Macroscopic Characteristics of Minerals Part 2: Cleavage and Hardness

Summary

This video explores key methods for identifying minerals, focusing on characteristics like luster, color, cleavage, and fracture. It highlights how the crystalline structure of minerals influences their breakage, such as basal cleavage in graphite and conchoidal fracture in quartz. The video also touches on the Mohs Hardness Scale, explaining how minerals are ranked based on their resistance to scratching, with examples like calcite, halite, and quartz. The discussion ties atomic structures to macroscopic properties, setting the stage for a deeper dive into mineral classification and crystal chemistry.

Takeaways

• 💎 Minerals can be identified by their luster, color, cleavage, and fracture.
• 🔍 Cleavage occurs when minerals break along planar surfaces due to their crystalline structure.
• ⚛️ Graphite has basal cleavage due to weak van der Waals bonds between carbon sheets.
• 📏 Minerals like quartz lack cleavage but exhibit conchoidal fractures resembling broken glass.
• 📐 The angle between cleavage planes is useful in identifying minerals; calcite has cleavage at 60- and 120-degree angles.
• 🔨 Minerals like halite break into cubes, while calcite breaks into rhombohedrons due to cleavage plane differences.
• 📊 Quartz and plagioclase may look similar, but plagioclase has two cleavage planes at 90 degrees.
• 🪙 A fingernail's hardness is about 2.5 on the Mohs scale, which ranks minerals by scratch resistance.
• 💡 Pennies and metal files are used to approximate hardness: pennies (3.5), files (6.5).
• 🧪 The Mohs Hardness Scale helps classify minerals from softest (talc) to hardest (diamond).

Q & A

What is cleavage in minerals?

Cleavage refers to the tendency of a mineral to break along planar surfaces in one or more directions. It is determined by the mineral's crystalline structure and the relative strength of bonds along different crystallographic directions.

How does cleavage differ between graphite and quartz?

Graphite has basal cleavage, meaning it breaks along weak van der Waals bonds between sheets of carbon atoms. Quartz lacks cleavage due to its equal-strength silicon-oxygen bonds in all directions and instead fractures with a characteristic conchoidal fracture.

What is basal cleavage, and which minerals exhibit it?

Basal cleavage occurs when a mineral breaks along one plane, such as in graphite. Phyllosilicates, or sheet silicates, are also known for their perfect basal cleavage, allowing them to break apart like layers of baklava.

What is conchoidal fracture, and which mineral is known for it?

Conchoidal fracture is a fracture pattern that appears as a concave, scoop-like surface with parallel arc-like steps. Quartz is known for exhibiting this type of fracture.

How can you differentiate between quartz, calcite, and halite based on cleavage?

Quartz has no cleavage, calcite has three directions of cleavage with planes 60 and 120 degrees apart, and halite also has three cleavage planes but at 90 degrees. Halite breaks into cubes, while calcite breaks into rhombohedrons.

What is the relationship between cleavage and crystallographic directions?

The way a mineral breaks is controlled by its crystalline structure and the relative strength of the bonds along different crystallographic directions. For example, graphite breaks easily along weak bonds between sheets of carbon atoms but breaks irregularly along stronger covalent bonds.

What is the Mohs Hardness Scale, and how is it used to classify minerals?

The Mohs Hardness Scale ranks minerals based on their ability to scratch one another, from the softest (talc at 1) to the hardest (diamond at 10). It helps geologists estimate mineral hardness by comparing how they scratch or are scratched by objects of known hardness.

What is the hardness of a typical human fingernail, and why is it useful in mineral identification?

A human fingernail has a hardness similar to the mineral calcite, around 2.5 on the Mohs scale. Scratching a mineral with a fingernail can help geologists quickly identify softer minerals like calcite.

How can you estimate the hardness of an unknown mineral using common objects?

You can estimate a mineral's hardness by seeing whether it can be scratched by a penny (hardness 3.5) or a metal file (hardness 6.5). If the mineral can be scratched by a file but not by a penny, its hardness is between 3.5 and 6.5.

How do crystal chemistry and crystal structure relate to a mineral's macroscopic properties?

Crystal chemistry and structure determine a mineral’s macroscopic properties like cleavage, fracture, and hardness. For example, the strength and arrangement of atomic bonds dictate how a mineral breaks and how resistant it is to scratching.