GG101 Elements and Crystals

GEOLOGY/GEOPHYSICS 101 Program 14

ELEMENTS AND CRYSTALS

These materials seem very different from one another; on one hand there's a piece of coal; on the other hand, graphite, and, finally,a pile of diamonds.

These are actually different substances, and they have different properties. The diamond, for example, can scratch glass. You probably hear the glass being scratched as I drag the diamond across it. Here's a nice deep scratch. On the other hand, the piece of graphite is soft enough to write on a piece of paper. In fact, pencil lead is made of graphite.Even though these are different substances, they're all made out of the same kinds of atoms. They're all various forms of the element carbon.

How can the same atoms form materials that are so different, have such different properties? The answer is in the arrangement of the atoms.

We can build many different substances out of the same atoms in the same way that we can build many different houses out of the same pile of rocks, but we can't see the atoms with our eyes, just like we can't see waves on the ocean from outer space. They're simply too small to see, but we can use varous kinds of indirect methods to study the arrangements of the atoms, just like Xrays, and in recent years people have even developed ways to take pictures of the atoms themselves,so we can use various indirect methods to learn the structure of substances, and we can use chemical analyses in the laboratory to learn the composition.

What we're interested in today, of course, is minerals, but before we study minerals, we need to learn a little bit about atoms, so let me remind you of the lesson assignment.

This lesson will actually be split into two different programs. Today we'll look at atoms, and elements, and minerals, and in the next program we'll look at the structure and classification of minerals.

The text assignment for this lesson is Chapter 9, pages 193 to 213, and you should

Okay, I'll review the objectives with you briefly. For this lesson, we want to be able to

Well, we don't have a video today. The video is actually a single lesson, but I want to take a little bit of extra time today to demonstrate for you some of the physical properties of minerals, but first we want to take a look at atoms.

This section begins our study of Earth materials, and this particular lesson is also the last one before the second exam, so you might want to go back and review some of the material from the previous lesson to be included in the second exam.

The study of Earth materials is a basic area of geology, and it's really worthy of study in its own right for several reasons. For one thing, it can help us to explain the variation and the similarities between different substances that we find on the Earth's crust, and it also helps us to understand the concept of crystals and symmetry; in fact, the study of crystals, which is called "crystalography" is an area really that can be studied without reference to any other geology.

Crystals, in fact, are beautiful things, and many people study them and they've inspired much poetry over the years. We're also interested, of course, in the origin of currents of ores. Ores provide for us the natural materials that we use to build things, most notably metals, but also non-metals, and it also helps us to understand the surface and near surface processes, such as volcanism and igneous rocks, weathering, and the formation of soils, the erosion transportation and deposition of sediments, and the formation of both sedimentary and metamorphic rocks.

See, the study of atoms relies upon an atomic model. The atomic model has been derived over hundreds of years of experiments in the chemistry laboratory. We don't need to understand the details of atoms and their structure in the same way that you would for a Chemistry course, so if you haven't had a Chemistry course before, or if the stuff is all brand new to you, don't worry too much about it but be able to get a sense of what atoms are, and how one atoms differs from another, and, most importantly, to understand how different atoms conform in different ways to form the variety of substances that we see in the Earth's crust, the minerals, for example, so let's start by clarifying what we mean by "matter".

Everybody knows what matter is, right? But how would you define it If you had to write an answer to the question "What is matter?" It's not really an easy question. I mean it's so common that you could probably recognize matter when you see it, but knowing exactly what it is, it's hard to know. Scientists define matter very simply, saying that it occupies space and has mass; in other words, it has a shape, and it has a weight. Matter can be classified in several different ways, and I'll go through some of these with you. We need to distinguish between an "element," and a "compound," and a "mixture".

"Elements" are pure substances, which are composed of only one kind of atom. I have several examples of elements for you. The things I showed you at the beginning, the diamond, graphite, and coal are examples of elements, but also some other common substances are elements like iron, for example.

This is a piece of iron that's been cut out of a block, and it even has a rust stain on there, which points out the fact that iron is a particularly reactive metal; that iron very rarely, in fact, never, I think it's safe to say, occurs in the uncombined form in nature. It usually occurs combined with oxygen in the form of iron ore, commonly called "rust", and fairly extensive chemical methods are necessary to extract it in refineries, and so forth.

Another example of an element is one that does occur naturally. It's the element sulphur. Okay, sulphur always looks sort of this yellow color. It's often associated with volcanoes but often also associated with other types of ore deposits, has many uses in industry. In fact, sulphuric acid, which is made from sulphur, is probably the most common chemical used in industry.

A couple of other things which occur naturally. Graphite, as I've already shown you, which does not occur naturally in this form. This is a piece of silicon. Actually, it's several crystals of silicon. Silicon, although very common in the Earth's crust, has to be refined in order to be used in computers and electronic components.

Though, another example, of course, is the diamond. This is not a particularly valuable diamond; in fact, it's not a very good quality diamond at all, but diamonds are another example of the natural occurrence of an element, in this case, carbon. Diamonds, outside of their use in jewelry, are also used in industrial purposes such as for grits, and for grinding, and for nail files, and that sort of thing.

