Artificial chemical weathering of gabbro under earth surface conditions- - : The influence of acid rain on rocks and minerals (part 4)

Abstract

In order to predict the effect of acid precipitation on building stone, artificial chemical weathering of polished plates of Muroto Gabbro from Kochi, Shikoku Island, Japan, was conducted. An improved Soxhlet extraction apparatus was used with distilled water and HNO 3 , H 2 SO 4 and HCl solutions at pH 4 at 55°C for different periods of time up to 780 days. The gabbro was composed mainly of olivine (Fo 66 Fa 34 ), plagioclase (Ab 33 An 66 Or 1 ), and clinopyroxene (En 47 Fs 13 Wo 40 ). Morphological and chemical changes of each mineral surface were studied by SEM, EPMA and optical microscopic techniques. Alteration products were collected from the surface of each mineral on the polished plates by hand picking and examined using XRD. The leached sample solutions were analyzed for major and minor elements using ICP-AES, ICP-MS and ion-chromatography. It is evident that olivine changed into smectite, chlorite and talc by exposure to the various acid solutions used in the experiment. Smectite appeared to be formed from plagioclase during the artificial chemical weathering. However, altered products from clinopyroxene were not detected. Molar ratios of each element in the leached solutions to those in the fresh gabbro varied depending on the experimental period. The ratios of elements such as Mg, Mn, Fe, Ni, Zn and Co, which reflect the dissolution of olivine, were relatively high. Similarly, the ratios of Ca, Na, Sr, Ce, Sm and La, which reflect the dissolution of plagioclase, were relatively high. It seems, therefore, that olivine and plagioclase are easily weathered. It was also estimated from the molar ratios that the reactivity order of the acid solutions for olivine and plagioclase is H 2 SO 4 , HNO 3 and HCl.