[SOUND] [MUSIC] How do you tell one mineral from another? You can't actually see the arrangement of silica tetrahedra, or what anions are present. Are there any clues that you can use to be able to recognize and identify minerals? Now sometimes you may know somebody that actually can look at a picture of a mineral and say this is such and such or they may see a specimen of a mineral in a museum and identify what it is before they've looked at the label. Is there anything that you can do to do that as well? At least with some of the more common minerals. Well yes. There are many, specific, physical properties. Things that you can see, observe, feel, touch, smell, whatever, that will allow you, to identify and distinguish among different mineral specimens. Let's look at a few examples of these. For example, one of the most common features that allows you to distinguish one mineral, from another, is its luster. Now by luster, we basically mean how does that mineral interact with light? Is it shiny like a metal? Is it dull? Does it look like a pearl? Does it look like satin? All these are adjectives that are self-explanatory. Metallic luster means that it looks kind of like a metal. Non-metallic luster means that it doesn't. Within the category of non metallic we can distinguish between satin like luster or pearly luster, or glassy luster so forth. All of these are terms that we can use to characterize the luster. Another term that we can look at is the crystal habit, the characteristics of the crystal structures. Now, I haven't really talked about what a crystal is yet, but in geological discussion, when we say that we have a crystal of a mineral, we mean that it's been able to grow so that the faces, or the surfaces of the mineral are determined by the internal arrangement, or the internal architecture of the crystal lattice. Now different minerals grow in different way, because they have different internal structures. Some minerals will grow into long needles, some will grow into blocky face, that characteristic is called the crystal habit. Here are a couple of examples, on the left is kyanite. Kyanite tends to grow into knife like blades so it has a bladed habit. On the right is an example of Asbestos. Asbestos tends to grow into little hair like fibers. Different kinds of minerals have different crystal habits. Now another key characteristic that we often use to identify minerals is the way in which the mineral sample or the mineral specimen fractures. Now some minerals fracture like a piece of glass, in the sense that you hit them with a hammer, they'll splay and form a big curving surface that's called a conchoidal fracture. Looks like the surface of a clam shell. But some minerals, if you were to look at how they break, they break into a very orderly arrangement. So you can see a whole series of parallel planes that have exactly the same orientation. Sometimes there's one set of parallel planes. Sometimes there are two sets of parallel planes. Sometimes there are three sets of parallel planes. Sometimes the planes, the different sets, are at right angles to on another. Sometimes they're at different angles than right angles to one another. All of these are examples of the way in which crystals fracture. Some minerals have very distinctive characteristics that are unique to that mineral. For example, calcite, the mineral that we've already introduced as having the chemical formula calcium carbonate, CaCO3. If you were to put a drop of acid on a crystal of calcite it would fizz and release bubbles of carbon dioxide gas. Now let's address the question of how do minerals form. Why do we have minerals on the surface of the Earth. Well, they form in a variety of different ways. For example, let's imagine that we have lava come out of a volcano or we have magma injected into the ground beneath the volcano. That molten rock does not contain minerals. Because the atoms are not arranged into an orderly lattice. Remember that part of the definition of a mineral is that the atoms are arranged into an orderly lattice. But when that molten rock starts to cool, if it cools slowly enough, the atoms within it will begin to organize. And as they organize they form an orderly lattice, the result is the production of distinct minerals. So, one way of forming minerals is by the solidification from a melt. You can have a similar process happen, If you start to evaporate a solution that contains dissolved ions. Those ions, they bond together and grow new crystals of minerals and those new minerals are going to have an orderly structure, they're minerals. So another way of forming minerals is by precipitation from a solution. Finally, you can have situations, where you actually precipitate minerals directly from a gas. You can imagine, a very hot gas, being emitted in a volcano, when that That hot gas enters the atmosphere, it suddenly cools and minerals may precipitate onto the rock surface around that vent. In some cases you have minerals precipitate freely in an open cavity underground. The result is the production of a geode, such as this example from Brazil. Typically on the surface of the geode if it hasn't filled entirely The minerals will have nicely formed crystal faces. As we'll see later on during the formation of rocks, however, that if you're having say, solidification of minerals out of a melt, there is no room for nice crystal faces to form in many cases. And you end up with irregular grains as the grains interlock with one another. Okay, so now we've seen what minerals are, how they form, let's finish this topic by talking about a particular kind of mineral that is very popular. Gemstones. So what is a gemstone? Well, simplistically, a gem is simply a particularly valuable mineral specimen. Now, some gems are just particularly clear or shiny versions of more common minerals. For example, emeralds are particularly clear, shiny versions of a more common mineral called beryl. Some gems are unique, there isn't a more common version of them, and when you find them, they're particularly valuable. But when people take gems and turn them in to jewelry, often what you actually see, for example, the diamond that you may see in your ring has many, many shiny faces. What are those faces? Well, what they're not is they're not crystal faces and they're not cleavage faces. Turns out that to form the shiny faces or facets of a gemstone, you produce them by polishing an originally rough stone. So when diamonds occur in nature, they tend to be irregular. Sometimes they have a crystal shape, but they don't have all the different faces that make them shine as they do in a ring. To make those, you use a lap, which is a rotating surface that's usually covered with a kind of grit. You take the gem stone, fix it to a stick called a doping arm and then press it down against that revolving lap, and as the gem comes in contact with that it grinds away and forms a single shiny face. Well to get a diamond to have the kinds of facets that you see in a ring, often that requires the production of maybe as many as 50, or more, different facets. So to get each one of those facets, you grind away for on the first one. Then rotate the stone by a certain angle, then grind the next one. Rotate the stone again by a certain angle, grind the next one, and so forth. Very precise process that requires quite a bit of skill. [MUSIC]