So elements represent pure substances, and for the most part,we rarely find elements in the natural form as uncombined atoms. There are, in fact, 103 different elements. Of these, 88 are found on Earth, 4 are so short lived; in other words, are radioactive, that we don't find them naturally on Earth, and 11 elements haves been created by man in nuclear reactors.

Some common substances do occur naturally in addition to the carbon and sulphur are the coinage metals called copper, gold, and silver, and also, the metal called "platinum". Most substances naturally occur as what we call "compounds", and "compounds" are substances which are formed from atoms in combined form.

One example is water, which is a chemical combination of atoms of hydrogen and oxygen, and also, of course, minerals, which are combined forms of several different elements.

I want to distinguish between "compounds" and "mixtures" briefly. A "mixture" is a combination of substances which can be separated easily by physical means, things like seawater, or a mixture of salt and sugar, and even rocks, for example. That rocks can be seen to be composed of minerals, and these minerals can be separated very easily by physical means.

Okay, another thing we need to note about matter, is that matter can occur in three different forms. The names of these common terms: solid, liquid, and gas. I might note here that water is the only substance which occurs naturally in all three states in the range of temperatures that we find here on Earth. Most substances like sulphur occur either as a solid, or a liquid, or a gas. Gas is like oxygen or nitrogen in the atmosphere, for example.

Okay, when we talk about substances, we can classify them based upon what we might call "properties". "Properties" are simply characteristics of a particular substance, which can be used in one way or another to identify it, and in the second half of this program, we'll come back and talk about physical properties a little bit. By physical properties, we mean those characteristics or properties which are detectable with the senses. These are things like color and the way light reflects off of the surface that we call "luster". Even something like taste or specific gravity, or electrical conductivity.

Now, I said earlier that physical properties can be detectable with the senses. We sometimes consider instruments like electrical equipment to be extensions of our senses. On the other hand, though, are chemical properties. Chemical properties are a little bit harder to categorize without some experience in the chemistry lab, but basically chemical properties have to do with how a particular substance combines with another substance. We might note, for example, that iron has a chemical property of reacting with oxygen to form iron oxide or rust. We might also note that a chemical property is the reaction of a raw material to an acid, or the ability of sodium and chlorine to combine together to form the substance that we call "salt". These are all chemical properties and a little bit more abstract, and as we'll see in the later part of today's program, it's much easier for us usually to classify substances based upon their physical properties than chemical properties because we can easily do tests to determine the physical properties.

Okay, so let's take a look now at the concept of atoms. The concept of the atom is a difficult one for some people to grasp because it's kind of abstract, but basically we want to think of atoms as a model; in other words, the atomic model is a way of helping us visualize how matter is put together so that we can understand the physical properties and understand how minerals and other substances differ one from the other.

The atoms are too small to be seen directly. We can't see atoms; in fact, we can't look at atoms with light because the atoms are smaller than the waves of light that we use to look at them, but many experiments, both in the chemistry laboratory and the physics laboratory show the existence of atoms. The concept is a model again, which can be used to explain the properties and the nature of substances, and that this model is really very consistent. What that means is that various areas of study from the various features of matter point to the existence of atoms, and

so the lesson here is that atoms are usually combined either with themselves or with another type of atom. You'd never see an individual atom because it's too small, so that something we call "substance" is actually uncountable atoms mixed together, and these atoms, again, may be pure in the case of an element, or they may be mixed together of various types in the case of a compound or a mixture.

So atoms combine with each other by exchanging or sharing electrons. We might think of compounds as atoms which are glued together in various ways by electrical forces, so in order to get a picture of how this works, we need to delve into the structure of the atom a little bit, and see if we can see what the atom is actually made out of.

Now, again, if you haven't seen this material before, don't be too confused by it. This is background material, but we need to have the sense of how atoms combine even though for our purposes we're going to look at them as kind of fuzzy little balls.

Basically atoms are made of fundamental particles: protons, neutrons, and electrons, and atoms of different elements basically are different arrangements of these different fundamental particles. The fundamental particles may be electrically charged. To explain this I need to review what we know about electricity a little bit. I'm not going to go into detail here, but basically we don't know very much about electricity. Nobody really knows what electricity is. What we do know is that there are particles inside the atom that carry electricity of various kinds. Well, not various kinds, two different kinds.

On one hand we have positive charges; on the other hand we have negative charges. The words "positive" and "negative" here are sort of arbitrary terms. I could just as easily have called them Fred and Ethyl. Benjamin Franklin chose positive and negative as a way of distinguishing between the two types of charge. All we really know about these two types of charge is that opposite charges attract each other, and like charges repel each other, and it's this understanding of the attractive forces between positive and negative charges that allow us to understand how the atom is constructed.

Okay. The fundamental particles carry electric charge. They have protons, which carry positive charge, and electrons, which carry negative charge. The atom can actually be thought of as a two-tiered structure.

At the center of the atom is a nucleus. The nucleus contains the positive charges and also the neutral electric charges called "neutrons".

The electrons, on the other hand, are in orbit around the nucleus and what we call an "electron cloud", sort of like fruit flies flying around a papaya where the papaya represents the nucleus, and the fruit flies represent a cloud of electrons surrounding the nucleus. The structure of the nucleus; in other words, which particles and how many particles are in the nucleus determines what kind of atom it is, and that determines which element. The number of protons in the nucleus also determines the number of electrons in the electron cloud. In turn, the number and arrangement of electrons in the electron cloud determines the properties of the atom, and, again, the properties of the atom mean how that particular atom will combine with other atoms of the same type or of different type, and it determines what type of reactions will take place when atoms are glued together.

