MINERALS

     A mineral is a naturally occurring inorganic solid substance which has a definite chemical composition (within certain limits), a specific internal arrangement of atoms and/or molecules (i.e., a particular internal crystalline structure), and characteristic physical properties.  Most minerals are a combination of two or more elements; however, some minerals are composed of only one element.  For example, diamond is composed entirely of carbon.  Graphite is another mineral that is composed entirely of carbon, but in graphite the atoms of carbon are arranged in a different manner; thus diamond and graphite have quite different and distinct physical characteristics.  Some naturally occurring, inorganic solid substances have specific chemical compositions and characteristic physical properties, but lack a definite crystalline structure (they have an amorphous structure).  These substances are referred to as mineraloids.  Opal (silicon dioxide with a variable amount of water) and limonite (iron hydroxide) are examples of mineraloids.  Sometimes fossil fuels (coal, crude oil, and natural gas) are referred to as mineral resources, although according to the definition above they are not minerals.  Fossil fuels are of organic origin, being formed from the remains of past organisms.  Also, mineral supplements that are added to vitamins are not true minerals, they are generally chemical elements, like iron, calcium, magnesium, etc. (but most of these elements are obtained from true minerals).

     Minerals generally form by precipitation (crystallization) from a fluid.  The fluid may be hot molten magma or lava, as the magma or lava cools, the minerals form by crystallization from the liquid phase.  The fluid may also be water or brine.  For example, if seawater becomes restricted from the ocean (such as in an inland sea or behind a barrier reef) and if the climate is arid (if evaporation is greater than rainfall), salt (the mineral halite, NaCl) will crystallize from the seawater.

     Although there are over 4000 minerals known, most common rock forming and ore minerals can be easily identified by the beginning student using simple physical (check out his web site for physical characteristics of minerals: http://mineral.galleries.com/minerals/physical.htm, Mineral Gallery - The Physical Properties of Minerals ) and/or chemical tests.  However, for some minerals it is necessary to determine the internal atomic structure by x-ray diffraction studies for positive identification.
 

ATOMIC STRUCTURE

     All matter is composed of elements (here is a site that contains an interactive periodic table of the elements: A Periodic Table of the Elements at Los Alamos National Laboratory ).  Sometimes matter consists of atoms of a single element (gold, for example).  In other cases, atoms of two or more elements combine to form a compound, such as water (H₂O) or salt (NaCl).  In a few cases, atoms of the same element combine to form molecules of the element, for example the diatomic form of gases such as oxygen (O₂), nitrogen (N₂), and hydrogen (H₂).  However, most compounds (and also minerals) are a combination of two or more elements.  Compounds (and thus minerals) do not have the same physical and chemical properties as the elements which make up the compound.  For example, the mineral halite (table salt) is composed of atoms of sodium (Na) and chlorine (Cl).  The atoms are bonded together to form the compound sodium chloride.  Sodium is a white, powdery metal that will react violently with water, whereas chlorine is a greenish colored poisonous gas (Chlorine was used as a chemical weapon in World War I).  However, table salt (sodium chloride) is essential for life.

     Most of the minerals in the rocks of the Earth's crust are silicate minerals, primarily composed of some combination of only eight elements (Table 1-1).  Oxygen and silicon are by far the most abundant elements by weight percent in rocks of the Earth's crust (see Table 1-1).

     Because all matter is composed of elements (in most cases combined to form compounds), all matter is composed of atoms. An atom is the smallest particle of an element that still retains the characteristics of a particular element.  In the model atom (Figure 1-1),  the   dense   nucleus  contains positively charged (electrically) particles called protons and neutrally charged particles called neutrons (try this web site:  All About Atoms ).  The mass of a proton and a neutron is about equal and most of the mass of an atom is the combined mass of the protons and neutrons contained in the nucleus.  Orbiting around the nucleus are negatively charged particles called electrons, with a mass of about 1/1800^th that of a proton or neutron.  The number of protons that an atom contains is the atomic number of the element and is the basis for what the element is named.  For example, hydrogen  (H) has one proton, helium (He) has two protons, lithium (Li) has three protons, etc.  The atomic mass number is the number of protons plus the number of neutrons contained in the nucleus of an atom.  Normal hydrogen has only a single proton in its nucleus, thus its atomic mass number is the same as its atomic number (namely, one), but in the atoms of the most common form of calcium there are 20 protons and 20 neutrons in the nucleus, so the atomic mass number of calcium is 40 a.m.u (a.m.u = atomic mass units; each proton or neutron has a mass of approximately 1 a.m.u.).

Figure 1-1.  Two dimensional model of an atom.  This example has 8 protons, 8 neutrons, and 8 electrons; and is the atomic structure of the most common isotope of oxygen (take a look at these interactive models of atoms  http://www.colorado.edu/physics/2000/periodic_table/index.html ).

