Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa
Indurated, light-coloured 'sepiocrete' horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quanti...
| Autores: | , , , |
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| Formato: | artículo |
| Estado: | Versión publicada |
| Fecha de publicación: | 2012 |
| País: | España |
| Recursos: | Universitat de Lleida (UdL) |
| Repositorio: | Repositori Obert UdL |
| OAI Identifier: | oai:repositori.udl.cat:10459.1/59295 |
| Acesso em linha: | https://doi.org/10.3232/SJSS.2012.V2.N1.01 http://hdl.handle.net/10459.1/59295 |
| Access Level: | acceso abierto |
| Palavra-chave: | Sòls Edafologia Soils Soil science |
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Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa |
| title |
Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa |
| spellingShingle |
Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa Francis, M. L. Sòls Edafologia Soils Soil science |
| title_short |
Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa |
| title_full |
Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa |
| title_fullStr |
Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa |
| title_full_unstemmed |
Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa |
| title_sort |
Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South Africa |
| dc.creator.none.fl_str_mv |
Francis, M. L. Fey, M. V. Ellis, F. Poch, Rosa M. |
| author |
Francis, M. L. |
| author_facet |
Francis, M. L. Fey, M. V. Ellis, F. Poch, Rosa M. |
| author_role |
author |
| author2 |
Fey, M. V. Ellis, F. Poch, Rosa M. |
| author2_role |
author author author |
| dc.subject.none.fl_str_mv |
Sòls Edafologia Soils Soil science |
| topic |
Sòls Edafologia Soils Soil science |
| description |
Indurated, light-coloured 'sepiocrete' horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quantities of sepiolite in the bulk-soil was confirmed by XRD analysis. The degree of induration in some these horizons suggested cementation by silica, and so in this paper the slaking properties, bulk chemistry, mineralogy and micromorphology of these horizons are compared with the typical silica-cemented, reddish-brown petroduric/duripan (dorbank) encountered in the region. 'Sepiocrete' horizons are chemically, mineralogically and morphologically distinct from the petrocalcic and petroduric horizons with which they are commonly associated. Micromorphology of the petroduric horizons revealed prominent illuviation in the form of oriented clay parallel to grains and crescent coatings on voids, a red matrix due to iron oxides, and translucent, isotropic amorphous silica coatings on grains and voids. In the 'sepiocrete' horizons, sepiolite appeared as a matrix of interlocking, sub-parallel fibres while the amorphous material was localised. The amorphous material was silica-rich with prominent aluminium and lesser magnesium; light brown under plane polarised light; not completely isotropic and had a lower birefringence than the sepiolite. The calcite was usually micritic, but also appeared as loose granules and as elongate crystals in a sepiolite matrix. The presence of the laminar Si-Al -rich areas on the sections suggested at the least localised duric properties and so mutual reinforcement of sepiolite and silica is possible. However, the 'sepiocrete' horizons did not meet the slaking requirements of the petroduric (dorbank) horizons and are distinct in appearance to the typical petroduric horizons in the region. They contained more MgO than the region's typical petroduric, and too little SiO2 to be silcrete. While the '-crete' terminology provides a useful expression of the cemented nature of the horizon, in order to fit existing soil classification and description schemes the terms 'sepiolitic' and 'petrosepiolitic' (in the same sense as 'calcic' and 'petrocalcic') are proposed and defined. The term 'sepiolitic' would be useful in the adjectival form in petrocalcic or petroduric horizons where sepiolite is significant but not the primary cement. The genesis of the 'petrosepiolitic' horizons is likely to be essentially similar to that of petrocalcic and petroduric horizons, except for chemical differences in the matrix solutions from which secondary minerals were precipitated, dictated by the pH and evaporative evolution of the soil solution. |
