MINERALS

Mineralogy is the science or study of minerals--minerals can be used as gemstones and sources of chemical elements--since minerals are chemical substances it would be prudent to initiate a study or review of some concepts in chemistry to aid in understanding the nature of minerals

I. Atoms, elements, and ions

A. Atom

is the smallest unit of all substances and comprised of protons (plus charge) and neutrons (no charge) located in the nucleus (center) of atom, and electrons (minus charge) which orbit the nucleus
the structure of an atom resembles the solar system--the protons and neutrons are located at the center of the nucleus as the sun is in the solar system and the electrons orbit the nucleus similar to the planets which orbit the sun
click here to see the atom model

B. Element

1. Definition

· is a substance comprised of atoms, each atom with the same number of protons but not necessarily the same number of neutrons or electrons

· there are many different elements (see periodic table of elements)

2. Atomic number of an element

· is the common number of protons in the nucleus of each atom of the element

· examples: (see periodic table) there is 1 proton in each atom of the element hydrogen (H) and 92 for the same in uranium (U)

3. Atomic mass of an element

· is the sum of the number of protons and neutrons in the nucleus of each atom of an element

· an oxygen atom with an atomic mass of 16 has 8 neutrons and 8 protons in its nucleus and that for uranium with an atomic mass of 235 has 92 protons and 143 neutrons

· the aforementioned atoms are given a designation of O16 and U235, respectively

4. Isotope of an element

· atoms of an element which have the same atomic mass belong to the same element isotope

· stable isotopes are atoms which are not radioactive which means they do not break down or decay to form atoms of other elements

· examples of stable isotopes are O16, O17, and H1

· unstable isotopes are atoms which are radioactive and decay to form atoms of other elements

· examples of unstable isotopes are U238, C14, or any isotope which has an atomic number greater than that for bismuth (Bi)

· U238 decays or breaks down in a series to form Pb206 which is stable while C14 decays to stable N14

5. Atomic weight of an element

· is the average of the atomic masses of all the stable and radioactive isotopes of an element

6. Periodic table of elements---see page 41 in text

· is a listing of the elements in order of increasing atomic number left to right and top to bottom

C. Ion

is a charged atom of an element caused by the atom having an unequal number of protons and electrons

1. Cation

· is a (+) plus charged atom resulting from the number of protons exceeding the number of electrons

· if a sodium atom with its 11 protons has lost an electron from the original 11 it then has only 10 electrons, and becomes a cation because it has more + than -

2. Anion

· is a (-) minus charged atom caused by the number of electrons exceeding the number of protons

· if a chlorine atom with its 17 protons has acquired an extra electron from the original 17 then having 18 electrons, it becomes an anion because it has more - than +

click here to see more on atoms, elements, etc.

II. Chemical bonds

are forces which hold atoms together
type of bond between atoms can determine mineral properties
there are 3 main types of bonds in minerals

A. Ionic bond

is an attraction force caused by a transfer of electron(s) from one atom to another
an example is atoms in halite (NaCl); each sodium (Na) atom transfers one electron to a chlorine atom
this causes Na atoms to become plus charged and chlorine atoms to become minus charged thus attracting each other
click here to see the ionic bond

B. Covalent bond

is a force caused by a sharing of electron(s) between atoms
an example is atoms of graphite (C); each atom of carbon shares electrons with another carbon atom
since one C atom has all of the shared electrons at one instance it is minus charged while the other atom is deficient in electrons and is plus charged
click here to see the covalent bond

C. Metallic bond

is a force caused by electrons moving freely between atoms
an example is electrons of iron (Fe) atoms moving between iron atoms
this flow of electrons results in good electrical conduction

often a bond between atoms is a mixture of more than one bond type

III. Minerals

A. Definition and explanation

1. naturally occurring

2. inorganic solid substance

3. having an orderly arrangement of atoms

4. having a consistent chemical composition that may vary within specific limits

man made diamonds are not naturally occurring therefore not minerals
natural petroleum and coal are referred to as mineral resources but are not minerals because they are organic
ice is a mineral if formed in nature
a mineraloid is a substance which conforms with 1, 2, and 4 above but does not have an orderly arrangement of its atoms
differences between minerals and rocks are:

minerals can have crystal form but rocks cannot (any mineral can form a crystal shape if it has adequate space in which to grow)
minerals occur in smaller or limited amounts while rocks occur in large massive concentrations

