually forms in well oxygenated water where dissolved iron and oxygen react and precipitate around sand grains, eventually forming red sandstone. Evaporation of sea or lake water triggers precipitation of an important group of minerals called "evaporites".
Halite is an example. Other minerals precipitate directly from gases through a process known as "sublimation". The sublimation process usually happens when you have a very hot volcanic gas like a sulphur dioxide, which can come out literally by tons per minute in a large volcanic eruption. When these gases start to cool, they'll go directly from the gaseous vapor state to individual crystals of sulphur and build a yellow mass around the volcanic vents.
Minerals also form by biologic processes as when an oyster makes a pearl. In addition, sponges and corals make their shells out of calcium carbonate taken from seawater and precipated as the minerals calcite or aragonite. As we've seen, minerals can form in many ways. Most are relatively uncommon, while a few dozen are quite plentiful, but no minerals on the planet are more abundant than the silicates.
Silicates constitute more than 90 percent of all mineral varieties on Planet Earth. Most silicates possess neither the political and financial power of gold, nor the exquisite beauty of diamonds, but their economic value as construction material is enormous, and one of their common ingredients, the element silicon, is used extensively in a very specialized type of modern technology: computers.
Pure, solid silicon is crystallized and hard, so it can be sliced to a thickness of only a fraction of a centimeter. It's also a semi-conductor, which means it can be made to conduct electricity. These properties make silicon the ideal raw material for the manufacture of microchips used in computers. These days computer technology is so widespread that we tend to take it for granted, but without the thin silicon wafers made from common silicon minerals, the awesome processing power of the computer age might never have come about.
Minerals have played a fundamental role in the political, economic, and technological evolution of human civilization. Wars have been fought and empires created over the geographic distribution of precious metals, of gems, and industrial minerals, and today mineral resources are more important than ever before.
The primary concern of the petrologist, however, is purely scientific. Ultimately, the lure of studying minerals for these geologic detectives is to unravel the geologic history, not only of rocks, but of the Earth itself.
Funding for this program was provided by the Annenberg C.P.B. Project.
That's an interesting video. Now these videos show us things that I couldn't even show you in the classroom. In some ways seeing this on television is better than actually just being in the classroom. There are some things I want to note that I think the video doesn't cover very well; for example, we use these idealized models of minerals and talk about the composition and so forth, but minerals are not really necessarily pure substances.
You see, an ion that's similar in size and charge can substitute for another. Calcium and magnesium can intersubstitute in the mineral calcite and dolomite. Now, many times the atoms that substitute are actually impurities, and they don't significantly alter the properties of the mineral although they may give it the different color.
We also note that there are certain types of minerals which can have what we call solid solutions, and a "solid solution" is simply a variable composition within known limits where substitutions take place. A good example of this is the mineral "olivine", which normally is either iron or magnesium mixed with silica, and the amounts of iron and magnesium are variable, and the properties are well known, and the compositional ranges are known within fairly good limits.
We'll be talking about solid solutions in later programs, especially in relation to the feldspar when we talk about igneous rocks and the formation of igneous rocks.
Okay, I also want to note as far as the categories of minerals go that you should review the descriptions of these main categories in the textbook. There are several different ways that minerals can be classified.
We can classify them by
and it's not that the classifications themselves are important because there are always many different ways to classify things, so you don't really need to memorize the classifications, but if you read later on that a particular mineral is an oxide or a sulfide, you should have some idea what that means, so you might want to review that section in the textbook.
I want to focus for the rest of the program on a special category of minerals called the "silicate minerals". Since silica comprises 75 percent of elements in the crust, and remember that silica is the special word we use for silicon and oxygen comprises 75 percent of the elements of the crust, it's not surprising that minerals formed out of silica that we all the "silicates" are the most common and the most important rock forming minerals. In fact, more than 90 percent of the rock forming minerals are silicates.
The silicates also form another category of minerals called the "clay minerals", which we'll come back to when we discuss weathering in a later program. The silicates, because they're the most common types, they're also the most complex type. Their are many solid solutions and many different substitutions that take place in the silicate minerals, and there are many varieties of crystal structures, so what we need to look at here is trying to understand how these silicate minerals are constructed, and it's really very simple. It has to do with the size and charge of the silicon atom and the oxygen atom.
I have a model here on the table in front of me that I think I can use to illustrate this. Okay, the oxygen ion is a relatively large ion; in fact, if we look at the crustal abundance of the elements by volume; that is, by how much space they take up, we find that oxygen occupies almost 95 percent of all crustal rocks. Basically, we can think of rocks as oxygen atoms with silicon and metal atoms arranged in between them.
The model here on the table, the white balls represent oxygen. The orange ball in the center represents silicon. Now, it's quite a coincidence that the silicon atom happens to have four bonding sites; that is, it's sticky in four different places. It's also interesting that oxygen has two bonding sites, and you notice here that the silicon fits almost exactly in between three of the oxygen atoms; in other words, if I put three oxygen atoms together edge to edge, there's just enough space in between for the silicon atom to fit. Now, that takes care of three of the sticky sites or the bonding sites of silicon. The fourth one is taken care of by another oxygen atom which fits very nicely on top, so that the whole package has a shape that looks like this, and you see here as I rotate this around that the silicon atom is almost completely concealed within this pyramid of oxygen atoms.
It's this grouping of silicon and oxygen atoms that forms the basis for all of the silicate minerals. We call this particular grouping the "silica tetrahedron". The word "tetrahedran" is a geometric term, and it comes from the Greek word that means four-sided figure.
