World of the Rockhound; 7

Fact is that the people who. Seem to make the fastest progress and study in crystallography are the people rather the knack of visualizing things in three dimensions. The second largest craft hobby in America today is said to be Rock County with all of its branches of interest some of which are Jim cutting making jewelry and the collection of Jim stones rocks minerals and fossils. This universal hobby has grown to astonishing proportions in the last 25 years and shows signs of even more rapid growth as the need develops for more people to pursue more leisure time activities due to the fact that the working few hours have more days off and get more vacation time. Rocks and minerals are specimens of extreme importance to the raw count by over.

Most people do not realize the important role rocks and minerals play in everyday living. For example fluorite used in toothpaste and clothing. This series of programs is designed to give an overall picture of the rock on offer encouragement to the newcomer in this hobby. And present information of general interest to everyone. All of this will be examined as we explore the world of the rock our own. Today's program is entitled. Crystallography blossoms under the ground. The narrator is Len felt crystallography is said by many people to be the most difficult aspect of mineral study. It takes great imagination to visualize crystal forms and even with the aid of models of the various shapes and types of crystals it is still not easy. In this discussion today with Dr Tom Wilcox assistant professor of geology at

Vanderbilt University we will present some of the fundamentals of crystallography. Dr. Wilcox has several Crystal models for illustration purposes. Bees of course cannot be seen nor touched by means of radio. We will talk about them however and hope that imaginations will stretch. Since we are dealing primarily with minerals in this discussion of Christos and Crystal and it is important to establish exactly what a mineral is. The problem here is that the word means different things to different people. For instance someone interested in nutrition has a very different notion of what minerals are than. Someone interested in mineralogy. To someone interested in nutrition or in. The elements in soils that are necessary for life. Minerals are elements. Such as iron phosphorus potassium

and the elements necessary for life they were called minerals. Someone else might think of a mineral as being. Something of economic importance that we extract from the ground. Many people would think of coal oil minerals. AS. Copper is a mineral. And. All that matters here is that to be in the ground and be of economic significance but then that that leaves us with what are we going to. Call mineralogy we being people interested in rocks and minerals. And here we have a rather rather specialized and Rather. We have a definition of minerals that looks rather complex at first because there are really several things that are required in order for something to be a mineral. First of all it must be a solid substance that it forms naturally. That would perhaps be the most most important thing.

But beyond that. Besides being solid and being formed by natural processes and mineral should be mineral to be a mineral a substance must have a uniform composition or it must be homogeneous we say. That is it must have exactly the same composition from one. Part to another of the particle of a mineral. A further requirement for something to be a mineral that. It should be. It must have an orderly arrangement of the atoms and this is perhaps the most important aspect of being a mineral. This very neat orderly arrangement that the atoms in the mineral must tap. We say that minerals are crystal and which mean that which refers to this orderly arrangement of the atoms. Another very very important part of being a

mineral is that the composition is definite. Generally this means that minerals or chemical compounds chemical compounds have definite composition. The composition is when we say the definite that means escape capable of being defined. For it doesn't vary only within limits and that that makes minerals compounds and that that sets them ah that's one of the things that sets minerals off from rocks which are mixtures of these minerals. I've already exceptions to this definition never know what you are to give them. Well yes there are several exceptions. To various parts of the definition I just game. For instance I said that it was very very important that a substance be a solid to be considered a mineral and that it almost always true but not always. For instance certain

substances such as mercury exist and in rocks in the liquid state they exist as metallic mercury which at ordinary temperatures is a liquid as you people know. And we still would call Mercury a mineral even though it is an exception to this rule that substances should be be sour. But another. Sort of exception we said that perhaps one of the most important requirements for a substance in order that it be considered a mineral is that it have this orderly arrangement or crystal in it to be crystal in. And again that's almost always true but there are exceptions. The mineral ople the material that we call Opal is usually considered a mineral and it has a rather sloppy arrangement of the atoms. It's not completely Helter-Skelter arrangement to be sure but it's.

