[SOUND] [MUSIC] We're going to turn our attention now to the bulk of what makes up planet Earth, and the bulk of that are the minerals. Minerals, in effect, are the building blocks of our planet. So, what is a mineral? Mineral in everyday English is used in a variety of different ways, but to a geologist, it has a very specific meaning. Specifically, a mineral is a naturally occurring, solid material that has an organized internal arrangement of atoms and has a definable chemical formula. Let's pull apart that definition and understand what each component of it means, because it's kind of important if you want to understand what minerals are. Naturally occurring, well, that's a little bit of semantics. By that we simply mean that for a material to be categorized as a real mineral, it has to occur in nature. It's not produced in a factory. Now, that said, there are minerals that are produced by humans in factories in some ways. There are ways you can squeeze carbon together to form diamonds. There are ways you can have chemical reactions that produce different kinds of substances, which are exactly the same as minerals. But we call those synthetic minerals, and we actually make a subtle distinction between those and real minerals. Secondly, what do we mean by a crystalline solid? In a crystalline solid, the atoms are not randomly distributed, or randomly arranged. If they were, it would be a glass. In fact, a glass is a solid in which the materials are not arranged in a specific order. In a crystalline solid, the atoms are arranged like rows and columns of a regime of soldiers so that there's a specific structure, and that structure is called a crystalline lattice. Glasses are different because in a glass there is not a crystalline lattice. Finally, what do we mean by definable, chemical formula? You can write out a chemical formula that specifies what different kinds of elements are within the mineral, and if there are more than element, what are the proportions of the different elements. For example, a very common element is quartz, SiO2. That means that quartz is composed of atoms of silicone and atoms of oxygen, and that for every atom of silicon, there are two atoms of oxygen. Another common mineral is calcite, CaCO3. In calcite, there are calcium atoms, there are carbon atoms, and there are oxygen atoms. For every atom of calcium, there's on atom of carbon, and there are three atoms of oxygen. Now these are some of the more simple formulas of minerals that exist. There are some formulas that are much more complicated. Here's a formula of intermediate complexity, the formula of the mineral muscovite, which is a kind of mica. It occurs in very thin sheets. It has the formula, KA2(AlSi3O10)(OH2). In other words, it's long. There's potassium. There's aluminum, more aluminum, silicon, oxygen, hydrogen, lots of different atoms in different proportions. But within this mineral they're arranged in a certain rigid structure, but at least we can write out the formula for that particular mineral. Mineralogists, the scientists that study minerals, are able to characterize minerals into different classes based on their chemical composition. Now, to understand the basis for this, we have to introduce one more term, ion. An ion is an atom that has a different number of electrons than it does protons. So remember, physicists define atoms as having protons, neutrons, and electrons. For a given element, all the atoms within that element have a specific number of protons characterized by the atomic number. Now sometimes, there are excess electrons, and if that happens, the atom has an electrically negative charge. Sometimes, it has a deficit, it has too few electrons, and in that case, the atom has an overall positive charge. Atoms that have an electrical charge, either positive or negative, are called ions. Those ions, in turn, that have a negative charge are called anions. Those that have a positive charge are called cations. So with that background, we can now look at how mineralogists classify these 4,000 different minerals into relatively few categories or mineral classes. They based their classification on the nature of the anion, the negative charged ions, that occur within the mineral. Specifically, silica, which is the SiO4 ion, which has a negative four charge. It's missing four electrons relative to the number of protons that it has within it. Sometimes ions are just single atoms, I should say, and sometimes, ions are molecules, more than one atom. Oxides are minerals that contain the oxygen anion. The carbonate ion, which is CO3. And so forth. There's sulphide minerals, there are halide minerals, and several others. By and large, there are a handful of mineral classes compared to the total number of minerals that are present on the planet. The most common minerals on earth are the silicate minerals. Silicates have a very interesting internal architecture. They're made up of tiny little building blocks called the silicon-oxygen tetrahedron, or more commonly called the silica tetrahedron. You can think of the silica tetrahedron as being a tiny little pyramid. Each corner of the pyramid is an oxygen atom, and in the middle is a silicone atom. Even though we've said that silicates are a single class of mineral, there are actually many different kinds of silicate minerals. Each of these different types of silicates is defined based on the way in which the silica tetrahedra are bonded to each other. Specifically, in the olivine category, if you looked inside an olivine, you would see that the silica tetrahedra are not bonded together at all. They don't share any oxygen atoms. If you go to the next category, those are called the pyroxenes, and in the pyroxenes, the atoms are bonded together in single chains. So, each silica tetrahedra shares an oxygen atom with its neighbor. In the amphibole category they are double chains, in the mica category they are two dimensional sheets, and in the final category, which includes quartz and feldspars, they form three dimensional networks where each silica tetrahedron is sharing all of its oxygens with its neighbors. [MUSIC]
