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Kaolinite and halloysite derived from sequential transformation of pedogenic smectite and kaolinite-smectite in a 120 ka tropical soil chronosequence

P. C. Ryan, F. J. Huertas, F. W. C. Hobbs and L. N. Pincus
Kaolinite and halloysite derived from sequential transformation of pedogenic smectite and kaolinite-smectite in a 120 ka tropical soil chronosequence (in Clays in critical zone, Paul A. Schroeder (editor))
Clays and Clay Minerals (October 2016) 64 (5): 639-667

Abstract

Tropical soils range from nutrient-depleted lateritic soils rich in halloysite or kaolinite to Inceptisols rich in interstratified kaolinite-smectite (K-S), smectite, or related 2:1 clays. Given the strong influence of clay minerals on tropical soil quality, better understanding of factors influencing their occurrence is important for modeling and managing tropical environments. This study examines the alteration of smectite to kaolinite by way of intermediate K-S and halloysite in a 120 ka moist tropical chronosequence. Iron-rich smectite (11.6 + or - 2.2% Fe (sub 2) O (sub 3) ) is the dominant mineral in Holocene soils (1-8 ka) originating from sediments rich in plagioclase and clinopyroxene. The cation exchange capacity (CEC) of smectite is 54-84 cmol (sub c) /kg and pH is 6.1 to 7.4. Within 50 ka, smectite fixes Al-hydroxy complexes into interlayers, K (super +) is retained preferentially over Ca (super 2+) , and 2:1 layers are stripped of tetrahedral sheets; the resulting K-S inherits flaky smectite crystal habit and the 2:1 layers - which only expand partially - include Al-hydroxy smectite and some illite-like layers. After 50 ka, the dominant mineral is K-S, the CEC is 18-28 cmol (sub c) /kg, and the pH is 5.3. Flaky Fe-kaolinite with approximately 10% residual smectite layers and halloysite (7.4% Fe (sub 2) O (sub 3) ) also occur in 50 ka soil. The 120 ka soils are dominated by flaky Fe-kaolinite (<10% residual smectite layers) and halloysite (4.9% Fe (sub 2) O (sub 3) ), and Fe-poor hexagonal kaolinite also occurs (5-10% of soil). The CEC is 11-16 cmol (sub c) /kg and the pH is 4.7-5.3. Changes in crystal chemistry of the soil clays (decreasing Fe, Mg, Ca, and K; increasing Al) over time reflects two reaction mechanisms: (1) cell-preserved transformation of smectite layers to kaolinite layers that accompanies conversion of smectite to K-S and eventually kaolinite; this results in the formation of flaky Fe-rich kaolinites after 50 ka; and (2) dissolution of K-S followed by crystallization of halloysite. Neoformation of hexagonal kaolinite and/or halloysite with low Fe (<3% Fe (sub 2) O (sub 3) ) follows dissolution of Fe-kaolinite or halloysite after 100 ka. This sequence is probably common in moist tropical soils and these findings may inform modeling of soil composition in tropical landscapes where tectonic, volcanic, or geomorphic activity periodically exposes unweathered parent material, producing a range of soil ages.


ISSN: 0009-8604
EISSN: 1552-8367
Coden: CLCMAB
Serial Title: Clays and Clay Minerals
Serial Volume: 64
Serial Issue: 5
Title: Kaolinite and halloysite derived from sequential transformation of pedogenic smectite and kaolinite-smectite in a 120 ka tropical soil chronosequence
Title: Clays in critical zone
Author(s): Ryan, P. C.Huertas, F. J.Hobbs, F. W. C.Pincus, L. N.
Author(s): Schroeder, Paul A.editor
Affiliation: Middlebury College, Department of Geology, Middlebury, VT, United States
Affiliation: University of Georgia, Department of Geology, Athens, GA, United States
Pages: 639-667
Published: 201610
Text Language: English
Publisher: Clay Minerals Society, Chantilly, VA, United States
Meeting name: 2015 EuroClay conference
Meeting location: Edinburgh, GBR, United Kingdom
Meeting date: 20150705July 5-10, 2015
References: 135
Accession Number: 2017-014455
Categories: Quaternary geologySedimentary petrology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, sketch map
N09°30'00" - N10°15'00", W85°45'00" - W84°52'00"
Secondary Affiliation: Universidad de Granada, ESP, SpainUniversity of Wisconsin, USA, United StatesYale University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Clay Minerals Society. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201710
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