GG101 Minerals: The Materials of Earth (2024)

GEOLOGY/GEOPHYSICS 101 Program 15

MINERALS: The Materials of Earth

Now, more than 2,000 minerals have been identified, and many ofthese are very rare; in fact, some are found only in one isolation location somewhereon Earth. Luckily for us, the common minerals only number about 50, so as a geologistworking 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 usuallytell them apart pretty easily, but of those 50, most rocks are made of only about20 different minerals, and of those 20, only about 10 are really important for ourpurposes for understanding crustal processes, so in the last program we learned aboutatoms and the physical properties of minerals, and this program will focus on thestructures 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

• read the text assignment. You should have already read it anyway,but we're still in Chapter 9, pages 193 to 213, and you should be sure to
• study the photographs and diagrams carefully and pay special attentionto those diagrams which show you the silicate structures and don't forget to
• follow the study plan and the study guide and be sure to go backand review the learning objectives to make sure you've learned each one. I won'tgo over the objectives with you this time because they're same as the last lesson,and they are written out for you in the study guide.

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 ionsthat make up crystals to see if we can relate that to the larger scale propertiesof the physical properties of minerals, so let's take a look at crystal growth brieflyfor 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 magmasor lavas. The other way is the deposition from a solution, much in the same way thatsalt water creates salt crystals as the water evaporates. In both cases the crystalgrows ion by ion or atom by atom usually at the corners because, as we'll see fromthe crystal models later, it's the corners where the weaknesses occur that allowthe ions to grow. When the ions arrange themselves, they produce different patternswhich 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, isthe type of atoms that are involved, and the structure is the way that those atomsare arranged or the pattern that they are arranged in.

Every crystal consists of atoms in a repeating three- dimensionalarrangement. Now, keep in mind that the atoms are really quite small, and there aresomething like 4 million atoms per millimeter. A millimeter is very small. If youpull out your ruler and look at the metric scale, the millimeter is the smallestdivision on there, so try to imagine, if you can, 4 million atoms fitting in thesize of one millimeter.

Within a crystal, the smallest repeating unit in this three- dimensionalstructure is called the "unit cell". You might want to take a lookat Box 9.4 on page 207 to get a sense of what we mean by "unit cell". Crystalographershave 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 basicallyit depends upon how many ions can fit around a particular other ion, and I'll showyou 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 differentsizes 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 toeach other, or they may be at different angles to each other. The crystal actuallygrows 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 bysliding, rotating, and various other symmetry operations.

Okay, and by the way, the symmetry is reflected in the arrangementof the faces on a particular crystal. Okay, so with this in mind, we note here thatdifferent 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 thedifferent sites in the crystal. A good example of this are two minerals. I showedyou 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 saythey have the same cleavage. They both have the cubic cleavage, but they have verydifferent properties because the composition is different. Halite is a soft, clearsolid. Galena is a metallic silvery, very dense material.

I have an example of the sodium chloride crystal model here thatwe can use to see how this arrangement works. You'll note on this model first ofall that there are two different colors of balls. Now, since this model can representseveral 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 redballs represent sodium, and the white balls represent chlorine.

Now keep in mind that this is a model, and it doesn't show allof the features of the crystal, just like all models, and by nature they have tobe incomplete; in fact, this one's expanded a little bit, so we can see how the atomsrelate to each other. In a real crystal, the balls are different sizes, and they'repacked 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-dimensionalnature of it. One of the things you'll notice about this particular model is thatif you sort of peer down into the center of the crystal, and I may get the pencilto point here, you'll notice that a white ball down here is completely surroundedby six red balls, and by the same line of reasoning if you look at a red ball, you'llsee that it's completely surrounded by six white balls, so the nature of this crystalis that basically the sodium and the chlorine are both completely surrounded by eachother in the most efficient way possible.

