Petroduric and ‘petrosepiolitic’ horizons in soils of namaqualand, South Africa | Horizontes petrodúricos e “petrosepiolíticos” em solos de Namaqualand, África do Sul | Horizontes petrodúricos y “petrosepiolíticos” en suelos de Namaqualand, Suráfrica

M.L. Francis, Martin Fey, F. Ellis, M. Poch

Research output: Contribution to journalArticle

7 Citations (Scopus)

Abstract

© 2012, Universia. All rights reserved. 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.
Original languageEnglish
Pages (from-to)8-25
JournalSpanish Journal of Soil Science
Volume2
Issue number1
DOIs
Publication statusPublished - 2015

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sepiolite
soil horizons
silica
South Africa
slaking
matrix
micromorphology
soil
void
microstructure
coatings
coating
silcrete
illuviation
birefringence
calcrete
Crete
soil classification
polarized light
iron oxides

Cite this

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title = "Petroduric and ‘petrosepiolitic’ horizons in soils of namaqualand, South Africa | Horizontes petrod{\'u}ricos e “petrosepiol{\'i}ticos” em solos de Namaqualand, {\'A}frica do Sul | Horizontes petrod{\'u}ricos y “petrosepiol{\'i}ticos” en suelos de Namaqualand, Sur{\'a}frica",
abstract = "{\circledC} 2012, Universia. All rights reserved. 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.",
author = "M.L. Francis and Martin Fey and F. Ellis and M. Poch",
year = "2015",
doi = "10.3232/SJSS.2012.V2.N1.01",
language = "English",
volume = "2",
pages = "8--25",
journal = "Spanish Journal of Soil Science",
issn = "2253-6574",
publisher = "Universia Espana",
number = "1",

}

TY - JOUR

T1 - Petroduric and ‘petrosepiolitic’ horizons in soils of namaqualand, South Africa | Horizontes petrodúricos e “petrosepiolíticos” em solos de Namaqualand, África do Sul | Horizontes petrodúricos y “petrosepiolíticos” en suelos de Namaqualand, Suráfrica

AU - Francis, M.L.

AU - Fey, Martin

AU - Ellis, F.

AU - Poch, M.

PY - 2015

Y1 - 2015

N2 - © 2012, Universia. All rights reserved. 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.

AB - © 2012, Universia. All rights reserved. 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.

U2 - 10.3232/SJSS.2012.V2.N1.01

DO - 10.3232/SJSS.2012.V2.N1.01

M3 - Article

VL - 2

SP - 8

EP - 25

JO - Spanish Journal of Soil Science

JF - Spanish Journal of Soil Science

SN - 2253-6574

IS - 1

ER -