Really all we need to understand about the nuclear particles is that first of all the positive charges are concentrated in the nucleus, and that the positive charges although they have the same magnitude of electric charge have much greater mass; in other words, protons are very big compared to electrons. The electron weighs about one two-thousandth as much as a proton, and this is about the same relationship as between you and a bus. You, being the electron; the bus representing the proton.

So most of the mass of the atom is concentrated in the center; in fact, an analogy here is if you were to imagine a large stadium like Aloha Stadium, put a basketball in the center of the 50 yard line in center field, then take a pea and run up to the top layer of the stadium, this represents about the size and distance relationships of the nucleus of the atom and the electron cloud. So you see the atom is really mostly empty space.

Inside the nucleus as well along with the protons are neutrons. The neutrons do various things, have various functions inside the atom, and we don't need to concern ourselves with this although we do need to note that it's the number of protons in the nucleus which determine which element that atom represents, and it's the number of neutrons, which determine which isotope of that element, so the simplest atom, for example, is hydrogen, which has a single proton and a single electron, but isotopes of hydrogen may have one or two or no neutrons in the nucleus, and we talked a little bit about isotopes in a previous lesson.

Okay, it's kind of hard to imagine that if the atom is really empty space, why is it the material seemed so solid in the first place. We know that matter's solid. We can't walk through walls, after all. The reason for this is very simple. The electrons are moving so fast they appear to occupy the entire volume surrounding the atom, very much in the same way that an airplane propellor when the engine's running seems to occupy the entire circle of the propellor. You can't walk through the propellor when the engine's running.

So let's review this for a second. The number of protons in the nucleus determines which element it is, and we use the number of protons called the "atomic number" to designate a particular element. Hydrogen is number one. Uranium, the heaviest, is number 92.

Okay, a nucleus of an element may contain different numbers of neutrons. Generally, the numbers of protons in the nucleus and the number of electrons in the electron cloud are the same for a given atom or of a given element. Okay, I want to also note that the elements are arranged in what we call a "periodic table" by atomic number, and you might want to take a look at the period table in Appendix D in your textbook on page 512.

Really what we need to understand about the periodic table is that this is a way of referring to a location of the elements, and the periodic table is arranged in such a way that elements in the same column have similar chemical properties, and we'll see how this applies when we start talking about minerals, and substitutions, and crystal structures, and various sorts of things.

Okay, another thing to note now, as I mentioned earlier, that the number of protons and electrons in a typical atom is usually equal. The number of positive charges balances the number of negative charges, and, of course, it's the attractive charges between the positive and negative charges that hold the whole thing together, but some atoms have the ability to either gain or lose electrons.

The reasons for this are more obscure than I want to get into for this course, but some atoms are capable of gaining electrons, and some are capable of losing electrons. Those atoms which gain electrons tend to become what we call "negative ions". The word "ion" simply means an atom which has simply gained or lost one or more electrons, so a negative ion has taken on an extra electron, so, therefore, has one extra negative charge, one extra meaning one more negative charge than positive charges.

On the other hand, an atom which loses an electron has one more positive charge than negative, so we say this is a positive ion. The ability to form either positive or negatives ions is characteristic of a particular atom, and in many ways helps us to understand why atoms combine in the way they do.

The other thing I want to note is that atoms are all basically the same size, but ions have very different sizes. The reason for this is simple. Take for example a negative ion. A negative ion is an atom which has lost an electron. Because there's an extra positive charge in the nucleus, that means that the negative charges are held more tightly to the nucleus, and so they're compressed inward toward the center which makes the ion, the size of positive ions much smaller.

Negative ions, on the other hand, are usually very large ions. Okay, one thing we need to understand, and the reason for going into all this stuff about ions is that we're looking or trying to understand what it is that holds the ions together in chemical bonds in the first place. It's this transfer of electrons or loss of electrons that holds atoms together. You see if you have one atom, which has lost an electron to become a positive ion, and you have another atom, which has gained an electron ion to become a negative ion, the positive and negative charges then attract to each other and form a glue. This is called "ionic bonding," and this is contrasted with another type of bonding called "covalent bonding", in which the electrons are not completely transferred from one atom to another but are sort of shared in between, sort of like two people tied together by a rope who are sharing the same rope are bonded together.

Another type of bonding that I'll mention later on is called a "vanderwaals bond", and this has to do with electron fluctuations and movements, electron dances, if you like, within the atoms. When a few minerals, such as graphite are held together by these weak vanderwaals bonds, which explains why graphite is so soft compared to diamond.

I might note here that diamond is also held together by covalent bonds, which are extremely strong. Ionic bonds are usually fairly weak, and many minerals which have ionic bonds are fairly soft and cleave very easily. Ionic bonds are also easily disturbed by water, so ionic substances like salt dissolve very easily in water.

I don't know if this really explains atoms enough. You might have to refer to the textbook, or even go to the library and look up a chemistry textbook, but I want to note here that although there are 103 known elements, only a very few of these elements are important in our study of the Earth; in fact, the crust of the Earth consists of only about eight elements.