Okay, I have a couple of other models here just to note here. I'llcome back and look at these models in a little more detail in a second. Whoops! LiveTV again! This is a model of calcite. Now calcite has the chemical formula calciumcarbonate, which is "Ca", that's 1 calcium, "CO3", that's 1 carbonand 3 oxygen, and you can see represented in the model, here's a calcium ion. Here'sa carbon atom attached to 3 oxygen atoms. Now, in this particular model, you'll notethat 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 dissolvesin water, for example, the carbon and the oxygen stay attached to each other, andthe 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 althoughthe mineral "dolomite" is related to calcite except that some of the calciumions are replaced by magnesium ions instead.

Okay, we also note here that different minerals may have the samecomposition but different structures. These are called "polymorphs",and in this case, the arrangement depends upon the composition; that is, which atomsare involved and also the temperature and pressure at the time of formation of aparticular mineral.

For example, there are several different varieties of silica. Silicais silicon dioxide. The most common is quartz, but there are other varieties thatare 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 intothe Earth, and this quartz in the rocks was reformed into a different crystal structureto form the new mineral called "coesite". Even ice, which we'reall familiar with, has many different structures. The most common, of course, isthe one that you find in your freezer, but under different conditions of temperatureand pressure, ice can actually form several different crystal structures.

I also have some models here of graphite and diamond. I noted inthe last program that graphite and diamond are both examples of carbon. This particularmodel is the graphite structure, and, you see there's something very distinctiveabout graphite. On one hand, if we turn this to the top, you see that the graphiteconsists of these rings of six carbon atoms. Okay, 1, 2, 3, 4, 5, 6, and the ringsare all connected together, but on the other hand, these rings are in layers. I'llcome back to look at this model again in a minute, but let me pull the diamond modelout here. Here you see again the black balls represent carbon atoms but notice thistime 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'rein a little bit different shape and a little bit different configuration than theywere before, so with these crystal models, which, again, are models. They don't reallyrepresent reality completely, but they represent the models well enough that we canget a sense of how these things work.

I want to go back to the halite models, and I'll come back to thesein 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 crystalmodel again. You may remember in the last program I cleaved a piece of halite, andwe 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 backto 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 wantto break a crystal of halite, what you're actually doing is breaking the bonds betweenadjacent sodium and chlorine ions. So which way do you think it would be easiestto break this? If your job was to cut this thing apart into pieces, how would itbe easiest to break it? Well, you can see breaking it along here along this planewould 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 beeasy, 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'mdoing 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 thiscrystal halite with the calcite crystal and remember from last time that the calcitecrystal has this distinctive rhombehedral shape. The rhombehedral is sort of likea squashed cube. Go back to the calcite structure for a second. Here's the calcitecrystal 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'scalled a "hexagonal crystal system", and if you look at it fromthe end like this it is actually a six-sided figure, a six-sided shape, but whenyou 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 thatwhen you cleave the calcite, what you're doing is cleaving it along this plane, againbreaking the bonds between the calcium and the carbon, or the same thing in thisdirection, or the same thing in this direction.

Now, in this particular model, the bonds between the carbon andthe oxygen are covalent bonds and they're quite strong. The bonds between the calciumand 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 thatthey tend to stay together as a unit even through the weathering process and eventhrough 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 pieceof paper with it. I think we can see why from the crystal model. Now, in this particularmodel, again turning it end on like this, these rings of carbon atoms that form theindividual sheets are covalently bonded, so these bonds between the adjacent carbonatoms are quite strong, and it tends to resist breaking, so if I try to break thisby cutting it this way, I'm dealing with breaking these covalent bonds, which arequite 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 arecalled "Vander Waals bonds". I mentioned them briefly in yesterday'sprogram, and these bonds are extremely weak, so if I pick up a piece of graphiteand rub it between my fingers, what I'm doing is rubbing off these individual layersand 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 theselayers off, so graphite because of these weak Vander Waals bonds between the layersis 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 diamondis the hardest known substance. First of all, look at this model. Look how denseand compact it is. In fact, if you examine this a little bit closer, you see thatthere 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 diamondatom. You can see that the unit cell is actually a cube. Now, diamond is very hardbecause this particular type of bonding is what some people have described as the"perfect covalent bond". They say it's the perfect covalent bondbecause the bond is between atoms of the same type, so neither one of the atoms hascontrol over the electrons, and the electrons are equally shared between all of theatoms. The diamond is very hard, and it's very difficult to break these bonds, butas I pointed out in the last program, diamond does actually have cleavage, and, Ithink, you can see this if I turn this around this way, you can see that there aregaps in here. Here, for example, along this line. I'll put the pencil in front. Yousee 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 thatthe cleavage direction is oriented at an angle to the unit cell, so when a diamondcutter wants to cut a diamond, he moves the diamond around and analyzes under polarizedlight until he finds that particular cleavage direction in the actual mineral, andthen he can cleave it along that direction, and we see that the diamond breaks verynicely along those lines.

