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Description
In the Santa Margarita Ecological Reserve (SMER), southern California, U.S.A., spheroidally weathered corestones are common components within the regolith. At SMER, we undertook a study aimed at documenting the three-dimensional element redistribution patterns around a single spheroidally weathering corestone. In saprock surrounding the spheroidally weathered corestone, biotite, the most severely weathered mineral, commonly is split parallel to {001}, and seams of Fe and Mn oxides or oxyhydroxides parallel this cleavage direction. Additionally, pleochroism is anomalous. XRD data indicate that throughout the section of regolith containing the spheroidally weathered corestone, biotite has been converted mostly to vermiculite. Utilizing Ti as an immobile element, calculations show that the mass of K, relative to the corestone, is depleted. In contrast, only minor and irregular losses of Si, Al, Ca, and Na mass are evident, while no redistribution of P, Fe, Mn, and Mg mass occurred. The above observations are consistent with the idea that the oxidation of Fe produces a charge imbalance within the biotite structure, and as a result interlayer K is replaced with hydrated cationic complexes, and (001) d expands from 10 Å to ~14 Å. Complete transformation of a biotite grain to vermiculite would thus involve an ~40% volumetric expansion. Given that biotite makes up ~9% of saprock samples, average volumetric strains derived from the transformation of biotite to vermiculite would have been 40 * 0.09 = 3.6%. Such strains exceeded the breaking strength of the host rock, and are the main causes of spheroidal weathering in the studied outcrop.