You can refer to Table 9.1 on page 196 in the textbook to see the abundance of elements in the Earth's crust. I want to note here that the abundance of elements in the crust is not the same as the proportions in the Earth as a whole. The layering of the Earth that we discussed in an earlier program tends to concentrate the heavier elements toward the center and the lighter elements toward the surface, but generally the crust of the Earth is mainly composed of only two elements; in fact, 75 percent of it is only two elements. Together these are called "silica". Oxygen comprises 47 percent; silicon about 28 percent.

Again, the combination of these two together is called "silica", which basically means silicon and oxygen.

Twenty-five percent is mainly the six other metallic elements. These are aluminum, which is about eight percent, fairly abundant, but not concentrated enough to really be useful except in certain concentrations that we call "bauxite," which are used for ore deposits. Iron is the next most abundant, about five percent. Calcium and sodium are about the same: four percent and three percent. Potassium is about three percent. Magnesium comprises only about two percent. All together this makes up about 99.5 percent.

The remaining half percent consists of all the rest of the elements, and a good portion of that is the element titanium, which in its purified form is a strategic metal used in aircraft construction and so on, and, finally, hydrogen, the tenth most abundant, occupies only about one-tenth of one percent, so even with all the water that we have on the crust of the Earth, the element hydrogen is still not very abundant. It only comprises about one- tenth of one percent of all of the crustal elements.

Okay, with this background of atoms and elements, let's turn our attention to minerals and their characteristics. First of all, what is a mineral? A "mineral" is a naturally occurring solid with a specific physical and chemical structure. Notice there are several things that a mineral has to have in order to qualify in the geological definition of a mineral. First of all, "naturally occurring". This leaves out artificial gemstones that are made in a laboratory. "Inorganic" means that it's made by non-biological processes. "Solids", this leaves out things like water although ice, which is a solid, is considered to be a mineral. "Specific physical and chemical properties". "Physical properties" we'll come back to in a second, and "chemical structure" means that we know the composition of that particular mineral within fairly definite limits.

One thing we need to understand about minerals is that the minerals may actually take different forms, but even though they may look slightly differently on the large scale, the physical and chemical structures are identical regardless of how the mineral was formed, so the quartz that's formed in an igneous rock from cooling magma is the same mineral; that is, it has the same structure and composition as quartz that's formed from precipitation from a solution.

Okay, minerals, then, you see, are building blocks of rocks, and minerals may consist of a single element. On one hand, certain naturally occurring substances like sulphur and carbon occur as minerals in a natural state containing only one element, but most minerals consist of one or more elements combined in a particular crystal structure.

In the next program, we'll take a look at some of these crystal structures, but for now we just want to look at the characteristics of the minerals on the large scale or what geologists call a "hand specimen".

Okay, so the characteristics of minerals are determined by their internal arrangement of the atoms, and also by what those atoms are; in other words, by both composition and structure, and we identify minerals by testing their properties. See, like people, no two minerals have exactly the same set of properties. Now, how do we recognize people anyway? Different people might have certain characteristics in common. Two people might be tall and have black hair, but even two people who look similar, we can identify one from the other by noticing which things are not similar about the two people, so minerals although they may be similar in some ways have different sets of properties that allow us to distinguish between them if we can identify enough properties.

Usually, if we can identify only one or two properties of a mineral, we can decide what it is and sort out which mineral it is from a whole list of possibilities. We can use either physical or chemical properties to identify the minerals, but chemical properties require laboratory analysis in the chemical lab and requires a bit more work than the geologist usually wants to do or has at his disposal in the field.

Physical properties are much easier, and generally physical properties are those things which we can distinguish with our senses. These are things like the crystal shape, and cleavage, and twinning, striations, hardness, specific gravity, color. The list goes on.

I have a collection of minerals here from the box. Let me dump these out on the table to get a look at some of these things. Here's a whole collection of various types of minerals, and one of the first things you notice when you look at a pile of minerals like this is the variety of colors. Now there's some things that appear to be the same color; for examples there are a bunch of white things here that all look pretty much the same, but if we were to rely on color alone, we wouldn't be able to tell the difference between these various minerals, but there are certain things that we can use the color to identify specifically; for example, sulphur.

The yellow color of sulphur is very distinguishable, and, in fact, there's nothing else that has that exact same color. Don't get me wrong. There are other minerals that are yellow, but none of them are quite the sames color yellow as sulphur.

Another one is the mineral called "hematite". This sort of rusty red color is very characteristic of hematite, which is an iron oxide, and, in fact, has the same composition as the rust that forms on a rusty nail.

Another one that color is easily identifiable is the mineral called "malachite". The green color of malachite is very distinctive, and you may have seen malachite sometimes cut into jewelry and polished as a semi- precious stone. In some cases, color is not diagnostic at all.

The mineral, quartz, for example, can come in almost any color, and I'll show you some examples of quartz here in a couple of seconds. Okay, so we can sort of sort through things here.

Some other minerals that, the green color of olivine, for example. We don't get a really good view of the color green in olivine, I think, but olivine comes in a couple different colors of green, and although the two colors are different, there's only a very narrow range of colors that the mineral occurs in.

