Studies were made of giant haloes in two biotite samples. Microphotometry at known wavelengths in the range 350–1100 nm gave comparative transmission data for material outside the haloes, in the bleached haloes, and within their inner edges. Mössbauer spectra were obtained for similar non-halo biotite samples. All results are consistent with the theory (Vance, 1978) that the bleached haloes formed through the reducing action on ferric iron of atomic hydrogen, produced by radiolysis.

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