SILICA-RICH GARRONITE-Na FROM HIRADO ISLAND, NAGASAKI PREFECTURE, JAPAN

Silica-rich garronite-Na was found together with epistilbite in Miocene basaltic rock from Shiratobana, Hirado Island. Nagasaki Prefecture in Japan for the first time. Garronite-Na occurs as an anhedral crystal that covers the center of a small cavity in altered basaltic rock, whereas the epistilbite covers the inside of the cavity. Electron probe microanalysis of the garronite-Na gives an empirical formula of (Na1.99K0.27)(Sigma 2.26)Ca-1.61(Fe0.01Al5.31Si10.64)Sigma O-15(.96)32.14.3H(2)O on the basis of O = 32. Its Na/Ca molar ratio varies from 1.00 to 1.53, and its unit-cell parameters (space group I2) calculated from X-ray powder diffraction data are alpha =9.983(11) angstrom, b = 10.089(14) angstrom, c = 10.070(10) angstrom, and beta =90.223(3)degrees with a calculated density of 2.183 g/cm(3). Garronite-Na from Hirado Island formed from an alkaline high-silica solution in the later stages of hydrothermal zeolitization associated with volcanic activity.