Some of the other kinds of things are fairly identifiable simply by color alone; for example, biotitee mica, the black mica has a very distinctive appearance mainly because, not only of its color because it also comes in these thin waferlike sheets. Its cousin is white mica or muscovite, which because it's very transparent and very clear was often used as a window glass in pioneer houses when the industry wasn't available for making glass.

Even though color isn't particularly diagnostic, there are some other properties of minerals that we can use. Let me clear these things off the table for a second, put them back into the box. So even though the color of a mineral may not tell us everything we need to know, very often the color of a powdered mineral is somewhat different.

I have some examples of a couple of things here. On one hand, I have the mineral called "pyrite" Pyrite is often known as "fool's gold", and it's called fool's gold for obvious reasons because it looks like gold, and many people have been confused thinking they've struck gold when what they've really found is pyrite. Pyrite is actually an iron sulfide mineral very similar in appearance to gold, but there's one very easy way to tell the difference. The property of a mineral called "streak" is basically due to the fact that when the mineral is finally powdered, it may have a different color; for example, if I take this unglazed porcelain tile called a "streak plate" and scrape it with a piece of pyrite, you'd see that the color of the streak is black. Now, if this was gold, the color of the streak of gold is actually golden, so if you're prospecting or you're walking around someplace and you find something you think is gold, you streak it on something white, and if it has a black streak, it's fool's gold, and if it has a gold streak, then it's real gold.

Another mineral where the streak is somewhat different color from the actual mineral is the mineral "hematite". This piece of red hematite has a very distinctive sort of brick red or rusty streak, but hematite appears in other varieties as well. Here, for example, is a large piece of hematite has a metallic luster and looks somewhat different from the other piece of hematite.

This is one example where the same mineral may occur in slightly different forms, but even in this form if I make a streak on the piece of unglazed tile, yet I'm not sure how well you see the color, but if I sort of smooth it out, though, you can see that it has a very definite red tint to it. So streak can often be used to identify a particular mineral even if the color isn't reliable, and, by the way, streak is much more reliable in identifying a particular mineral than color is.

Okay, another way that we can identify minerals is by the crystal habit. Now, most minerals have a particular crystal form. Some minerals may occur in more than one form, but, in general, the particular form of a crystal, if it's apparent, can give us a hint of what the mineral is.

See, the crystal structure of a particular mineral expresses this orderly internal arrangement or symmetry of the atoms that make up that particular mineral. It was discovered quite some time ago back in the 1600s that a particular mineral if it's crystal form is present, has a given shape even though the crystals may be different size, they're basically the same shape. Here, for example, is a quartz crystal, and you can see the crystal faces as I rotate this into the light. You see the light reflecting off of the various crystal faces, and you can see the very distinctive shape of the quartz crystal here, and if we were to take a protractor and measure the angle between these crystal faces, we would find that quartz always has the same angles between the same faces. This is called the law of constant interfacial angles. Here, for example, is a larger piece of quartz, larger quartz crystal, and, again, notice the size of the crystal faces on here, but in this particular mineral, the angle between these two faces: this face and this face is the same as it is in the smaller black crystal.

Here's another piece of quartz. This is the purple variety called amethyst, and, again, the crystals are basically the same shape. You can see on some of the crystal faces as they glow in the light that all the crystals have basically the same shape. While I have these out here, I might as well note, look at the color variation in the various pieces of quartz. Clear quartz on one hand; the amethyst on another hand; and then here, black.

Now, it turns out that quartz actually can occur in almost any color, and quartz is one of those minerals where the color doesn't tell you anything at all. In fact, the color in quartz is caused by small amounts of impurities. One more example of some quartz here. It's the geode, and here you see the quartz has grown inside a hollow, and it's kind of hard to see down in here, but you'll see the glistening shape of the quartz crystals down inside, and if you were to examine these crystals closely even though they're extremely small, they still have that same basic shape with the constant interfacial angles.

Well, I want to note here before we get out of this that in this case some of the crystals that are formed don't display crystal faces at all, and this happens when crystals grow in restricted environments, and when crystals grow into a cavity, for example, they often can't express their crystal faces and simply may take on the shape of the cavity that they're growing in, and sometimes, by the way, the crystals are so small that they can't even be seen with a microscope, and we have to rely upon other techniques like Xrays, for example, to figure out what the crystal structure of the mineral actually is.

Okay, let me put this aside. Even in these small microscopic crystals, the internal arrangement that's expressed in crystals is still present, it's just that we can't see it either because they're intergrown or because the crystals are too small.

I also want to note here that sometimes crystals can have different shapes. What I mean by that is sometimes the same elements, the same atoms, can arrange themselves in more than one way to form different crystal structures. This is called "polymorphism," and we'll come back and look at this in the next program.

Okay, another example of physical property of mineral has to do with hardness. Now, hardness can be defined in several different ways, but geologists use the term "hardness" to mean "scratchability", so that something which is hard can scratch something which is softer. Now, you might think that it's kind of hard to come up with a scale of hardness because it's hard to figure out, you know, how to attach numbers to hardness, but it's really not that difficult, and geologists over the years have developed a fairly simple scale of hardness, which is based on commonly occurring minerals. This is called the "Moh's Hardness Scale," named after a geologist, a mineralogist named "Moh," who invented the scale.

