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Differences between lateritic and podzolic weathering
Geochimicaet CoemochimicaActa, 1970,Vol. 34, pp.
1361to 155% PergamonPreaa.
Printed in Northern bland
NOTES DifEerencesbetween late&c and podzolic weathering Department
c. D. CURTIS* of Geology, The University, SheffieldSl 3JD
(Received 1 June 1970; accepted ira rev&d
form 7 July 1970)
Abstract-A thermochemicalanalysis of gibbsite-kaolinite stability relations provides a sound basis for appreciatingthe development of podzols at higher latitudes and lateritesin the tropics.
IN HIS recent text, KRAUSKOPF(1967) comments thus upon laterite genesis: “The chemical reason for the breakdown of clay and removal of silica under conditions of high temperature and high rainfall has occasioned much argument, but remains an unsolved riddle”. Similar leaching conditions in colder climates result in removal of everything but quartz and iron hydroxides (podzols). One approach to the problem is to consider the reaction of pure water (initial precipitation) with kaolin& and quartz at near neutral pH values. Reactions under this heading (free energy data taken from GARRELS and-Cnmsr, 1965 and REESMAN and KELLER, 1968) are: SiO zc.wartz + 2H,O,
= H4SiOShq
log [H,SiO,] = -3.99 Al(OH)Sc.gibi,site + H,O, = AJ(OH),, log lN(OH),-I
=
-15.06
(1) + 2H,,+ + pH
= 2Al(OH)&
LWH),-I A~,S~,WW,, + 5JW,
= - 19.40 - log [H,SiO,] + pH
log [H,SiO,]
= 2Al(OH)ac.,bb*it,+ =
+
2H,SiOl,
Al,SiZ0,(OH)4, + 7&O, log
+ 2H,+
(2)
(3)
2H4f3i04,
-4.34
(4)
Below about pH 5, behaviour according to the reaction: Al,Si,O,(OH),,
+ 6H,,+ + 2SiOz,.,,,,, + 2aaqs+ + 5Hz0, log [AP+] = 6.92 - 3 pH
(5)
must be considered. ROBERSON and HEM (1969) have analysed in great detail factors influencing the solubility of gibbsite. Figure 1 here is a modification of their Fig. 1 with quartz solubility superimposed. Reaction of pure water with kaolinite at near neutral pH proceeds via ‘congruent’ dissolution (equation 3) until gibbsite saturated (equation 2). Thereafter ‘incongruent’ dissolution with gibbsite precipitation occurs until kaolin& is eventually stabilised. Within a soil pro&, this would imply that * Present address: California 90024.
Department of Geology, University of California, Los Angeles, 1351
135%
Notes
gibbsite should develop above kaolinite in response t,o increasing (downward) silica activity. This is quite typical of latcrite profiles where kaolinittt is often found to be stable below water table level, gibbsi& above it. !I’he same type of behaviour is to be anticipated over the M-8.0 pH range. In acid solutions the solubility of gibbsite increases rapidly with decreasing pL1 whereas the solubility of quartz shows no such dependence (Fig. 1). Below about pH 4.5 (no precise prediction is advisable due to uncertainties in free energy measurements), the solubilit’y of gibbsite exceeds that of quartz. Equation 5 i:‘, then more
art2 saturation ----------
-4
-5
4
5
6
7
8
9
IO
II
PH
Fig. 1. The dependence of solubility on pH for quartz and microcrystalline gibbsite (ROBERSONand HEM, 1909).
relevant than equation 4. Leaching under these conditions should result in the development of quartz residues in the upper part of the profile. This ia podzolization. LOUGHNAN (1969) gives the pH range of podzols as 3.5 to 5.0 whilst laterites lie in the 5.0 to 7-O region. KRAUSKOPF (1967, p. 194) observes “Some geologists have supposed that special processes of weathering must be operative in the tropics. Silica, for example, has been thought to be more soluble because soil solutions in the tropics are less acid than those of temperate climates; this guess has been proved wrong by measurements showing that silica is no more soluble in near-neutral solutions than in acid”. Silica aolubility may not vary with pH but that of aolid alumino-silicates and aluminium hydroxides certainly is enhanced in acid conditions relative to near neutral ones. It is the ratio of solubilities that is important, not their absolute values. Tropical humid climates favour one type of plant community whereas temperate and higher latitude climates favour others. There is a definite link between prevalent plant community and soil water pH which dictates that high latitude humid profiles are more acid than their tropical counterparts. Enhanced aluminium mobility can also be envisaged to result from chelative dissolution by organiu compounds. This would effectively extend the pH range of podzolic leaching. It is not necessary,
Notes
1363
however, to invoke such direct organic involvement in order to explain the essential features of lateritic and podzolic weathering, REFERENCES GARRELS R. M. and CJXRIST C. L. (1966) Solutione, Mineral.9and Equilibria. Harper & Row. K.RAUSKOPF K. B. (196’7) Introductory to Beochemkt~. McGraw-Hill. LOUGHNAN F. C. (1969)Chmnical Weathering of Silicate Minerals. American Else&r. REESMAN A. L. and KELLER W. D. (1969) Aqueous solubility studies of high alumina and clay minerals. Anzer. Mineral. 58, 929-942. ROBERSON C. E. and HEM J. D. (1969) Solubility of aluminium in the presence of hydroxide, fluoride and sulphate. U.S. Beol. Sure. Water Supply Paper 1827-C.
