Geochemistry Study Notes

Geochemistry

Definition of Geochemistry

Geochemistry is the study of the chemical composition of the Earth and its rocks and minerals.
Key questions addressed in this unit include:
- What is the composition of the Earth?
- What are minerals?
- How are rocks formed?

Composition of the Earth

Relative Abundance of Elements

The Earth’s composition can be studied in terms of the relative abundance by weight of elements in both the whole Earth and the Earth’s crust.
Differentiation has resulted in a light crust that is depleted in iron but enriched in:
- Oxygen
- Silicon
- Aluminum
- Calcium
- Potassium
- Sodium

Layers of the Earth

The Earth’s interior has distinct layers formed by differentiation, which is defined as the “separation of material by density.”
Key concepts include:
- Dense materials sink, while lighter materials float.
- The properties of layers change with temperature and pressure; deeper layers have higher temperature and pressure.

Earth's Interior Structure

Layers of the Earth

Lithosphere:
- The rigid outer part of the Earth, encompassing the crust and the uppermost part of the mantle.
Asthenosphere:
- The “plastic” upper mantle characterized by semi-molten flowing rock; described as pliable and malleable yet capable of hardening.
Mantle:
- The mostly solid bulk of the Earth’s interior composed of silicate rocks that “flows” on a scale of billions of years.
Outer Core:
- A fluid layer primarily made of molten iron and nickel.
Inner Core:
- A solid sphere of iron and nickel with an estimated temperature of 5700 Kelvin.

Earth's Heat Sources

Reasons for High Temperatures

The heat within the Earth cannot be attributed to the Sun and must originate from other processes, including:

Primordial Heat (≈40%)
- The residual heat from the initial accretion of the Earth and impacts (e.g., the formation of the Moon).
- The Earth is insulated by space, losing heat primarily through radiation, retaining much heat over 4.5 billion years.
Internal Friction (Maximum 10%)
- Caused by differentiation, as materials grind against one another.
- Additional heating occurs from tidal forces exerted by the Moon and Sun.
Nuclear Decay (≈50%)
- The presence of radioactive elements like Uranium and Thorium contributes to ongoing heat generation through the decay of their nuclei.
- Involves a radioactive parent isotope decaying into a daughter isotope, producing energy.

Introduction to Basic Chemistry

Properties of Metals and Non-Metals

Metals

Characteristics:
- High density
- High melting point
- Good conductors of heat and electricity
- Tend to lose electrons, forming positive ions

Non-Metals

Characteristics:
- Low density
- Low melting point
- Poor conductors
- Tend to gain electrons, forming negative ions

Ions and Their Properties

Definition of an Ion

An ion is defined as an atom that has gained or lost electrons, resulting in a net charge.
- Example:
- Lithium (Li) has an atomic number of 3:
  - Neutral: $( ext{Li}_0)$
  - Cation: $( ext{Li}^+)$ (loses 1 electron resulting in a +1 charge)
Mass Number: The total count of neutrons and protons.
Atomic Number: The number of protons that defines the element.
Charge: The difference between the number of protons and electrons.

Examples of Ions

Magnesium: $( ext{Mg}^{2+})$
Gallium: $( ext{Ga}^{3+})$
Oxygen: $( ext{O}^{2-})$
Chlorine: $( ext{Cl}^-)$

Ionic Bonds

Definition

An ionic bond is formed through the positive-negative attraction between ions.
Example: Lithium ions $( ext{Li}^+)$ and Fluoride ions $( ext{F}^-)$

Properties

Ionic bonds lead to the formation of crystalline structures.
Physical behavior:
- Metals change shape upon being struck (malleable) while ionic crystals will shatter (brittle).

Compounds and Their Formation

Ionic Compounds

The process of combining ions involves criss-crossing their charges to form neutral compounds.
Example:
- Sodium and Chlorine: $( ext{Na}^+ + ext{Cl}^- ightarrow ext{NaCl})$
- Magnesium and Chlorine: $( ext{Mg}^{2+} + ext{Cl}^- ightarrow ext{MgCl}_2)$

Naming Compounds

Examples include:
- Gallium Chloride: $( ext{GaCl}_3)$
- Barium Phosphide: $( ext{Ba}3 ext{P}2)$
- Sodium Sulfide: $( ext{Na}_2 ext{S})$
- Magnesium Oxide: $( ext{MgO})$

Minerals

Definition

The Earth’s crust primarily consists of:
- Minerals (individual crystals of the same compound)
- Rocks (aggregates of separate minerals).