Basically, the scale uses common minerals and runs from the softest, number 1, up to the hardest, number 10. On this scale, the softest mineral is a mineral called "talc". It's the mineral from which talcum powder is made, and the hardest is "diamond." "Diamond," of course, being the hardest material basically means that nothing on Earth can scratch diamond; that's one of the reasons why diamond is so valuable because of its durability.

I have an example here to demonstrate hardness for you. The minerals on the Moh's Hardness Set, you should look in the textbook and get a sense of what those minerals are. I'll read them for you.

I have two of the things from the Moh's Hardness Scale here on the desk in front of me. I have a piece of quartz over here, a piece of calcite over here, and I also have a glass plate. Now, quartz is number 7 on the hardness scale. Calcite is number 3, so that means that quartz should scratch calcite. I take the piece of calcite and scratch it with the quartz and see that the quartz makes a nice deep scratch in the calcite. It's a scratch that doesn't wear off, but if I try the reverse; that is, if I try to scratch the piece of quartz with the piece of calcite, see what happens.

See what happens is the calcite is so soft that it actually powders and breaks leaving the quartz almost entirely unscratched. Now, a lot of times if you're out in the field picking up specimens of minerals, you might not have a hardness set with you, but you can also use various common materials to estimate hardness; for example, glass has a hardness of about five and a half. What that means is that something that has a hardness of six will scratch it, and something that has a hardness of five won't. If I take the piece of quartz and rake it along the glass, you can probably hear this.

Okay, you can see that the quartz leaves a nice deep scratch in the glass. On the other hand, if I tried the same thing with calcite, rubbing the calcite, the calcite again powders on the glass. It leaves a streak, which brushes off.

If the geologist can identify minerals, then, based upon their hardness, then other common devices that you can use, your fingernail, for example, has a hardness of about two and a half. A copper penny has a hardness of about three. A steel nail has a hardness about the same as glass, about five and a half. Okay, let me clear this stuff off of here.

Okay, another physical property that we can use to identify minerals is the property of cleavage. Geologists use the term "cleavage" to mean the tendency for a particular material to break along planes or more or less flat surfaces, and this reflects the orderly internal arrangement of the atoms that make up that particular mineral, and, again, I'll show you some examples of this in the next program. A specific mineral may show different numbers of cleavage planes that may show anywhere from no cleavage all the way up to six different directions of cleavage.

Some minerals also show a distinctive, a way of breaking that we call fracture, which doesn't split along flat planes necessarily but breaks in a particular way that helps identify that mineral. I can show you some examples of cleavage. I just happen to have some things here that I can use to illustrate this. Okay, let's do the mica first. Probably the best example of cleavage is in the mineral that we call "mica". This is a larger piece of the biotite mica. Its light colored counterpart was the white mica. They both have the same type of cleavage, but I can illustrate it better, I think, on the big piece. You'll notice first of all that this particular piece is very flat. Okay, it's almost like the pages of a book; in fact, if I take the knife.

Let's see if I can get this started. Once I get it started with the knife it should break fairly easily. Let me find a good place to do it. Let's see. Okay, if I flip the knife into here and break this apart, pretty much like peeling off a layer. You see that the mica breaks along this flat surface and notice the shine on that broken surface, so we say that mica has a perfect cleavage in one direction. The one direction refers to that direction of the plane along which the mica tends to break. As I said, mica is one of the best examples of cleavage, and it illustrates what we call "perfect cleavage" in one direction.

Okay, there are other examples of cleavage. I have here a couple of other pieces of things. Let's see. Let's do the halite next. I want to brush some of the mica out of the way so we can work with a clean piece here. The halite is common table salt. Halite actually has the chemical composition of sodium and chlorine, its ionic bonds that are fairly easily broken, so this one's a little harder to break. The cleavage is good but not perfect, so I have to work a little harder at it, so I'm going to take the razor blade here. If you try this at home, be careful. The razor blade's very sharp. I'm going to take the razor blade. I'm going to whack it with the hammer, and let's see what happens to the piece. Uh, oh, sometimes the razor blade breaks, and sometimes it doesn't,but you see what happened to the piece of calcite. I'm sorry, the piece of halite. It broke along almost a flat surface, and if I look at the piece that remains, you'll note that the piece itself is blocky. It's almost perfectly square. This is because halite actually has three directions of cleavage that intersect each other at right angles. Each of the two sides represents one plane of cleavage, two sides in the same direction, two parallel sides of the box, and if I were to cut this in different directions, maybe I can try this with the knife. Let's see if the knife will work. Oh, it actually works better than the razor blade. You'll see that it breaks once again into nice blocky fragments, so I can keep cutting it into smaller and smaller pieces. Every time I cut it, it breaks into these blocky pieces.

The cleavage of halite is good enough that when rock salt is mined and ground into fragments to use as table salt, the halite tends to break along these planes. In fact, if you're at home you can try this. Dump some table salt onto the table and look at it with a magnifying glass. If you don't have a magnifying glass, just look at it very closely. You'll see that the individual grains of salt are actually little tiny cubes reflecting this cubic cleavage, and, again, this reflects the internal arrangement of the sodium and chlorine ions that make up the salt, and I'll show you examples of the crystal structure in the next program.

