Abstract
A new analytical method has been developed for the determination of the oxygen isotopic compositions of the Si 8O 20 framework of apophyllite and the Al 2Si 4O 12 framework of wairakite. The method consists of the preferential oxygen isotopic exchange of the water of crystallization with isotopically different external water, followed by the determination of the bulk isotopic composition of the minerals. The δ 180-value of the silicate framework can be obtained by subtracting the δ 18-O-value of the water of crystallization from that of the bulk mineral in proportion to the oxygen atomic fraction of the water of crystallization. Optimum conditions of the exchange for apophyllite have been found to be ∼ 160° C for ∼ 10 days during which the water of crystallization is almost completely exchanged with the external water, whereas the framework oxygen remains unchanged. For wairakite analysis, a graphical method utilizing a three-isotope plot ( 17O/ 16 vs. 18O/ 160) has been used. The advantage of the three-isotope method is that the δ 18O-value of the silicate framework can be graphically estimated with an accuracy better than ± 2‰, even if the complete exchange of external and crystallization waters is not attained.
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