G 208 Lecture Notes 04/09/2005

• A mineral is:

1) Naturally occurring

2) Inorganic

3) Crystalline- has a regular internal structure (atoms are ordered)

4) Solid

5) Has a definite chemical formula (chemical composition)

• A rock is made up of multiple mineral grains

1) Monomineralic rock (made of 1 type of mineral): rock salt

2) Polymineralic rock ( made of many types of mineral): granite

• How to make minerals:

1) Form bonds

2) Preserve electrical neutrally

3) Fit ions in 3-D structure

• Polymorphs

- Minerals with the same chemical formula but different internal atomic structures

- Diamond and Graphite- two polymorphs of carbon

1) Diamond is strong

2) Graphite is weak

• Minerals in nature are never pure

- Other elements can substitute into a site as long as 2 things are satisfied

a) Size of substituting atom fits the size of the site

b) Charge of the substituting atom fits the charge of the sites

· Si-tertrahedron

- Basic building block of silicate minerals

• Major rock-forming minerals

1) Feldspars- most abundant minerals on the Earth’s surface (50% of overall crust)

a) Plagioclase feldspars: complete solid solution

b) Alkali feldspars: partial solid solution

2) Quartz- abundant in continental rocks

a) Framework silicate

b) Can accommodate only small amounts of other elements

c) Low temperature, low pressure form of SiO₂

3) Muscovite- “white mica”

4) Biotite- “black mica”

a) Both are sheet silicates

b) Both contain OH_1- (hydrol anion)

c) From the presence of water

5) Amphiboles (ex: hornblende)

a) Double- chain silicates

b) Contain OH_1-

6) Pyroxenes- common in oceanic crust and many continental rocks

a) Single-chain silicates

7) Olivine- only common in certain igneous rocks; but the main mineral in upper mantle

a) Isolated tetrahedral silicate

b) Usually pale green (gem quality= peridot)

• Mineral properties

- Properties depend on crystal structures

- Minerals have well-defined chemical composition but different minerals can have the same composition (polymorphs)

- Minerals have well-defined properties: luster, hardness, cleavage, streak, and specific gravity

1) Luster- how a substance reflects light

a) Metallic vs. Nonmetallic

2) Hardness- resistance to scratching

a) Moh’s scale (H)

3) Cleavage- tendency for minerals to break along flat surface

a) Cleavage surfaces reflect light back towards on direction (so a good cleavage surface will look shinier than a broken/ fractured surface)

b) Quartz: crystal growth faces (framework structure) not cleavage

4) Streak- color of mineral after powdered

5) Specific Gravity- measure of density

a) S.G.= weight substance/ weight water

6) Magnetism- magnetic attracts a magnet

7) Reaction to acid- calcite “fizzes” in dilute HCl acid

8) Striations- often shown by feldspars

9) Iridescence (“play of colors”)- often shown by plagioclase feldspar

• Igneous Rocks

- Rocks that formed by the solidification of magma (molten rock)

• Lava

- Magma that reaches the Earth’s surface

- Liquid vs. amorphous solid vs. crystalline solid

• Magma

- Molten rock material from Earth’s interior

• Classification of Igneous Rocks

1) Minerals can crystallize from magma underground

a) Plutonic or Intrusive

b) Texture: Phaneritic (coarse- grained because of slow cooling)

2) Mineral can crystallize from magma aboveground

a) Volcanic or Extrusive

b) Texture: Aphanitic (fine-grained because of fast cooling)

3) Minerals can begin to crystallize from magma at depth, and then magma can move higher, where more minerals crystallize

a) Texture: Porphyritic (2 grain sizes)

4) Magma can solidify without growing any minerals

a) Glassy texture: obsidian

• Crystalline igneous rocks

- Classified on the basis of their texture and chemistry

• Phaneritic

- Coarser grained

• Aphanitic

- Finer grained

• Frothy (generally pyroclastic)

- Pumice (felsic to intermediate): light color

- Scoria (mafic): red, usually black in color