Shape and surface area measurements using scanning electron microscope stereo-pair images of volcanic ash particles

The shape and surface area of fi ne particles are traditionally measured using projected or two-dimensional (2D) sections or with nitrogen gas adsorption using the BET (Brunauer, Emmett, and Teller) method. However, 2D sections are incomplete shape descriptors and nitrogen gas adsorption analysis is precluded when the amount of sample is limited, for example, in direct ash cloud sampling. In this study we present a technique for measuring the shape and surface area of individual grains of volcanic ash using scanning electron microscope (SEM) stereo pairs. The application we discuss is the stereoscopic analysis of 25 ash particles in the 4–130 µm size range from the August 1992 Crater Peak–Spurr eruption, located 130 km west of Anchorage, Alaska. Surface area data are presented from stereo measurements of glass microspheres in the same size range as validation of the technique’s accuracy. Differences in surface area values between our technique, 2D shape data, and nitrogen gas adsorption are presented and discussed