|Table of Contents for Caveman Chemistry: 28 Projects, from the Creation of Fire to the Production of Plastics|
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Now, the crystal is the most beautiful, most powerful, and most mysterious thing in the whole world. Way after the end of the radioactive cockroach civilization, way after the stars have petered out, crystals will still be around, like gold nuggets in the cosmic garbage dump. Pure gold. And not just a metaphor for pure, either. No matter whether it's in a cave or a quarry or a chemical laboratory, a crystal is your best and most visible sign that you've got a homogeneous chunk of matter with a fixed composition, that you've got one kind of stuff rather than lots of different kinds of stuff all mixed up together. Put it another way; you've got yourself a pure substance, either an element or a compound.
Unktomi is right to stress the importance of crystals. The process of crystallization can be visualized with a simple demonstration. Fill a shallow dish with identical balls; in Figure 2-1 I have used bb's. The balls pile randomly over each other, but if the dish is gently shaken they begin to arrange themselves into rows, and rows of rows, until a definite pattern emerges. The rows are aligned at angles of sixty degrees to one another, an angle characteristic of the way that spheres pack together. Similarly, crystals are formed from atoms and molecules, with angles between the crystal faces characteristic of the ways that the atoms and molecules pack together.
While a mineral is a pure substance with a characteristic crystal shape, a rock contains more than one kind of mineral. This situation can be modeled by mixing balls of different sizes in our dish. Because the distance between rows would be different for balls of different sizes, they can't pack together in rows and rows of rows when the dish is shaken. Isolated pockets may form which contain, for example, only small balls or only big ones, and rows may form in these pockets. Similarly, a rock may have no overt crystal structure, or it may be a heterogeneous mixture of tiny crystals.
There are four basic ways for crystals to form. First, molten rock may cool slowly enough that its constituent minerals crystallize into igneous rock, just as water (molten ice) may freeze into ice crystals. You may witness the growth of crystals from molten glass in Chapter 13. Second, hot aqueous solutions may cool slowly enough for crystals to form. A familiar example of this is rock candy, made by dissolving sugar in boiling water and allowing the solution to slowly cool. Third, an aqueous solution may slowly evaporate, precipitating the least soluble materials first and the most soluble materials last. If you allow a dish of salt water to evaporate, for example, you may grow crystals of sodium chloride. Finally, crystals may precipitate from a cooling gas. Familiar examples include snowflakes and frost. Whether by cooling or evaporation, the slower the process the larger the crystals which may grow and the greater their purity.
One of the most abundant and beautiful crystals is the mineral quartz, which is composed of pure silicon dioxide, SiO2, or silica. Quartz has a very characteristic crystal structure, as shown in Figure 2-2(L). It is important to stress that the faces of this crystal have not been cut by human hands. No, the crystal grew that way naturally, with flat faces and sharp angles between them. Any quartz crystal from anywhere in the world will have the same angles between the faces and the same general shape. Some quartz crystals are big, some little, some long, some short, but they are all recognizably and unambiguously quartz. While the purest quartz is clear and transparent, even the tiniest impurity will lend it color, producing rose quartz, orange citrine, and purple amethyst. Despite their colorations, these minerals have the characteristic crystal structure of quartz because they are quartz, albeit with minuscule amounts of materials other than silicon dioxide. Given enough such impurities, however, quartz becomes translucent rather than transparent and its crystals grow less regularly. Figure 2-2(R) shows such a less-than-perfect quartz crystal.
Quartz crystals may grow from the slow cooling of molten rock, from the slow cooling of hot silica-rich water, or from the evaporation of such a solution. When cooling or evaporation is too rapid, however, there is no time for large crystals to grow. The rocks which result contain either very small crystals or no crystals at all and any impurities are trapped in the resulting rocks. The rapid cooling of molten silicate rock produces obsidian, a glassy material prized by Unktomis the world over. The rapid cooling or evaporation of silica-rich water may also produce "cryptocrystalline" silicates, those without evident crystals. Whereas pure quartz is transparent and colorless, these materials are opaque, with colors derived from the impurities they contain. Chert, flint, agate, jaspar, and petrified wood share with obsidian a glassy texture suitable for making stone tools.
Wind and water gradually reduce silicate rock to rubble and rubble to sand. Sand accumulates and when it is crushed and compacted by later sediments it becomes sandstone, a sedimentary rock. When sandstone is compressed and heated, but not to the point of melting, it may become quartzite, a metamorphic rock. The texture of quartzite is more granular than obsidian or flint, but it shares the glassy texture common to most all of the silicates. Quartzite may be used to make stone tools, though it is not valued as highly as its cryptocrystalline cousins.
Lucifer got one thing right. Folks have gotten out of touch with nature. They don't know where things come from or how to make anything from scratch or how lucky they are to have the good things nature gives them. Seems to me, that's just downright ungrateful. So if you want to learn how to make arrowheads, for example, I can help you out.
In these early projects, the potential hazards are less chemical than mechanical. This project will involve making arrowheads from broken glass, which is, of course, very sharp. Before you begin, you ought to think of all the ways you might get cut making arrowheads. Losing your eyes wouldn't be good, so you ought to get some kind of glasses, either prescription glasses, sunglasses, or safety glasses. If you do get a flake in your eye you ought to go immediately to the hospital. It's pretty easy to cut your hands open, and so you ought to wear work gloves and a couple of leather pads would be good for protecting your shins. But no matter how safe you are, you could still goof and it would be good to have some band-aids on hand. Above all, you should take responsibility for your own safety, recognizing that playing with broken glass can be dangerous. Be as careful as you can and if you mess up, get some help, preferably medical help, not legal help.
|Research and Development|
It would be downright foolish to push ahead with making arrowheads if you didn't know anything about the silicates. So I guess you better study a bit: