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Reading: Mineral Basics

INTRODUCTION

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.

WHAT ARE 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.

THE CHEMISTRY OF MINERALS

Why study the chemistry of minerals? Minerals are made of atoms. To understand, explain, and predict the behavior of minerals, and rocks, which are made of minerals, we must understand some basic facts about atoms and how they behave. This requires understanding some chemistry. We will begin by constructing atoms in our thinking in terms of the three sub-atomic particles of which atoms are made.

Atoms

Atoms consist of protons, neutrons, and electrons. Protons have a positive (+) electrical charge. Neutrons are electrically neutral. Electrons have a negative (-) charge that is exactly equal and opposite to the electrical charge of a proton. /

Most of the mass of an atom is packed into its tiny nucleus. An atomic nucleus is made of protons and neutrons. Arranged in specific orbitals around the nucleus of an atom are the electrons. Even though the mass of an electron is a tiny mass compared to the mass of a proton or a neutron, the electrons occupy most of the volume of an atom.

Ions

A neutral atom has the same number of electrons as it does protons. An atom that has lost or gained any electrons is no longer an electrically neutral atom. That type of atom, which is not electrically neutral and has an electrical charge associated with it, is called an ion. Atoms that have gained electrons are negatively (-) charged ions, or anions. Atoms that have lost electrons are positively (+) charged ions, or cations.

It is also possible to have ions that are actually small groups of atoms bonded together. These are known as polyatomic ions. One example of a polyatomic ion is the carbonate ion, (CO3)2-, which has two extra electrons, giving it the net electrical charge of 2-.

The Periodic Table

Naturally occurring atoms found in the earth range from hydrogen, with just one proton in its nucleus, to uranium, with 92 protons in its nucleus. These are the naturally occurring chemical elements, which includes such commonly known elements as carbon, oxygen, iron, and so on. The periodic table lists all the chemical elements in a way that tells us how many protons each of them has, how its electrons are arranged, and what the general chemical behavior of each element is.

Follow this link to a large version of the Periodic Table. The link will open in a new window so you can easily refer to it as you read through this Basics page.

Each chemical element is distinguished by the number of protons in its nucleus. The atomic number of an element tells you how many protons it has. For example, every atom of the element oxygen has eight protons in its nucleus. That is why the atomic number of oxygen is 8. If an atom has greater or fewer than eight protons in its nucleus, it is not oxygen, it is some other chemical element. In the periodic table, the atomic number of each element is listed above the chemical symbol of the element.

The chemical elements in the periodic table form columns that are called groups. All the elements in a group have a similar chemical behavior. That is because all the elements in a group have a similar arrangement of electrons in their atoms, and it is the electron arrangement that determines the chemical behavior of an element.

Isotopes

An atom of a specific chemical element must have in its nucleus the number of protons given by its atomic number. However, there is a range of possible numbers of neutrons it can have in its nucleus. The fact that atoms of a chemical element may have different numbers of neutrons results in each chemical element having several isotopes. Isotopes are atoms of a given chemical element that have different numbers of neutrons in their nuclei.

For example, while all atoms of the element oxygen have eight protons in their nuclei, those oxygen atoms may have eight, nine, or ten neutrons. The different numbers of neutrons in the nucleus distinguishes the three isotopes of oxygen. Oxygen-16 is the isotope of oxygen with 8 neutrons in its nucleus. The number 16 is called the atomic mass number. The atomic mass number is the total number of protons and neutrons in the nucleus of an isotope. From this definition, and knowing that all oxygen atoms have 8 protons in the nucleus, you can deduce that oxygen-17 is the oxygen isotope with 9 neutrons and oxygen-18 is the oxygen isotope with 10 neutrons. Abbreviated into symbols, the three isotopes of oxygen are written as 16O, 17O and 18O.

Isotopes are not very important for understanding minerals, but are important in understanding how to apply chemistry and nuclear physics to geology, such as how to use measurements of radioactive isotopes to measure the ages of rocks and minerals and how to use oxygen isotopes from layers of glacial ice to determine what the temperature of the earth was during an ice age.

