Abstract
IN the course of a series of determinations by the rubidium-87 → strontium-87 method of the ages of pegmatitic micas, it has been found that the method of separating and sampling such materials involves a possible source of error which we feel should be known to workers in this field and to those who prepare and select material for them. The effect is illustrated by measurements on a lepidolite from Pope's Claim, near Salisbury, Southern Rhodesia, of which the hand sample consists of a roughly elliptical ‘book’ of mica with axes 6 in. and 4 in. and 1 ½ in. thick. The book contains three sharply differentiated types of mica : (a) purple biaxial (2V ≈ 30°), (b) greenish-white biaxial (2V ≈ 40°) and (c) purple uniaxial. The uniaxial mica lies between the two biaxial forms and in two isolated patches at the edge of the hand sample. The boundaries between the micas are perpendicular to the plane of the mica sheets and penetrate the full thickness of the book.
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Aldrich, L. T., Doak, J. B., and Davis, G. L., Amer. J. Sci., 251, 377 (1953).
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SCHREINER, G., JAMIESON, R. & SCHONLAND, B. Age Measurements on a Pegmatitic Mica from the Rhodesian Shield. Nature 175, 464 (1955). https://doi.org/10.1038/175464a0
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DOI: https://doi.org/10.1038/175464a0
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