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Micromorphology and mineralogy of weathering and neoformation phenomena in a quaternary terrace sequence of the Allier, Limagne, France
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd I N T E R N A T I O N A L SYMPOSIUM July, 2-8, 1990, Aix en Provence, France.
83
M I C R O M O R P H O L O G Y AND MINERALOGY OF W E A T H E R I N G AND N E O F O R M A T I O N PHENOMENA IN A QUATERNARY T E R R A C E SEQUENCE OF THE ALLIER, LIMAGNE, FRANCE.
JONGMANS A.G., VAN DOESBURGL J.D.J. and VAN BREEMEN N. Department of Soil Science and Geology, Agricultural University P.O. Box, AA Wageningen, The Netherlands.
INTRODUCTION Soil formation is influenced by the weathering of primary minerals and rockfragments and by formation of secundary minerals. We studied the micromorphological and mineralogical aspects of mineral weathering in a Quaternary Allier terrace sequence, to characterize primary mineral weathering in relation to neoformation of secundary products. M A T E R I A L AND METHODS Nine gravelly terraces with various amounts of volcanic and granitic fragments and without calciumcarbonate were studied. Parent material in terms of textural and mineralogical composition of all terraces was found to be essentially similar (Jongmans et al., 1990). Micromorphology was described using the terminology of Bullock et al., 1985. Mineral grains were quantified by point-counting 500-700 points in thin sections (Van der Plas and Tobi, 1965). X-ray diffraction of microquantities in situ and microchemical analysis (SEM-EDXRA) was performed according the methods described in Feijtel et al., 1989. R E S U L T S AND DISCUSSION Three major soil groups with increasing soil age were distinguished. Group I, Eutric cambisols on holocene terraces : All soil horizons contain fresh volcanic (mainly basaltic) and granitic fragments, and quartz, alkalifeldspars, plagioelase, hornblende, pyroxene, olivine, muscovite and biotite. Weathering is restricted to some exfoliation and ironstaining of biotite and pellicular alteration of olivine.
Group II, Haplic Luvisols on Late pleistocene terraces. The mineralogical composition is similar to that of Group I, but surface horizons of Group II contain more quartz, and less volcanic fragments, suggesting distinct weathering of volcanic fragments and relative enrichment of quartz (Fig.l). Isotropic coatings of amorphous silica(type 1) and anisotropic coatings of calcite (type 2), point to precipitation of dissolved silica and calcium, liberated by weathering of volcanic fragments above that zone (Table 1). Group II, Eutric/Distric Planosols, on Early pleistocene terraces : Biotite, plagioclase, olivine and pyroxene grains are absent in A and E horizons, indicating that these minerals have been removed by weathering. Volcanic fragments are absent in A horizons, and their content in E and B horizons decreases with soil age (Fig.l). Substantial amounts of volcanic fragments in B horizons have been fragmented, deformed and transformed into clay which show well-developed b-fabrics. Transformation into clay contributes to the increase of the clay content in B horizons with soil age. Quartz content in A and E horizons shows an inverse relationship with depth and age, indicating strong weathering over greater depths in soils of group III. (Fig.l). Granite fragments, almost absent in A horizons, increased in content with depth. Alteration has led to disintegration and formation of quartz, alkali-feldspar, muscovite and biotite sand grains. Three types of coatings were observed : Type 3 : Limpid, non- laminated, isotropic coatings, pale yellow in plain polarized light (P.P.L.), occur in lower B and C horizons up to 5 m depth. In these coatings no crystalline
84
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd INTERNATIONAL SYMPOSIUM July, 2-8, 1990, Aix en Provence, France.
Prof ]e
i
p#
!13','
~:#c2,1 ........ :r, i
J
Figure 1 : Mineralogical quantification by point-counting of quartz and volcanic fragments in thin sections.
Figure 2 : Alteration imagaes of compound mineral gains. I : Desintegration of a granite fragment into Quartz (1), ironstained Biotite (2), and Alkalifeldspar (3). Neoformation of sand. II : Totally altered volcanic fragment (1). Neoformation of clay.
GEOCHEMISTRY OF THE, EARTH'S SURFACE AND OF MINERAL FORMATION 2rid INTERNATIONAL SYMPOSIUM July, 2-8, 1990, Aix en Provence, France.
85
TABLE 1. Microchemicalelemental and mineralogical compositionof neoformedproducts. SEM - EDXRA (massfraction, %)
X-ray diL of in sire microdfilled samples
A1203 SiO2 Fe203 TiO2 CaO M~O K20 Type 1 Type 2 Type 3 Type 4 Type 5
38 31
55 54
6.8 10.0 n.d.
