Clay mineralogy of soils formed in tuffaceous greywacke, Southland, New Zealand, in relation to genesis, soil properties and classification

Abstract

The clay mineralogy of 12 soils (Dystrochrepts, a Eutrochrept, a Cryochrept and a Placaquept) formed in tuffaceous greywacke parent rocks is presented and discussed. In a New Zealand context, the soils are unusual because of their base-rich parent material which has been partly pre-weathered to smectite and kaolin minerals in geological time. Superimposed on this assemblage are the affects of present climate and soil drainage, which have altered smectite and vermiculite to dioctahedral (aluminous) chlorite. Conventional laboratory treatments cause dioctahedral chlorite to revert fully to smectite or vermiculite, or alternatively partially to interlayered hydrous mica. The labile nature of the interlayer Al is evident in high KCI-Al values. Allophane and gibbsite occur in acid upland soils that are also trace-element deficient. More intense leaching of upland soils with respect to lowland soils accounts for the upland soils' clay mineralogy and trace element deficiencies. The soils fall into three mineralogy classes of Soil Taxonomy and six classes of the proposed Whitton and Childs revision. The classes are not readily usable in the field, and subgroup or family distinctions based on simple chemical tests are suggested.