A New Route of Oxygen Isotope Exchange in the Solid Phase:  Demonstration in CuSO₄·5H₂O

Abstract

Temperature-programmed desorption mass spectrometry (TPD-MS) measurements on [(18)O]water-enriched copper sulfate pentahydrate (CuSO(4).5H(2)(18)O) reveal an unambiguous occurrence of efficient oxygen isotope exchange between the water of crystallization and the sulfate in its CuSO(4) solid phase. To the best of our knowledge, the occurrence of such an exchange was never observed in a solid phase. The exchange process was observed during the stepwise dehydration (50-300 degrees C) of the compound. Specifically, the exchange promptly occurs somewhere between 160 and 250 degrees C; however, the exact temperature could not be resolved conclusively. It is shown that only the fifth, sulfate-associated, anionic H(2)O molecule participates in the exchange process and that the exchange seems to occur in a preferable fashion with, at the most, one oxygen atom in SO(4). Such an exchange, occurring below 250 degrees C, questions the common conviction of unfeasible oxygen exchange under geothermic conditions. This new oxygen exchange phenomenon is not exclusive to copper sulfate but is unambiguously observed also in other sulfate- and nitrate-containing minerals.

Oxygen isotope exchange during thermal dehydration of copper selenate pentahydrate

“…It should be mentioned that the calculated probability for oxygen exchange in copper sulfate pentahydrate during thermal dehydration was 44% [1]. All together, the experimental results suggest that the oxygen isotope exchange, in either copper sulfate or copper selenate, occurs during the dehydration reaction, following desorption of the first four water molecules.…”

“…This value is much higher than the 32.9% average enrichment of the as-grown crystal. This result is in contrast to copper sulfate in which the 18 O average abundance of its O-containing decomposition products was equal to that of the as-grown crystal [1]. A possible explanation for this discrepancy may be the occurrence of oxygen exchange between the selenate anion and the enriched liquid water during the dissolution and recrystallization periods.…”

“…In a previous study a new route of oxygen isotope exchange was unambiguously revealed in copper sulfate pentahydrate [1]. The exchange was shown to occur in the solid state, within the crystal, between differently distributed oxygen atoms in the unit cell, during thermal dehydration of the compound.…”