teaching - Earth Science Teachers' Association

More documents

Recommendations

Info

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 www.esta-uk.org 100
TEACHING EARTH SCIENCES ● Volume 26 ● Number 3, 2001 Some equations that may be applied now follow... 1. Weathering and oxidation of pyrite Pyrite + oxygen + water = iron III ions + sulphate ions + hydrogen ions FeS2 (s) + 3 1 /2 02 (g) + H20(l) = Fe 2+ 2- (aq) + 2S04 (aq) + 2H + (aq) Iron II ions + hydrogen carbonate ions + oxygen + water = iron oxide + carbonic acid 2Fe 2+ - (aq) + 4HCO3 (aq) + 1 /2O2 (g) + 2H20(l) = Fe203 (s) + 4H2C03 (aq) 2. Formation of hydrated complex ions generating acidic conditions Iron III + water = hexaaqua iron III ions Fe 3+ (aq) + 6H20(l) = [Fe 111 (H20)6 ] 3+ (aq) Hexaaqua iron 111 + water = hydroxopentaaqua iron 111 + hydroxonium ion [Fe 111 (H20) 6] 3+ (aq) + H20(l) = [Fe 111 (0H)(H20)5] 2+ (aq) + H30 + (aq) 3. Acidic solutions attacking limestones Hydroxonium ions + calcium carbonate ion = calcium ions + hydrogen carbonate + water H30 + (aq) + CaCO 3 (s) = Ca 2+ - (aq) + HCO3 (aq) + H20(l) 4. Desiccation of hydroxoaquairon complexes to precipitateiron oxide Trihydroxotriaqua iron III = iron oxide + water 2[Fe 111 (H2O)3(OH)3] 0 (s) = Fe203 + 9H20(l) not always fully understood. Mineralogically more simple rocks often seem more susceptible to mineralization processes. The susceptibility of carbonate rocks to acid attack means that, not surprisingly, iron ore deposition has occurred in these rocks (Moseley 2000). Hematisation of silicic rocks is less easily explained. There is the possibility of attraction between positive dipoles on aqua~ions and negatively charged SiO4 4- units (Moseley 1994), an association hinted at in the variety of quartz called citrine. Much research remains to be done in this field. John Moseley Hutton Grammar School Liverpool Road Hutton Preston PR4 5SN Bibliography King, R.J. (1998) The History of Iron Mining in the Forest of’ Dean and Origin of the Ores. Bedrock Newsletter. 4:1-2. Krauskopf, K.B. (1989) Introduction to Geochemistry. Second Ed. McGraw- Hill. Lowe, DJ. (1993) The Forest of Dean Caves and Karst: Inception Horizons and Iron-Ore Deposits. Cave <strong>Science</strong>, 20(2):31-43. Lowe, D.J. and Gunn, J. (1995) The role of strong acid in speleoinception and subsequent cavern details Moseley, J.B. (1994) The Origin and Significance of the Hematisation of Silicie Rocks of Precambrian and Ordovician age in South Shropshire. Mercian Geologist, 13(3):111-115. Moseley, J.B. (2000) The Application of Geochemical Models to the Interpretation of the Forest of Dean Iron Ore Deposits. Proceedings of the Cotteswold Naturalists’ Field Club, Vol. Xli ((III) Forest of Dean Iron Ore Deposits. Proceedings of the Cotteswold Naturalists’ Field Club. Vol.XLI (III). Rose, W.C.C., and Dunham, K.C. (1977) Geology and Hematite Deposits of South Cumbria. Geol.Surv. of G.B. H.M.S.O. 101 www.esta-uk.org
Page 1 and 2: teaching EARTH SCIENCES Earth Syste
Page 3 and 4: Journal of EARTH SCIENCE TEACHERS A
Page 5 and 6: ESTA? Websearch Book Reviews Diary
Page 7 and 8: TEACHING EARTH SCIENCES ● Volume
Page 11 and 12: Journal of EARTH SCIENCE TEACHERS A
Page 17: TEACHING EARTH SCIENCES ● Volume
Page 23 and 24: WINTER 2001 ● Issue 35 VOLCANOES
Page 25 and 26: WINTER 2001 ● Issue 35 ● VOLCAN
Page 47 and 48: Key Stage 3 Science of the Earth 11

teaching - Earth Science Teachers' Association

Create successful ePaper yourself

Delete template?

Save as template?