L2 - Chemical weathering

Question 1

what is chemical weathering

Answer 1

chemical weathering is a process where minerals in rocks exposed at the Earth’s surface interact with water and atmospheric gases to form new stable minerals

Question 2

what are the main types of chemical weathering

Answer 2

chemical weathering can occur through reactions with water, oxygen, acids and organisms

Question 3

what happens during carbonate weathering

Answer 3

carbonate weathering involves the reaction of carbonate rocks, such as calcite, with carbonic acid, leading to the release of Ca2+ and HCO3- into solution

Question 4

write the equation for the weathering of calcite by carbonic acid

Answer 4

CaCO₃(s) + H₂CO₃(aq) ⇌ Ca²⁺(aq) + 2HCO₃⁻(aq)

Question 5

what is acid mine drainage (AMD) and does it form

Answer 5

AMD is pollution from the oxidation of pyrite, releasing acid and heavy metals into streams, often from abandoned coal and metal mines

Question 6

what is role of pyrite in acid mine drainage

Answer 6

pyrite (FeS2) reacts with oxygen and water, producing sulfuric acid and iron ions, which contribute to acidic nature of AMD

Question 7

how does biologically mediated oxidation of pyrite

Answer 7

Acidophilic bacteria, such as Acidithiobacillus ferrooxidans, accelerate pyrite oxidation by producing Fe3+, which oxidises pyrite faster than oxygen

Question 8

what is the natural attenuation process of AMD?

Answer 8

it is includes neutralisation of acidity, ppt of metal hydroxides, dilution of metal concentration and sorption of trace elements onto solid surfaces

Question 9

what is the difference between congruent and incongruent dissolution

Answer 9

congruent dissolution involves the complete dissolution of a mineral into solution

incongruent dissolution produces dissolved ions and new solid minerals such as clay

Question 10

provide an example of congruent dissolution

Answer 10

Mg₂SiO₄ (forsterite) + 4H⁺ → 2Mg²⁺ + H₄SiO₄

Question 11

provide an example of incogruent dissolution

Answer 11

2KAlSi₃O₈ (K-feldspar) + 2H⁺ + 9H₂O → 2K⁺ + H₄SiO₄ + Al₂Si₂O₅(OH)₄ (kaolinite)

Question 12

why are silicate minerals less affected by chemical weathering compared to carbonates

Answer 12

silicate minerals are more stable and abundant in the Earth’s crust and their dissolution rates are slower than carbonates

Question 13

what are clay minerals

Answer 13

clay minerals are hydrous aluminosilicates formed by the incongruent dissolution of silicate minerals, often in soils and sediments

Question 14

what are the main types of clay mineral structures

Answer 14

2 main types:
1:1 layer clays e.g. kaolinite
2:1 layers clays e.g. smectite and vermiculite

Question 15

what is the key difference between 1:1 and 2:1 layer clays

Answer 15

1:1 layer clays consist of one silica tetrahedral sheet and one alumina octahedral sheet
2:2 clays have 2 silica sheets sandwiching 1 alumina sheet

Question 16

why do some clays expand in water

Answer 16

Clays with interlayer ions like Na2+ or Ca2+ allow water to fit between the layers causing expansion e.g. smecitite, while clays with K+ e.g. illite do not expand

Question 17

how does ion exchange in clays affect water composition

Answer 17

clays can exchange ions with water, modifying the water composition through adsorption or ion exchange on the mineral surface

Question 18

why is quartz stable during weathering

Answer 18

quartz crystallizes at lower temps and is highly resistant to chemical weathering, making it a dominant constituent in soils and sediments