Quartz, olivine, garnet, pyroxene, hornblende, the micas and the feldspars – what unites them? There is an underlying order in their composition. They are all share chemically identical molecular silicate skeletons. It is said that if you get to know the minerals just named and recognize them when they appear in more complex rocks, you should be able to identify most of the rocks on the face of the earth.
As a start, from his own collections Rob handed out samples of the minerals pictured at right. He explained that the molecular building blocks of all of them are tetrahedrons – triangular pyramids – each with a single silicon atom surrounded by four of oxygen. In mica, the tetrahedron units are very strongly connected together in broad sheets, while adjacent sheets are very loosely held to each other; mica peels away thinner than paper. Clay particles are layered like this, too, allowing a mass of clay to absorb water and become squishy and capable of being molded into pottery. Rob passed around portions of prehistoric Iroquoian pots made more than 500 years ago.
In quartz (amethyst is a variety) the tetrahedra are bound to each other in three dimensions. A large specimen can be a single crystal. In chert and jasper, also considered varieties of quartz, there are millions of crystals, all microscopic. in garnet, the tetrahedra are isolated from each other.
Rob’s father was an archaeologist, so he has specimens of prehistoric stone tools. He showed everyone Iroquoian arrowpoints made from chert, together with similar points he himself had made out of broken window glass. Glass is another silicate.
For hundreds of thousands of years, until metals came into use, humans shaped naturally occurring silicates into their tools. Now, several kinds of artificial silicon-based materials are used all through our material world. Pure silicon, a metalloid, is the basis for integrated circuit electronics. Zeolites are used in hair gel, non-clumping cat litter, and water filters. Silicone (silica-oxygen polymers with organic side chains) have been designed to serve as lubricants, rubber sealants, and kitchen utensils.