Rocks

To expand somewhat on the definition of rock, the term may be said to describe an aggregate of minerals or organic material, which may or may not appear in consolidated form. Consolidation, which we will explore further within the context of sedimentary rock, is a process whereby materials become compacted, or experience an increase in density. It is likely that the image that comes to mind when the word rock is mentioned is that of a consolidated one, but it is important to remember that the term also can apply to loose particles.

The role of organic material in forming rocks also belongs primarily within the context of sedimentary, as opposed to igneous or meta-morphic, rocks. There are, indeed, a handful of rocks that include organic material, an example being coal, but the vast majority are purely inorganic in origin. The inorganic materials that make up rocks are minerals, discussed in the next section. Rocks and minerals of economic value are called ores, which are examined in greater depth elsewhere, within the context of Economic Geology.

The definition of a mineral includes four components: it must appear in nature and therefore not be artificial, it must be inorganic in origin, it must have a definite chemical composition, and it must have a crystalline internal structure. The first of these stipulations clearly indicates that there is no such thing as a man-made mineral; as for the other three parts of the definition, they deserve a bit of clarification.

At one time, the term organic, even within the realm of chemistry, referred to all living or formerly living things, their parts, and substances that come from them. Today, however, chemists use the word to describe any compound that contains carbon and hydrogen, thus excluding carbonates (which are a type of mineral) and oxides such as carbon dioxide or carbon monoxide.

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