- What are minerals?
- The chemistry of minerals
- Why study minerals?
- Classes of minerals
- Open Source Web links
The solid earth is made of rocks, which are made of minerals. To understand rocks you need to become familiar with minerals and how they are identified. This basics page gives you the background needed to understand the terms used in the minerals classification table, which contains information for identifying minerals.
All rocks except obsidian and coal are made of minerals. (Obsidian is a volcanic rock made of glass and coal is made of organic carbon.) Most rocks contain several minerals in a mixture characteristic of the particular rock type. When identifying a rock you must first identify the individual minerals that make up that rock.
Minerals are naturally occurring, inorganic solids with a definite chemical composition and a crystal lattice structure. Although thousands of minerals in the earth have been identified, just ten minerals make up most of the volume of the earth's crust--plagioclase, quartz, orthoclase, amphibole, pyroxene, olivine, calcite, biotite, garnet, and clay.
Together, the chemical formula (the types and proportions of the chemical elements) and the crystal lattice (the geometry of how the atoms are arranged and bonded together) determine the physical properties of minerals.
The chemical formula and crystal lattice of a mineral can only be determined in a laboratory, but by examining a mineral and determining several of its physical properties, you can identify the mineral. First, you need to become familiar with the physical properties of minerals and how to recognize them.
See the Mineral Chemistry Basics page for a detailed discussion of the chemistry of minerals.
Why study minerals? Because the solid earth is made almost entirely of minerals, to understand the earth we must understand the nature of minerals, how they form, and how they can be analyzed as sources of information about the earth and its history. Most rocks are made entirely of minerals, so to understand rocks and the rock cycle in depth requires being able to analyze, identify, and interpret minerals.
Each mineral contains information about the chemistry, pressure, and temperature that was present, in or on the earth, at the place and time the mineral formed. For example, diamond is a mineral, made of pure carbon, which only forms under the high pressures that occur deeper in the earth than the bottom of the crust, in places where unusually high concentrations of carbon are present in the earth's mantle. We can analyze diamonds, and the other minerals with which they co-existed, to get at the temperatures, pressures, and chemistry of these special sites in the earth's mantle. Diamonds thus act as probes of the earth, bringing us geological information from far greater depths in the earth than we could ever dig or drill.
Minerals commonly grow in layers that accrete onto the surface of earlier-formed parts of the mineral. If a mineral has a variable chemical composition that changes as the chemistry, pressure, and temperature of its environment changes, the layers of mineral growth can be analyzed to track the changing conditions in which the mineral grew. For example, analyzing the layers in a crystal of feldspar in a volcanic rock may reveal that the mineral grew as the magma was cooled and then re-heated as it mixed with an intruding batch of hotter magma with a differing chemical composition, which may have occurred just before the magma erupted to the earth's surface and rapidly cooled and solidified as a lava flow.
A mineral may incorporate a radioactive element into its atomic structure as it crystallizes, and the decay of the element into its stable daughter product, which may remain trapped inside the crystal, along with the decay rate of the radioactive element, allows an analysis of the elements in the mineral to be used to measure the age of the mineral. This is how many of the ages of geological materials are measured.
Minerals, of course, provide resources for construction, industry, and technology, from quartz to make silicon chips out of for computers (or to make glass out of for windows), to calcite for making cement for concrete mix, to clay for making ceramics - in fact, there are hundreds of minerals necessary for production of productions and construction of houses, roads, and buildings. Therefore, some geologists, known as economic geologists, specialize in certain minerals that are valuable as resources, and explore the earth to locate places where the mineral is concentrated and accessible.
Gemstones and semiprecious stones, such as emeralds, diamonds, and rubies (which are gemstones) or garnets (which are semiprecious) are all minerals, as is gold. The beauty and durability of such minerals, along with the limitations to their abundance, makes them valuable to people. Even common minerals such as quartz are collected by some people and put on display, if they are found in the form of beautiful or unusually colorful crystals.
The physical properties of a mineral are controlled by its chemical composition (which types of atoms it consists of, and in what proportions) and its crystal lattice (the three-dimensional geometric pattern in which those atoms are arranged and bonded together).
It is no coincidence that crystals of quartz (SiO2) are six-sided, while crystals of halite (NaCl) are cubic. This is because of the geometry of their crystal lattices. It is also no coincidence that quartz is hard enough to scratch glass and will not dissolve in water to any visible extent, whereas halite will not scratch glass and will easily dissolve in water. These differences are due to the different chemical compositions of the minerals. The sodium (Na) and chlorine (Cl), by their chemical nature, readily break their bonds and become dissolved ions in water. The silicon (Si) and oxygen (O) in quartz are linked by strong bonds, which do not yield easily to the dissolving force of water.
Each mineral exhibits a unique set of physical properties. Therefore, the main task in identifying a mineral is to determine its physical properties. The physical properties that we will consider are color, luster, streak, cleavage, fracture, hardness, crystal shape and selected special properties.
Color is often useful, but should not be relied upon. Some minerals come in many different colors. Quartz, for example, may be clear, white, gray, brown, yellow, pink, red, or orange. So color can help, but do not rely on color as the determining property.