Most people would not consider it Christiane they wouldn't consider the orderly arrangement orderly enough to make it a crystal in substance. Sometimes we call these these exceptions these materials that are not Christian to call them a Marcus. And there are some minerals like opal that are amorphous. Sometimes minerals. Don't have this orderly arrangement because something it happened to destroy it to destroy the orderly arrangement. Some minerals that had radioactive elements in them like uranium and thorium originally formed with this orderly arrangement and they were good crystal and materials and then by radioactive bombardment from the Gori and where the uranium the structure was destroyed the orderly arrangement was destroyed. And so it is now an amorphous material that we still call these things minerals.

What is different between a man. And a rock. The mineral as we said. And defining what a mineral is is a homogeneous substance with a. That has the same composition from one part to the next. Rocks are mixtures of minerals. And therefore they may vary in composition from one point to another within the rock. Rock would have consist of one mineral particle one mineral in one place and next door to it. Another grain of perhaps the same mineral but perhaps a different mineral with composition is variable from place to place not homogeneous neighborhood don't know about these things you get two terms. They get the next step down and that's certainly true and in fact it's true even among people who have been exposed to this difference in an introductory geology course. It's often the. Different somewhat difficult takes a little while to get students to make the distinction between a mineral

and a rock and sometimes to mix up the two terms. How much crystallography actually affect the amateur Iraq and how much of it should he know. Lynn since crystallography and mineralogy are so intimately related we said before that all of almost all minerals are crystalline and a good many of them or at least many times there are crystals. Why it is important that a rock ound or anyone interested in minerals rocks in any way understand something about crystallography. But he doesn't necessarily need to go very into it very deeply he doesn't need to understand these patterns in any great detail. But he should understand that the things we talked about what the difference is between a crystal and a crystal in material. He should understand in a very general way

what kinds of patterns are involved in it in Crystal and substances or in crystals. If you had think you identified material very that's very very important point B. When the crystalline materials do form crystals then the shapes of these crystals are a very important means a very helpful means of identifying the substance since the shapes are usually characteristic of a particular substance like out of Baghdad if necessary to understand it. Septic crystallography most people when they begin to look into crystallography have had NO NO background that formally. Or specifically prepares them to understand that. Fact is that the people who. Seem to make the fastest progress and study in crystallography are the people who have the knack of

visualizing things in three dimensions the person that did well and solid geometry in high school will probably enjoy and do astronomy because solid geometry and crystallography are certainly akin to each other they know how to correct out the fact that they're going to they're between a mineral and a growth OK. Crystal but let's backtrack a minute. We mentioned I mentioned a minute ago that. Minerals in almost all cases. Have this orderly arrangement of atoms. We speak of that as being a being crystalline we say that materials are crystal and if they have this orderly arrangement. So since we said minerals have this property that means most in almost all minerals are crystalline in that they are almost without exception have these very neat orderly arrangement of the

atoms. Sometimes these crystal and particles mineral particles grow in such a way that the arrangement shows up on the outside of the particle so that the mineral fragment has very neat geometric shapes. We said bounded by plane surfaces. That's where text but not in this case we call it a crystal. Crypto had a definite shape correct re. If it took if a material is a crystal that it grows in such a way that it surfaces smooth plains and the overall shape generated is of. It is peculiar to that substance. The minute minerals and I've actually always crystal and by which you mean to have this orderly arrangement. A man may sometimes grow in

such a way that this arrangement shows up externally and then it's a crystal. So. Mineral may or may not grow as a Crystal Wright actually usually does not because. You know it's usually crystal and usually circumstances don't permit it to grab this neat ex-general shape that we call Crystal. Do you mean there is a difference between Crystal and Crystal yet crystal and refers us to this orderly internal arrangement of the atoms. But a crystal is a is a crystal in substance in which this arrangement shows up on the outside surface in the form of this neat shape that we can say Crafters or are not always visible to the naked eye man. Now that's that's correct. Crystal. Well let's put it this way the fact of being. The property of being

crystalline. Not at all apparent to the naked eye and many cases if it's if the crystalline material happens to grow as a crystal. It may or may not be apparent to the naked eye just depending on how big the crystal is it might be a nice crystal with these smooth faces. But be so small that we need a microscope to see it. What might go is a reasonably big crystal sometimes inches long or even feet long and in rare cases tens of feet. There are such things as crystals that are 10 20 feet long. I've heard Christopher doing the. Flowers or the mineral king down laughing. Did they seem that way to you. I can I confess that I. Never thought of that analogy before. Thank you Crystal that way. They can be very beautiful.