Qeochimicn etCosmochimica Acta, 1970,Vol. 34.pp.1353 to1356.Pergamon Press. PrintedIn Northern Ir&nd
JOIDES cores: Organic geochemical analyses of four Gculfof Mexico and western Atlantic sediment samples CHARLES B. KOONS Esso Production Research Company, Houston, Texas 77001 (Received 25 June 1970; accepted 17 July 1970) Abstract-Four abyssal sediment samples collected during the JOIDES Leg 1 cruise of the drilling vessel Qlomar Challengerin 1968 contain low amounts of organic carbon and hydrocarbons, as compared with miscellaneous sediment samples reported in the literature. This suggests that these abyssal sediments are not likely source sediments for petroleum.
FOUR abyssal sediment samples collected on Leg 1 of the JOIDES* Deep Sea Drilling Project do not appear to be likely petroleum source sediments. These samples, from the Gulf of Mexico and the western Atlantic Ocean, contain low amounts of organic carbon (range from O-1 1 to 0.84 per cent) and heavy hydrocarbons (range from <5 to 46 ppm). C,-C, light hydrocarbons could not be detected in any of the samples. The saturated hydrocarbons extracted from the two richer samples show compound type distributions similar to those observed on other Recent and ancient sediments. Stable carbon isotope ratios, CY3/Cla,on the total organic carbon in the samples are similar to ratios reported on other deep water sediments. Table 1 gives the location, lithologic description, and age for the four samples, according to EWING et al. (1969). Three of the four samples came from Hole 3, drilled about 20 miles southeast of the Challenger Knoll where a petroleum-stained core (Hole 2) was recovered in August 1968, as reported by GEALY and DAVIES (1969). At the beginning of this study, it seemed possible that one of the three samples from Hole 3 might represent the source sediment for the Challenger petroleum. * Joint Oceanographic Institutions for Deep Earth Sampling. Leg l-the cruise of the Drilling Vessel GlonzarChallenger from Orange, Texas to Hoboken, New Jersey, Aug._Sept. 1968.
1361to 155% PergamonPreaa.
Printed in Northern bland
NOTES DifEerencesbetween late&c and podzolic weathering Department
c. D. CURTIS* of Geology, The University, SheffieldSl 3JD
(Received 1 June 1970; accepted ira rev&d
form 7 July 1970)
Abstract-A thermochemicalanalysis of gibbsite-kaolinite stability relations provides a sound basis for appreciatingthe development of podzols at higher latitudes and lateritesin the tropics.
IN HIS recent text, KRAUSKOPF(1967) comments thus upon laterite genesis: “The chemical reason for the breakdown of clay and removal of silica under conditions of high temperature and high rainfall has occasioned much argument, but remains an unsolved riddle”. Similar leaching conditions in colder climates result in removal of everything but quartz and iron hydroxides (podzols). One approach to the problem is to consider the reaction of pure water (initial precipitation) with kaolin& and quartz at near neutral pH values. Reactions under this heading (free energy data taken from GARRELS and-Cnmsr, 1965 and REESMAN and KELLER, 1968) are: SiO zc.wartz + 2H,O,
= H4SiOShq
log [H,SiO,] = -3.99 Al(OH)Sc.gibi,site + H,O, = AJ(OH),, log lN(OH),-I
=
-15.06
(1) + 2H,,+ + pH
= 2Al(OH)&
LWH),-I A~,S~,WW,, + 5JW,
= - 19.40 - log [H,SiO,] + pH
log [H,SiO,]
= 2Al(OH)ac.,bb*it,+ =
+
2H,SiOl,
Al,SiZ0,(OH)4, + 7&O, log
+ 2H,+
(2)
(3)
2H4f3i04,
-4.34
(4)
Below about pH 5, behaviour according to the reaction: Al,Si,O,(OH),,
+ 6H,,+ + 2SiOz,.,,,,, + 2aaqs+ + 5Hz0, log [AP+] = 6.92 - 3 pH
(5)
must be considered. ROBERSON and HEM (1969) have analysed in great detail factors influencing the solubility of gibbsite. Figure 1 here is a modification of their Fig. 1 with quartz solubility superimposed. Reaction of pure water with kaolinite at near neutral pH proceeds via ‘congruent’ dissolution (equation 3) until gibbsite saturated (equation 2). Thereafter ‘incongruent’ dissolution with gibbsite precipitation occurs until kaolin& is eventually stabilised. Within a soil pro&, this would imply that * Present address: California 90024.
