ASPECT RATIO DETERMINATION THROUGH SURFACE AREA ANALYSIS FOR MICA This method utilizes surface area measurement with sample volume information generated during the surface area analysis to determine average flake thickness (t). The calculated t is ratioed with mean particle size values as determined by both mass and particle number distributions for two different aspect ratio values. The total surface area of a flake is the total of the top, bottom and four edge surfaces. This method assumes that the sample flakes are sufficiently thin so that the surface area of the edge surfaces is negligible. The top and bottom surfaces are also assumed to be equal, making the measured surface area approximately 2A. The volume of a flake which has equal top and bottom surfaces would be equal to the top surface area multiplied by the flake thickness - At. With these assumptions, the surface area to volume ratio would be 2A/At or 2/t. In determining the surface area, the analyzer uses helium to determine sample container free space or conversely, sample volume. If the specific gravity of the sample is known, the volume can be calculated from the sample weight. The surface area measurement is expressed as square meters per gram and should be multiplied by the sample weight for strictly an area value. Area 2 (measured surface area) x (sample weight) ------ = --- = ---------------------------------------------------- Volume t (measured volume) or (sample weight)/(specific gravity) 2 x (measured volume) t = ----------------------------------------- (measured surface area) x (sample weight) or 2 t = -------------------------------------------- (measured surface area) x (specific gravity) For mica samples with thick or poorly delaminated flakes, volume per unit weight of sample will not change, but the surface area will decrease significantly. The V/A ratio will increase, thereby increasing the calculated t. With the calculated thickness, the particle size distribution can be used to further describe the sample. The ratio of the measured mean particle size to t would give the aspect ratio based upon the mass distribution. The Microtrac mass distribution can be entered into Lotus and defined by particle size interval (mass by interval). Using the assumption that the flakes are square and using the average interval diameter, the minimum number of flakes per interval can be computed. The relative particle number distribution can easily be determined and data regression analysis is used to determine the mean particle size diameter from this distribution. This mean particle size will be considerably smaller than the mass mean, but the ratio of the number mean to t gives an aspect ratio based upon the relative number of flakes throughout the size range. Eric Westbrook July 1992