OK that's that's a fact they they are very often are very beautiful the. Basic gesture something very orderly about nature when you see these perfect shapes perfect geometric shapes that crystals often have. Why that does suggest that is indicative of or suggestive of this. Orderly ness about nature that many people think of and sort of in a philosophical way. And there's a certain beauty in that order. What about the hardness. I've been there I know that you know it's very in the degree of hardness. Now did I have anything to do with the formation of critical information or the formation of crystals. It certainly does the hardness of the mineral is most closely related to how strongly the atoms are held together. And that in turn goes back to this orderly arrangement of the atoms that we mentioned before. Certain minerals and certain orderly arrangements that amuse the

atoms are held together very tightly in this arrangement and so mineral is hard. In other cases the minerals are held who are the atoms held to adjacent atoms by very weak bonds and so this makes makes that mineral soft want to mention a couple of examples Diamond is the very hardest mineral that we know of the carbon atoms that make up Diamond are very closely arranged very closely packed together in this orderly arrangement and that is what makes the diamond so hard the atoms are close together and held. Together very tightly in another mineral that happens to have the same chemical composition as Diamond and mineral graphite. We're still talking about an arrangement of atoms of carbon but in this arrangement the atoms are close together

in certain directions in the crystal and material and the orderly arrangement but they're spaced far apart in other directions and in this direction where the atoms are spaced farther apart. The bombs holding them together are weaker and so it's so soft. This accounts for the softness of graphite and the Platy habit that graphite has at the same time the Platy or sea like form that crystals of graphite have is accounted for by the fact that the carbon atoms exist in layers. What effect do crystals have Coit age are fracturing. Again here there is a very close relationship between the orderly arrangement of the atoms and a crystalline substance and cleavage cleavage. The we defined of the tendency for something to

break along certain preferred directions more easily than a break long other direction than the substance. It certainly does have a very definite relationship to. The orderly arrangement of atoms that. Crystalline materials have mineralogy we ordinarily say cleavage is the tendency for a mineral to break along certain preferred directions through the mineral more readily more easily than it breaks through other directions and substance in the mineral. And this these preferred directions are. Very closely related to the orderly arrangement of the three dimensional pattern. Of the arrangement of atoms in the mineral. We might cite the same two examples that we mention and talk about differences in the hardness of diamond and graphite here we have the hardest mineral known. It has one cleavage pattern. It has certain directions on which it breaks easily.

The graphite the one of the softest minerals known has a different cleavage pattern it has just ONE DIRECTION of easy braking and it corresponds to the layers of atoms in the pattern. There is still another property other than the shape of crystals which is closely related to the crystal. The orderly arrangement of the atoms the density density or the you might say the heaviness of. Have a mineral. Define a little bit more rigorously than let's say the weight of a unit volume or the weight of a given volume of the mineral is dependent partly on this orderly arrangement of the atoms. Sometimes we're inclined to think that density. Depended mostly on what kinds of atoms are present and that is a very important lead minerals are heavy. Dense minerals usually because late is an

element with a high of comic weight so the weight of the atoms in the substance is important but there's more to it than met the orderly arrangement and also has a big effect on whether mineral be dense or not. Two minerals might might have the same composition but one will be denser than the other because the orderly arrangement of atoms involves a closer packing of these atoms and that will make it a denser substance even though it. Has the same composition as another one. Well let's let's talk about that you can have these. They add in the Crystal Wright from the sun. We mentioned before that. Adams that minerals rather are orderly arrangements of atoms. Crystal and substance or substances or orderly arrangement of atoms and therefore minerals are also these orderly arrangements. Way way back. Or in the 1840s.