     Some elements have atoms with a variable number of neutrons in the nucleus, thus they have differing atomic mass numbers.  These different forms of the same element are referred to as isotopes of that particular element.  For example, oxygen (O) has three natural isotopes, oxygen with 8 neutrons (₈O¹⁶), oxygen with 9 neutrons (₈O¹⁷), and oxygen with 10 neutrons (₈O¹⁸).  However, all oxygen atoms have eight protons.

     Electrons are arranged in energy levels (shells) around the nucleus of an atom.  The first energy shell can hold a maximum of 2 electrons, the second energy shell can hold a maximum of 8 electrons, the third shell can hold 18 electrons, and the fourth shell 32 electrons.  The general formula for the maximum number of electrons in a major energy (quantum) shell is 2N², where N is the number of the shell outward from the nucleus.  The outermost electrons, called the valence electrons, are the electrons that are responsible for bonding one atom to another.  All atoms try to put either 2 electrons (only for hydrogen, helium, lithium, beryllium, and boron) or 8 electrons (all other atoms) in their valence shell.  All atoms try to attain the electron configuration of the stable noble gases.  The octet rule states that atoms will attempt to become more stable when reacting with other atoms chemically by putting eight electrons in their outermost (valence) shell.  This is accomplished by a transfer of electrons from one atom to another or by the sharing of electrons between atoms.
 

BONDING OF ATOMS TO FORM COMPOUNDS

     The valence electrons in an atom are involved in the bonding of atoms together to form compounds.   If an atom (like Cl) captures an electron from another atom (like Na) to complete its octet, then the atom capturing the electron has an excess negative charge and becomes a negative ion (an ion is a charged atom or radical).  The atom that loses electrons would have a deficiency of electrons, and would become a positive ion. Negative ions are called anions and positive ions are called cations.  Thus, for example, when chlorine captures an electron from sodium, chlorine becomes a negative chloride ion (Cl^-1) and sodium becomes a positive sodium ion (Na⁺¹).  Oppositely charge ions are attracted to each other and may bond together to form an ionic bond.  Some atoms do not readily give up their electrons, but may share their valence electrons with another atom, thus becoming bonded to the other atom by a covalent bond.
 

MINERAL STRUCTURES

     Minerals are composed of elements bonded together into a crystalline structure.  A crystalline structure has a repeating pattern consisting of the atoms which make it up.  The pattern may be cubic, as in Halite (NaCl) (Figure 1-2), single or double tetrahedral chains, as in some silicate minerals, etc.  Sometimes two or more minerals have the same chemical composition, but different crystalline structures.  This will result in the minerals having different physical characteristics.  Such minerals are called polymorphic minerals.  Calcite and aragonite are examples of carbonate minerals that both have the composition calcium carbonate (CaCO₃), thus they are polymorphic forms of calcium carbonate.  As previously mentioned, diamond and graphite are other examples of polymorphic minerals.
 

MINERAL GROUPS

     Minerals are placed into major groups according to their elemental composition and crystalline structure.  More specifically, minerals groups are named according to the common negative ion (anion) that occurs in the group.  For example, sulfides (like pyrite  [FeS₂] and galena [PbS]) contain the sulfide anion (S^-2) and carbonates [such as  calcite  (CaCO₃)  and  dolomite
[Ca, Mg (CO₃)₂] contain the carbonate anion (CO₃^-2).

     Over 4000 minerals have been identified in the Earth's crust and new ones are being discovered fairly often.  However, only a few dozen are important as mineral resources and as rock forming minerals.  The most abundant rock forming minerals are the silicates.  Silicate minerals are primarily composed of oxygen and silicon, with other elements included into the crystalline structure to produce electrical neutrality.  The silicon-oxygen tetrahedron (refer to your textbook) is the basic building block of the silicate minerals.  The silicon-oxygen tetrahedron consists of four oxygen atoms covalently bonded to a centrally located silicon atom.  Some silicate minerals consist of single tetrahedra (plural of tetrahedron) linked with other elements [for example olivine, (Mg,Fe)₂SiO₄ ].  The silicon-oxygen tetrahedron can also be linked with other silicon-oxygen tetrahedra by sharing oxygen atoms to form single chains, double chains, sheet structures, or complex three dimensional networks.  For example, the mica minerals and the clay minerals (hydrous aluminum silicates) are silicate minerals with a sheet silicate structure; positive ions (cations) are sandwiched between the silicate sheets to produce electrical neutrality.  On the other hand, quartz (SiO₂) has a three dimensional network of linked silicon-oxygen tetrahedra.

