teaching - Earth Science Teachers' Association

TEACHING EARTH SCIENCES ● Volume 26 ● Number 3, 2001
Table 1:
Names and
Formulae of
Chemical Species
Formula
Fe 3+ (aq)
Fe 3+ (aq)
Fe203 (s)
FeS2 (s)
[Fe 111 (H20)6] 3+ (aq)
[Fe 111 (OH) (H20)5] 2+ (aq)
[Fe 111 (OH)3 (H20)3] 0 (s)
FeCl3
H2CO3 (aq)
HCO3 (aq)
CaCO3 (S)
H2O (1)
Iron (Ill) ion
Iron (Ill) ion
Iron (Ill) oxide (hematite)
Iron sulphide (pyrite)
Hexaaqua iron III.complex
Hydroxopentaaqua iron Ill complex
Trihydroxotriaqua iron III complex
Iron chloride
Carbonic acid
Hydrogen carbonate ion
Calcium carbonate
water
02 (g) oxygen
C02 (g)
2
S04 (aq)
Name
carbon dioxide
sulphate ion
Species states
Abbreviation
aq
g
l
s
State
aqueous solution
gas
liquid
solid
models. Crushed weathered pyritous Coal Measure
shale and red Triassic sandstones stirred thoroughly
with water can provide iron containing solutions.
Aqua-ion chemistry (see Table 1) shows that iron
in solution generates and maintains acid conditions.
The pH and iron (III) concentration of the solutions
described above can be determined by titration (see
Photograph). As iron (III) concentration controls
acidity (pH) rather than anion concentration (e.g.
sulphate ion from oxidation and weathering of pyrite)
iron (III) chloride can be employed at appropriate
concentrations to replicate natural solutions. Samples
of crushed pyritous shales produced solutions where
hydroxonium ion concentration is 10 -4 M to 10 -5 M, or
pH of 4-5, so 0.027g to 0.0027g of hydrated iron chloride
dissolved in 1000 cm 3 H 2 O provides an appropriate
concentration range for iron (III) and
hydroxonium iron concentration.
Different rock types and minerals can then be
immersed in various iron-containing solutions. Even
very dilute solutions (e.g. 10 -5 M iron (III) ions) over a
few days react with carbonate rocks which develop a
hydrated iron oxide coating. Unexpectedly (but much
more slowly) sandstones can also develop iron oxide
coatings (see discussion). Non-pyritous shales and
mudstones show no reaction.
This investigation is appropriate for laboratory based
‘A’ level geology coursework. Depending on the range
of concentrations for iron-bearing solutions, one to two
hours should be allowed for initial setting up. Small
hand specimens can be allowed to stand in solutions in
250 ml. beakers. Reactions should be observed regularly
over a one to two week period.
Safety: Protective goggles should be worn at all
times, due to acidic solutions used.
Discussion
Various discussion topics can be developed from the
model described above. For example: Does the model
replicate natural processes? If so, what are the geochemical
reactions? This question may also appeal to ‘A’
level chemistry students who have some appreciation of
aqua-ion activity.
The role of weak acids in speliogenesis is well
known (Lowe 1993). Strong acids are those almost
completely dissociated to give a high hydroxonium
ion concentration. Weak acids undergo a small degree
of dissociation to give low hydroxonium concentration.
The aqua-ion model implies the development of
strong acid conditions (Lowe and Gunn 1995) as
limestone was rapidly attacked with the subsequent
deposition of iron oxides. This can promote discussion
on the possibility of speliogenesis not always
being a slow process where limestones are overlain by
pyritous sediments.
Preferential mineralization is well documented if
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