I think that gives a fairly good background, at least for watchingthe video. The video shows some of these principles, and it talks about some of theother aspects of minerals as well, so we'll come back after the video and specificallytalk about the types of minerals and the structure of the silicate minerals, so inthe 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 aboutthis 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 ofthese items, as well as those that fill our everyday lives, are made at least inpart of minerals, the natural minerals of which the Earth is composed. Geologistsdefine minerals as "solid substances that are naturally occurring and inorganic".Minerals also have a definite chemical composition in which the atoms are arrangedin an orderly pattern called the "crystalline structure". Thousandsof different chemical compositions in crystalline structures occur in nature, andcombinations of these result in thousands of different mineral varieties. If we wereto take away the objects from this campsite around me that require minerals in theirmanufacture, 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 manufactureof consumer goods and that are a part of virtually any man-made object you can nameare also found here in the rocks that make up the Earth's crust.

In this open pit mine, for example, iron ore is extracted fromthe Earth. It is smelted and combined with other mineral products to form steel whichis molded to make automobiles, ships, and skycrapers.

From these sand dunes, quartz grains are separated, then meltedand molded to form glass, which is used to fill the windows of the world. Rocks aresimply aggregates of mineral grains. Many granitic rocks, for example, contain mostlyorthoclase, quartz, and plagoclase. While basalt typically contains plagoclase, pyroxeneand olivine, and so apart from their value as components of everyday objects, mineralsare also useful tools for classifying rocks.

The minerals contained in rocks provide hidden clues about theconditions under which the rocks formed. A mineral is like a little fossil. It'sa story of a past time. Fossils to us tell us about past living conditions and wherethat 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 ownhistory.

Different geologists use minerals in different ways. A chronologistuses minerals to determine the age of a rock, whether it be in millions or billionsof years. He uses the radioactive elements that are in each mineral. The sedimentarypetrologist and stratigrapher use minerals in a sediment to determinehow that sediment was formed into a sedimentary rock, and some of those mineralsin the sediment tell that geologist about mountains that were once there eroded todeformed sedimentary rock. The igneous petrologist and the metamorphicpetrologist used minerals to determine the pressure and temperature recordedduring a rock's crystallization from a molten magma or deformation during metamorphism.

In plate tectonics, a structural geologist uses mineralsas 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 recordedthat 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 haveonly been found at a single location on the planet. In fact, the common types ofminerals number only about 200. Examples include quartz, olivine, orthoclase andplagoclase. These common mineral varieties are called rock forming minerals becausethey comprise most of the rocks on earth and also serve as the basis for classifyingthem.

Of course, before a rock can be classified, its minerals must beidentified. This is one of the most fundamental tasks in all of geology. The differencesbetween mineral varieties are related to their atomic structure. The atoms that makeup a mineral are perfectly and symmetrically arranged in an almost infinite three-dimensional crystal lattice work. This structure is held together by a variety ofchemical bonds. Individual atoms often occur as electrically charged particles called"ions". One important bond is formed when these ions combine toneutralize their charges. This results in the more stable configuration of a crystalstructure. A somewhat analogous situation might be the relationship between loosecinder blocks and a cinder block wall that has been carefully constructed and morteredtogether. Both are composed of the same raw material, but the wall is strong andstable because of the way the individual blocks are mortered together.

