Porosity and focused dissolution of granitic rocks in two study areas in southern Finland. Aspects of methodology: Dissertation

This document is principally a description of methods for classifying the effective porosity of rock and modeling water rock interaction in pore spaces in rock.Two Proterozoic granitic intrusions in southern Finland, a synkinematic tonalite and a postkinematic rapakivi granite were examined.The effective porosity of granites is due to fractures, microcracks and tubelike diffusion pathways.Fracture zones and fractures are classified according to fracture spacing, frequencies, and the nature of infillings.Microfractures are classified according to their locations and extension within mineral grains.A rectangular geometry for rock fractions, such as rectangular 'fracture fractals', was identified at both mesoscopic and microscopic scales.Natural groundwater equilibrium and mineral saturation states of the intrusions were calculated with an EQ3NR program.Rocks were artificially dissolved at 250 °C (0,l N HCl) in a Parr type (autoclave) reaction bomb.Both groundwater and the final solution in the dissolution experiments were supersaturated with respect to boehmite, montmorillonite and kaolinite, which were also identified as secondary minerals amongst the solids remaining in the autoclave.Dissolution of rock took place preferentially at mineral defects, such as microfractures and pores.Alkali metals and iron were the most mobile elements in hornblende, biotite, and plagioclase, which were also the most extensively dissolved minerals.Dissolved components were transported along grain boundary fractures and in cleavage planes, utilizing discrete diffusion pathways.This was especially the case with sheet and chain silicates.A reaction surface area of rock can be considered equal to a surface of effective rock pore space.A new procedure for studying the diffusion porosity profiles of natural fractures (porosity profiling, fracture profiling) is proposed.The method is based on microscopy, porosity measurements and chemical analysis along profiles perpendicular to the fracture surfaces.The purpose of such profiling is the determination of diffusion capacity (diffusion depth), sorption capacity (adsorbing mineral surfaces), and redox capacity (reducing mineral surfaces) for fractures in granitic rocks.