Luster is how the surface of a mineral reflects light. It is not the same thing as color, so it crucial to distinguish luster from color. For example, a mineral described as "shiny yellow" is being described in terms of luster ("shiny") and color ("yellow"), which are two different physical properties. Standard names for luster include metallic, glassy, pearly, silky, greasy, and dull. It is often useful to first determine if a mineral has a metallic luster. A metallic luster means shiny like polished metal. For example cleaned polished pieces of chrome, steel, titanium, copper, and brass all exhibit metallic luster as do many other minerals. Of the nonmetallic lusters, glassy is the most common and means the surface of the mineral reflects light like glass. Pearly luster is important in identifying the feldspars, which are the most common type of mineral. Pearly luster refers to a subtle irridescence or color play in the reflected light, same way pearls reflect light. Silky means relecting light with a silk- like sheen. Greasy luster looks similar to the luster of solidified bacon grease. Minerals with dull luster reflect very little light. Identifying luster takes a little practice. Remember to distinguish luster from color.
Streak is the color of the mineral as a powder. It is determined by scratching a mineral against a streak plate and checking the color of the streak left behind. The streak, the color of the mineral as a powder, may be different from the whole mineral color.
A mineral that naturally breaks into perfectly flat surfaces is exhibiting cleavage. Not all minerals have cleavage. A cleavage represents a direction of weakness in the crystal lattice. Cleavage surfaces can be distinguished by how they consistently reflect light, as if polished, smooth, and even. The cleavage properties of a mineral are described in terms of the number of cleavages and, if more than one cleavage, the angles between the cleavages. The number of cleavages is the number or directions in which the mineral cleaves. A mineral may exhibit 100 cleavage surfaces parallel to each other. Those represent a single cleavage because the surfaces are all oriented in the same diretion. The possible number of cleavages a mineral may have are 1,2,3,4, or 6. If more than 1 cleavage is present, and a device for measuring angles is not available, simply state whether the cleavages intersect at 90° or not 90°.
To see mineral cleavage, hold the mineral up beneath a strong light and move it around, move it around some more, to see how the different sides reflect light. A cleavage direction will show up as a smooth, shiny, evenly bright sheen of light reflected by one set of parallel surfaces on the mineral.
All minerals have fracture. Fracture is breakage, which occurs in directions that are not cleavage directions. Some minerals, such as quartz, have no cleavage whatsoever. When a mineral with no cleavage is broken apart by a hammer, it fractures in all directions. Quartz is said to exhibit conchoidal fracture. Conchoidal fracture is the way a thick piece of glass breaks with concentric, curving ridges on the broken surfaces. However, some quartz crystals have so many flaws that instead of exhibiting conchoidal fracture they simply exhibit irregular fracture. Irregular fracture is a standard term for fractures that do not exhibit any of the qualities of the other fracture types. In introductory geology, the key fracture types to remember are irregular, which most minerals exhibit, and conchoidal, seen in quartz.
Hardness is the strength with which a mineral resists its surface being scraped or punctured. In working with hand samples without specialized tools, mineral hardness is specified by the Mohs hardness scale. The Mohs hardness scale is based 10 reference minerals, from talc the softest (Mohs hardness of 1), to diamond the hardest (Mohs hardness of 10). It is a relative, or nonlinear, scale. A hardness of 2.5 simply means that the mineral is harder than gypsum (Mohs hardness of 2) and softer than calcite (Mohs hardness of 3). To compare the hardness of two minerals see which mineral scratches the surface of the other.
|Mohs Hardness Scale
|3.5-pure, untarnished copper
|5 to 5.5-stainless steel
|5.5 to 6-glass
|6 to 6.5-hard steel file
All minerals are crystalline, but only some have the opportunity to exhibit the shapes of their crystals, their crystal forms. Many minerals in an introductory geology lab do not exhibit their crystal form. If a mineral has space while it grows, it may form natural crystals, with a crystal shape reflecting the geometry of the mineral's internal crystal lattice. The shape of a crystal follows the symmetry of its crystal lattice. Quartz, for instance, forms six-sided crystals, showing the hexagonal symmetry of its crystal lattice. There are two complicating factors to remember here: (1) minerals do not always form nice crystals when they grow, and (2) a crystal face is different from a cleavage surface. A crystal face forms during the growth of the mineral. A cleavage surface is formed when the mineral is broken.
There are some properties that only help to distinguish a small number of minerals, or even just a single mineral. An example of such a special property is the effervescent reaction of calcite to a weak solution of hydrochloric acid (5% HCl). Calcite fizzes or effervesces as the HCl solution dissolves it and creates CO2 gas. Calcite is easy to identify even without testing the reaction to HCl, by its hardness, luster and cleavage.
Another special property is magnetism. This can be tested by seeing if a small magnet responds to the mineral. The most common mineral that is strongly magnetic is the mineral magnetite. A special property that shows up in some sample of plagioclase feldspar is its tendancy to exhibit striations on cleavage surfaces. Striations are perfectly straight, fine, parallel lines. Magnification may be required to see striations on plagioclase cleavage surfaces. Other special properties may be encountered on a mineral to mineral basis.