On an atomic level, each type of mineral has its own unique crystalframework based on an orderly arrangement of bonded atoms. Crystal growth occursatom by atom, layer by layer, in exactly the same pattern repeated over and overagain. This regular internal structure has a great deal to do with the shape andphysical properties of the resulting mineral.

As it turns out, a mineral's physical properties are usually quitedifferent from those of the elements that compose it. A good example is halite. Haliteis a mineral that forms when sodium and chlorine atoms join during the evaporationof 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 joinedtogether, sodium and chlorine combine to form something that most of us use all thetime: ordinary table salt.

Another mineral with physical properties that are different fromthose 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, andoxygen, which isn't a solid at all, but an important atmospheric gas that also behavesflammably. Silicon and oxygen are very different individually. They don't combineto form quartz under ordinary surface conditions, but inside the Earth quartz formingreactions are common. Quartz is harder than steel due to the three-dimensional bondingof its individual silicon and oxygen ions.

It's usually transparent, forms beautiful crystals, very differentfrom its pure separated elements. With a few simple tools: a steel knife, hydrochloricacid, a rock hammer, geologists in the field can perform tests to identify minerals.

Each mineral has a distinctive set of physical properties basedon 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 areused to help identify the mineral. It's easy to see that this rock is made of differentminerals because there are four different colors of mineral crystals. Color is afundamental 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 quiteeasily. 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 themineral.

This pink mineral is "feldspar". Unlike muscovite,it has cleavage, but there are two directions of cleavage about 90 degrees to oneanother. The hardness of minerals is another identifying characteristic. Quartz isquite hard. It can't even be scratched by this steel hammer. Calcite looks similarto quartz but is much softer and scratches easily. Like cleavage, hardness is a physicalproperty that's determined by the crystalline structure of the mineral and is a goodway of differentiating between these two minerals.

Another physical property of calcite is that it dissolves in diluteacid. Calcite is a carbonate mineral, and the acid releases the carbon as carbondioxide gas. Quartz is a silicate mineral. It doesn't dissolve in acid, and so there'sno obvious chemical reaction. The way in which of the minerals reflect light is thephysical property called "luster".

Feldspar has a dull luster. It doesn't shine at all. But comparethat to muscovite, which has a glassy luster. Metallic minerals likes galena reflectlight like a polished metal surface. Pyrite also has a metallic luster but is a differentcolor than galena. One useful way to distinguish between some metallic minerals isa physical property called "streak". When we rub a mineral againsta porcelain plate, we powder the mineral, and by comparing the color of the mineralin its powdered form to the coarse crystalline form, we can distinguish some typesof minerals. Hematite is reddish brown in its powdered form and grey metallic inits coarse crystalline form. Compare this to galena, which is grey, both the powderedform and the coarse crystalline form.

Geologists in the field use simple tests like these to help identifyminerals in rocks, but this is only the first step. Some minerals are only presentas microscopic crystals in rocks. Others, in only extremely small quantities, andsome materials can't be identified by physical properties alone.

Petrologists, the geologists that study the compositionand origin of different types of rocks need to know much more about a rock samplelike this and the minerals it contains. Once a sample is collected and identifiedin the field, it's taken back to the laboratory for a much more thorough analysisof the minerals. Petrologist Lawford Anderson is analyzing a piece of granite fromthe Whipple Mountains, which lie along the Colorado River in Southeastern California.

The purpose of the investigation is to determine the age of thegranite, 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 ofEarth's history. We've got to open that rock up like opening up a book and startto 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 rockthat's removed, we have a piece of the rock here, and from the slab, we break itdown 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 passlight through it in a microscope to look down and see how the different mineralsare arranged, be they sedimentary, igneous, or metamorphic minerals, the nature ofthe way they're intergrown, their composition tells us about the conditions of thatrock's history, that part of Earth's history.