     The silicates are the most important rock forming minerals.  All igneous rocks are composed of silicate minerals.  Many metamorphic and sedimentary rocks consist primarily of silicate minerals.  Other important rock forming minerals are the carbonates (particularly calcite and dolomite); the sulfate minerals, gypsum (CaSO₄ .2H₂O) and anhydrite (CaSO₄); and the halide mineral, halite.  Several of the other mineral groups, although less common, are important for their economic value.  The oxides, hematite (Fe₂O₃) and magnetite (Fe₃O₄), are important ores of iron; and the sulfides, galena (PbS), sphalerite (ZnS), and chalcopyrite (CuFeS₂), are important ores of lead, zinc, and copper, respectively.  Of the halides, halite (NaCl) is mined for use as salt and fluorite (CaF₂) is used as a flux in the steel industry, in chemicals, and in ceramics.  The native elements (which are also minerals) gold (Au), silver (Ag), copper (Cu), diamond (C), sulfur (S), and graphite (C) are important for their intrinsic value and/or industrial uses.  Many other minerals are of economic value.

Figure 1-2.  The crystal lattice structure of the the mineral halite (NaCl).  The lines drawn between the ions represent the ionic bonds.  Halite has a cubic crystalline structure and also has cubic cleavage, with 3 directions at 90^o to each other.  Note that each Cl^-1 ion is surrounded by Na⁺¹ ions and each Na⁺¹ ion is surrounded by Cl^-1 ions.

MINERAL IDENTIFICATION

     A beginner can shortly learn to identify the common rock-forming and industrial minerals by a few simple observations and tests of physical and chemical characteristics.  The following are some common physical and chemical characteristics that can be used by the student to identify the common minerals:

COLOR - Color is often a diagnostic property for some minerals.  However, color should never be used as the major identifying characteristic.  Many minerals may vary in color due to impurities in the chemical composition.  For example, the mineral quartz commonly occurs as several different colors; such as rose, milky white, smoky gray, yellowish, transparent, and purple.

CRYSTAL FORM - The crystal form of a mineral is the external reflection of the internal crystalline structure.  The shape of the crystals are diagnostic for many minerals and can be used as an aid in identification.  However, in many cases (particularly in rock samples), large crystals are not developed.  It is often necessary to use a hand lens or low power microscope for magnification to observe the crystal form of minerals.  In some cases, the mineral specimen is microcrystalline, thus not allowing the observation of crystal form.  In addition, when rocks form, minerals compete for space.  Large crystals with all the crystal faces well developed (euhedral crystals) are rare.  Typically, only some of the crystal faces for a particular mineral grain will be evident (a subhedral condition).  Sometimes the crystals are so crowded together in a rock that no natural crystal faces are evident in a mineral grain (an anhedral condition).  There are several crystal classes that minerals may belong to; however, the beginning student need not worry about these.  Although, it is helpful to learn the crystal habit of some of the common minerals.  For example, halite (NaCl) and fluorite (CaF₂) typically display a cubic crystal form; whereas, quartz (SiO₂) forms six-sided prisms terminated with six-sided pyramids (Figure 1-3).

Figure 1-3.  Prismatic crystals of quartz (photos by E. L. Crisp).

HARDNESS - The hardness of a mineral is its resistance to abrasion or scratching.  Ten minerals have been chosen as a scale for relative hardness tests.  This scale is called Mohs scale of hardness.  The minerals in Mohs scale are numbered in order of increasing hardness as follows:
 

MOHS SCALE OF HARDNESS

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

     If the minerals on Mohs Scale are not available for comparison, a little ingenuity on the part of the student will suffice to determine the approximate Mohs scale hardness by comparison with some common objects:

Fingernail .............................2.0 to 2.5
Copper Penny.......................2.5 to 3.5
Knife Blade..........................5.0 to 5.5
Steel Nail.............................5.0 to 5.5
Glass Plate...........................5.5
Steel File..............................6.5 to 7.0
 

     In addition, once an unknown mineral has been identified so that its hardness is then known, that mineral can be used to compare its hardness with an unknown mineral.

CLEAVAGE - Cleavage is the tendency of a mineral to split along planes of weakness in the crystalline structure.  If these  planes of weakness are present in a particular mineral, the mineral will tend to break repeatedly along these planes.  Some minerals have better cleavage in one direction than another direction.  For example, selenite gypsum has perfect cleavage (very smooth clean break mostly on one plane) in one direction, but poor (imperfect, very stair-stepped cleavage) cleavage in another direction.  A mineral may have one, two, three, four, or six directions of cleavage (GO TO THIS SITE TO SEE EXAMPLES OF DIFFERENT DIRECTIONS OF CLEAVAGE -  Mineral cleavage ).  The number of cleavage directions and the angular relationship between different cleavage planes is characteristic for certain minerals. For example,the mineral fluorite (Figure 1-4) has perfect cleavage in four directions. This results in octahedral cleavage.  However, keep in mind that fluorite crystals are cubic.