I have a geometric figure of a tetrahedron here that you can get a sense of how this relates to this pile of oxygen atoms with the silicon in the center. You'll notice that this forms sort of a pyramid shape. Here's one flat side, another flat side. In fact, we can use this tetrahedron; I'll rotate it here a little bit so you can get a sense of the shape of it. You can use this tetrahedron as a way of modeling how the silicate minerals behave. You see that if I turn the tetrahedron in different directions, there's a very symmetrical sort of shape. It's basically a four-sided pyramid. Each side of the pyramid is an equalateral triangle. No matter how I turn it, it looks the same way. We say that it has a high degree of symmetry. If I turn it this way, it looks the same as if I turn it this way, and so on, so what makes the silicate minerals so interesting is that, on one hand, silicon is very similar to carbon; that is, it has four bonding sites, and you may know that it's the particular shape of the carbon atom and it's ability to bond with itself that's responsible for the organic chemicals that make up the variety of life on Earth.
Silicon is very similar to carbon with one important difference: Carbon tends do bond easier to hydrogen; whereas, silicon has this great affinity for oxygen because of the particular size of the oxygen atoms related to the silicon atom, so we can see here if I take these off, and I'll put some smaller models of the tetrahedron on, we can see how we can build the structures of the various silicate minerals.
Okay, here's a single tetrahedron. Now some of the silicate minerals consist of individual silica tetrahedron, which are not connected to each other, but rather are each connected to another atom of one type, so that this whole combination of one silicon and four oxygen atoms actually has an overall charge of minus 4,and that means that it attracts positive ions, so metal ions, which are positively charged tend to stick, so we can construct a mineral in many different ways by putting tetrahedrons together and putting positive ions in between them in various shapes.
Things are rolling around here a bit, so this doesn't necessarily represent any real crystal, but you can see how the tetrahedron are used as building blocks. We might also note that there are lots of different ways to put the tetrahedra together; for example, in the mineral "olivine", the tetrahedron are arranged in alternate rows, and one tetrahedron points upward, and the next one points downward, so if you can imagine rows of these, you'd get a picture of this.
There is, by the way, a picture of the olivine structure in the textbook. It's Figure 9.8 on page 199. You might want to take a look at this structure of olivine, but, you see, the tetrahedra can combine in other ways as well.
The tetrahedron can share oxygen atoms at the corner. What that means is that if I take the large tetrahedron and pull one of the oxygen atoms off and stick another one together, the two tetrahedra can share corners, like this, so the silicon tetrahedra have the ability to form many different types of structures. One example is a chain. You can arrange tetrahedra in a chain like this. There are many different rock forming minerals that have this chainlike structure.
The most noteable one is the mineral that we call "pyroxene", which is one of the common rock forming minerals, very common in volcanic rocks in generally and especially here in the rocks in Hawaii. In these so-called chain silicates, the blank spaces like here may be occupied by several different types of atoms. In the case of pyroxene, these blank spaces may be filled up with iron, or magnesium, or calcium, or in some cases, even sodium atoms, and, again, which atoms fit in depend upon the exact temperature and pressure and the available atoms at the time the mineral was forming, but this is not the only way you can put the silica tetrahedra together; in fact, another type of minerals that we call "amphibole" has what we call a double chain structure.
I can build another chain like this. The chain looks exactly like the first one. The difference is that it's a mirror image of the first chain. I can put them together in a second chain like this, and now we have a structure that is one chain on one side and another chain on the other side. You'll also notice here that this chain structure also can be modified to form another structure of a ring. The silica tetrahedra can arrange themselves in a ring. The mineral "beryl", of which is a gem variety mineral, has this ringlike structure. Well, I can't show the other models because it would require me to hang things in mid-air, and if we could get rid of gravity, I could do that, but as long as we have gravity, it's rather difficult for me to do this, but you can also have a set of only double chains.
You can have whole sheets of silica tetrahedra. The mica minerals, for example. Remember the cleavage of the mica minerals in one direction. The mica minerals are sheets of these silica tetrahedra that are held together by metallic ions sort of like a sandwich. You might think of it as a metal sandwich on silicate bread.
Okay, the two most commonly occurring silicates are quartz and feldspar, and both of these minerals represent a three dimensional framework with the tetrahedra arranged in a three dimensional framework very similar to the way the atoms are arranged in the carbon atoms, so I'll call your attention again to see the Figures 9.7 through 9.11 on page 199 and 200 in the textbook and also Box 9.3 on page 203, which gives you a sense of the clay structure.
Well, it seems that, I think anyway, that crystallography is a fascinating subject. It combines geometry, symmetry, and art with geology. Now, there's much more that we can learn about crystals in terms of symmetry and the various ways in which the atoms can be arranged, and the different types of crystal structures, and symmetries, and so forth, but we don't have time to do that in this course.
There are courses available in crystallography and mineralogy that would take you much deeper into this if you're interested in this, one of the those areas of geology that sort of can stand alone without even understanding the rest of it. What we have learned in these two programs will help us to understand, I think, many aspects of future lessons, things like igneous rocks, and the formation of sedimentary rocks, and why certain minerals occur, and also metamorphic rocks, and what sort of changes take place when rocks formed at the surface are exposed to high temperature and pressure deep in the Earth. It also helps us to explain weathering and soils, and, as we'll see, weathering and soil formation involves rearrangement of these various atoms from one place to another as they're adjusting to new equillibrium at the Earth's surface, so for the next lesson, you should
That's it for today, and I'll see you next time.