0.3 2.0
0.0 0.0 0.6 0.8
Ca Qu Ka Sm Mi Am
0.1 0.7
* (*)
*
(*) (*)
Ca = Calcite, Qu = Quartz, Ka = Kaolinite, Sm= Smectite, Mi = Mica, Am = X-Ray amorphous
material could be detected by X-ray diffraction. (Table 1) and the A1/Si ratio (0.81) suggest that they consist of allophane-like material (Feytel et al., 1989). Liberation of A1 and Si predominantly from volcanic fragments, may be the source of type 3. Type 4 : Limpid, non-laminated, anisotropic, strong continuous, yellow clay coatings, occur mainly in the upper B horizon. X-ray diffraction shows the presence of kaolinite (Table 1). Type 5 (Fig. 3-V): Limpid non-laminated, anisotropic, speckled oriented, greenish yellow clay coatings, occur at a depth of 5m. X-ray diffraction shows the presence of smectite. Coatings of type 3 plus 4 or type 3 plus 5 may occur within one coating and are indistinguishable in P.P.L., suggesting an identical genesis. Crystallisation of coatings of type 3 apparently results in the formation of types 4 and 5. We explain the transformation to kaolinite at relatively shallow depth and to smectite at greater depth by differences in hydrochemical conditions. Stronger
leaching in the upper B horizons led to lower solute concentrations, more conductive to transformation of Al/Si gels into kaolinite. Higher solute concentrations, associated with longer residence time and decreased percolation rates of drainage water in the C horizon on 6 m depth, may have caused transformarion of A1/Si gels into smectite. REFERENCES
Bullock, P., N. Fedoroff, A. Jongerius, G. Stoops, and T. Tursina. 1985. Waine Res. Pub., Albrighton, England. pp 152. Feijtel, T.C., A.G. Jongmans, and J. van Doesburg. 1889. Soil Sci. Soc. Am. J. VoL 53 no 3: 876-882. Jongmans, A.G., T.C. Feijtel, R. Miedema, N. v Breemen and A. Veldkamp.1990. Subm. to Geoderma. Van der Plas, L and A.C. Tobi, 1965. Am. J. Sci., 263 : 8790.
83
M I C R O M O R P H O L O G Y AND MINERALOGY OF W E A T H E R I N G AND N E O F O R M A T I O N PHENOMENA IN A QUATERNARY T E R R A C E SEQUENCE OF THE ALLIER, LIMAGNE, FRANCE.
JONGMANS A.G., VAN DOESBURGL J.D.J. and VAN BREEMEN N. Department of Soil Science and Geology, Agricultural University P.O. Box, AA Wageningen, The Netherlands.
INTRODUCTION Soil formation is influenced by the weathering of primary minerals and rockfragments and by formation of secundary minerals. We studied the micromorphological and mineralogical aspects of mineral weathering in a Quaternary Allier terrace sequence, to characterize primary mineral weathering in relation to neoformation of secundary products. M A T E R I A L AND METHODS Nine gravelly terraces with various amounts of volcanic and granitic fragments and without calciumcarbonate were studied. Parent material in terms of textural and mineralogical composition of all terraces was found to be essentially similar (Jongmans et al., 1990). Micromorphology was described using the terminology of Bullock et al., 1985. Mineral grains were quantified by point-counting 500-700 points in thin sections (Van der Plas and Tobi, 1965). X-ray diffraction of microquantities in situ and microchemical analysis (SEM-EDXRA) was performed according the methods described in Feijtel et al., 1989. R E S U L T S AND DISCUSSION Three major soil groups with increasing soil age were distinguished. Group I, Eutric cambisols on holocene terraces : All soil horizons contain fresh volcanic (mainly basaltic) and granitic fragments, and quartz, alkalifeldspars, plagioelase, hornblende, pyroxene, olivine, muscovite and biotite. Weathering is restricted to some exfoliation and ironstaining of biotite and pellicular alteration of olivine.
