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Micromorphological classification of gypsiferous soil materials
321
Micromorphological classification of gypsiferous soil materials G. Stoops1 and R.M. Poch2 'Lab. Mineralogy, Petrography and Micropedology, University of Ghent, Belgium 2Dept, Environment and Soil Science, University of Lleida, Spain ABSTRACT Stoops, G. and Poch, R.M., 1994. Micromorphological classification of gypsiferous soil materials. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Roc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 327-332.
The present approach to soil thin section description, based on a morpho-analytical study, deals with the lowest level of classification of materials, i.e. with the basic components of the soil and their fabric. The relation of such descriptions to soil characterization and classification is difficult and therefore a higher level of classification, using a more morpho-synthetic approach, is necessary to obtain insight into the diversity of soil materials and in the relationships between some specific features. An application of these new concepts to gypsiferous soils is presented. The micromorphological characteristics of gypsiferous soil materials have several features in common which often appear together and can be related to genetic, climatic and plant growth conditions. For these reasons an attempt has been made to classlfy the materials in terms of the proposed criteria. Initially, materials are classified according to their volumetric gypsum content. A further subdivision depends on the shape and appearance of gypsum and the composition of the micromass. The main crystal habits of gypsum in the soil (lenticular, fibrous, microcrystalline) and the degree of intergrowth are taken into account. Finally the particle size class and hydromorphic or decarbonation features are also considered. INTRODUCTION
This paper is an example of the classification of microfabrics of Stoops (1994), applied to soils with gypsum. The process of gypsification, or progressive enrichment of gypsum in the soil, has been recognized as such by several researchers (Barzanji and Stoops, 1974 Herrero et al., 1992), consisting of the crystallization of gypsum in pores and subsequent mixing within the gypsumfree groundmass. This process gives rise to soil materials with increasing amounts of gypsum showing characteristic fabrics, depending on the particle size distribution and structure characteristics of the host material. The last stage consists of horizons made almost completely of sand-sized lenticular gypsum crystals, which correspond to hypergypsic horizons (ICOMID, 1989; Eswaran and Zi-Tong, 1991). In other cases, when the soils are formed on gypsum rock, in margins of playa-like enviroments or in some gypsum desert crusts another type of
G . STOOPS & R.M. POCH
328 Table 1. Micromorphologicalclassification of gypsiferous materials.
Formation
Subformation
Phase
Syndrome
Eogypsic
lenticular petric fibrous lenticular petric fibrous lenticular petric microcrystalline lenticular microcrystalline petric
calcareous non-calcareous
decarbonated hydromorphic
id.
id.
id.
id.
Gypsic Hypergypsic Hologypsic
Texture: sand, silt, loam, clay, gravelly. hypergypsic horizon consisting mainly of silt-sized microcrystalline gypsum may occur (Warren, 1982; Watson, 1988; Herrero et al., 1992). Higher degrees of crystal intergrowth leading to cementation, decarbonatation of the gypsum-free groundmass, or hydromorphic features are phenomena frequently encountered in these materials. The micromorphological classification presented here takes into account all of these factors in a hierarchical way, classifying them into formations, subformations, texture and phases. Other processes recognized as overprints are designated as syndromes.
THE CLASSIFICATION SYSTEM The proposed classification system is summarized in Table 1 and additional details are presented below.