First, you need good light and a hand lens or magnifying glass. A hand lens is a small, double-lens magnifying glass that has a magnification power of at least 8X and can be purchased at some bookstores and nature stores.
Minerals are identified on the basis of their physical properties, which have been described in the the previous section. To identify a mineral, you look at it closely. At a glance, calcite and quartz look similar. Both are usually colorless, with a glassy luster. However, their other properties they are completely different. Quartz is much harder, hard enough to scratch glass. Calcite is soft, and will not scratch glass. Quartz has no mineral cleavage and fractures the same irregular way glass breaks. Calcite has three cleavage directions which meet at angles other than 90°, so it breaks into solid pieces with perfectly flat, smooth, shiny sides.
When identifying a mineral, you must:
- Look at it closely on all visible sides to see how it reflects light
- Test its hardness
- dentify its cleavage or fracture
- Name its luster
- Evaluate any other physical properties necessary to determine the mineral's identity
In the minerals tables that accompanies this section, the minerals are grouped according to their luster and color. They are also classified on the basis of their hardness and their cleavage or fracture. If you can identify several of these physical properties, you can identify the mineral.
Minerals are classified according to their chemical properties. Except for the native element class, the chemical basis for classifying minerals is the anion, the negatively charged ion that usually shows up at the end of the chemical formula of the mineral. For example, the sulfides are based on the sufur ion, S2-. Pyrite, for example, FeS2, is a sulfide mineral. In some cases, the anion is of a mineral class is polyatomic, such as (CO3)2-, the carbonate ion. The major classes of minerals are:
- native elements
SILICATES--Based on the polyatomic anion, (SiO4)4-, which has a tetrahedral shape. Most minerals in the earth's crust and mantle are silicate minerals. All silicate minerals are built of silicon-oxygen tetrahedra (SiO4)4- in different bonding arragements which create different crystal lattices. You can understand the properties of a silicate mineral such as crystal shape and cleavage by knowing which type of crystal lattice it has.
- In nesosilicates, also called island silicates, the silicate tetrahedra are separate from each other and bonded completely to non silicate atoms. Olivine is an island silicate.
- In sorosilicates or paired silicates, such as epidote, the silicate tetrahedra are bonded in pairs.
- In cyclosilicates, also called ring silicates, the silicate tetrahedra are joined in rings. Beryl or emerald is a ring silicate.
- In phyllosilicates or sheet silicates, the tetrahedra are bonded at three corners to form flat sheets. Biotite is a sheet silicate.
- In single-chain inosilicates the silicate tetrahedra are bonded in single chains. Pyroxenes are singele-chain inosilicates.
- In double-chain inosilicates the silicate tetrahedra are bonded in double chains. Amphiboles are double-chain inosilicates.
- In tectosilicates, also known as framework silicates, all corners of the silicate tetrahedra are bonded to corners of other silicate tetrahedra, forming a complete framework of silicate tetrahedra in all directions. Feldspar, the most common mineral in earth's crust, and quartz are both framework silicates.
SULFIDES--based on the sulfide ion, S2-. Examples include pyrite, FeS2, galena, PbS, and sphalerite, ZnS in its pure zinc form. Some sulfides are mined as sources of such metals as zinc, lead, copper, and tin.
CARBONATES--based on the carbonate ion, (CO3)2-. Calcite, CaCO3, and dolomite, CaMg(CO3)2, are carbonate minerals. Carbonate minerals tend to dissolve relatively easily in water, especially acid water, and natural rain water is slightly acid.
OXIDES--based on the oxygen anion, O2-. Examples include iron oxides such as hematite, Fe2O3 and magnetite, Fe3O4, and pyrolusite, MgO.
HALIDES-- have a halogen element as the anion, whether it be fluoride, F-, chloride, Cl-, bromide, Br-, iodide, I-, or astatide, At-. Halite, NaCl, is a halide mineral.
SULFATES--have the polyatomic sulfate ion, (SO4)2-, as the anion. Anhydrite, CaSO4, is a sulfate.
PHOSPHATES--have the polyatomic phosphate ion, (PO4)3-, as the anion. Fluorapatite, Ca5(PO4)3F, which makes your teeth hard, is a phosphate mineral.
NATIVE ELEMENTS-- made of nothing but a single element. Gold (Au), native copper (Cu), and diamond and graphite, which are made of carbon, are all native element minerals. Recall that a mineral is defined as naturally occurring. Therefore, elements purified and crystallized in a laboratory do not qualify as minerals, unless they have also been found in nature.
Follow this link to the minerals classification table.
For pictures of the common minerals, go the U.S. Geological Survey Web page: http://geomaps.wr.usgs.gov/parks/rxmin/mineral.html
For a discussion of mineral classes and the chemical properties of minerals, go the Wikipedia Web page: http://en.wikipedia.org/wiki/Mineral#Chemical_properties_of_minerals
Created by Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, including figures unless otherwise noted
Unless otherwise specified, this work by Washington State Colleges is licensed under a Creative Commons Attribution 3.0 United States License.