Minerals

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Contents 1 Introduction 2 Minerals defined 3 Silicate minerals 4 Non-silicate minerals 5 Mineraloids 6 Identifying minerals 7 Related articles

[edit] Introduction

In this article we shall define minerals and discuss their properties, their classification, and their identification.

It is impossible to discuss these subjects without referring to some basic facts about chemistry; readers who have forgotten everything they learned about chemistry at school can catch up by looking at our Appendix on Chemistry for Geologists, which contains a short review of everything you need to know.

[edit] Minerals defined

A mineral is defined by its chemical composition and crystal structure. For example halite (rock salt) has the chemical formula NaCl (sodium chloride) with the atoms arranged in a cubic lattice, as shown to the right, so that each sodium is bonded to six chlorines, and each chlorine to six sodiums.

With some chemicals, the same molecules can form crystals in different ways: for example, molecules of calcium carbonate (CaCO₃) can stick together either in a trigonal lattice or in an orthorhombic lattice; in the first case, the mineral is calcite, in the second case it is aragonite. Minerals with the same chemical composition but different crystalline structures will have different physical properties and different histories, so it is important to distinguish between them.

A mineral may be made of different sorts of molecules, so long as they behave in the same way when crystalization takes place. For example, the chemical formula for pyroxine is written as (Mg,Fe)SiO₃. The meaning of writing (Mg,Fe) in this formula is that the molecules that form the crystal can be either MgSiO₃ or FeSiO₃. When a molten mix of these molecules crystalizes, these two chemicals are sufficiently similar in their properties that, instead of separating out into distinct crystals of MgSiO₃ and FeSiO₃, they form a crystal lattice in which the atoms of metal in the crystal are sometimes magnesium (Mg) and sometimes iron (Fe), at random. Such a substance is known as a solid solution.

With the caveat that we shall count such solid solutions as minerals, we may say that what distinguishes a mineral from just any old rock is that a mineral is one substance (with a given crystalline structure) whereas rocks may be, and usually are, composed of several different minerals.

In Appendix A of this text, we list some of the commoner minerals, their composition, crystal structure, and properties. The reader who intends to become a geologist will have to learn this stuff; but we expect that the average reader of this textbook is simply trying to get a good understanding of geology, and so can just use this appendix for reference as required.

[edit] Silicate minerals

The commonest elements in the Earth's crust are silicon and oxygen. These bond together to form ions consisting of one atom of silicon and four of oxygen (SiO₄^4-), with the oxygen atoms arranged equally in three dimensions around the central atom of silicon so that the oxygen atoms lie at the corners of a regular tetrahedron. This structure is known as a silica tetrahedron or as an SiO₄^4- unit; we shall prefer the latter term. Because silicon and oxygen are so abundant, most common and important minerals have these units in their composition; these are known as silicate minerals.

The SiO₄^4- units can stick together by sharing the oxygen atoms at their corners. So, for example, they can form chains in which each SiO₄^4- unit (except the ones at the end of the chain) shares an oxygen atom with each of its two neighbors, as shown in the diagram to the right. Or they can form rings of three, four, or six SiO₄^4- units; which are like chains but with their ends joined together. Double chains and sheets are also illustrated in the diagram to the right. They can also form three-dimensional structures in which every silicon atom shares each oxygen atom with one other silicon atom, as in quartz; or similar structures involving both SiO₄^4- units and similar units based on aluminum (AlO₄^4-), as is seen in feldspar. Finally, we should note that in some minerals the SiO₄^4- units appear in isolated form, without sharing their oxygen atoms with any other such unit.

For each oxygen shared between silicon atoms this reduces the negative electrical charge by 1 for each unit involved. Those oxygen atoms which are not shared between silicon atoms must bond with some other atom, since a mineral cannot have a net negative charge.

The way in which the SiO₄^4- units stick together (or don't stick together) shows up in the chemical formulae for the minerals. For example, quartz is made up entirely of SiO₄^4- units, but because every oxygen atom is being shared between two silicon atoms, this means that there are two oxygen atoms for every silicon atom, giving quartz the formula SiO₂. Beryl, meanwhile, has its SiO₄^4- units joined in rings of six units; as you should be able to figure out from the diagram above and to the right, this means that there are three oxygen atoms for every silicon atom, hence six silicon atoms and eighteen oxygen atoms in each ring; and so its chemical formula is BeAl₂(Si₆O₁₈).

Conversely, from the chemical formula you can work out the structure: if we tell you that olivine has the formula (Mg,Fe)₂SiO₄, then you know that the SiO₄^4- units must be isolated from one another: there are four atoms of oxygen for every atom of silicon, so none of the oxygen atoms attached to one silicon atom are being shared with another silicon atom.