Formations Materials (whole thin sections or parts of it) are first classified into formations according to their gypsum content, since this is a parameter that is associated with other characteristics. The presence of gypsum restricts the range of possible formation processes of soil to certain genetic conditions and it constitutes a major constraint for plant growth. Furthermore it can be correlated with the results of chemical analyses. The indicated percentages of gypsum are given in volume, i.e. surface estimations over the area of the thin section occupied by solid material. It is a direct estimator of the bulk content of gypsum in the soil. Eogypsic gypsum content 4 0 % gypsum content 10-60% Gypsic gypsum content 60-90% Hypergypsic gypsum content >90% Hologypsic
CLASSIFICATION OF GYSIFEROUS SOIL MATERIALS
329
The criteria for these limits are: 10%: A smaller content is considered not to affect the plant growth. 60%: Limit for the Hypergypsic Horizon (ICOMID 1989) although it is referred to as a weight percentage by that committee. 90%: Separates hypergypsic materials from pure gypsum crusts. Subformations
Subdivision of formations depends on the morphology of the gypsum. Eogypsic: Infillings and/or coatings of loose lenticular crystals, coarse to fine Lenticular: sand in size; occurs in channels and chambers or as poikilotopic aggregations of gypsum and sand (Fig. la). I n f ~ g of s pores by xenotopichypidiotopic gypsum intergrowths Petric: (Fig. lb). Fibrous gypsum (satin spar) occurs as continuous infi'igs in planar Fibrous: voids (mainly in clayey soils). Gypsic: Idiomorphichypidiomorphic lenticular gypsum is found in channels, Lenticular: chambers and fissures, as i n f i i g s andor coatings, and also in the groundmass; irregularly distributed. Some regions of the groundmass may show isles fabric (Herrero et al., 1992) i.e., fragments of groundmass, deformed by biological activity, completely surrounded by loose gypsum. Gypsum crystals often have a crescent, bow-like distribution. Nests of celestite needles may be found (Fig. lc). As above, but with xenotopic/hypidiotopic crystal intergrowths. In the Petric: case of gravels they may form pendants of xenotopic lenticular gypsum "in palisade" (Fig. Id). As for eogypsic (Fig. le). Fibrous: Hypergypsic: Lenticular: As for Gypsic, but isles fabric (Herrero et al., 1992) more developed. Microcrystalline gypsum (silt size) may be found as nodules or infiilings (Fig. 2a). Petric: As for gypsic, but with more gypsum. Microcrystalline gypsum may be present as nodules or infillings (Fig. 2b). Microcrystalline: Silt-sized microcrystalline gypsum constitutes most of the thin section, appearing as faint yellow masses in PPL and almost isotropic in XPL. The isotropic quality is due to the presence of low amounts of very fme dispersed insoluble material and to the superposition of the small crystals (Fig. 2c). Hologypsic: Lenticular: Sand-sized lenticular gypsum, idio- or hypidiotopic, predominant in the thin section (Fig. 2d). Microcrystalline: Silt-sized microcrystalline gypsum predominant (Fig. 2e). Petric: Gypsum crystals, mostly sand size, are hypidio- or xenotopic, forming frequent intergrowths (Fig. 20.
330
G. STOOPS & R.M. POCH
Fig. 1. a) Lenticular Eogypsic, loam, calcareous (Spain); b) Petric Eogypsic, loam, calcareous (Spain); c) Lenticular Gypsic, loam, calcareous (Spain); d) Petric Gypsic, loam, calcareous, decarbonated syndrome (Syria); and e) Fibrous Gypsic loam, non-calcareous (Egypt). Frame length 10.3 mm.
CLASSIFICATION OF GYSIFEROUS SOIL MATERIALS
33 1
Fig. 2. a) Lenticular Hypergypsic, sand, calcareous (Spain); b) Petric Hypergypsic, noncalcareous (Syria); c) microcrystalline Hypergypsic, sand, calcareous (Spain); d) Lenticular Holoypsic, sand (Syria); e) Microcrystalline Holoypsic, loam (Spain); and 9 Petric Hologypsic (Syria). Frame length 10.3mm.
332
G. STOOPS AND R.M. POCH
Texture
The nomenclature for texture classes as suggested by Stoops (1994) may be applied. It can be used for the groundmass in the case of eogypsic and gypsic, and for a whole range of materials, including gypsum crystals, in hypergypsic and hologypsic types. It gives a better approximation to field texture than laboratory results. Moreover, in these cases an indication of the texture of the gypsum-free material would not have any meaning. Especially in the eogypsic and gypsic materials, a clear difference can be observed between clayey and loamy texture at one side and sandy at the other. In the former gypsum crystallizes in the biopores forming rather pure crystals, whereas in the latter larger crystals are found in the packing pores, often enclosing the sand grains (poikilotopic crystals). Phases
Calcareous: Non-calcareous:
The gypsum-free groundmass consists of a mixture of micritic calcite, clay and silt. The coarse elements are often polymictic. Micritic calcite is missing.