In addition to microscope work, Anderson also uses X-ray analysisto provide important imformation about the composition of minerals and rocks. Wehad 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 smallerand 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 adisk.

Finally, the disk is subjected to X-ray bombardment that yieldsthe precise composition of the rock element by element. Another important analysisinvolves shooting beams of electrons at thin sections of rock to determine the individualmineral compositions within the rock.

The data derived from these procedures is vital. It enables Andersonand 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 thatthey originated from the middle crust of the Earth some 25 or perhaps even more that30 kilometers down, those minerals were crystallizing fom a magma that was in placein that level deep in the Earth crust at their age of 89 million years, so todaywe brought the rocks back.

They're at the surface now, but they were once deep, and they recordin their composition and in their minerology how their inner crust originates. Theconditions under which a mineral is created may be clearly reflected in its atomicstructure, and, therefore, in its physical properties. Diamonds and graphite areperfect 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 andprecious of all gems. Graphite, which is used in pencils is extremely commonplaceand 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 differingstructural 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 thatholds the different carbon atoms so tightly that they cannot be scratched.

In contrast, graphite, also a carbon mineral, the same carbon atomsare held with a very different kind of bond, and it's a very soft bond, and thatmineral becomes soft, and that's why we can use graphite in pencils. So hardnessis one aspect, and it's directly related to the bonding that holds the structuretogether. The covalent bond gives a strongly interlocking atomic arrangement to thecarbon atoms in diamond. The weak bonds of carbon in graphite, however, develop alayered crystal structure.

Graphite is formed under low pressure conditions near surface,while diamond is formed under tremendously high pressures, in fact, needs great depthsin the Earth to form; depth that are well within the mantle. It is these depths andpressures that give a diamond its covalent bonding and dazzling beauty and make itthe 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 copperand silver, which have various industrial uses, gold has only limited practical value.The considerable value that gold does possess is based on its historical functionas kind of a universal currency in a world where countries have little faith in eachother'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, sparselypopulated region. Then came the discovery of gold at Sutter's Mill, and practicallyovernight, thousands of people from all walks of life pulled up stakes and convergedon the area hoping to strike it rich. The same kind of frenzied activity was thenrepeated at the end of the Nineteenth Century.

Following a gold strike in the Klondike, 30,000 adventurers pouredinto what is now the Yukon Territory. Gold and other pure minerals are relativelyuncommon. What makes them so rare is that unusual conditions are required for themto concentrate within the Earth's crust.

Metallic minerals, such as gold, and silver, and copper all formthe same way; they're precipitated from very hot water solutions called "hydrothermal"solutions that percolate up through cracks, up through fissures in the Earth, andas 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 specialwaters, and that's why they're all very rare. These waters are generally deriveddirectly from crystallizing magma or from hot ground water circulating through therock overlying an igneous intrusion. Rocks containing economically viable concentrationsof metallic minerals are called "ore deposits", and hydrothermcalore deposits are among the more important sources of metal known.

Most of the elements that make up ores don't have a home in everydayminerals, don't fit into the structures of quartz, or feldspar or mica, and as alava or a magma begins to crystallize the common minerals, the elements are bunchedup together and concentrated being dislodged away from the growing crystals.

Water is also building, and at some late stage in the crystallizationof almost all magmas, water begins to boil off, and as it boils and rises out ofthe magma system, it takes with it all those elements that didn't have a home, andthese 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 oneof several ways that minerals commonly form. A number of minerals crystallize directlyfrom water. This occurs under certain conditions that favor chemical reactions betweenelements 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 dissolvediron and oxygen react and precipitate around sand grains, eventually forming redsandstone. Evaporation of sea or lake water triggers precipitation of an importantgroup of minerals called "evaporites".

Halite is an example. Other minerals precipitate directly fromgases through a process known as "sublimation". The sublimation processusually happens when you have a very hot volcanic gas like a sulphur dioxide, whichcan come out literally by tons per minute in a large volcanic eruption. When thesegases start to cool, they'll go directly from the gaseous vapor state to individualcrystals of sulphur and build a yellow mass around the volcanic vents.