[edit] Non-silicate minerals

The minerals other than silicates can be divided into various different classes according to their chemistry.

First of all, we should mention native elements: minerals that contain just one sort of element, such as a nugget of pure gold. In native elements, the atoms are stuck together by sharing some of their electrons between them, which is known as covalent bonding.

You might remember from your chemistry classes that the other way for molecules to stick together is ionic bonding. In this case, the molecule consists of two parts, a cation and an anion, and the anion "borrows" some electrons from the cation. Since electrons have a negative charge, this means that the cation ends up with a positive electrical charge, and the anion with a negative electrical charge, and so they are bonded together by the principle that opposite charges attract. By convention, the chemical formula for such a molecule is always written with the cation first, then the anion; so, for example, in calcite, with the formula CaCO₃, the cation is Ca²⁺ (calcium which has donated two electrons, and therefore has a charge of plus two) and the anion is CO₃^2- (which has borrowed two electrons from the calcium, and therefore has a charge of minus two). This same order is also reflected in the chemical name of the molecule: in this case calcium carbonate.

This allows us to classify minerals according to their anions. So, to continue with the same example, calcite is classed as a carbonate, according to its anion, and is placed in the carbonate class of minerals.

• The carbonate class consists of carbonates, logically enough, and of similar chemicals: nitrates, iodates and borates.
• The halide class consists of chemicals with anions from the halogen group of elements: fluorides, chlorides, bromides and iodides.
• The oxide class consists of oxides and hydroxides.
• The sulfide class contains sulfides and similar chemicals: selenides, tellurides, antimonides, and arsenides.
• The sulfate class consists of sulfates and similar chemicals: sulfites, chromates, molybdenates, the selenates and selenites, tellurates, tellurites and tungstates.
• The phosphate class consists of phosphates and similar chemicals: the arsenates, the vanadates and antimonates.

Most non-silicates are fairly rare and of little importance in understanding historical geology: among the more important ones that we shall have need to mention in this text are calcite and aragonite, the two crystal forms of calcium carbonate; halite (sodium chloride, i.e. salt); gypsum (calcium sulfate dihydrate); and hematite (iron(III) oxide). We might also add ice to this list: it is technically a mineral, though probably not the first example a mineralogist would think of if asked.

[edit] Mineraloids

A mineraloid is a substance which is a bit like a mineral but lacks one or more of a mineral's defining features, such as crystal structure. Well-known mineraloids include coal, opal, amber, and obsidian.

[edit] Identifying minerals

As minerals are defined by their composition and structure, the ideal way to identify a mineral would be to subject it to chemical and structural analysis. In most cases there are much easier ways; this is particularly useful in the field, as it saves one the trouble of getting an X-ray machine up a mountain or down a cave.

Some of the criteria used are as follows:

• Color. Unfortunately, many minerals come in a variety of different colors, because in many cases the color of a mineral is caused by the chemical impurities in it, which can vary. For example, the gemstones we know as rubies and sapphires are corundum with different metallic impurities.

• Streak is the color you get by scraping the mineral on an unglazed porcelain plate. Different-colored varieties of the same mineral can still produce the same colored streak.

• Luster is the way the mineral shines: metallic, greasy, resinous, vitreous, pearly, silky, and so forth.

• Crystal habit is the overall shape typically formed by the mineral. Shown to the right are the hexagonal pointed prisms of amethyst (a sort of quartz). However, the same mineral may occur in different shapes, or in a lumpy shape with no particular form, depending on the conditions under which the crystal formed.

• Hardness is usually measured on Mohs' hardness scale, which places minerals on a scale from one to ten based on which will scratch which. Standard minerals determining the scale are:

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond

• Specific gravity is a measure of density given by dividing the weight of a chunk of mineral by the weight of water (at 4ºC) that it displaces. You may remember that according to legend, it was thinking up the basis for this sort of mineralogical assay that led Archimedes to jump out of his bath shouting "Eureka".

• Cleavage. The crystalline structure of a mineral may make it easy to split evenly along certain planes; the number of these planes and the angles between them can be used to identify a mineral. It is by exploiting the existence of cleavage that jewelers cut faceted stones.

• Fracture. Besides cleavage, any mineral will break it you hit it hard enough, and the way it fractures can be classified as splintery, earthy, hackly, conchoidal, and so forth.

• Refraction is the way a transparent mineral will bend light passing through it, the refractive index being characteristic of the mineral.

Besides this there are other features that can help identify specific minerals: for example, halite, being salt, will taste salty;.

Our overview of this topic has been brief, because this is not a good subject to teach over the internet: what the reader really needs is a selection of minerals (and a small hammer, some goggles, and some dilute hydrochloric acid).

[edit] Related articles

• Rocks
• Geology Glossary
• Science and Pseudoscience