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2012 |
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2012 |
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info:eu-repo/semantics/article info:eu-repo/semantics/publishedVersion |
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article |
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https://doi.org/10.3232/SJSS.2012.V2.N1.01 http://hdl.handle.net/10459.1/59295 |
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https://doi.org/10.3232/SJSS.2012.V2.N1.01 http://hdl.handle.net/10459.1/59295 |
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Inglés |
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Inglés |
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Reproducció del document publicat a: https://doi.org/10.3232/SJSS.2012.V2.N1.01 Spanish Journal of Soil Science, 2012, vol. 2, núm. 1, p. 8-25 |
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cc-by-nc (c) Francis, M. L. et al., 2012 info:eu-repo/semantics/openAccess http://creativecommons.org/licenses/by-nc/3.0/es/ |
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cc-by-nc (c) Francis, M. L. et al., 2012 http://creativecommons.org/licenses/by-nc/3.0/es/ |
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openAccess |
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reponame:Repositori Obert UdL instname:Universitat de Lleida (UdL) |
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Petroduric and 'petrosepiolitic' horizons in soils of Namaqualand, South AfricaFrancis, M. L.Fey, M. V.Ellis, F.Poch, Rosa M.SòlsEdafologiaSoilsSoil scienceIndurated, light-coloured 'sepiocrete' horizons have been found in Namaqualand Calcisols and Durisols. These horizons resembled calcrete but were non- to only mildly calcareous, resisted slaking in acid and alkali, and often broke with a conchoidal fracture. The presence of elevated quantities of sepiolite in the bulk-soil was confirmed by XRD analysis. The degree of induration in some these horizons suggested cementation by silica, and so in this paper the slaking properties, bulk chemistry, mineralogy and micromorphology of these horizons are compared with the typical silica-cemented, reddish-brown petroduric/duripan (dorbank) encountered in the region. 'Sepiocrete' horizons are chemically, mineralogically and morphologically distinct from the petrocalcic and petroduric horizons with which they are commonly associated. Micromorphology of the petroduric horizons revealed prominent illuviation in the form of oriented clay parallel to grains and crescent coatings on voids, a red matrix due to iron oxides, and translucent, isotropic amorphous silica coatings on grains and voids. In the 'sepiocrete' horizons, sepiolite appeared as a matrix of interlocking, sub-parallel fibres while the amorphous material was localised. The amorphous material was silica-rich with prominent aluminium and lesser magnesium; light brown under plane polarised light; not completely isotropic and had a lower birefringence than the sepiolite. The calcite was usually micritic, but also appeared as loose granules and as elongate crystals in a sepiolite matrix. The presence of the laminar Si-Al -rich areas on the sections suggested at the least localised duric properties and so mutual reinforcement of sepiolite and silica is possible. However, the 'sepiocrete' horizons did not meet the slaking requirements of the petroduric (dorbank) horizons and are distinct in appearance to the typical petroduric horizons in the region. They contained more MgO than the region's typical petroduric, and too little SiO2 to be silcrete. While the '-crete' terminology provides a useful expression of the cemented nature of the horizon, in order to fit existing soil classification and description schemes the terms 'sepiolitic' and 'petrosepiolitic' (in the same sense as 'calcic' and 'petrocalcic') are proposed and defined. The term 'sepiolitic' would be useful in the adjectival form in petrocalcic or petroduric horizons where sepiolite is significant but not the primary cement. The genesis of the 'petrosepiolitic' horizons is likely to be essentially similar to that of petrocalcic and petroduric horizons, except for chemical differences in the matrix solutions from which secondary minerals