Gypsum Analysis with the Polarizing Microscope

The fastest and most accurate method for the qualitative analysis of gypsum is use of the polarizing microscope. Five bits of optical data can be used for identification of gypsum and its most common impurities such as natural anhydrite, calcite, dolomite, and silica. The optical data are morphology, refractive index, birefringence, angle of extinction, and dispersion staining. Gypsum can be identified by its refractive index of 1.521 and 1.530, its oblique extinction angle, its birefringes of 0.009, and a blue dispersion staining color when mounted in a refractive index liquid of 1.528. Natural anhydrite is normally seen as blocky crystals with a refractive indices of 1.570 and 1.614. The birefringence of 0.044 gives it much higher order polarizing colors than gypsum, and it has parallel extinction. Silica has the same birefringence as gypsum; so it is hard to distinguish with crossed polars. However its refractive index of 1.544 is higher than gypsum and has a yellow dispersion color to contrast with the blue of gypsum when mounted in 1.528. Limestone may be either calcite or dolomite. Limestone may be distinguished mainly by its very high birefringence of 0.172, which renders even very fine particles colorful with crossed polars—larger particles are high order white. Calcite may be distinguished from dolomite by mounting in a refractive index liquid of 1.660 rendering the dolomite orange with the calcite blue. The phases of gypsum can be distinguished also. Beta hemihydrate has the same shape as the dihydrate from which it was made but is porous and cloudy rather than clear and solid. Alpha hemihydrate may be blocky or acicular and has a refractive index of 1.558 and 1.586. The birefringence is 0.028. When mounted in 1.564 liquid, the dispersion colors will change from orange to blue as the stage is rotated. Soluble anhydrite is difficult to identify, but dead-burned gypsum has a refractive index close to natural anhydrite, and the dispersion colors are red and blue when mounted in a liquid of 1.596. The qualitative analysis of natural anhydrite may be done by direct estimation, or by estimating a series of fields and averaging the results. A more precise method is to count and measure the diameters of all particles then calculate the weight percent of each.