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Chemical Bonding

Atoms are normally grouped in molecules.
Bonding of atoms occurs because of interactions between electrons of neighboring atoms.
Most free atoms have too many or too few electrons.
This results in an atom that is either negatively or positively charged - an ion.
Ionic Bonds result when one atom gives up an electron to another atom - weak bonds.
Covalent Bonds result when two atoms share an electron - very strong bonds.
Metallic Bonds result when electrons are shared by a large number of atoms - electrons are free to move.
Van der Waals Bonds develop because the zone around an atom in which the electrons are found is not spherical - electrical charges develop

Minerals

Mineral - A naturally occurring, inorganic solid with a definite chemical composition and a regular internal crystalline structure.

Naturally Occurring

Inorganic

Solid

Chemical Composition

Some minerals contain only certain elements:
Halite - NaCl
Quartz - SiO₂
Orthoclase - KAlSi₃O₈
Some minerals have a range of compositions:
Olivine - (Mg,Fe)₂SiO₄
Hornblende - (Ca,Na,K)_2-3(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂

Regular Internal Crystalline Structure

Atoms and molecules are arranged in a regular, repeating pattern.
• Example 1
• Example 2
Crystal structures form because the arrangement is the most stable for those atoms or compounds.

Mineral Physical Properties

Physical aspects of a mineral.
Based on chemical composition and crystalline structure.

Physical Properties

Color

Most obvious physical property.
Most deceptive physical property.
Most minerals can have more than one color.
Controlled by chemical composition - different chemicals result in different colors.

Streak

The color of a mineral that is powdered.
Streak color of a mineral shows less variation than the Color.
Most minerals have only one streak color.
All minerals have a streak color.
Controlled by the chemical composition.

Hardness

Ability of a mineral to resist abrasion.
Controlled by the crystalline structure and the type of chemical bonds.
stronger bonds = harder mineral
Mohs Scale of Hardness
Minerals are ranked on a scale of 1 to 10
1 is the softest; 10 is the hardest

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

Cleavage

Tendency of a mineral to break along a plane of weakness within crystalline structure.
Controlled by crystalline structure and chemical bonding.
0, 1, 2, 3, 4, and 6 cleavage planes can exist.
Most difficult physical property for students to understand and identify.

Other Physical Properties

Crystal Form
Fracture
Luster
Specific Gravity - ratio of the mass of a given volume of mineral to the mass of an equal volume of water
Magnetism
Taste
Hydrochloric acid - HCl - Click here to see an short animation (MPG file - 351 Kb)
Radioactivity - Example showing the detection of radioactivity using a Geiger Counter in a sample containing the radioisotope thorium.
Fluorescence

Mineral Systems

Minerals can be grouped together based on similarities in chemical composition and crystalline structure

Polymorphs

Polymorphs are minerals which have a single chemical composition but more than one crystalline structure.
Graphite and Diamond same chemical composition - different crystalline structure.

Pseudomorphs

Pseudomorphs are minerals that take the form of another mineral.
Quartz has a hexagonal crystal.
Quartz will form a cubic crystal if it replaces halite.

Mineral Systems

Minerals can be grouped together based on similarities in chemical composition and crystalline structure.

Silicate Minerals
Nesosilicates
Single Chain Silicates
Double Chain Silicates
Sheet Silicates
Framework Silicates

Non-Silicate Minerals
Carbonates
Sulfates
Sulfides
Oxides
Hydroxides
Halides
Phosphates
Native Elements

Rock Forming Minerals

Augite - Dark or dull green color, 2 cleavages at ~90 degrees, similar properties to Hornblende

Biotite - Black color, one perfect direction of cleavage resulting in the mineral pealing into thin, flexible sheets, similar properties to Muscovite

Calcite - H=3, reacts with HCl, 3 directions of cleavage (rhombic cleavage)

Fluorite - H=4, 4 directions of cleavage, often purple in color (can be white, clear, yellow, green)

Garnet - Typically reddish brown color, no cleavage, commonly found in twelve-sided crystals (dodecahedrons)

Graphite - "Pencil lead", soft metallic mineral, gray streak

Gypsum - H=2, can be scratched with a fingernail

Halite - "Salt", H=2.5, cannot be scratched with a fingernail, 3 directions of cleavage (cubic), salty taste

Hematite - Reddish brown streak, "rust"

Hornblende - Black to dk. green color, 2 directions of cleavage at 120 or 60 degrees, similar properties to Augite

Magnetite - Magnetic, metallic mineral

Muscovite - Clear or translucent color, one perfect direction of cleavage resulting in the mineral pealing into thin, flexible sheets, similar properties to Biotite

Olivine - Apple green or yellowish green color, H=7 (often difficult to determine), conchoidal fracture, no cleavage

Orthoclase - H=6, salmon pink color is typical, perthitic intergrowths are common, 2 directions of cleavage at 90 degrees, similar properties to plagioclase

Plagioclase - H=6, white or gray color, striations may be seen on cleavage surface, 2 directions of cleavage at 90 degrees, similar properties to orthoclase

Pyrite - "Fool's Gold", gold metallic color

Quartz - H=7, conchoidal fracture, no cleavage, color is typically white or clear but can be pink, red, purple, black

 

Mineral Identification

Additional information and identification exercises can be found at:
Mineral Physical Properties and Mineral Identification

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