plinthite

(from the article `Plinthosol`) ...conditions. They are defined by a subsurface layer containing an iron-rich mixture of clay minerals (chiefly kaolinite) and silica that hardens on ...
Found on http://www.britannica.com/eb/a-z/p/80

Plinthite

Plinthite (from the Greek plinthos, brick) is an iron-rich, humus-poor mixture of clay with quartz and other minerals. Plinthite is a redoximophic feature in highly weathered soil. The product of pedogenesis, it commonly occurs as dark red redox concretions that usually form platy, polygonal, or reticulate patterns. Plinthite changes irreversibly ...
Found on http://en.wikipedia.org/wiki/Plinthite
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