Minerals also form by biologic processes as when an oyster makesa pearl. In addition, sponges and corals make their shells out of calcium carbonatetaken from seawater and precipated as the minerals calcite or aragonite. As we'veseen, minerals can form in many ways. Most are relatively uncommon, while a few dozenare quite plentiful, but no minerals on the planet are more abundant than the silicates.

Silicates constitute more than 90 percent of all mineral varietieson Planet Earth. Most silicates possess neither the political and financial powerof gold, nor the exquisite beauty of diamonds, but their economic value as constructionmaterial is enormous, and one of their common ingredients, the element silicon, isused extensively in a very specialized type of modern technology: computers.

Pure, solid silicon is crystallized and hard, so it can be slicedto a thickness of only a fraction of a centimeter. It's also a semi-conductor, whichmeans it can be made to conduct electricity. These properties make silicon the idealraw material for the manufacture of microchips used in computers. These days computertechnology is so widespread that we tend to take it for granted, but without thethin silicon wafers made from common silicon minerals, the awesome processing powerof 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 empirescreated over the geographic distribution of precious metals, of gems, and industrialminerals, 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 unravelthe 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 thatI couldn't even show you in the classroom. In some ways seeing this on televisionis better than actually just being in the classroom. There are some things I wantto note that I think the video doesn't cover very well; for example, we use theseidealized models of minerals and talk about the composition and so forth, but mineralsare not really necessarily pure substances.

You see, an ion that's similar in size and charge can substitutefor another. Calcium and magnesium can intersubstitute in the mineral calcite anddolomite. Now, many times the atoms that substitute are actually impurities, andthey don't significantly alter the properties of the mineral although they may giveit the different color.

We also note that there are certain types of minerals which canhave what we call solid solutions, and a "solid solution" is simplya variable composition within known limits where substitutions take place. A goodexample of this is the mineral "olivine", which normally is eitheriron 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 withinfairly good limits.

We'll be talking about solid solutions in later programs, especiallyin relation to the feldspar when we talk about igneous rocks and the formation ofigneous rocks.

Okay, I also want to note as far as the categories of mineralsgo 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

• composition,
• by crystal structure,
• by origin, or
• how they occur,

and it's not that the classifications themselves are importantbecause there are always many different ways to classify things, so you don't reallyneed to memorize the classifications, but if you read later on that a particularmineral is an oxide or a sulfide, you should have some idea what that means, so youmight want to review that section in the textbook.

I want to focus for the rest of the program on a special categoryof minerals called the "silicate minerals". Since silica comprises75 percent of elements in the crust, and remember that silica is the special wordwe 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 than90 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 weatheringin a later program. The silicates, because they're the most common types, they'realso the most complex type. Their are many solid solutions and many different substitutionsthat take place in the silicate minerals, and there are many varieties of crystalstructures, so what we need to look at here is trying to understand how these silicateminerals are constructed, and it's really very simple. It has to do with the sizeand charge of the silicon atom and the oxygen atom.

I have a model here on the table in front of me that I think Ican 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 muchspace they take up, we find that oxygen occupies almost 95 percent of all crustalrocks. Basically, we can think of rocks as oxygen atoms with silicon and metal atomsarranged 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 thatthe silicon atom happens to have four bonding sites; that is, it's sticky in fourdifferent places. It's also interesting that oxygen has two bonding sites, and younotice 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 enoughspace in between for the silicon atom to fit. Now, that takes care of three of thesticky sites or the bonding sites of silicon. The fourth one is taken care of byanother oxygen atom which fits very nicely on top, so that the whole package hasa shape that looks like this, and you see here as I rotate this around that the siliconatom is almost completely concealed within this pyramid of oxygen atoms.

