Rocks and Minerals

Lessons

Senior 6 · Geography

Rocks and Minerals

4 lessons

Lesson 4 of 4

Rocks, Minerals and People

25 min

Three metals in your pocket

A mobile phone contains tantalum in its tiny capacitors, tin in the solder that joins its circuits, and often tungsten in the small weight that makes it vibrate. All three are mined in Rwanda as the "3Ts": tin from cassiterite, tantalum from coltan and tungsten from wolframite. Rwanda Mines, Petroleum and Gas Board (RMB) lists areas such as the pegmatite fields around the Gitarama–Kayenzi granite in Kamonyi District, which host cassiterite and coltan, and quartz veins at Bugesera–Nkanga, near the border with Burundi, which carry cassiterite, coltan and wolframite (RMB, 2025). This lesson looks at how rocks shape landscapes and lives, and at minerals as resources.

Rocks and the landscape

Rock	Typical landforms	Why
Granite	Rounded domes, inselbergs and tors; uplands	Hard, but weathers deeply along joints; the unjointed cores are left standing
Basalt	Plateaus, stepped lava flows, columnar cliffs	Stacked, runny lava flows spread widely and cool into columns
Quartzite	Ridges, steep hill crests	Very hard and chemically resistant
Limestone	Karst: caves, sinkholes, gorges, pavements	Dissolved by weak carbonic acid (unit 4)
Shale and clay	Lowlands, vales, gentle slopes; landslides	Soft and impermeable; weak when wet
Sandstone	Escarpments, plateaus, cliffs	Fairly resistant and permeable, so little surface runoff erodes it

Rocks and human life

Soils and farming: rock is the parent material of soil. Basalt and volcanic ash weather to fertile soils; quartzite and granite give sandy, less fertile soils (unit 7).
Water: porous and permeable rocks such as sandstone and jointed limestone are aquifers; clay and unjointed granite hold little water.
Building materials: granite and quartzite for stone and aggregate, clay for bricks and tiles, limestone for cement and lime, sand and gravel for concrete.
Minerals and energy: ores of metals, gemstones, and fuels such as coal, oil and natural gas, all held in particular rocks.
Tourism and hazards: volcanic and limestone landscapes attract visitors; weak, steep rock slopes are prone to landslides.

Definition

Ore: a rock containing a mineral from which a metal can be extracted at a profit. Example: pegmatite rich in cassiterite is a tin ore.

Ore mineral: the mineral that carries the metal (cassiterite for tin). Gangue: the worthless minerals around it (often quartz).

Grade: the concentration of the valuable mineral or metal in the ore, as a percentage or in grams per tonne. Concentrate: the product left after the gangue has been removed.

Types of minerals

Type	Examples	Uses
Metallic: ferrous	Iron ore (haematite, magnetite)	Steel for construction, vehicles, tools
Metallic: non-ferrous	Cassiterite (tin), coltan (tantalum and niobium), wolframite (tungsten), copper, aluminium (bauxite)	Solder, tinplate, electronics, alloys, cables
Precious	Gold, silver, gemstones	Jewellery, electronics, stores of value
Non-metallic (industrial)	Limestone, clay, sand, gypsum, kaolin	Cement, bricks, glass, plaster, paper
Energy minerals	Coal, oil, natural gas, uranium; methane dissolved in the deep water of Lake Kivu	Electricity, transport fuels, heating

Rwanda's 3Ts and their products

Ore mineral	Formula	Metal	Manufactured products
Cassiterite	$\mathrm{SnO_2}$	Tin	Solder for electronics, tinplate for cans, bronze and other alloys
Coltan (columbite-tantalite)	$\mathrm{(Fe,Mn)(Nb,Ta)_2O_6}$	Tantalum and niobium	Tantalum capacitors in phones and computers; niobium in high-strength steel
Wolframite	$\mathrm{(Fe,Mn)WO_4}$	Tungsten	Tungsten carbide for drill bits and cutting tools, heavy alloys, vibration weights

Figure 4: from ore to manufactured product.

Worked example 1: tantalum in a coltan concentrate

Problem. RMB reports coltan from the Mamfu–Rwasama block in Eastern Province averaging 62.68% $\mathrm{Ta_2O_5}$ by mass (RMB, 2025). How much tantalum metal does one tonne of such concentrate contain? (Ta = 180.95, O = 16.00.)

