Spheroidally weathered boulders occur in Deccan Basalt near Sagar, India. A simplified weathering model was developed to depict the alteration of the three primary minerals bytownite, augite and magnetite to an assemblage of beidellite, kaolinite and goethite. It was concluded that the thin concentric shells characteristic of the boulders resulted from an increase in volume during weathering. An iterative computer program showed that maximum expansion can be achieved through an appropriate combination of initial modal composition, fraction of each mineral altered and the relative proportions of beidellite, kaolinite and goethite in the weathered rock.

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