Gonnardite from Higashi-Matsuura Peninsula, Saga Prefecture, Japan

Abstract

Gonnardites found in the altered hydrothermal veins or druses in the Higashi-Matsuura basaltic rocks have cell parameters a=13.45 Å, b=13.42 Å and c=6.67 Å ; the chemical composition determined by Electron probe micro-analyser, (Na, Ca, K)_{3, 62}(Al, Si)_10.05O₂₀•5.47 H₂O. By heating at 350°C for one hour, the mineral turned to amorphous state and by heating at 900°C for one hour, it changed to nepheline, with cell parameters a=9.978 Å and c=8.365 Å (hexagonal system) and chemical composition determined by Electron probe micro-analyser K_0.02Na_4.45(Ca, Mg, Al)_1.18[Si_9.09Al_6.91O₃₂]. The DTA curve shows a large sharp endothermic peak at 345°C and an exsothermic peak at 940°C. The former is due to the dehydration of structural water and the latter to the formation of nepheline. The infra-red absorption spectrum shows absorption bands at 3600, 3400, 1640 and 1000 cm^-1.