Syndromes (tentative)
Hydromorphic: Decarbonated:
Nodules, hypocoatings or punctuations of Fe-oxihydroxides, intercalations or hypocoatings of Fe-depleted groundmass. CaC03-depleted patches of groundmass, as intercalations or hypocoatings, showing stipple-speckled or granostriated b-fabric.
REFERENCES Barzanji, A.F. and Stoops, G., 1974. Fabric and mineralogy of gypsum accumulations in some soils of Iraq. Proc. 10th Int. Congress Soil Science (Moscow), Vol. VII:271-277. Eswaran, H. and Zi-Tong, G., 1991. Properties, genesis classification and distribution of soils with gypsum. In W.D. Nettleton (Editor), Occurrence, Characteristics and Genesis of Carbonate, Gypsum and Silica Accumulation in Soils. Soil Sci. SOC.Am. Publ., 26, pp. 89119. Herrero, J., Porta, J. and Fedoroff, N., 1992. Hypergypsic soil micromorphology and landscape relationships in Northeastern Spain. Soil Sci. SOC.Am. J., 56: 1188-1194. ICOMID - International Committee in Aridisols, 1989. Aridisols, Version 6.0. Draft. Stoops, G., 1994. Soil thin section description: higher levels of classification of microfabrics as a tool for interpretation. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology. Developments in Soil Science, 22. Elsevier, Amsterdam, pp. 317325. Warren, J.K., 1982. The hydrological setting, occurrence and significance of gypsum in Late Quaternary Salt Lakes in South Australia. Sedimentology, 29: 609-637. Watson, A., 1988. Desert gypsum crusts as palaeoenvironmental indicators: A micropetrographical study of crusts from Southern Tunisia and the Central Namib Desert. J. of Arid Environments, 15: 19-42.
Micromorphological classification of gypsiferous soil materials G. Stoops1 and R.M. Poch2 'Lab. Mineralogy, Petrography and Micropedology, University of Ghent, Belgium 2Dept, Environment and Soil Science, University of Lleida, Spain ABSTRACT Stoops, G. and Poch, R.M., 1994. Micromorphological classification of gypsiferous soil materials. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Roc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 327-332.
The present approach to soil thin section description, based on a morpho-analytical study, deals with the lowest level of classification of materials, i.e. with the basic components of the soil and their fabric. The relation of such descriptions to soil characterization and classification is difficult and therefore a higher level of classification, using a more morpho-synthetic approach, is necessary to obtain insight into the diversity of soil materials and in the relationships between some specific features. An application of these new concepts to gypsiferous soils is presented. The micromorphological characteristics of gypsiferous soil materials have several features in common which often appear together and can be related to genetic, climatic and plant growth conditions. For these reasons an attempt has been made to classlfy the materials in terms of the proposed criteria. Initially, materials are classified according to their volumetric gypsum content. A further subdivision depends on the shape and appearance of gypsum and the composition of the micromass. The main crystal habits of gypsum in the soil (lenticular, fibrous, microcrystalline) and the degree of intergrowth are taken into account. Finally the particle size class and hydromorphic or decarbonation features are also considered. INTRODUCTION
This paper is an example of the classification of microfabrics of Stoops (1994), applied to soils with gypsum. The process of gypsification, or progressive enrichment of gypsum in the soil, has been recognized as such by several researchers (Barzanji and Stoops, 1974 Herrero et al., 1992), consisting of the crystallization of gypsum in pores and subsequent mixing within the gypsumfree groundmass. This process gives rise to soil materials with increasing amounts of gypsum showing characteristic fabrics, depending on the particle size distribution and structure characteristics of the host material. The last stage consists of horizons made almost completely of sand-sized lenticular gypsum crystals, which correspond to hypergypsic horizons (ICOMID, 1989; Eswaran and Zi-Tong, 1991). In other cases, when the soils are formed on gypsum rock, in margins of playa-like enviroments or in some gypsum desert crusts another type of
G . STOOPS & R.M. POCH
328 Table 1. Micromorphologicalclassification of gypsiferous materials.
Formation
Subformation
Phase
Syndrome
Eogypsic
lenticular petric fibrous lenticular petric fibrous lenticular petric microcrystalline lenticular microcrystalline petric
calcareous non-calcareous
decarbonated hydromorphic
id.
id.
id.
id.
