Pore structures induced by water vapor adsorbed on nonporous lunar fines and ground calcite

Abstract

Lunar fines and the ground calcite are both highly defective materials and because of this there are marked similarities between their interactions with adsorbed water vapor. The solar radiation and meteorites incident on the lunar surface produce soil particles, or fines, which are nonporous to adsorbed inert gas molecules. Many crystalline grains contain very high densities of damage tracks (>10 11 tracks/cm 2), and show an amorphous coating. The adsorption isotherm of water shows that the fines interact strongly with water vapor. The subsequent adsorptions of nitrogen and argon show that the specific surface area has increased as much as threefold, and that pores have been generated. The probable explanation for these changes is that water interacts with the amorphous coating and diffuses along the damage tracks leaching a system of microchannels. During the ball milling of calcite the crystals suffer processes of flow and plastic deformation which leave the grains nonporous to adsorbed nitrogen. The structural degradation caused by milling, which was examined by X-ray and infrared techniques, develops in three discrete steps and causes a marked enhancement in the reactivity towards water. The changes in reactivity were examined by measuring the solubility in water, the pH of aqueous suspensions and the buried water content by thermogravimetric analysis. In the most reactive sample of ground calcite, the adsorption isotherm of water reveals massive uptake at high relative pressure and shows high pressure hysteresis overlying a hysteresis which extends over the entire range of relative pressure. The subsequently measured adsorption of nitrogen shows that the grains are now porous and that the specific surface area has increased fourfold. Dissolution occurs within the dislocated calcite to form a pore system of fine channels and crystallization of the leached calcium carbonate forms whiskers of nail-head spar calcite.