The Effect of Grinding on Micas

The general effect of grinding upon various minerals and the manner in which time of grinding affects cation-exchange capacity, etc., have been known for some time, but relatively little information is available upon the micas (Jackson and Truog, 1939; Kelley and Jenny, 1946; Parkert et al., 1950; Mackenzie and Milne, 1953). This, coupled with an observation made here on the pronounced effect of short-period grinding on muscovite, has led to a more detailed examination of the effects induced by such grinding upon some micas. Since the data for muscovite are completely discussed elsewhere (Mackenzie and Milne, 1953) only preliminary experiments upon biotite and vermiculite are recorded here. Materials and Methods . -The biotite employed occurred as segregations of small crystals in a nepheline-syenite pegmatite from Sutherland and is probably slightly impure*. The optical data were: = 1.584, #---1.626, y = 1.627, 2V small. The vermiculite, a very pure sample from Kenya, has been previously described (Walker and Milne, 1950). Each sample was divided into two parts, one of which was filed to pass 200-mesh, thus giving a powder of unaltered material, while the other was dry-ground in a mechanical agate mortar, samples being removed at various times up to 24 hours and sieved through 200-mesh. X-ray data were obtained with FeK ~ radiation and intensities were measured visually. Examination of some selected samples by differential thermal analysis was performed by standard methods using 0.20 g. samples (Mackenzie, 1952). A micro method was employed for determination of cation-exchange capacities (c.e.c.) (Mackenzie, 1951).