Gypsic Hypergypsic Hologypsic
Texture: sand, silt, loam, clay, gravelly. hypergypsic horizon consisting mainly of silt-sized microcrystalline gypsum may occur (Warren, 1982; Watson, 1988; Herrero et al., 1992). Higher degrees of crystal intergrowth leading to cementation, decarbonatation of the gypsum-free groundmass, or hydromorphic features are phenomena frequently encountered in these materials. The micromorphological classification presented here takes into account all of these factors in a hierarchical way, classifying them into formations, subformations, texture and phases. Other processes recognized as overprints are designated as syndromes.
THE CLASSIFICATION SYSTEM The proposed classification system is summarized in Table 1 and additional details are presented below.
Formations Materials (whole thin sections or parts of it) are first classified into formations according to their gypsum content, since this is a parameter that is associated with other characteristics. The presence of gypsum restricts the range of possible formation processes of soil to certain genetic conditions and it constitutes a major constraint for plant growth. Furthermore it can be correlated with the results of chemical analyses. The indicated percentages of gypsum are given in volume, i.e. surface estimations over the area of the thin section occupied by solid material. It is a direct estimator of the bulk content of gypsum in the soil. Eogypsic gypsum content 4 0 % gypsum content 10-60% Gypsic gypsum content 60-90% Hypergypsic gypsum content >90% Hologypsic
CLASSIFICATION OF GYSIFEROUS SOIL MATERIALS
329
The criteria for these limits are: 10%: A smaller content is considered not to affect the plant growth. 60%: Limit for the Hypergypsic Horizon (ICOMID 1989) although it is referred to as a weight percentage by that committee. 90%: Separates hypergypsic materials from pure gypsum crusts. Subformations
Subdivision of formations depends on the morphology of the gypsum. Eogypsic: Infillings and/or coatings of loose lenticular crystals, coarse to fine Lenticular: sand in size; occurs in channels and chambers or as poikilotopic aggregations of gypsum and sand (Fig. la). I n f ~ g of s pores by xenotopichypidiotopic gypsum intergrowths Petric: (Fig. lb). Fibrous gypsum (satin spar) occurs as continuous infi'igs in planar Fibrous: voids (mainly in clayey soils). Gypsic: Idiomorphichypidiomorphic lenticular gypsum is found in channels, Lenticular: chambers and fissures, as i n f i i g s andor coatings, and also in the groundmass; irregularly distributed. Some regions of the groundmass may show isles fabric (Herrero et al., 1992) i.e., fragments of groundmass, deformed by biological activity, completely surrounded by loose gypsum. Gypsum crystals often have a crescent, bow-like distribution. Nests of celestite needles may be found (Fig. lc). As above, but with xenotopic/hypidiotopic crystal intergrowths. In the Petric: case of gravels they may form pendants of xenotopic lenticular gypsum "in palisade" (Fig. Id). As for eogypsic (Fig. le). Fibrous: Hypergypsic: Lenticular: As for Gypsic, but isles fabric (Herrero et al., 1992) more developed. Microcrystalline gypsum (silt size) may be found as nodules or infiilings (Fig. 2a). Petric: As for gypsic, but with more gypsum. Microcrystalline gypsum may be present as nodules or infillings (Fig. 2b). Microcrystalline: Silt-sized microcrystalline gypsum constitutes most of the thin section, appearing as faint yellow masses in PPL and almost isotropic in XPL. The isotropic quality is due to the presence of low amounts of very fme dispersed insoluble material and to the superposition of the small crystals (Fig. 2c). Hologypsic: Lenticular: Sand-sized lenticular gypsum, idio- or hypidiotopic, predominant in the thin section (Fig. 2d). Microcrystalline: Silt-sized microcrystalline gypsum predominant (Fig. 2e). Petric: Gypsum crystals, mostly sand size, are hypidio- or xenotopic, forming frequent intergrowths (Fig. 20.
