Mineralogy and Crystallography: Key Concepts and Properties

Mineralogy and Crystallography

1. Define Polymorphism

Polymorphism refers to the phenomenon where the same chemical substance can exhibit different physical and even chemical properties due to variations in formation conditions, such as pressure and temperature. These variations lead to different arrangements of atoms within the crystal structure.

2. Define Isomorphism

Isomorphism occurs when two or more chemical compounds adopt the same crystalline structure.

3. Define Pseudomorphs

Pseudomorphs are minerals where the internal structure does not correspond to the external crystal form, resulting in a "false form."

4. Define Solid Solution and Mixing

• Solid solution: A homogeneous solid with a variable composition.
• Mixing: A type of solid solution where two elements are fully interchangeable at elevated temperatures but not at lower temperatures. The structure must unfold as it cools.

5. Mineral Classification

Minerals are classified into the following crystallographic groups:

1. Native Elements
2. Sulfates
3. Sulfides
4. Nitrates
5. Phosphates
6. Halides
7. Oxides and Hydroxides
8. Silicates
9. Carbonates
10. Tungstates

6. Define and Explain the Mohs Hardness Scale

Hardness is the resistance a mineral exhibits to being scratched or penetrated.

The Mohs Hardness Scale is used to measure a mineral's relative hardness:

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

7. Define Toughness, Malleability, Sectility, Elasticity, and Flexibility

• Toughness: A mineral's resistance to breaking, crushing, bending, or tearing. It is a measure of cohesion.
• Malleability: The ability of a mineral to be hammered into thin sheets due to high cohesion.
• Sectility: The ability of a mineral to be cut with a knife without powdering, due to high cohesion. An example is cerargyrite.
• Elasticity: The ability of a mineral to bend and regain its original shape once the applied force is removed.
• Flexibility: The ability of a mineral to bend but not recover its original shape.

8. Describe the General Properties of Gold Group Minerals

Minerals in the gold group are isostructural, with face-centered cubic lattices. They have identical atoms and a coordination number of 12.

• Hardness: Low
• Toughness: Sectile and malleable, with high cohesion
• Luster: Metallic
• Melting Point: Low
• Specific Gravity: High, due to the type of atomic packing
• Crystallography: Isometric system, hexoctahedral class

9. Define Patina

Patina is a thin layer that forms on the outer surface of minerals. It is composed of other minerals and is usually a product of oxidation, sulfidation, or other chemical reactions. Patina can often conceal the true color of the underlying mineral.