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Types of Rocks Study Guide

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Updated on Sep 25, 2011

Introduction

The dynamic earth churns new matter up to its surface and returns other matter back down to the mantle. Through all this activity, including through the actions of water and wind as well as deposition of sediments to form new types of rock, continents are created and changed. Continents are, in essence, made of rock, spanning scale of sizes from dust particles to mountains and the underlying bedrock of the continental masses themselves. Here we into look the science of rock.

Elements, Minerals, and Rocks

Different elements exist because of the different kinds of atoms that can be made from the same basic three atomic building blocks. Atoms are the most finely divisible parts of matter that possess the characteristics of a particular element—elements such as copper, gold, carbon, or hydrogen. Atoms alone (not in molecules or ions) are electrically neutral and contain equal amounts of positive and negative electrical charges. The positive charge is concentrated in a tiny central massive region called the nucleus. The negative charge is in one or more tiny electrons (the first of the atomic building blocks). Electrons whirr around the nucleus, bound to it by electrical attraction.

The nucleus of atoms contains protons and neutrons, which are the other two atomic building blocks. Protons are the carriers of the positive charge. Neutrons, as their name suggests, are neutral. Now, all the atoms of a particular element have the same number of protons in their nuclei (which determine the charge of the nucleus, thus the number of electrons around the nucleus, and thus the chemistry of the element). But atoms of the same element can vary in the number of neutrons in their nuclei. These variants are called isotopes and will be discussed more in other lessons in this book. In summary, the atoms of elements are made from the atomic building blocks of protons, neutrons, and electrons.

Table 7.1 outlines the amounts of different kinds of elements in Earth's continental crust.

Table 7.1 Elements in the Continental Crust

Note that nearly of Earth's rock is oxygen and silicon. Furthermore, the top eight elements (down to and including potassium) make up 98% of all rock. What is element number 3? Number 4? From the table, we know that rocks will be mostly oxygen and silicon, and we can surmise that differences between kinds of rock will be found in how the oxygen and silicon are arranged and what kinds of small amounts of other elements are in the rock. Is calcium and manganese there? Is iron and phosphorus there?

This discussion already assumes that rocks are made of several elements. Note that it is rare to find elements by themselves, such as a vein of pure gold. We don't dig up elements as pure veins of calcium or aluminum, for example. So to understand more about the structure of rocks, we have to review a bit of chemistry.

The science of chemistry studies the interactions of atoms, how they form molecules, and the interactions of those molecules. Molecules range from simple ions, such as sodium dissolved in seawater, to complex organic molecules of life, such as proteins and lipids. The key concept in how atoms form molecules is the atomic bond, that is, the connection that joins atoms into molecules.

The atomic bond occurs between electrons of atoms. For example, electrons can be shared in what is called a covalent bond. In another type of bond, called the ionic bond, electrons are taken from one atom and are joined to another. The bonds between metals, say in copper or gold, are unique and are named the metallic bond. Whatever it is that allows certain types of bonds to form between particular kinds of atoms has to do with the number of electrons the atoms of an element possess. And, as we have seen, the number of electrons depends on the number of protons in the nucleus of an atom of an element. So the bonds among atoms in molecules depend directly on the atomic structure of the atoms.

Now, even though we have been discussing the fact that rocks are made of elements, an intermediate level of organization is crucial to the study of rock. In that level, we find what are called minerals. Geology is mainly concerned with the broad type of molecular organization of minerals. Minerals are always larger than a single molecule. Indeed, crucial to the existence of minerals is that they have crystalline structure. Having a crystalline structure means that the atoms create a network of repeating units, a crystal.

Common table salt is an excellent example of a crystal. The salt crystal is a network (or lattice) of sodium and chlorine atoms. Many types of crystals exist, and the terminology and geometry can get quite complex. These topics won't concern us here. Just be aware that rocks are made of minerals, and that minerals are solids made of certain elements drawn from the list of elements in Earth's crust. Minerals have specific chemical compositions and have crystal structures.

Now let's return to the fact that the two most abundant elements are oxygen and silicon. Silicon and oxygen alone can form the mineral called quartz. The chemical formula of quartz is SiO2. It has one atom of silicon (Si) and two atoms of oxygen (O). Of course, this basic unit of SiO2 is linked with others of the same formula to create the gorgeous crystals of quartz we have all seen.

Many other minerals are combinations of silicon and oxygen along with other elements. In general, these minerals are silicon oxides, also known as silicates. Other elements join in to create different kinds of silicates, such as magnesium–iron silicates, magnesium–aluminum silicates, and so forth. Geologists have given the most common minerals names. Perhaps you have heard, for instance, of feldspar. The chemical formula for the abundant mineral called feldspar is KAlSi3O8. It has one atom of potassium (K), one atom of aluminum (Al), three atoms of silicon (Si), and eight atoms of oxygen (O). If you are shown the chemical formula for a mineral and know the symbols for the different elements in the formula, you should be able to say how many atoms of each element are in the basic molecular unit of the mineral. (As an example of a symbol, the chemical symbol for magnesium is Mg.) We just went through an example for feldspar.

Geologists have developed ways to tell minerals apart by classifying them according to a number of basic properties. Here is the classic list of the basic properties of minerals used in geology.

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