ORIGINS OF THE SURFACE-ROUGHNESS OF UNWEATHERED ALKALI FELDSPAR GRAINS

The origins of surface roughness (lambda) of powdered feldspar grains, the ratio of the total surface area quantified by gas adsorption to t he surface area estimated by assuming an average grain geometry, has b een investigated by a detailed study of alkali feldspars from a range of igneous rocks and from pegmatites. The lambda of 125-63 mu m powder s of ten different feldspar samples ranges from 5.16-22.00 and correla tes well with mineral microtexture. Powders of feldspars which are hom ogeneous or have straight lamellar exsolution microtextures yield mean lambda values of 5.59, whereas powders of vein macroperthites give a mean lambda of 6.64 and powders of braid microperthites yield mean lam bda values of 12.15. Microtexture influences lambda by partly controll ing the manner in which feldspar grains break during grinding, and thi s affects the frequency of steps on grain surfaces. Feldspars which ar e homogeneous or have straight lamellar exsolution microtextures break to produce a relatively high proportion of grains with smooth (step-p oor) external surfaces, whereas vein macroperthites and braid microper thites break to produce grains with much rougher (step-rich) surfaces. Microtexture is able to influence style of breakage because the prese nce, size, and crystallographic orientation of exsolution lamellae has a significant influence on the trajectories of fractures propagating through grains as they fragment during grinding. We found little corre lation between internal surface area due to micropores, expressed as e ither microporosity or mean perimeter/area of micropores, and lambda, indicating that microporosity is a significantly less important contri butor to lambda than external surface area due to steps. This conclusi on is supported by an analysis of the lambda of a number of different size fractions of two of the feldspar powders. The origin of lambda is only one part of the more important question of how closely measured surface areas are related to the proportion of grain surfaces which ar e reactive during experimental dissolution and natural weathering. Cop yright (C) 1997 Elsevier Science Ltd.