Coupled substitutions in the tourmaline group |
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Authors: | Franklin F Foit Jr Philip E Rosenberg |
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Affiliation: | (1) Department of Geology, Washington State University, 99163 Pullman, Washington, USA |
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Abstract: | Statistical analysis of 136 natural tourmaline compositions from the literature reveals the presence and extent of coupled substitutions involving several cations and structural sites. In schorls and dravites these are a dehydroxylation type substitution (1) (OH)–+R2+ = R3++O2– and an alkali-defect type substitution (2) R++R2+ = R3++, Al3+ being the predominant R3+ action. Substitution (1) which represents solid solution towards a proton-deficient end-member, R+ R
3
3+
R
6
3+
(BO3)3 Si6O18O3(OH), accounts for three times as much of the observed compositional variability as does (2) which represents substitution toward a hypothetical alkali-free end-member, (R
2
2+
R3+) R
6
3+
(BO3)3Si6O18(OH)4. The occurrence of both of these substituions produces intermediates between end-member schorl/ dravite, R+ R
3
2+
R
6
3+
(BO3)3Si6O18(OH)4, and a new series within the tourmaline group, R
1–x
+
R
3
3+
R
6
3+
(BO3)3Si6O18O3–x (OH)1+x.In addition to dehydroxylation type, 2(OH)–+Li+ = R3++202–, and possibly alkali-defect type, 2R++Li+ = R3++2, substitutions, a third type Li++O2– = (OH)–+, occurs in the elbaites giving rise to Li-poor, proton-rich species. All three substitutions serve to reduce the Li-content of natural elbaite which, as a result, does not attain the composition of the ideal end-member, Na(Li1.5Al1.5)Al6(BO3)3Si6O18(OH)4. Substitution from elbaite and schorl/dravite toward R
1–x
+
R
3
3+
R
6
3+
(BO3)3Si6O18O3–x(OH)1+x is very extensive and may be complete.Substitution toward R
1–x
+
R
3
3+
R
6
3+
(BO3)3Si6O18O3–x(OH)1+x results in improved local charge balance. The mean deviation
from oxygen charge saturation
is at a maximum in end-member schorl, dravite and elbaite. Substitutions (1) and (2) progressively decrease
but substitution (1) does so more effectively, which may explain its predominance in nature. However, alkali-defective end-members appear to be unstable regardless of
. Substitution (3) in the elbaites cannot be discussed on the basis of charge balance considerations at present due to the lack of structural information on proton-rich species. |
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Keywords: | |
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