It's this grouping of silicon and oxygen atoms that forms the basisfor all of the silicate minerals. We call this particular grouping the "silicatetrahedron". 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 geta sense of how this relates to this pile of oxygen atoms with the silicon in thecenter. 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 littlebit so you can get a sense of the shape of it. You can use this tetrahedron as away of modeling how the silicate minerals behave. You see that if I turn the tetrahedronin different directions, there's a very symmetrical sort of shape. It's basicallya four-sided pyramid. Each side of the pyramid is an equalateral triangle. No matterhow 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, sowhat makes the silicate minerals so interesting is that, on one hand, silicon isvery similar to carbon; that is, it has four bonding sites, and you may know thatit's the particular shape of the carbon atom and it's ability to bond with itselfthat's responsible for the organic chemicals that make up the variety of life onEarth.

Silicon is very similar to carbon with one important difference:Carbon tends do bond easier to hydrogen; whereas, silicon has this great affinityfor oxygen because of the particular size of the oxygen atoms related to the siliconatom, so we can see here if I take these off, and I'll put some smaller models ofthe tetrahedron on, we can see how we can build the structures of the various silicateminerals.

Okay, here's a single tetrahedron. Now some of the silicate mineralsconsist 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 combinationof one silicon and four oxygen atoms actually has an overall charge of minus 4,andthat means that it attracts positive ions, so metal ions, which are positively chargedtend to stick, so we can construct a mineral in many different ways by putting tetrahedronstogether and putting positive ions in between them in various shapes.

Things are rolling around here a bit, so this doesn't necessarilyrepresent any real crystal, but you can see how the tetrahedron are used as buildingblocks. We might also note that there are lots of different ways to put the tetrahedratogether; for example, in the mineral "olivine", the tetrahedron are arrangedin 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 thetextbook. It's Figure 9.8 on page 199. You might want to take a look at this structureof olivine, but, you see, the tetrahedra can combine in other ways as well.

The tetrahedron can share oxygen atoms at the corner. What thatmeans is that if I take the large tetrahedron and pull one of the oxygen atoms offand 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 aremany 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 ingenerally 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. Inthe 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 dependupon the exact temperature and pressure and the available atoms at the time the mineralwas 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 whatwe call a double chain structure.

I can build another chain like this. The chain looks exactly likethe first one. The difference is that it's a mirror image of the first chain. I canput them together in a second chain like this, and now we have a structure that isone chain on one side and another chain on the other side. You'll also notice herethat 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 showthe other models because it would require me to hang things in mid-air, and if wecould get rid of gravity, I could do that, but as long as we have gravity, it's ratherdifficult 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 micaminerals are sheets of these silica tetrahedra that are held together by metallicions sort of like a sandwich. You might think of it as a metal sandwich on silicatebread.

Okay, the two most commonly occurring silicates are quartz andfeldspar, and both of these minerals represent a three dimensional framework withthe tetrahedra arranged in a three dimensional framework very similar to the waythe atoms are arranged in the carbon atoms, so I'll call your attention again tosee the Figures 9.7 through 9.11 on page 199 and 200 in the textbook and also Box9.3 on page 203, which gives you a sense of the clay structure.

Well, it seems that, I think anyway, that crystallography is afascinating subject. It combines geometry, symmetry, and art with geology. Now, there'smuch more that we can learn about crystals in terms of symmetry and the various waysin 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 thatwould take you much deeper into this if you're interested in this, one of the thoseareas of geology that sort of can stand alone without even understanding the restof 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 sedimentaryrocks, and why certain minerals occur, and also metamorphic rocks, and what sortof changes take place when rocks formed at the surface are exposed to high temperatureand 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 thesevarious atoms from one place to another as they're adjusting to new equillibriumat the Earth's surface, so for the next lesson, you should

• read Chapter 11, the chapter on volcanism and extrusive rocks.I want to note here that we get out of sequence a little bit. For some reason, we'regoing to do Chapter 11 first on extrusive igneous rocks and volcanism. Then we we'llcome back and do Chapter 10 on intrusive igneous rocks, so I hope this has helpedyou visualize some of these things.

That's it for today, and I'll see you next time.