Crystallographer figured out that there were a fairly small number of ways in which. Anything. Could be. Repeated in an orderly way in space. In fact a man by the name of they figured out that there. Could only be fourteen different ways of repeating a pattern in three dimensions. And the models in this case here the upper models. Here and there represent some of these fourteen different ways of repeating a pattern space. These three dimensional repeat patterns are called. Sometimes are called Space lattice is sometimes called the brow of a lattice is after this man who figured out there were just fourteen of them. And the important thing to realize about these models. Is that they are. Models of just of patterns. And he.

Would rob them on the corners of the models do not represent Adams they just represent points in a pattern. Now what does that have to do with with minerals or with Crystal and substances that I reall atoms and them. With each of these points in these patterns. There are associated atoms. Maybe with one atom or maybe hundreds of atoms. But the. Space lattice is. Overbroad by lattices are just the repeat patterns of the three dimensional. Regular arrangements of points for three dimensional repeat patterns. And. Then we hang atoms on this pattern we put a cluster of atoms associated with each point in the pattern. Then we get a crystalline substance. And the model is down. Below in this case. Our atomic structure models. And in these the. Balls of the spheres are they do represent atoms.

According to the book there are six fifty of crystals. Is that OK now. Upright I think. Christensen How did they tie in here. Might I try and rephrase the question slightly for convenience sake you ask what. What is the relationship between these things we call crystal systems and these. Repeat patterns that I have called lattices. And there is a definite relationship. If we. Pigeonhole these 14 lattices. On the basis of the special shapes that they have we get six groups of them and these we call the six crystal systems. Well. When I when we say that we can group these fourteen lattices into six crystal systems. We group them on the basis of these. Special properties these special lengths or special angles. For instance all the lattices. That have three equal edges in the pattern. And. Have each edge at right

angles to the plane of the other two we group in the isometric system they happen to be three. Lattices in this system. I'm going. To go back to these these crystal systems these six kinds of. Six groups of lattices. You might notice here that I have. A crystal real. Crystal. Sitting underneath each of the models that represent the six. Six. Crystal systems. And. For many of these you connect you can see a sum relationship between the shape of the crystal and the. Shape of the lattice model. You know the variety in the flower I. Do. You mean the size of the Christos. It's. Quite. A trick to find when you start looking for crystals. That are perfect enough to actually resemble the lattice they belong to. Why it's hard to

find any of any size and so some of these are. Example crystals are pretty tiny. Now one could find bigger crystals but they would. Be less perfect examples of their crystal system. Are all the crystals ever found. In me. Six shapes that represent the six systems and the 14 paintings. I know the answer the real explanation for that is that there are different ways of stacking these. Units that we call unit cells. Different ways of stacking these six different shapes of unit cells together to make crystals. We can take the cube shaped unit cells of the isometric system and stack them in such a way that we make a. Cube shaped crystals but we can also stack them such a way that we make. Eight faced crystals that we call an octahedron. Or. Twelve faced. Crystal that we call dodecahedron. And. And many others. So there are. Buy stock in these.

Units and these building blocks and different. Stacking patterns we get the many different shapes of crystal. I have a model here that shows how one can stack the cube shaped unit cells of the isometric system in such a way as to form an octahedron an eight faced. Right place Crystal and if you. Look at this close model closely you can see that if the cubes are stacked in a stair step fashion one up and one over. Long three. Long directions proud of the three different edges of the cube. Why you get this. Octahedron. Looks like a two pyramid put together back to back. It certainly does have that. Shape. And. In fact with the if we had made a pattern shaped like this by stacking. Blocks of a summer building blocks of a somewhat different shape than the cube we might call it a pair of pyramid

or a parabola or crystal. Knowing the difference between rocks and minerals heading in a quite nice with the orderly arrangement of atoms and possessing the ability to visualize in three dimensions all of these are of great importance to the serious rock and we wish to thank Dr Tom Wilcox assistant professor of geology at Vanderbilt University for his comments in this discussion of crystallography. This has been another in the series of programs exploring the world of the rock. The narrator this series is produced by the service of the public library of Nashville and Davidson County in Nashville Tennessee. Next week Mrs. folk will discuss the fascinating aspects of this hobby and a program entitled travel bites cephalopods and

others. This is Charles Mitchell. This is in our national educational radio network.