Manganiferous soil concretions have been located in pineapple
and sugar cane plantations of the Schofield Plateau, Oahu, and black manganiferous
coatings on the surface of soils have been observed on the crests of
embankments next to pineapple plantations on Oahu, Molokai, and Lanai.
The concretions and coatings are found inoxisols derived from volcanic
parent material. The moisture regime of the soils is characterized by alternate
wet and dry periods. This alternation facilitates remobilization and reprecipitation
of manganese and to a much lesser extent iron and associated trace metals
in the soils. Optimum conditions for manganiferous soil concretion development
are encountered on the Schofield Plateau, where mean annual rainfall exceeds
1000 mm/year but where there is a net evaporation loss from the soils during
the dry period (May through October). Remobilization of manganese is less
pronounced in the oxisols of Molokai and Lanai, where mean annual rainfall
is lower. Thin manganiferous coatings rather than concretions therefore form
in these islands.
The contents of Mn, Fe, Cu, and Ni of the soils on which Hawaiian concretions
form are higher than those of New Zealand soils in which manganese
soil concretions are found. This is reflected by the much redder color of the
Hawaiian concretion-bearing soils compared with their New Zealand counterparts.
Rare earths show an enrichment sequence: parent rock ---+ soil ---+ soil
concretion. A small positive Ce anomaly is noted in the soil and is more marked
in the concretions. This may be due to the increasing oxidation of Ce in the
concretions relative to the soil, although contamination of the soil by concretionary
material may also playa role. The parent rock shows no Ce anomaly.
Barium and Pb are enriched in concretions relative to the surrounding soil,
whereas D, Th, Hf, Nb, Zr, and As are present in similar or higher concentrations
in the soil relative to the concretions. Arsenic may follow Fe in the
soils. Todorokite is the predominant manganese mineral in the Hawaiian soil
concretions, not pyrolusite as previously reported. Scanning electron microscope
studies show the crystalline nature of the manganese oxide minerals
in the concretions