B. Origin and occurrence of minerals

1. Form during the rock forming process and comprise an essential part of the rock

2. Formed in cracks, fractures, or holes within the rock after formation of the rock

C. Mineral classification

minerals are classified into classes (groups) based on chemical composition
based primarily on the anion type

1. Non-silicate minerals

· oxides-oxygen (O) is major anion with examples as hematite (Fe2O3), and magnetite (Fe3O4)

· sulfides-sulfur (S) is major anion with examples as galena (PbS), pyrite (FeS), and sphalerite (ZnS)

· sulphates-sulfur (S) and oxygen (O) together form the major (complex) anion with examples as barite (BaSO4) and gypsum (CaSO4.2H2O)

· native elements-atoms of one element with examples as sulfur (S), and graphite (C)

· halides-Cl, Br, F or I is major anion with examples as halite (NaCl) and fluorite (CaF2)

· carbonates-carbon (C) and oxygen (O) together form the major (complex) anion with examples as calcite (CaCO3) and dolomite-CaMg(CO)2

· click here to see the nonsilicate mineral groups (classes) or page 56 in text

2. Silicate minerals

· silicon (Si) and oxygen (O) together form the major (complex) anion

· the single most abundant class (group) of mineral

· all igneous rocks, most sedimentary rocks and all but one metamorphic rock are comprised of silicate mineral aggregates

· the tetrahedron is the basic "building block" of the silicate mineral structure-- individual tetrahedral---see page 49 in text can comprise the structure of some minerals as olivine, or tetrahedra are directly connected or shared in other minerals--this configuration of tetrahedra can explain cleavage in minerals

· click here to see the silicate structures or page 52 in text

C. Identification of minerals by name

1. Use of physical properties

· color---color may vary from one specimen of a mineral to another hence is not a very reliable property to help in mineral identification

· luster---a mineral with a metallic luster will have a black or blackish colored powdered streak on a porcelain streak plate---that for a mineral with nonmetallic luster will have a light colored and mostly a non-prominent streak

· streak color---see luster above

· hardness---a streak plate, glass, copper penny, and fingernail are hardness tools which can be used to test mineral hardness---also a set of minerals called Moh's relative hardness set can be used to specify more accurately the relative hardness of a mineral

· cleavage and fracture---the ability of a mineral to break apart in a consistent way (cleavage) or inability of a mineral to do so (fracture or to shatter) can aid in mineral identification

· other special characteristics---the taste of halite (salty) and the smell of powdered sphalerite or sulfur (rotten eggs or sulfur) on a streak plate are examples

2. Use of chemical analysis

· the element composition of a mineral gives important information in the determination of the name of a mineral

3. Use of xray analysis

· the atomic arrangement in a mineral is like a genetic code for that mineral and X-ray analysis can determine that code

D. Radioactivity in minerals

needs to be present for the determination of the absolute age of a rock
some atoms of elements comprising the chemical makeup of minerals in rocks are radioactive

1. Radiation particles in minerals—see page 44 in text

· when atoms of elements decay to form other elements, particles are emitted

· most common are alpha particles ( plus charged), beta particles (minus charged), and gamma particles (no charge)

2. Radioactive parent and daughter elements

· parent atoms are radioactive and decay eventually to stable atoms of an element called a daughter

· examples of radioactive parent and daughter are: U238 and Pb206; K40 and Ar40; C14 and N14

3. Absolute age determination method

· the amounts (number of atoms) of parent and daughter are determined by a mass spectrograph. Based on the concentrations of each the age of the rock can be determined

· the daughter present must represent the amounts produced since the rock formed

· many sedimentary rocks are comprised of accumulated particles from previously formed rocks and the amounts of parent and daughter represent the time of formation of the previous rock

· most absolute age determinations are performed with igneous rocks

E. Important uses of minerals

1. gemstones

· a rare variety of a mineral

· possesses a quality of beauty, and is usually hard

· rubies (red) and sapphires (blue) are gemstone varieties of corundum

· emeralds (green) and aqua marine are gemstones of beryl

· amethyst and cat's eye are gemstones of quartz

2. Elements extracted from minerals

· barium (Ba) from barite (BaSO4)

· iron (Fe) from hematite (Fe2O3)

· lead (Pb) from galena (PbS)