Chemical anomalies of lunar plagioclase,described by substitution vectors and their relation to optical and structural properties |
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| Authors: | H -R Wenk W R Wilde |
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| Institution: | (1) Department of Geology and Geophysics and Space Sciences Laboratory, University of California at Berkeley, USA |
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| Abstract: | Lunar plagioclase posed an enigma because optical properties indicate slow cooling while structural properties suggest rapid
quenching when compared directly to terrestrial plagioclase. If, however, the An-content is inferred from the Al/Si ratio
rather than from the Ca/Na ratio, lunar plagioclase corresponds more closely to normal volcanic plagioclase. The differences
between An=4(Al/(Al+Si))-1 and An=Ca/(Ca+Na+K) is caused by substitutions which appear so far to be unique to lunar feldspars.
A vector representation of chemical composition makes it possible to single out several substitutions which account for the
chemical peculiarities of lunar plagioclase. It permits to assign sites in the structure to various minor elements simply
from the chemical analysis without having to rely on sophisticated spectroscopic techniques or an X-ray structure determination.
1. The deficiency in Al+Si (up to 0.06 per formula unit) is compensated for by Fe, Mg, and small amounts of Na or Ca occupying
tetrahedral sites. The Ca/Na ratio is increased accordingly to balance the electric charge. This substitution accounts for
most of the differences. 2. The vacancycoupled substitution Ca+□ → 2 Na (Weill et al., 1970) is another cause for the increased Ca/Na ratio. 3. The amount of alkali evaporation from plagioclase, if it occurred,
is probably small. These unusual substitutions are an expression of special conditions on the moon during the crystallization
of plagioclase. Crystallization at high temperatures, rapid cooling, and peculiar magma composition seem to be important factors,
but experimental work and detailed study of terrestrial volcanic plagioclase are necessary to interpret the chemical composition
quantitatively. |
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