Composition of Clinoptilolite Formed from Volcanic Glass During Burial Diagenesis

Abstract

The composition of clinoptilolite formed from volcanic glass was investigated from cuttings collected from the offshore MITI-Somaoki borehole, 52 km east of Soma City, Japan. Unaltered volcanic glass and clinoptilolite-replaced glass were found coexisting in the same silicic tuff at 1000 m depth, but unaltered volcanic glass and clinoptilolite did not coexist in individual glass shards, and each material analyzed in this study was pure. The SiO₂ and Al₂O₃ contents of the pure clinoptilolite and the pure, unaltered glass were 69.32 and 13.05 wt. %, and 70.03 and 11.54 wt. %, respectively. The SiO₂ content was almost the same and the Al₂O₃ content was greater in the clinoptilolite-replaced glass compared with the unaltered volcanic glass. As a result, the SiO₂/Al₂O₃ ratio in the clinoptilolite-replaced glass (SiO₂/ A1₂O₃ = 5.31) was slightly lower than that of the unaltered volcanic glass (SiO₂/Al₂O₃ = 6.07). The extra-framework cation composition of the clinoptilolite-replaced glass was three times richer in K (K₂O = 7.84 wt. %) than the unaltered volcanic glass (K₂O = 2.59 wt. %). The cation selectivity of clinoptilolite, which shows a preference for K to Ca and Na, played an important role in the concentration of K in clinoptilolite during clinoptilolite formation from volcanic glass, although the initial pore waters and the volcanic glass were not especially relative K.

Clinoptilolite-replaced glass in the calcite-cemented tuff at 1100 m depth was not enriched in Ca, suggesting that the exchangable-cation composition of clinoptilolite did not change by exposure to Ca-rich solutions at this depth. Apparently, the exchangable-cation composition of clinoptilolite, once formed, is relatively stable and difficult to change, even under conditions in which calcite is precipitating on clinoptilolite crystals.