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Minerals

Building Blocks of Rocks and Economic Resources

GLY 2010 - Summer 2015

Lecture 4

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Minerals

• Minerals are a major building block of most rocks

• Their properties determine a good part of the physical behavior of the earth

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Mineral Definition

• Naturally occurring

• Inorganic

• Crystalline

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Crystal Structure Examples

Halite Fluorite

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Formation of Halite

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Atoms

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Atomic Terminology

• The atomic number equals the number of protons. Thus Hydrogen, with one proton, has

atomic number 1, and iron, with twenty-six protons, has atomic number 26.

• The atomic weight equals the number of protons plus the number of neutrons. If an iron atom has 26 protons, and 30

neutrons, it has an atomic weight of 56.

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Chemical Elements

• An element is composed of atoms with the same atomic number

• Each element has a unique chemical symbol

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Isotopes

• An isotope of an element is an atom with the correct number of protons for that element, plus a fixed number of neutrons Example: Carbon has three isotopes, each

with six protons, and with 6, 7, or 8 neutrons

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Stable or Radioactive

• An isotope may be stable or radioactive Carbon isotopes with 6 or 7 neutrons are stable, while the

isotope with 8 neutrons is radioactive

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Chemical Symbols

• Atomic number is shown as a subscript before the element symbol - 1H

• The atomic weight is shown as a superscript before the symbol - 56Fe

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Examples of Chemical Symbols

• Particular isotopes are shown using a superscript in front of the symbol 1H is normal hydrogen, with one proton

and no neutrons 2H is deuterium, with one proton and one

neutron 3H is tritium, with one proton and two

neutrons - it is radioactive

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Ions

• Ions are charged particles

• Cations: Atoms that lose one or more electrons become positively charged

Anions: Atoms that gain one or more electrons are negatively charges Ionic charge: Shown by a superscript after the chemical symbol, O2-

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Use of Isotopes

• Chemical tracers Study topics such as:

• Pollution • Formation temperature • The path of volcanic emissions, etc

• Radioactive isotopes are used in estimating the age of materials

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Compounds

• Combination of two or more atoms

• Combination is called a molecule Water H2O

Carbon dioxide CO2

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Molecules

• Molecules may consist of just one element Oxygen in the atmosphere is O2

• Molecules may consist of several elements, in various amounts Example: Plagioclase feldspar, the most common

mineral on earth NaAlSi3O8 - one sodium (Na), one aluminum (Al), three silicons (Si), and eight oxygens (O)

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Chemical Bonds

• The “glue” that holds materials together

• Responsible for the properties of matter On an atomic scale At the scale of the earth

• When two atoms combine to form a chemical bond, energy is released

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Types of Bonds

• Ionic

• Covalent

• Metallic

• Hydrogen

• Van der Waals

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Ionic Bonds

• Bonds between a cation and an anion

• They occur when a cation donates one or more electrons to an anion

• They are strong

• Dissolve in water Halite, or table salt, for example

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Covalent Bonds

• Equal sharing of electrons by two atoms• Very strong bonds• Compounds usually have limited

solubility• May create molecules that do not readily

combine to form larger particles Ex. Carbon dioxide is strongly bonded

within the molecule, but weakly bonded between molecules, so it is a gas

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Metallic Bonds

• Outer electrons are loosely held

• Properties: Opaque, may have a metallic luster

• Bond strength is moderate

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Hydrogen Bonds

• Secondary bond between oxygen on one water molecule and hydrogen on another

Hydrogen bonding in water

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Van der Waals Bonds

• Residual, extremely weak bonds form by distortion of electron clouds by the presence of a nearby atom

Diamond Graphite

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Molecular Properties

• Molecules - as strong as the weakest bonds within themselves

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Mineral Properties

• Depend on the type and strength of bonds and number of bonds (bond density) within themselves

• Minerals will be examined in the laboratory, and most properties will be taught there

• Examples of mineral properties: hardness, cleavage

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Hardness

• A mineral’s hardness is measured by the ability of a surface to resist abrasion

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Moh’s Scale of Hardness

Any higher number mineral will scratch any lower number. Moh’s scale is not linear – the difference in hardness between low numbers is much less than between high numbers

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Cleavage

• When a mineral always or usually breaks along a particular plane, it is said to have a cleavage plane

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Two-directional Cleavage

• Selenite, a variety of the mineral gypsum, shows cleavage in two directions

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Angle Between Cleavage Planes

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Three-directional Cleavage• Halite, common

table salt, shows three directions of cleavage at right angles

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Three-directional Cleavage• Calcite shows

three directions of cleavage, not at right angles

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Four Directional Cleavage

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Crystal and Crystal Faces

Apatite, showing a hexagonal prism - these are crystal faces, not cleavageplanes

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Identification of Minerals

• Minerals are identified based on their physical and chemical properties

• A combination of properties are needed, just as no single line from a fingerprint can identify a person

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Mineral Classification

• Classification is based on anion type Minerals with the same anions have similar

properties, while those with the same cations often do not

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Anions

• Anions may be a single ion Ex. Oxygen O2-

• Anions are often groups of atoms, with the entire group having a negative charge Ex. Carbonates are CO3

2- , one carbon with three oxygens, and the whole group with a minus 2 charge

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Common Anion Groups

• Silicates, SiO44-

• Oxides, O2-

• Sulfides, S2-

• Carbonates, CO32-

• Phosphates, PO43-

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Occurrence of Minerals

• Over half of all known minerals are silicates, because oxygen is the most common element on earth, and silicon is the second most common.

• Silicates are the most important type of rock-forming minerals, those minerals that make up most of the earth’s rocks

• Most silicate minerals contain other elements in addition to silicon and oxygen

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Silicon Tetrahedron

• The SiO44-

tetrahedron is the basic building block of silicate minerals

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Silicate Structures

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Chemistry of the Continental Crust

• Eight elements account for almost all of the earth’s crust

• Chart is based on weight percent

• Oxygen is the most abundant, and silicon the second, which is why most minerals are silicates

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Felsic Minerals

• Minerals with a lot of aluminum and silicon are light in color, and are called Felsic

• Plagioclase feldspar, the most common mineral in the earth’s crust

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Mafic Minerals

• Minerals with more iron and magnesium, and less silicon, are dark in color and are called Mafic (from the first two letters of magnesium and the first letter of ferium)• Augite, a type of pyroxene