Details

Autor(en) / Beteiligte

• Kautz, Christopher Q
• Ryan, Peter C

Titel

The 10 Å to 7 Å halloysite transition in a tropical soil sequence, Costa Rica

Ist Teil von

• Clays and clay minerals, 2003-06, Vol.51 (3), p.252-263

Ort / Verlag

Boulder, CO: Clay Minerals Society

Erscheinungsjahr

2003

Link zum Volltext

Quelle

SpringerLink (Online service)

Beschreibungen/Notizen

• Soils developed on Pleistocene andesitic lava flows and fluvial detritus in the Atlantic coastal plain of Costa Rica display a clay mineral assemblage that includes 10 Å and 7 Å halloysite and lesser amounts of kaolinite and dioctahedral vermiculite. Other secondary minerals include gibbsite, goethite, hematite, maghemite, allophane and amorphous Al hydroxides. Active floodplain soils are dominated by 10 Å halloysite and contain less allophane, while soil clays from Pleistocene terraces consist of a mixture of 10 Å and 7 Å halloysite as well as less dioctahedral vermiculite, kaolinite, and amorphous Al hydroxides. Residual soils formed on Pleistocene lava flows are dominated by 7 Å halloysite with less abundant kaolinite, dioctahedral vermiculite, 10 Å halloysite and amorphous Al hydroxides. This sequence suggests transformations of 10 Å halloysite to 7 Å halloysite and allophane to amorphous Al hydroxides with time. The presence of 10 Å halloysite in Pleistocene terrace soils implies slow reaction rates or metastability. Quantitative X-ray diffraction (QXRD) analysis indicates a decrease in the amount of plagioclase feldspar from 34 wt.% in the 1-2 year floodplain to 0-1.6% in terrace and residual soils. Plagioclase weathering is paralleled by the formation of dioctahedral clay, allophane and Al hydroxides. Analysis by QXRD also indicates that crystalline minerals comprise 70-95% of the soil fraction, implying 5-30% X-ray-amorphous material. These data are verified by selective extraction using ammonium oxalate, which indicates 8-30% amorphous material. Chemical analysis of the extractant by inductively coupled plasma-atomic emission spectrometry indicates that allophane (A1:Si ratios of 0.92-3.82) occurs in floodplain and some terrace soils while amorphous Al hydroxides appear to coexist with allophane in Pleistocene terrace and residual soils with A1:Si ratios of 6.53-8.53. Retention of Mg to a greater extent than Na, Ca and K suggests Mg incorporation into hydroxide sheets in dioctahedral vermiculite as well as substitution into hydroxides.