Chemical Reactions

Minerals form as a result of chemical reactions. Chemical reactions are driven mainly by the arrangement and re-arrangement of electrons in atoms. In a mineral, the atoms are held together by chemical bonds, which derive from the electrons.

Electrons can be thought of as occupying energy levels, or shells, in an atom. The lowest-energy shell is closest to the nucleus. Each shell can accommodate only a limited number of electrons. The first shell can hold two electrons, the second and third shells can hold eight electrons, and the next two shells can hold eighteen electrons. Unless energy is added to an atom to excite it from its low-energy state, the electrons in the atom will occupy the lowest-energy shells available to them.

Chemical Bonds

If atoms interact with other atoms, they can gain or lose electrons to the other atoms, or share electrons with other atoms. In an individual atom, the most stable arrangement is a full outer shell of electrons. Therefore, chemical reactions will occur, and chemical bonds will form that hold atoms together to each other, when atoms encounter other atoms and change their electron configurations toward more stable, lower-energy arrangements, which generally involves achieving full outer electron shells in the atoms.

This stable configuration—a full outer shell of electrons—is exemplified by the inert gases. In the periodic table the inert gases are the elements of group 18 or VIIIA, the last column on the right. Inert gases do not have to undergo any chemical reactions or form any chemical bonds with other atoms in order to have a full outer shell of electrons. The inert gases already have full outer shells of electrons. That is why they are chemically inert. Their electrons are as stable as can be arranged. For this reason, inert gases are extremely unlikely to undergo any chemical reactions and it is almost impossible for them to bond with any other atoms. Because they do not bond with any other atoms to form a liquid, a solid, a molecule, or a mineral, the inert gases consist of atoms that stay separate from each other, in the gas state.

Individual atoms of all the other chemical elements, when they are neutral atoms, do not have full outer shells of electrons like the inert gases do. Therefore, they do not have the most stable arrangement of electrons that they possibly can. That is why most chemical elements have a strong tendency to either gain or lose electrons, or to enter into other arrangements of their valence electrons, the electrons in their outer shell. Chemical reactions and chemical bonds are generally a result of electrons being rearranged within and among atoms to give the atoms full outer electron shells.

For an atom to lose or gain one electron takes less energy than to lose or gain two, which in turn takes less energy than to lose or gain a third electron. For an individual atom to gain or lose four electrons will only occur in extremely high-energy environments such as in a star. In common chemical reactions on earth, and in the formation of chemical bonds, no element will completely gain or lose four electrons. This limits the charges of atomic cations to +1, +2 or +3 and the charges of atomic anions to –1, –2, or –3.

Reading this far, you have learned about one group of elements in the periodic table, group 18, the inert gases. Another group of chemical elements in the periodic table is the alkali elements. The alkali elements compose group 1 or IA, the left hand column, including the elements sodium (Na) and potassium (K).

Hydrogen is not usually considered as an alkali element because, even though it is shown in group 1 in the periodic table. Hydrogen is so light and small, with just a single proton in its nucleus, that it has some unique behaviors and is considered in a class by itself.

The alkali elements have a single electron in their outer electron shell. If an alkali element loses a single electron, it becomes an ion with a +1 charge and a full outer shell. If an opportunity arises, alkali elements will easily turned into +1 cations.

Ionic Bonds

Now look at group 17 or VIIA in the periodic table, which includes the chemical elements fluorine (F), chlorine (Cl) and so on. These are the halogen elements. If a halogen element gains a single electron, it becomes an ion with a -1 charge and a full outer electron shell. If an opportunity arises, halogen elements have a strong tendency to take in an extra electron and become -1 anions because by doing so they achieve a full outer shell of electrons, which is the most stable arrangement of electrons possible.

