1.2.2: Matter and Minerals

1.7 Different kinds of matter

All matter can be classified as being either an essentially pure substance or a mixture (Figure 1.7). Mixtures, in contrast with pure substances, are made of two or more substances that can differ in composition or properties, and that can be separated from each other. Some mixtures are homogeneous (like a drink in which gin and tonic are completely mixed) and some are heterogeneous (like a gin and tonic with ice cubes floating in it), but they can always be separated into individual and different substances using some sort of physical process. Most rocks are examples of mixtures – they can be separated into individual mineral components that have different properties.

1.8 Quartz crystals

Minerals and other pure substances have invariant compositions and distinctive properties. This means that although we can often divide a pure substance into portions, all portions are equivalent and have the same overall composition and properties. For example, we can take a large piece of quartz, perhaps any of the quartz crystals in Figure 1.8, and break it into smaller pieces – the pieces will all still be quartz, the atoms in every piece will all be arranged the same way, and all pieces will have the same hardness and other properties.

Pure substances, including minerals, may be made of a single element, for example argon gas (Ar) or the mineral copper (Cu). Or, they may be compounds made of two or more elements in fixed proportions. Quartz (SiO₂), consisting of two elements (silicon and oxygen), is a very common natural compound. Other minerals may consist of more than two elements, and some contain many elements.

1.9 Sand from South Point, Hawaii

Some sand is made of a single mineral, perhaps quartz or gypsum, but most sand is a mix of materials. The sand seen in Figure 1.9 is a good example of a geological mixture. Most sand is made predominantly of quartz, sometimes also containing heavy minerals such as magnetite. This sand is different. It contains several minerals (1 mm sized green olivine grains are most dominant), some black volcanic glass, and two kinds of shell fragments (one white and one with pinkish stripes). We can, in principle, separate all the different components from each other. (Although it would probably be quite tedious.) Note that, except for the shell fragments, all the grains in this sand are very well rounded. This is because they were abraded while being transported before deposition.

● Box 1-1 The Standard Definition of a Mineral Minerals are: • naturally occurring • crystalline solids • formed by geological processes • elements or compounds • homogeneous • characterized by a well-defined composition that can be described by a formula

So, although the definition of a mineral is sometimes debated today, for most purposes it is sufficient to say that minerals are natural crystalline solids that generally form by geological processes. They must also be elements or compounds with a well-defined chemical composition that can be described by a chemical formula. The few exceptions to these criteria are dealt with on a case-by-case basis by the International Mineralogical Association (IMA) and others who are tasked with keeping track of all approved minerals.