Minerals

This is not a spider web. It is a fractal; a repeating

geometric design

What is a mineral• A mineral is a natural, inorganic, crystalline solid• Ask these questions:

– Is it organic? (is it alive or was it once alive)• Coal is made from dead plants and is organic

– Does it occur naturally ?• Steel is man-made and is not a mineral

– Does it have a crystalline form?• Petroleum occurs naturally and is inorganic but is not a solid so

it has no crystalline form and is not a mineral– Does it have a definite chemical composition?

• Mixtures are not minerals; compounds can be minerals

Types of Minerals• The most common

minerals are rock-forming

• All minerals fall into two categories:– Silicate minerals– Non-silicate minerals

Silicate Minerals• All silicate minerals contain silicon

(Si) and Oxygen (O) in varying combinations– Quartz contains only Si and O

• Feldspar is the most common silicate mineral– Different types of Feldspar are formed

when other elements (K, Na, Ca, etc) are combined with the Si and O

• Feldspar and quartz make up 50% of the earth’s crust

• Silicate minerals make up 96% of the Earth’s crust

Non-silicate Minerals

• Only 4% of the Earth’s crust• 6 groups:

– Sulfides– Sulfates– Oxides– Halides– Carbonates– Native Elements

Crystalline structure• A crystal is a natural solid with a

definite shape– A large mineral crystal will display the

characteristic geometry of its crystalline structure

– Crystals have a specific geometry of atoms that repeats (like a trelis)

– If a mineral is allowed to develop unrestricted then it will form one, huge crystal

Crystal Shapes• Draw table 9.3

Mineral identification• A Mineralogist is a scientist who identifies minerals• Characteristics:

– Color– Luster– Streak– Cleavage/fracture– Hardness– Crystal shape– Density

Color

• This is probably the worst characteristic to classify minerals because it is inconsistent– Some minerals have distinctive colors (Sulfur is yellow,

azurite is blue, serpentine is green)– Many minerals are similar in color– Very small amounts of certain element can greatly affect

the color • Corundum is a colorless mineral made from Aluminum and

Oxygen but with a little bit of chromium it forms a ruby• Sapphire is corundum with traces of cobalt and titanium• Amethyst is quartz that has a purple color because of small

amount of Manganese and Iron

Luster• Light reflected off the

surface of a mineral is called luster– Minerals will either have

metallic luster (shiny) or nonmetallic luster (not as shiny)

– Minerals that have no shine have a dull, Earthy luster

Streak• The color of the mineral

in its powdered form is called the streak– This is fairly reliable for

identification• Metallic minerals have a

dark streak– Ex. Pyrite (fool’s gold) has

a black streak• Nonmetallic minerals

have a streak that is colorless or a very light shade of the mineral’s normal color

Cleavage and Fracture• Cleavage: when a mineral splits evenly along flat

surfaces– Ex. Micas which split in sheets– The cleavage runs parallel to a plane in the crystal

where the bonding is weak– Fracture: when a mineral breaks unevenly

• Curved surfaces are called conchoidal• Fibrous surfaces look like broken wood• Rough surfaces are called uneven or irregular

Hardness• Hardness is the ability of a mineral to resist

scratching– Ex. A diamond is extremely hard but can easily

be split along cleavage planes– The hardness of an unknown mineral can be

determined by scratching it against other minerals of a known hardness on the Mohs hardness scale

– The hardness of the mineral determines the strength of the bonds between the atoms• Graphite and diamonds are made from carbon. Graphite

has no crystal structure so it is not as hard as diamonds

Crystal Shape• A certain mineral

always has the same general shape because the atoms or ions that form its crystals always combine in the same geometric pattern

Density• Density = Mass ÷

Volume• The density depends on

what type of atoms the mineral contains and how closely they are packed together– Most of the common

minerals have densities between 2 and 3 g/cm3

– Heavy metals (gold, uranium, lead) can have densities from 7 to 20 g/cm3

Special Properties• Magnetism

– Magnetite is the most common• Lodestone is a type of magnetite

that acts like a magnet• Double Refraction

– Clear minerals still fracture light that passes through them creating double images

• Radioactivity– Some atoms have unstable

protons and neutrons. This nuclear decay releases energy

• Uranium and Radium are some radioactive elements that occur in minerals

• Pitchblende is the most common uranium containing mineral

Special Properties• Florescence (ability to glow under UV light)

– Calcite is white under normal light but red under UV light

– Florescent minerals absorb UV light and then produce visible light of various colors

• Phosphorescence (ability to glow under UV light even after UV light is shut off)