If sodium and chlorine atoms get together in the right conditions, such as in an evaporating solution of salt water, each sodium atoms will give up an electron to a chlorine atom. This turns the sodium atoms into sodium ions, Na+, and the chlorine atoms into chloride ions, Cl. Opposite electrical charges attract, so the sodium ions and chloride ions will tend to stick together with each other, joined by what are called ionic bonds.

Not only will the sodium and chloride ions have a very strong tendency to join together with each other via ionic bonds, in most situations they will naturally arrange into a configuration where there is no wasted space and no wasted energy. This leads them to form the crystal lattice of the mineral halite. Halite is a mineral with the chemical formula NaCl, sodium chloride, in which the bonds between the atoms are all ionic bonds.

Look at the diagram of halite showing the sodium and chloride ions arranged into the crystal lattice. All the ionic bonds are at the same angle and the same distance, so they are all of equal strength. This is the lowest-energy arrangement of the ions, the most stable arrangement. If any of the ions were spaced located at different angles or at different distances, there would be extra energy available. This extra energy would drive the ions toward equal angles and distances from each other, until the extra energy is used up and the ions are arranged into their lowest energy state. That is why minerals form, as a natural way for atoms to arrange themselves into the lowest energy state currently available to them.

Covalent Bonds

Some elements, such as carbon (C) and silicon (Si) have a half-full valence shell. (The valence shell is another name for the outer shell, where the most reactive electrons are.) If an element such as carbon were to gain 4 electrons or lose 4 electrons, it would have a full valence shell. However, it is very difficult for an atom to gain or lose four electrons – the energy barrier becomes too strong. Therefore, carbon and silicon, along with a few other elements, tend to form a different type of bond in which they share their outer electrons with other atoms, which in turn share their outer electrons with the carbon (or silicon) atom. The atoms all end up with a full outer shell of electrons, even though some or all of those electrons are being shared with neighboring atoms. This electron sharing keeps the atoms bonded together. This type of chemical bond is called a covalent bond.

It is not uncommon for covalent bonds to be relatively strong. An extreme example can be in diamond. Diamond is a mineral consisting of nothing but carbon atoms, so its chemical formula is simply C. Each carbon atom in the diamond crystal lattice is covalently bonded to – sharing its valence electrons with – four neighboring carbon atoms. A diamond crystal is held together by nothing but extremely strong covalent bonds in all directions, which makes diamond a very hard mineral, the hardest known.

Metallic Bonds

Gold forms a naturally occurring mineral of more or less pure gold, Au, held together by another type of bond, the metallic bond. Metallic elements such as gold and copper, when they bond with other metallic elements, are sharing some of their electrons not just with adjacent atoms, but throughout the whole substance. That is why metallic substances such as copper, gold, and aluminum make such good electrical conductors, because it is so easy to get the “loose” electrons to respond through the whole extent of the metal.

Hydrogen Bonds

Another type of chemical bond that occurs in some minerals is the hydrogen bond. Hydrogen bonds are caused by the positive and negative ends of polar molecules attracting each other strongly enough to hold each other in fixed positions. For example, water molecules can join together through hydrogen bonds to form the mineral known as ice. In a water molecule, H2O, each of the hydrogen atoms forms a covalent bond with the oxygen atom. To form the covalent bonds, each hydrogen atom shares a pair of electrons with the oxygen atom. (This gives all the atoms in the molecule a full outer electron shell.) In the molecule the two hydrogen atoms are bonded to the oxygen atom toward one side of the oxygen atom, rather than at opposite ends. Because the oxygen atom is so electronegative, the pairs of electrons it shares with the hydrogen atoms are skewed toward the nucleus of the oxygen. This leaves a net positive charge on the hydrogen side of the molecule and a net negative charge on the oxygen end of the molecule, resulting in water molecule being a polar molecule, a molecule with a positive end and a negative end. Such molecules can form hydrogen bonds.

