This chapter is about the minerals based on silicon and oxygen, the silicates, the ones that make siliceous rocks. And because this means most rocks, apart from limestone and the other calcareous materials, they are often referred to as the “rock- forming minerals.” Let’s be clear at the outset that with these siliceous rocks we’re talking about silicate compounds, which involve the element silicon and a subgroup of silica minerals, which we’ll come to at the end of the chapter. None of this has anything to do with silicone, the synthetic polymer of multifarious uses. The principles discussed here are the same as those developed in the previous chapter, but the silicates present special challenges. Indeed, for a long time they were very tricky things to understand at all. The early geologists had at their disposal new ways of chemically analyzing minerals, and they applied them with gusto. They made impressively rapid progress, but they were baffled by the silicates. Their analyses showed that these minerals were dominated by silicon and oxygen but beyond that, well, they seemed too numerous, wildly varied, and inconsistent. It turned out to be well into the twentieth century before there was a breakthrough in understanding these perplexing compounds. The breakthrough took place when it dawned that X- rays could be used to study the structure of crystals—any crystals: geological, metallurgical, biological, and so on. Today, over twenty Nobel prizes have been awarded for work in this field, most recently in 2012. It’s even relevant to wine itself, through elucidating the structure of enzymes, proteins, and the like. (Incidentally, the use of X-rays to analyze crystals is quite different from its use in producing the familiar X-ray pictures of the human body.) In July 1912, the Bragg family rented a house, Whin Brow, at Cloughton, high above England’s Yorkshire coast. The father of the family, William, was a Professor of Physics at Leeds, and his 22-year-old son, Lawrence, was a precocious physics student at Cambridge. With an impending war, they were eager to escape the grimness of city life for a while, but even so, William took some work with him.