Solution. Mass fraction of Ta in $\mathrm{Ta_2O_5}$ :

\frac{2 \times 180.95}{2 \times 180.95 + 5 \times 16.00} = \frac{361.90}{441.90} = 0.8190

\text{Ta per tonne} = 0.6268 \times 0.8190 \times 1000\,\mathrm{kg} = 513\,\mathrm{kg}

About half the mass of the concentrate is tantalum, which is why high-grade coltan is so valuable.

Worked example 2: how much rock for a tonne of tin?

Problem. An ore contains 0.5% cassiterite by mass (illustrative grade). Pure cassiterite is $\mathrm{SnO_2}$ (Sn = 118.71, O = 16.00). How many tonnes of ore must be mined to produce 1 t of tin, and what does this mean for the environment?

Solution. Mass fraction of Sn in $\mathrm{SnO_2} = \frac{118.71}{150.71} = 0.7877$ . Cassiterite needed $= \frac{1}{0.7877} = 1.27\,\mathrm{t}$ .

\text{ore} = \frac{1.27\,\mathrm{t}}{0.005} = 254\,\mathrm{t}

About 253 t of waste rock and tailings are left for every tonne of tin. Unless tailings are stored safely and pits are backfilled and replanted, mining leaves bare, eroding land and sends silt into rivers.

Problems of mining and their solutions

Problem	Possible solution
Land degradation: open pits, spoil heaps and bare ground	Backfilling and replanting worked-out sites; mine closure plans
Silt and chemicals washing into rivers and wetlands	Settling ponds for washing water; buffer zones along rivers
Accidents in unsupported pits and tunnels	Training, protective equipment, supported workings, inspection
Low value added when raw concentrates are exported	Local processing and smelting, skills training
Minerals from conflict areas entering trade	Certification and traceability: exporters in Rwanda apply for Mineral Export Certificates or ICGLR certificates through RMB's online system

Warning

Common mistakes

"Coltan is a metal." Coltan is an ore mineral; tantalum and niobium are the metals extracted from it.
Confusing ore and mineral. Every ore contains minerals, but a rock is only an ore if the metal can be extracted at a profit, which changes with prices and technology.

Tip

Exam technique. For "evaluate the economic importance of minerals", balance benefits (exports, jobs, government revenue, raw materials for industry) against costs (environmental damage, price swings, a finite resource) and finish with a judgement about how the benefits can be kept.

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Practice

Define an ore, gangue and grade.
Name the ore minerals of tin, tantalum and tungsten, and one product made from each metal.
Explain how two different rock types influence the landscape.
Calculate the mass fraction of tungsten in ferberite, $\mathrm{FeWO_4}$ (Fe = 55.85, W = 183.84, O = 16.00).
A concentrate is 70% $\mathrm{SnO_2}$ . How many kilograms of tin does 1 t contain?
Explain two ways in which rocks affect farming.
Evaluate the importance of mining to Rwanda's economy and environment.

Show answersHide answers

Ore: rock from which a metal can be extracted at a profit; gangue: the worthless minerals in it; grade: the concentration of the valuable mineral or metal.
Cassiterite (tin: solder, tinplate); coltan (tantalum: capacitors in phones); wolframite (tungsten: drill bits, cutting tools).
For example: quartzite is hard and forms ridges; clay is soft and impermeable and forms lowlands with landslides on steep slopes.
$\frac{183.84}{55.85 + 183.84 + 64.00} = \frac{183.84}{303.69} = 0.605$ , about 60.5%.
$0.70 \times 0.7877 \times 1000 = 551\,\mathrm{kg}$ .
Parent rock sets soil texture and fertility (basalt gives fertile clay soils, quartzite sandy poor soils); permeable rocks give groundwater for irrigation and wells.
Benefits: export earnings, jobs (including small-scale miners), raw materials for world industry. Costs: land degradation, river siltation, accidents, a finite resource. Mining is important, but its benefits last only with rehabilitation of mined land, safe working and more local processing.

Summary

Rock type shapes landforms, soils, water supply and building materials.
An ore is a rock from which a metal can be extracted at a profit; grade and processing decide its value.
Rwanda's 3Ts are cassiterite (tin), coltan (tantalum) and wolframite (tungsten), used in electronics, solder and cutting tools.
Mining brings income but must be managed to limit land, water and safety problems.

1Three metals in your pocket

2Rocks and the landscape

3Rocks and human life

4Types of minerals

5Rwanda's 3Ts and their products

6Problems of mining and their solutions