Figure 1.4. Fluorite, showing very good (almost perfect) four-directional octahedral cleavage (photos by E.L. Crisp).

     In some minerals the chemical bonding is equally strong in all directions.  Such minerals will not show cleavage when broken, but will form an even or uneven surface referred to as a fracture.  A fracture will not reflect light in any particular direction, as does cleavage, but will scatter the light in many directions.  However, some types of fractures are characteristic for particular minerals.  As an example, a conchoidal  fracture is often characteristic for quartz.
 

TENACITY - Tenacity is defined as the resistance of a mineral to breakage.  The
                            following terms are used to describe a minerals tenacity:

     Brittle - Breaks or powders easily.  Examples are quartz, sulfur, and
                     pyrite.

     Flexible - Will bend easily, but will not return to its original shape.
                         Example, selenite gypsum.

     Elastic - Will bend easily and will return to its original shape when
                       the stress is released.  Examples. biotite and muscovite
                       mica.

     Sectile - Can be cut into shavings with a knife.  Examples, talc and
                        galena.

     Malleable - Can be pounded into thin sheets or otherwise modified in
                             shape without breakage.  Examples, copper, gold, and silver.
 
 

STREAK -  The powdered form of a mineral usually has a more characteristic color than an unpowdered specimen.  The streak of a mineral is the color produc    ed when the mineral is powdered, usually by rubbing the mineral across an unglazed porcelain plate or any other fine, hard, abrasive surface.  Streak is most useful for minerals with metallic lusters.  Minerals with nonmetallic lusters generally give a white or pastel streak.  Minerals with a hardness greater than glass (about the same hardness as porcelain) will scratch the porcelain plate.

LUSTER - Luster refers to the manner in which a mineral reflects light.  There are two major types of luster, metallic and nonmetallic.  A mineral with a metallic luster has the appearance of a metal, a mineral with a nonmetallic luster does not.  Galena and pyrite are examples of minerals with metallic lusters.  Several adjectives are used to describe the luster of nonm   'etallic minerals:

     Vitreous - having the appearance of glass.  Example, quartz.
     Adamantine - a very bright luster.  Example, diamond.
     Pearly - having the appeararance of pearl.  Example, feldspar.
     Resinous or Waxy - having the appearance of resin or wax.  Example, Sphalerite.
     Silky - having the sheen of silk.
     Greasy - an oily or greasy appearance.
     Earthy - having a dull appearance due to no or little reflected light.
                  Example, the clay mineral kaolinite.
 

SPECIFIC GRAVITY

     Specific gravity is one of the most constant physical properties of minerals and can be of much aid in mineral identification.  Specific gravity is defined as the mass of a mineral divided by the mass of an equal volume of water:

SPECIFIC GRAVITY = MASS OF SAMPLE DIVIDED BY MASS OF AN EQUAL VOLUME OF WATER

However, for general purposes in mineral identification, the S.G. can be estimated by lifting and hefting a mineral specimen.  The common rock forming minerals (such as quartz, the feldspars, and calcite) are examples of minerals with relatively low S.G., in the range between 2.6 and 2.8.  Galena, with a S.G. of 7.5, is an example of a mineral with a high S.G.
 

DIAPHANEITY

     The manner in which a mineral transmits light is referred to as diaphaneity.  A mineral may be transparent (can see an image through the specimen), translucent (light travels through the specimen, but no image is formed), or opaque (no light will pass through the specimen).  S   ?ome minerals can be either transparent, translucent, or opaque; quartz and calcite are examples.
 

ACID TEST

     Calcite (CaCO₃), one of the more common minerals in nature, will effervesce strongly when treated with cold, dilute hydrochloric acid.  The effervescence is caused by the release of carbon dioxide gas (CO₂) during the reaction of the acid with the mineral:

                CaCO₃ + 2 HCl <------> CaCl₂ + H₂O + CO₂

     By this simple test, calcite can be distinguished from most of the other common minerals.  Dolomite [Ca,Mg(CO₃)₂], another carbonate mineral, will also react (fizz) with hydrochloric acid, but only in powdered form (by scratching-up powder on the surface and applying the the acid to the powder).
 

MAGNETISM

     Some minerals containing iron can be magnetic or will be attracted to a magnet.  Magnetite (Fe₃O₄) is probably the only common mineral that the begin   Rning student will encounter that will be either magnetic or attracted to a magnet.  Specimens of magnetite that are magnetic are commonly called lodestone.  However, some samples of magnetite are not themselves magnetic, but will be attracted to a magnet.  Other common iron containing minerals, that the student may encounter, are not magnetic and will not be attracted to a magnet.  Examples of nonmagnetic iron minerals are pyrite (FeS₂), hematite (Fe₂O₃), and the mineraloid, limonite (Fe₂O₃.nH₂O).  So if a dark, metallic mineral is attracted to a magnet, it is most likely magnetite.