Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Define the lithosphere and its composition.
• Describe chemical processes in mineral formation (gold, platinum, coal).
• Explain the environmental impacts of mining, like acid mine drainage.
• Outline the formation of energy resources (coal, oil, gas).
• Discuss the extraction and refining of two South African mineral resources.

Lesson summary

South Africa is incredibly rich in mineral resources; in fact, mining has been a cornerstone of our economy for over a century. Understanding the lithosphere – the rigid outer part of the Earth, composed of the crust and the uppermost part of the mantle – and the chemical systems within it that create and concentrate these resources is crucial. This knowledge allows us to understand how mining affects our environment, how energy resources are formed, and how we can manage these resources sustainably for future generations. Moreover, many South African communities depend on the mining industry for their livelihoods, making this a vital topic for understanding their context.

Lesson notes

The lithosphere is the solid outer section of Earth, which includes the Earth's crust and the uppermost portion of the mantle. It is divided into tectonic plates that move and interact, leading to various geological processes. These processes play a significant role in the formation and concentration of mineral and energy resources.

Mineral Formation: Mineral deposits are formed through a variety of geological processes.

Here are some key examples: Magmatic Processes: As magma cools and solidifies, minerals crystallize. The order in which minerals crystallize depends on their melting points and chemical compatibility. This can lead to the concentration of specific minerals. For example, chromitite layers (containing chromium) in the Bushveld Igneous Complex were formed by the fractional crystallization of magma. Platinum group metals (PGMs) also concentrate in this way.

Hydrothermal Processes: Hot, chemically active fluids (hydrothermal fluids) circulate through rocks, dissolving and transporting minerals. When these fluids encounter a change in temperature, pressure, or chemical environment (e.g., mixing with cooler, oxygenated water), the dissolved minerals precipitate out, forming veins or disseminated deposits. Gold deposits often form this way.

Sedimentary Processes: Minerals can be concentrated by sedimentary processes, such as the weathering and erosion of pre-existing rocks. For example, placer deposits of gold form when gold particles are eroded from veins and transported by rivers. The denser gold particles are deposited in areas of lower flow velocity. Banded iron formations (BIFs) are another example of sedimentary deposits; they consist of alternating layers of iron oxides and chert and formed under specific ocean conditions billions of years ago.

Metamorphic Processes: Changes in temperature and pressure can transform existing rocks and minerals, leading to the formation of new minerals. For instance, graphite, a form of carbon, can form from the metamorphism of organic-rich sediments.

Energy Resource Formation: Coal Formation: Coal is a sedimentary rock formed from the accumulation and compression of plant matter over millions of years. The process begins with the accumulation of plant debris in swampy environments. This material is then buried and subjected to increasing pressure and temperature, which transforms it into peat, then lignite, then bituminous coal, and finally anthracite (the highest grade of coal).

Petroleum and Natural Gas Formation: Petroleum and natural gas are formed from the remains of marine organisms (plankton and algae) that accumulate on the seafloor. These organic-rich sediments are buried and subjected to increasing pressure and temperature, which transforms them into hydrocarbons. The hydrocarbons migrate upwards through porous rocks until they are trapped by impermeable layers.

Environmental Impacts of Mining: Mining can have significant environmental impacts, including: Acid Mine Drainage (AMD): When sulfide minerals (e.g., pyrite, FeS2) are exposed to air and water, they oxidize, producing sulfuric acid and dissolved iron. This acidic water can leach heavy metals from the surrounding rocks and contaminate surface and groundwater.

The chemical reaction is: 4FeS2(s) + 15O2(g) + 14H2O(l) → 4Fe(OH)3(s) + 8H2SO4(aq) The sulfuric acid lowers the pH of the water, making it toxic to aquatic life and corrosive to infrastructure.

Soil Erosion and Water Pollution: Mining activities can disturb the soil and vegetation, leading to soil erosion and sedimentation of rivers and streams. Mining waste (tailings) can contain toxic chemicals that can pollute water and soil.

Air Pollution: Mining operations can release dust and harmful gases into the air, contributing to air pollution and respiratory problems.

Mitigation of Environmental Impacts: Several chemical methods can be used to mitigate the environmental impacts of mining: Neutralization of AMD: Lime (calcium oxide, CaO) or limestone (calcium carbonate, CaCO3) can be added to AMD to neutralize the acid and precipitate out the dissolved metals: CaO(s) + H2SO4(aq) → CaSO4(aq) + H2O(l)

Phytoremediation: Plants can be used to remove or stabilize heavy metals from contaminated soil and water.

Constructed Wetlands: Wetlands can be designed to treat AMD by using natural biological and chemical processes to remove pollutants.

Worked example

Example 1: Calculating the amount of lime needed to neutralize AMD.

A mining site generates 1000 L of AMD with a sulfuric acid concentration of 0.01 M. Calculate the mass of CaO needed to neutralize this AM

D. Step 1: Calculate the moles of H2SO4 in the AMD.

Moles of H2SO4 = Volume (L) x Concentration (M) = 1000 L x 0.01 mol/L = 10 moles