Abstract
IN the Archæan black mica of Yitterby, very small, colourless, spherical, halo-like forms occur. The mica is, as it were, bleached within these halospheres. Many show a central opaque particle or nucleus. In the case of others it is difficult to be sure of the presence of a nucleus, or it may take the form of a limpid refracting particle. The optical appearances suggest that in these spheres the refractive index of the mica has been raised.
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JOLY, J. Haloes and Earth-History: A New Radioactive Element. Nature 109, 517–518 (1922). https://doi.org/10.1038/109517a0
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DOI: https://doi.org/10.1038/109517a0
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