Another example of cleavage is the mineral calcite. Now, calcite also has three directions of cleavage, but in this case the directions of cleavage are not at right angles to each other, so that the shape of the cleavage fragments that are formed are not square, but rombehedral. If you look down here on the table, you can see there's a piece of calcite and notice that it's still blocky, but the cleavage planes don't intersect at right angles; in fact, the angle between one plane and another is not a right angle but rather at about 60 and 120 degrees. In geometry these are called rhombohedrons.

The calcite should still cleave. I'll try it with the knife first, and if that doesn't work, I'll come back and use the razor blade. The knife's a little bit less dangerous. Don't try this with your fine silverware at home. Use the cheap stainless stuff. I'm going to whack this and see what happens. Boy, that's fun! Okay, notice that the two pieces when they split, split into this nice flat surface, but, once again, the angle is not 90 degrees. It's a block, but it's a rhombohedral block.

If I cut this in the other direction. This is so much fun I don't want to stop. Cut it in the other direction, notice that each of the fragments has this very distinctive blocky shape. There are other examples of cleavage that I can show you here though in many cases cleavage is not quite as good as it is in the calcite and halite.

Here, for example, is a piece of feldspar. This one's harder to break, so I'm not going to try to cut this one, but you can see here's a cleavage surface, and you can see how it reflects the light. That's characteristic, by the way, when you see a surface like this that's shiny in the light, it reflects light like a mirrored surface. Now this particular mineral called "orthoclase feldspar" has two directions of cleavage, one of which is pretty good, and the other one if I can catch-- There's the flash of light. The other one's a little bit better, so this one has two directions of cleavage more or less at right angles.

You can see the shape, kind of blocky shape. Here's one cleavage plane; here's another cleavage planes, again, almost at right angles. This particular mineral, the orthoclase felspar, also breaks irregularly in the third direction, so on the third side you notice that it's much rougher, and you don't see the flash of the cleavage surface. You do see the blocky shape from the other two cleavage planes, so here's an example of a mineral that has two different planes of cleavage at right angles.

Okay, another one, the mineral fluorite, has four directions of cleavage and tends to form these little octahedral crystals very similar in shape to a diamond; in fact, diamond has octahedral Fluorite's only "4" on the Moh's Hardness. And when a diamond cutter wants to cut a diamond in the raw, the cutter may spend months living with that diamond trying to figure out exactly where the cleavage planes are, so that he or she can do exactly what I did with the knife on the calcite except that the stakes are a little higher.

The diamond may be a lot more valuable than these pieces, so this is a cleavage fragment, and this one displays four directions of cleavage, and it's kind of hard to see the directions here, but each of the two parallel sides where my fingers are is one direction of cleavage. As you look at the octahedron you find four different sets.

Okay, now, sometimes the cleavage isn't quite so good, and it's harder to recognize. Here's a piece of the mineral called "pyroxene". The pyroxene still has cleavage, but it's not quite as well developed, but you can see as I rotate this in the light, you get that distinctive flash of light off of the flat surfaces. You look at this on edge on, you see that it doesn't have that blocky appearance, but if you look very closely, and I'm not sure you can see this on the screen, if you look very closely you'll see that these ridges are actually like steps, very much like a staircase. As I rotate this in the light, you can see a flash off of the cleavage. As I rotate it, there's one. As I rotate it, you can see the cleavage flash on another set of planes, so there's one direction of cleavage. Whoops, I lost it. One direction of cleavage, and there's the other one.

So even on the mineral like the pyroxene where the cleavage isn't well developed, it's still possible to see the angle between the cleavages simply by rotating it until you find that flash of light in one direction, and then rotate it a little further until you find the flash of light the second time and try to estimate how far you've actually rotated the piece.

Okay, one last example here. This is a piece of the mineral called "topaz". Topaz is a semi-precious stone, and usually it's precious when it's in the blue variety, or the smoky, or the amber variety. This particular piece is very clear, but for some reason people don't value clear topaz as much for gemstones. Topaz is a very hard mineral. It's number 8 on the Moe's Hardness, but it still has one perfect direction of cleavage, and I think you can see the two flat surfaces where my thumb and finger are on it, and if I rotate this around, you can see this almost perfect mirror surface. I'm having a hard time finding the light. There's a flash. Almost perfect mirror surface. In this case, it almost looks like it's been polished, but this is actually a cleavage fragment. Now, it takes a little more work to cleave the hard minerals than it does to cleave the soft ones.

Okay, while I have the piece of calcite out here before we turn our attention to something else, I want to note one more physical property; that is, certain minerals have the ability to react with dillute hydrochloric acid. I happen to have a bottle of dillute hydrochloric acid here. Calcite is the only common mineral that does this, and usually a geologist will carry along in the field some of, a little vile of hydrochloric acid. If we can get a close up of this, I think we can see the reaction to acid. Now, calcite, as I mentioned, is the only common mineral that does this and watch what happens when I dump some of the acid on here.

Okay, see that very violent fizzing reaction. Now, this is because there's a chemical reaction happening, and this is actually something we would say actually a chemical property rather than a physical property, but if I try this on these various other clear pieces of things; for example, on the topaz, you see that there's no reaction at all on the topaz. The acid just sort of sits there on the topaz and the same with the piece of halite. This is the halite piece that I cleaved earlier, and, again, no reaction at all on the piece of halite.

