Siliceous deposits formed from geothermal water in Kyushu, Japan: II. Distribution and state of aluminum along the growth direction of the deposits

The distribution of elements and chemical state of aluminum along the growth direction of a siliceous deposit formed from geothermal water at the Hachobaru geothermal power station were examined by chemical analysis and ²⁷Al MAS NMR. The content of 4-coordinated Al, Al(4), was essentially constant along the growth direction. The content of Na was also constant because of Na⁺ ion is neutralized by a negative charge induced by the isomorphous substitution of Al(4) in silica. Elements except Al(4) and Na were concentrated outward in the siliceous deposit. Since the chemical property of geothermal water was essentially constant during the growth of the siliceous deposit, the other elements including Al(6) would be expelled outward due to a change in silica structure by condensation of silica particles during the growth of siliceous deposit.