ORIGINS OF THE SURFACE ROUGHNESS OF UNWEATHERED ALKALI FELDSPAR GRAINS

Abstract

The origins of surface roughness (λ) of powdered feldspar grains, the ratio of the total surface area quantified by gas adsorption to the surface area estimated by assuming an average grain geometry, has been investigated by a detailed study of alkali feldspars from a range of igneous rocks and from pegmatites. The λ of 125-63 μm powders of ten different feldspar samples ranges from 5.16-22.00 and correlates well with mineral microtexture. Powders of feldspars which are homogeneous or have straight lamellar exsolution microtextures yield mean λ values of 5.59, whereas powders of vein macroperthites give a mean λ of 6.64 and powders of braid microperthites yield mean λ values of 12.15. Microtexture influences λ by partly controlling the manner in which feldspar grains break during grinding, and this affects the frequency of steps on grain surfaces. Feldspars which are homogeneous or have straight lamellar exsolution microtextures break to produce a relatively high proportion of grains with smooth (step-poor) external surfaces, whereas vein macroperthites and braid microperthites break to produce grains with much rougher (step-rich) surfaces. Microtexture is able to influence style of breakage because the presence, 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 correlation between internal surface area due to micropores, expressed as either microporosity or mean perimeter/area of micropores, and λ, indicating that microporosity is a significantly less important contributor to λ than external surface area due to steps. This conclusion is supported by an analysis of the λ of a number of different size fractions of two of the feldspar powders. The origin of λ is only one part of the more important question of how closely measured surface areas are related to the proportion of grain surfaces which are reactive during experimental dissolution and natural weathering.