Department of Geology, University of California, Los Angeles, 1351
135%
Notes
gibbsite should develop above kaolinite in response t,o increasing (downward) silica activity. This is quite typical of latcrite profiles where kaolinittt is often found to be stable below water table level, gibbsi& above it. !I’he same type of behaviour is to be anticipated over the M-8.0 pH range. In acid solutions the solubility of gibbsite increases rapidly with decreasing pL1 whereas the solubility of quartz shows no such dependence (Fig. 1). Below about pH 4.5 (no precise prediction is advisable due to uncertainties in free energy measurements), the solubilit’y of gibbsite exceeds that of quartz. Equation 5 i:‘, then more
art2 saturation ----------
-4
-5
4
5
6
7
8
9
IO
II
PH
Fig. 1. The dependence of solubility on pH for quartz and microcrystalline gibbsite (ROBERSONand HEM, 1909).
relevant than equation 4. Leaching under these conditions should result in the development of quartz residues in the upper part of the profile. This ia podzolization. LOUGHNAN (1969) gives the pH range of podzols as 3.5 to 5.0 whilst laterites lie in the 5.0 to 7-O region. KRAUSKOPF (1967, p. 194) observes “Some geologists have supposed that special processes of weathering must be operative in the tropics. Silica, for example, has been thought to be more soluble because soil solutions in the tropics are less acid than those of temperate climates; this guess has been proved wrong by measurements showing that silica is no more soluble in near-neutral solutions than in acid”. Silica aolubility may not vary with pH but that of aolid alumino-silicates and aluminium hydroxides certainly is enhanced in acid conditions relative to near neutral ones. It is the ratio of solubilities that is important, not their absolute values. Tropical humid climates favour one type of plant community whereas temperate and higher latitude climates favour others. There is a definite link between prevalent plant community and soil water pH which dictates that high latitude humid profiles are more acid than their tropical counterparts. Enhanced aluminium mobility can also be envisaged to result from chelative dissolution by organiu compounds. This would effectively extend the pH range of podzolic leaching. It is not necessary,
Notes
1363
however, to invoke such direct organic involvement in order to explain the essential features of lateritic and podzolic weathering, REFERENCES GARRELS R. M. and CJXRIST C. L. (1966) Solutione, Mineral.9and Equilibria. Harper & Row. K.RAUSKOPF K. B. (196’7) Introductory to Beochemkt~. McGraw-Hill. LOUGHNAN F. C. (1969)Chmnical Weathering of Silicate Minerals. American Else&r. REESMAN A. L. and KELLER W. D. (1969) Aqueous solubility studies of high alumina and clay minerals. Anzer. Mineral. 58, 929-942. ROBERSON C. E. and HEM J. D. (1969) Solubility of aluminium in the presence of hydroxide, fluoride and sulphate. U.S. Beol. Sure. Water Supply Paper 1827-C.
Qeochimicn etCosmochimica Acta, 1970,Vol. 34.pp.1353 to1356.Pergamon Press. PrintedIn Northern Ir&nd
JOIDES cores: Organic geochemical analyses of four Gculfof Mexico and western Atlantic sediment samples CHARLES B. KOONS Esso Production Research Company, Houston, Texas 77001 (Received 25 June 1970; accepted 17 July 1970) Abstract-Four abyssal sediment samples collected during the JOIDES Leg 1 cruise of the drilling vessel Qlomar Challengerin 1968 contain low amounts of organic carbon and hydrocarbons, as compared with miscellaneous sediment samples reported in the literature. This suggests that these abyssal sediments are not likely source sediments for petroleum.
FOUR abyssal sediment samples collected on Leg 1 of the JOIDES* Deep Sea Drilling Project do not appear to be likely petroleum source sediments. These samples, from the Gulf of Mexico and the western Atlantic Ocean, contain low amounts of organic carbon (range from O-1 1 to 0.84 per cent) and heavy hydrocarbons (range from <5 to 46 ppm). C,-C, light hydrocarbons could not be detected in any of the samples. The saturated hydrocarbons extracted from the two richer samples show compound type distributions similar to those observed on other Recent and ancient sediments. Stable carbon isotope ratios, CY3/Cla,on the total organic carbon in the samples are similar to ratios reported on other deep water sediments. Table 1 gives the location, lithologic description, and age for the four samples, according to EWING et al. (1969). Three of the four samples came from Hole 3, drilled about 20 miles southeast of the Challenger Knoll where a petroleum-stained core (Hole 2) was recovered in August 1968, as reported by GEALY and DAVIES (1969). At the beginning of this study, it seemed possible that one of the three samples from Hole 3 might represent the source sediment for the Challenger petroleum. * Joint Oceanographic Institutions for Deep Earth Sampling. Leg l-the cruise of the Drilling Vessel GlonzarChallenger from Orange, Texas to Hoboken, New Jersey, Aug._Sept. 1968.