Characteristics of Minerals

A substance must meet the following criteria to be classified as a mineral:
1. Solid state
2. Natural occurrence
3. Composed of inorganic material
4. Definite chemical formula
5. Crystalline structure (not derived from living organisms)

Identifying Minerals

Properties

Luster: Refers to how light is reflected on the mineral’s surface.
Streak: The color of the mineral’s powdered form when scraped across a surface.
Form: Minerals exhibit various crystal shapes.
Hardness: Determined through a scratch test, using Mohs scale (1-10, with diamond being 10).
Color: While self-explanatory, the color can vary; does not always align with the crystal color.

Mineral Formation Processes

Methods of Crystal Formation

Evaporation:
- Mineral crystals form from materials dissolved in liquids as the water evaporates.
- Longer evaporation times lead to larger crystals.
- Example: Salt flats are remnants of evaporated ocean water.
Cooling of Molten Material:
- Intrusive Cooling:
  - Magma cools slowly, allowing for the growth of larger crystals.
- Extrusive Cooling:
  - Lava cools rapidly at the surface, forming smaller crystals.
  - Distinction made between lava (molten on the surface) and magma (molten beneath the surface).

Notable Crystallized Minerals

Gypsum crystals from Naica, Mexico exemplify large mineral growth through evaporation.

Mineral Structures

Types of Bonding

Ionic Bonds: Transfer of electrons primarily between metals and non-metals.
Covalent Bonds: Involve sharing of electrons and occur between non-metals.
Bonding impacts how minerals will behave, including their solubility, cleavage properties, and structural stability.

Crystalline vs. Non-Crystalline

Crystalline: Atoms arranged in repeating patterns.
Non-Crystalline/Acrystal: Atoms lack organized pattern, which may result from fast cooling processes.
Example: Quartz ( $ext{SiO}2$ ) is crystalline; glass (also made of $ext{SiO}2$ ) is amorphous.

Crystal Systems

Overview

Each crystal can be characterized by its unit cell—the smallest repeating unit in the crystal structure.
The arrangement affects the overall shape of the crystal, determined by symmetry and dimensions.

Types of Crystal Systems

Cubic: a=b=c and α=β=γ=90°
- Example structures: Pyrite, Galena.
Tetragonal: a=b≠c and α=β=γ=90°
- Example structure: Wulfenite.
Orthorhombic: a≠b≠c and α=β=γ=90°
- Example structure: Topaz.
Hexagonal: Involves unique angles (α=120°).

Polymorphism

Polymorphs refer to materials with the same chemical composition but differing atomic arrangements, leading to different physical properties (e.g., diamond vs. graphite).

Mineral Classification

Groups of Minerals

Minerals can be classified based on their chemical compositions.
- Sulfides: e.g., Pyrite ( $ext{FeS}_2$ ), Galena ( $ext{PbS}$ )
- Oxides: e.g., Hematite ( $ext{Fe}2 ext{O}3$ ), Ice ( $ext{H}_2 ext{O}$ )
- Halides: e.g., Halite ( $ext{NaCl}$ ), Fluorite ( $ext{CaF}_2$ )
Some minerals have fixed compositions (e.g., sulfides), while others show variability in their chemical compositions (e.g., silicates).

Silicate Minerals

Silicate minerals, such as pyroxene and feldspar, display a range of chemical compositions.
The variability is attributed to silicon’s capacity to form complex covalent bonds, resembling carbon in behavior.

Ternary Diagrams

Ternary diagrams are visual representations used to show the relationship and frequency of elements within similar minerals.
They facilitate the understanding of compositional shifts, especially among silicate minerals.

The Rock Cycle

The cycles through which rocks are formed, altered, and transformed, involving processes like weathering, erosion, and sedimentation.