330
G. STOOPS & R.M. POCH
Fig. 1. a) Lenticular Eogypsic, loam, calcareous (Spain); b) Petric Eogypsic, loam, calcareous (Spain); c) Lenticular Gypsic, loam, calcareous (Spain); d) Petric Gypsic, loam, calcareous, decarbonated syndrome (Syria); and e) Fibrous Gypsic loam, non-calcareous (Egypt). Frame length 10.3 mm.
CLASSIFICATION OF GYSIFEROUS SOIL MATERIALS
33 1
Fig. 2. a) Lenticular Hypergypsic, sand, calcareous (Spain); b) Petric Hypergypsic, noncalcareous (Syria); c) microcrystalline Hypergypsic, sand, calcareous (Spain); d) Lenticular Holoypsic, sand (Syria); e) Microcrystalline Holoypsic, loam (Spain); and 9 Petric Hologypsic (Syria). Frame length 10.3mm.
332
G. STOOPS AND R.M. POCH
Texture
The nomenclature for texture classes as suggested by Stoops (1994) may be applied. It can be used for the groundmass in the case of eogypsic and gypsic, and for a whole range of materials, including gypsum crystals, in hypergypsic and hologypsic types. It gives a better approximation to field texture than laboratory results. Moreover, in these cases an indication of the texture of the gypsum-free material would not have any meaning. Especially in the eogypsic and gypsic materials, a clear difference can be observed between clayey and loamy texture at one side and sandy at the other. In the former gypsum crystallizes in the biopores forming rather pure crystals, whereas in the latter larger crystals are found in the packing pores, often enclosing the sand grains (poikilotopic crystals). Phases
Calcareous: Non-calcareous:
The gypsum-free groundmass consists of a mixture of micritic calcite, clay and silt. The coarse elements are often polymictic. Micritic calcite is missing.
Syndromes (tentative)
Hydromorphic: Decarbonated:
Nodules, hypocoatings or punctuations of Fe-oxihydroxides, intercalations or hypocoatings of Fe-depleted groundmass. CaC03-depleted patches of groundmass, as intercalations or hypocoatings, showing stipple-speckled or granostriated b-fabric.
REFERENCES Barzanji, A.F. and Stoops, G., 1974. Fabric and mineralogy of gypsum accumulations in some soils of Iraq. Proc. 10th Int. Congress Soil Science (Moscow), Vol. VII:271-277. Eswaran, H. and Zi-Tong, G., 1991. Properties, genesis classification and distribution of soils with gypsum. In W.D. Nettleton (Editor), Occurrence, Characteristics and Genesis of Carbonate, Gypsum and Silica Accumulation in Soils. Soil Sci. SOC.Am. Publ., 26, pp. 89119. Herrero, J., Porta, J. and Fedoroff, N., 1992. Hypergypsic soil micromorphology and landscape relationships in Northeastern Spain. Soil Sci. SOC.Am. J., 56: 1188-1194. ICOMID - International Committee in Aridisols, 1989. Aridisols, Version 6.0. Draft. Stoops, G., 1994. Soil thin section description: higher levels of classification of microfabrics as a tool for interpretation. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology. Developments in Soil Science, 22. Elsevier, Amsterdam, pp. 317325. Warren, J.K., 1982. The hydrological setting, occurrence and significance of gypsum in Late Quaternary Salt Lakes in South Australia. Sedimentology, 29: 609-637. Watson, A., 1988. Desert gypsum crusts as palaeoenvironmental indicators: A micropetrographical study of crusts from Southern Tunisia and the Central Namib Desert. J. of Arid Environments, 15: 19-42.