Start of the notes for the second quiz

Discussion:

Silicates
Other Common Minerals
Rocks
Igneous Rocks

Hawai'i Minerals and Rocks

Silicates

There are approximately 90 naturally-occurring elements in the crust of the earth, so one could expect mineralogy to be a very complex field of study.
However, only about 30 minerals are abundant and commonly encountered.

99% of the weight of Earth's crust is composed of eight elements:

1. O
2. Si
3. Al
4. Fe
5. Mg
6. Ca
7. K
8. Na

O and Si dominate the composition of the crust, constituting about 74 wt% of the crust.
Therefore silicates are the most common minerals in the crust.

Iron is about 30% of the mass of the Earth, however.
Why is Fe a relatively small percentage of the crust?
Most Fe sank to core when Earth formed.

Because of the abundance of silicates, geologists commonly classify minerals as:

1. silicates
2. nonsilicates

The silicate tetrahedron is the basic structural unit of silicate minerals.
The silicate tetrahedron is composed of a silica ion surrounded by 4 oxygen ions.

Ion - an atom or molecule (group of atoms) with a net charge:

• cation - a positively-charged ion
• anion - a negatively-charged ion

SiO4^4- is an anion with a 4- charge (four more electrons than protons).
The silica ion has a 4⁺ charge and each oxygen ion has a 2^- charge for a net total of 4-.
The negatively-charged tetrahedra must be balanced by positive charges for a mineral to remain electrically neutral.

Two ways to remain electrically neutral:

1. bonding with cations
2. sharing oxygens with adjacent tetrahedra

Classification system of silicate minerals - based how the tetrahedrons are linked:

• Isolated tetrahedra
• Single chain
• Double chain
• Sheet
• Framework

Example - Olivine
Tetrahedra share no oxygens.
Tetrahedra are bonded together with cations.

Example - Pyroxene
Tetrahedra in single chains share two oxygens.
Single chains are bonded together with rows of cations.

Example - Amphibole
Tetrahedra in double chains share two or three oxygens.
Double chains are bonded together with rows of cations.

Example - Mica
Sheets of tetrahedra share three oxygens.
Sheets are bonded together with layers of cations.

Examples - Quartz and Feldspar
Tetrahedra share three or four oxygens.

In quartz, all four oxygens are shared, so no cations are required.
The formula for quartz is SiO₂.

Some of the tetrahedra in feldspar share four oxygens, while others share only three.
Therefore feldspar requires cations for the mineral to be electrically neutral.
The formula for feldspar is CaAl₂Si₂0₈.
(Note that feldspar is an Al silicate, where Al replaces Si in some tetrahedra)

The primary cations in silicates are Fe²⁺/Fe³⁺, Mg²⁺, Ca²⁺, K⁺, and Na⁺

Other Common Minerals

There are several other common mineral groups.
Two of the most common in Hawai'i are

1. Carbonates
2. Oxides

Carbonates

Calcite is primary carbonate mineral.
The chemical formula for calcite is CaCO₃ - calcium carbonate.
The basic structural unit of carbonate is CO₃^2-.
The primary occurrences of calcium carbonate in Hawai'i is in coastal regions where limestone and beaches form.
Both the coral and the shells of marine organisms that form reefs and beach sand primarily are composed of calcite.

Iron oxide forms when iron is released during the weathering of basalt (Fe, Mg silicate).
Fe cations bond with O anions to form iron oxide.
The chemical formula for iron oxide is Fe₂O₃.
The basic structural unit of iron oxide is O^2-.
Fe oxide give rocks and sediment in Hawai'i a red color.

Rocks

Rocks - aggregates of minerals.
Rocks can be composed of one mineral only; however most rocks contain several different minerals.

Rocks generally are classified into three groups:

1. Igneous
2. Sedimentary
3. Metamorphic

Igneous rocks form by the cooling and crystallization of magma.
Sedimentary rocks are formed from materials that are weathered and eroded from other rocks.
Metamorphic rocks are formed from the mineralogical and textural reordering of other rocks, when rocks are exposed to high P or T.

Igneous rocks account for the bulk of the rocks that makeup the Hawaiian Islands.
See only minor occurrences of sedimentary and metamorphic rocks.
Most sedimentary rocks in Hawai'i form near the coastline.

Igneous Rocks

Igneous rocks crystallize from magmas that are generated by melting rock deep within the crust or upper mantle.

Two basic classifications for igneous rocks:

1. Intrusive
2. Extrusive

Intrusive igneous rocks are formed within the crust.
Extrusive rocks are extruded onto the surface of the earth.

Intrusive igneous rocks are insulated and cool relatively slowly.
Extrusive igneous rocks cool more quickly.

The different cooling rates result in different rock textures.
Therefore igneous rock textures tell geologists where the rocks formed.

The relatively slow cooling rates of igneous rocks result in fewer, larger crystals;
Large crystals have more time to grow.
Intrusive igneous rocks typically have macroscopic crystals (phaneritic or coarse-grained)

Rapid cooling rates forces relatively more crystals to nucleate at the same time,
The rapid growth of many crystals results in relatively small crystals.
Small crystals have less time to grow.
Extrusive igneous rocks typically have microscopic crystals (aphanitic or fine-grained)

Very rapid cooling rates are too fast for crystals to grow.
Magma that cools very fast or instantaneously results in the formation of volcanic glass. (glassy texture)
Volcano can eject magma into atmosphere, or lava can flow into water.
Volcanic eruptions typically form volcanic glass (obsidian).

Many igneous rocks have two distinct sizes of crystals.
These rocks are called porphyritic.
Porphyritic rocks cool in two stages.
The first is a slow cooling stage in which a few large crystals.
This mixture of a few large crystals in a magma can be erupted.
Once extruded, the remaining magma cools quickly, to form fine-grained rock.

End of the notes for the second quiz