Okay, well, let's see that one more time. This is kind of a fun test. See the very violent fizzing there on the piece of calcite, so the reason this test is useful, the acid test, is because very often, in fact most of the time, calcite doesn't occur in these nice rhombohedral fragments but it occurs in rocks such as limestone, which are so fine grained that you can't see the individual crystals; in fact, it may not display any of the properties at all of calcite, and when you drop the acid on it, it fizzes that way.

Okay, let me move the stuff off the table and try not to get the acid on anything. By the way, if you don't have hydrochloric acid at home and you want to try this, vinegar works reasonably well to see the reaction to the acid.

Okay, another physical property of minerals is the property we call "luster". "Luster" is basically just the way in which the surface reflects light. Geologists use all sorts of different terms to describe luster, and most of these terms have to do simply with common English words that are descriptive.

The calcite, for example, we would describe as having a vitreous or glassy luster, and, again, this is a cleavage fragment of calcite, and it's transparent and has this very glassy sheen to it.

Another type of common luster is "metallic" luster. This is a piece of galena, which is a lead sulphide; it's an ore of lead. It still has a shiny appearance, but it's dark in color and has what we call metallic. Now, it's hard to describe exactly what we mean by metallic luster except to say that it looks like a piece of polished metal. Even metallic luster may have different forms. Here's a piece of hematite, which also has a metallic luster, but in this case, this is called a specular metallic luster because the hematite's actually made up of tiny little flakes, very much like the microflakes that I was cleaving off here earlier, so that you see that there's a difference even between the luster of two different metallic substances, and as I mentioned before, the pyrite, which also has a metallic luster has a slightly different color, so sometimes the combination of the two is enough to identify things.

I have some other examples here just to bring things out here. A piece of kaolinite, a piece of clay, sometimes called china clay because china like plates and dishes are made from this type of clay has what geologists often use, often describe as an earthy luster that basically looks like dirt.

Don't get me wrong here. It's not really dirt; in fact, the color distinguishes it from ordinary clay that you find it soils, but still it has that earthy luster. On the other hand, we may have what some people call a dull luster, and, I think, even on the TV screen, you can see the difference in the luster between the two.

The dull luster is a little bit brighter, a little bit shinier, but still not shiny enough to be called "glassy". Okay, a couple other examples here. Here, for example, is what is often referred to as a "silky luster". Notice that it's a little bit shinier as you catch a little bit of a glint of the light off of it as you rotate it, and I think you can see even on the screen the difference between the three.

One last example or actually, two last examples. A luster that's described sometimes as "pearly luster". "Pearly luster" is a little bit shinier. I think you can see some of the sparkles in there, and it's getting a little bit glassier looking but still not quite glassy enough to be called vitreous or glassy. There are some materials that have a glassy luster even if they're not white. Here, for example, the mineral called pyroxene again, which has the, we're seeing a cleavage plane there, but still it has the glassy luster, in many ways very similar to the luster of the calcite. Note, even though they have different colors, they both still have this sort of glassy appearance that we call the vitreous or glassy luster.

Okay, I think I'm done with this stuff for now. Let's throw this back in the box and get stuff off the desk. There are other physical properties that we can use to describe minerals. Not all of the them are quite as obvious as this; in fact, one of the identifying characteristics of many minerals is the property called "specific gravity" or "density". We learned about density in relation to crustal rocks versus mantle rocks earlier in the program. It's difficult in the field to get a sense of the density of something, but the human hand is a fairly good judge of density. We can very often get a sense of the difference of density simply by hefting something. If I take a piece of pyrite and a piece of hematite, for example, and heft them in my hands, you can't tell this from where you are, but I can definitely feel the difference in the weight of these two pieces that are about the same size, so density, as we defined earlier, is the amount of mass per unit of volume.

Okay, one last thing that has to do with the surface of the crystal, what we might call striations. If we can get another closeup here of on the quartz crystal. Okay, on this quartz crystal you see on, let's see if I can get this into the light. There we go. On this crystal face you see these very fine lines that run across the crystal this way. These are called striations, and they reflect again the orderly internal arrangement of the crystal. Not all crystal shows striations, but when they do show striations you usually can identify the mineral based upon the type and the way the striations look.

We'll learn later that one of the identifying characteristics of the mineral called plagioclase feldspar as we see it in igneous rocks is crystals which have striations on them.

Okay, let's see. One last thing. I keep saying one last thing, and I keep thinking of more things, but it's an occupational hazard here. Some crystals show twinning. I have an example here of a mineral called "staurolite". It's a mineral that's often found in metamorphic rocks. The crystal we're interested in is this one. Notice the cross shape here. What's happened here is that two crystals are actually growing together in different directions. This will make a little more sense, I think, next program when we look at the crystal structures, but basically what's going on here is two crystals began to grow in two different directions, and they share some common atoms. This particular mineral called "staurolite" very often shows off its twin crystals, and in this case if we can get back to a longer shot here, in this case the crystals are interpenetrating, growing in two different directions, this way.

Okay. Sometimes the crystals grow in alternating directions like a sandwich where the piece of bread is turned in opposite directions. Well, this gives us a pretty good idea, I think, of the physical properties of minerals.

In the next program we'll examine the structure of the minerals, so before we close I want to remind you that you should

So in the meantime, keep reading Chapter 9. Pay special attention to the diagrams, and that's it for this program, so I'll see you next time.