Group II, Haplic Luvisols on Late pleistocene terraces. The mineralogical composition is similar to that of Group I, but surface horizons of Group II contain more quartz, and less volcanic fragments, suggesting distinct weathering of volcanic fragments and relative enrichment of quartz (Fig.l). Isotropic coatings of amorphous silica(type 1) and anisotropic coatings of calcite (type 2), point to precipitation of dissolved silica and calcium, liberated by weathering of volcanic fragments above that zone (Table 1). Group II, Eutric/Distric Planosols, on Early pleistocene terraces : Biotite, plagioclase, olivine and pyroxene grains are absent in A and E horizons, indicating that these minerals have been removed by weathering. Volcanic fragments are absent in A horizons, and their content in E and B horizons decreases with soil age (Fig.l). Substantial amounts of volcanic fragments in B horizons have been fragmented, deformed and transformed into clay which show well-developed b-fabrics. Transformation into clay contributes to the increase of the clay content in B horizons with soil age. Quartz content in A and E horizons shows an inverse relationship with depth and age, indicating strong weathering over greater depths in soils of group III. (Fig.l). Granite fragments, almost absent in A horizons, increased in content with depth. Alteration has led to disintegration and formation of quartz, alkali-feldspar, muscovite and biotite sand grains. Three types of coatings were observed : Type 3 : Limpid, non- laminated, isotropic coatings, pale yellow in plain polarized light (P.P.L.), occur in lower B and C horizons up to 5 m depth. In these coatings no crystalline
84
GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 2nd INTERNATIONAL SYMPOSIUM July, 2-8, 1990, Aix en Provence, France.
Prof ]e
i
p#
!13','
~:#c2,1 ........ :r, i
J
Figure 1 : Mineralogical quantification by point-counting of quartz and volcanic fragments in thin sections.
Figure 2 : Alteration imagaes of compound mineral gains. I : Desintegration of a granite fragment into Quartz (1), ironstained Biotite (2), and Alkalifeldspar (3). Neoformation of sand. II : Totally altered volcanic fragment (1). Neoformation of clay.
GEOCHEMISTRY OF THE, EARTH'S SURFACE AND OF MINERAL FORMATION 2rid INTERNATIONAL SYMPOSIUM July, 2-8, 1990, Aix en Provence, France.
85
TABLE 1. Microchemicalelemental and mineralogical compositionof neoformedproducts. SEM - EDXRA (massfraction, %)
X-ray diL of in sire microdfilled samples
A1203 SiO2 Fe203 TiO2 CaO M~O K20 Type 1 Type 2 Type 3 Type 4 Type 5
38 31
55 54
6.8 10.0 n.d.
0.3 2.0
0.0 0.0 0.6 0.8
Ca Qu Ka Sm Mi Am
0.1 0.7
* (*)
*
(*) (*)
Ca = Calcite, Qu = Quartz, Ka = Kaolinite, Sm= Smectite, Mi = Mica, Am = X-Ray amorphous
material could be detected by X-ray diffraction. (Table 1) and the A1/Si ratio (0.81) suggest that they consist of allophane-like material (Feytel et al., 1989). Liberation of A1 and Si predominantly from volcanic fragments, may be the source of type 3. Type 4 : Limpid, non-laminated, anisotropic, strong continuous, yellow clay coatings, occur mainly in the upper B horizon. X-ray diffraction shows the presence of kaolinite (Table 1). Type 5 (Fig. 3-V): Limpid non-laminated, anisotropic, speckled oriented, greenish yellow clay coatings, occur at a depth of 5m. X-ray diffraction shows the presence of smectite. Coatings of type 3 plus 4 or type 3 plus 5 may occur within one coating and are indistinguishable in P.P.L., suggesting an identical genesis. Crystallisation of coatings of type 3 apparently results in the formation of types 4 and 5. We explain the transformation to kaolinite at relatively shallow depth and to smectite at greater depth by differences in hydrochemical conditions. Stronger
leaching in the upper B horizons led to lower solute concentrations, more conductive to transformation of Al/Si gels into kaolinite. Higher solute concentrations, associated with longer residence time and decreased percolation rates of drainage water in the C horizon on 6 m depth, may have caused transformarion of A1/Si gels into smectite. REFERENCES
Bullock, P., N. Fedoroff, A. Jongerius, G. Stoops, and T. Tursina. 1985. Waine Res. Pub., Albrighton, England. pp 152. Feijtel, T.C., A.G. Jongmans, and J. van Doesburg. 1889. Soil Sci. Soc. Am. J. VoL 53 no 3: 876-882. Jongmans, A.G., T.C. Feijtel, R. Miedema, N. v Breemen and A. Veldkamp.1990. Subm. to Geoderma. Van der Plas, L and A.C. Tobi, 1965. Am. J. Sci., 263 : 8790.