If the temperature is low enough, water molecules will move slowly enough to settle into fixed positions relative to each other, held together by hydrogen bonds, forming ice. In a crystal of ice, the water molecules have formed hydrogen bonds by arranging themselves so the negative ends of water molecules face toward positive ends of other water molecules. Ice is a mineral with a six-sided symmetry in its crystal lattice. The hydrogen bonds between water molecules in ice are relatively weak. That is why ice melts at a temperature that is not very high compared to the melting temperatures of minerals held together by ionic or covalent bonds.

Van Der Waals Bonds

There is one other type of chemical bond to consider, a weak type of bond which occurs in some minerals. It is called the van der Waals bond after its discoverer. van der Waals bonds form between parallel sheets or parallel lines of atoms, and are caused by the electric charges in the sheets or lines of atoms fluctuating and responding to the flux—for example, the electrical charge of part of a sheet of atoms may fluctuate and become more negative. In response, this may cause the adjacent sheet to fluctuate toward the positive. There will be an attraction between these positive and negative areas, tending to hold the sheets of atoms together. The electrical attractions shift and fluctuate as the electrical charges shift and fluctuate among the atoms, but even though the electrical attractions do not remain in one place for very long, they are occurring at all times in some parts of the parallel structure. The result is a weak bonding effect, the van der Waals bond.

Graphite is a mineral held together, in part, by van der Waals bonds. Graphite consists of sheets of covalently bonded carbon atoms. Each sheet of carbon atoms is strongly bonded in two dimensions throughout the sheet. But in the third dimension, only weak van der Waals bonds hold the sheets of carbon atoms together. It is very easy to get the sheets of graphite to flake apart from each other, as easy as writing on a piece of paper with a pencil. Under a microscope, you can see that pencil marks are made of many tiny flakes of graphite.

Silicate Minerals and the Silicate Tetrahedron

Most of the minerals in the earth are silicate minerals. The building block of silicate minerals – the essential component that makes them silicate minerals – is the silicate tetrahedron. The silicate tetrahedron consists of four oxygen atoms arranged as close as they can get around a central silicon atom. The result is a pyramidal shape known as a tetrahedron, with an oxygen atom at each of its four apices. (The apices are the points on the tetrahedron where three corners come together.)

The silicon atom by itself has four electrons in its outer shell. In the silicate tetrahedron each of those four electrons is being shared with one of the four attached oxygen atoms. In turn, each oxygen atom is sharing one of the 6 electrons it has in its outer shell.

The result is that the silicon at the center of the tetrahedron has, in effect, a full outer shell with eight electrons in it. Those eight electrons are shared, in pairs, with the four oxygen atoms of the tetrahedron. Each oxygen atom in the tetrahedron, in turn, will have seven electrons in its outer shell – if there is nothing more to the system than the one silicon atom bonded to the four oxygen atoms. This would leave each oxygen atom one electron short of having a full outer shell of electrons.

However, oxygen is a strongly electronegative element, which means that it has the strength to attract electrons from other elements to its nucleus in most situations. In a mineral, each oxygen atom in the silicate tetrahedron will actually have eight electrons: the six electrons that each oxygen atom had in its outer electron shell to begin with, the electron it gained by sharing a pair of electrons in a covalent bond with the silicon atom in the tetrahedron, and one more electron from another atom (or another small group of atoms) in the mineral, outside the tetrahedron.

Silicate tetrahedra are able to bond with many common elements in many different crystal lattice arrangements. In addition, silicate tetrahedra are able to bond with other silicate tetrahedra in a variety of geometric arrangements, including rings, sheets, chains, and three-dimensional networks.

WHY STUDY 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.

What are the Physical Properties of Minerals?

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 colorlusterstreakcleavagefracturehardnesscrystal shape, and selected special properties.

Color

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

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

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.

Cleavage

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.

Fracture

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

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.


Crystal Shape

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.

Special Properties

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 tendency 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.

How to Identify Minerals

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:

  1. Look at it closely on all visible sides to see how it reflects light
  2. Test its hardness
  3. Identify its cleavage or fracture
  4. Name its luster
  5. 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.

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