were precipitated, dictated by the pH and evaporative evolution of the soil solution.En Calcisols y Durisols de Namaqualand se encuentran horizontes endurecidos y de colores claros denominados “sepiocretas”. Estos horizontes son similares a calcretas pero no son calizos -o solo ligeramente-, resisten el colapso en ácidos y álcalis y tienen a menudo una fractura concoidal. Mediante análisis de difracción de RX se confirmó la presencia de elevadas cantidades de sepiolita. Debido a que la dureza de algunos de estos horizontes sugiere que están cementados por sílice, el objetivo de esta investigación es comparar su composición química, mineralógica y micromorfológica, así como su colapso, con las de los horizontes petrodúricos/duripanes (“dorbank”) pardo-rojizos cementados por sílice que se encuentran en la región. Los horizontes “sepiocreta” se distinguen de los horizontes petrocálcicos y petrodúricos con los que están comúnmente asociados desde los puntos de vista químico, mineralógico y morfológico. Los horizontes petrodúricos muestran una intensa iluviación en forma de arcilla orientada paralelamente a los granos y de revestimientos crecientes en huecos; una matriz rojiza debida a óxidos de hierro; y revestimientos isotrópicos amorfos translúcidos de sílice en granos y huecos. En los horizontes de “sepiocreta” la sepiolita se encuentra como una matriz de fibras entrelazadas subhorizontales, yuxtapuesta a un material amorfo compuesto principalmente de sílice, con considerables cantidades de aluminio y menos magnesio. Éste último es pardo claro en luz polarizada paralela, no completamente isotrópico y con una menor birrefringencia que la sepiolita. La calcita es normalmente micrítica, aunque aparece también como gránulos sueltos y cristales elongados en una matriz de sepiolita. La presencia de áreas con estructura laminar ricas en Si y Al sugiere que existen propiedades dúricas localizadas, por lo que no se puede descartar un refuerzo mutuo entre la sepiolita y la sílice. A pesar de ello, los horizontes de “sepiocreta” no cumplen los requisitos de colapso de los horizontes petrodúricos (“dorbank”) en la región, de los que difieren en aspecto. Contienen más MgO que los horizontes petrodúricos asociados, y no contienen suficiente SiO2 para ser clasificados como silcreta. Si bien la terminología “-creta” es útil para designar la naturaleza cementada de un horizonte, se proponen específicamente los términos “sepiolítico” y “petrosepiolítico” (análogos a “cálcico” y “petrocálcico”) para adaptarse a los sistemas de descripción y clasificación de suelos actuales. El término “sepiolítico” podría utilizarse como adjetivo en horizontes petrocálcicos o petrodúricos en que la sepiolita no es el principal agente cementante. Probablemente la génesis de los horizontes petrosepiolíticos es similar a la de los horizontes petrocálcicos y petrodúricos, excepto por las diferencias en la composición química de las soluciones matriz a partir de las que han precipitado los minerales secundarios, regidos por el pH y por la evolución de la solución del suelo a medida que se evaporaValuable comments were provided by J.J.N. Lambrechts and two anonymous referees. Technical assistance was received from M. Antúnez Pujol (University of Lleida, Spain), D. Wil son (University of Cape Town), B. White and V.J. Bandu (University of KwaZulu-Natal), R. Bucher (iThemba Labs), W. de Clercq, M. Gordon, R. O’Brien, E. Spicer, N. Steenkamp, N. van der Merwe and T. Vilakhazi (University of Stellenbosch). Financial and logistical support from De Beers Namaqualand Mines Limited is gratefully acknowledged. Financial support was also provided by grant number 2047381 from the National Research FoundationUniversia2012info:eu-repo/semantics/articleinfo:eu-repo/semantics/publishedVersionapplication/pdfhttps://doi.org/10.3232/SJSS.2012.V2.N1.01http://hdl.handle.net/10459.1/59295reponame:Repositori Obert UdL instname:Universitat de Lleida (UdL)InglésReproducció del document publicat a: https://doi.org/10.3232/SJSS.2012.V2.N1.01Spanish Journal of Soil Science, 2012, vol. 2, núm. 1, p. 8-25cc-by-nc (c) Francis, M. L. et al., 2012info:eu-repo/semantics/openAccesshttp://creativecommons.org/licenses/by-nc/3.0/es/oai:repositori.udl.cat:10459.1/592952026-06-24T12:42:17Z |
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