Identification of Minerals in Mine Dusts 



353 



Fig. 1. Diffraction pattern of PbTe layer. 



Fig. 2. PbTe layer exposed to intense electron beam during 

 10-20 seconds. Magnification 6000. 



Fig. 3. PbTe layer exposed to intense electron beam longer 

 than 20 seconds. Magnification 6800. 



rings of the NaCl type structure, the intensity of 

 which is determined by the difference of atom factors. 

 Thus, a crystal lattice of the kind proposed shows an 

 electron diiTraction pattern corresponding to simple 

 cubic structure with the lattice constant halved. 



Of the three analogous compounds PbS, PbSe and 

 PbTe, the properties of PbS are the best known. 

 Hxperimental data and a theoretical analysis (1) 

 prove this compound to be only slightly ionic in 

 character. The lesser difTerence in atomic numbers 

 of the components in PbTe should cause the ionic 

 properties to recede still further. These considerations 

 should make the authors' assumption of the Pb and 

 Te atoms replacing each other in the crystal lattice 

 points seem more plausible. 



The PbTe layers were also examined with the 

 electron microscope. When magnified 10,000 times 

 they appeared to be homogeneous. Following 

 illumination with an intense electronic beam for 10- 

 20 seconds the layer underwent changes: small 

 crystals formed, at first very minute in size, and 

 later on separate, scantily distributed crystals at- 

 taining 1000 A (fig. 2). Further illumination caused 

 gaps in the layer (fig. 3). 



The authors wish to express their gratitude to Professor 

 L. Sosnowski for his valuable discussions in interpreting 

 the results. 



References 



1. Bfll. D. C, Hum, D. M., Pinchfrle, L., Sciama, D. W., 



and Woodward, P. M., Proc. Roy. Soc. A, 217, 71 

 (1953). 



2. Checinska, H. and Sosnowski, L.. Bull. Acad. Polon. 



Sci., CI. Ill, 383 (1954). 



3. Wyckoff, Crystal Structures. Interscience. New York 



1953. 



I(dentification of Minerals Present in Mine Dusts by Electron DifTraction 



an(J Electron Microscopy 



J. H. Talbot 



Research Laboratories, Transvaal and Oranf;e Free State Chamber of Mines, 



Johannesbiiri^, South Africa 



Introduction. — An investigation of the dust content 

 of the air in Witwatersrand mines has revealed the 

 existence of large numbers of submicroscopic par- 

 ticles in the air leaving working places. X-ray diffrac- 

 tion methods have given no information concerning 

 the composition of these particles, since, although 

 they are present in very large numbers, their propor- 

 tion by weight of the sample is small (less than 5%). 

 Thus their contribution to the diffraction pattern 

 is obscured by that of the larger particles. 



The difficulty is overcome by using electron dif- 



fraction. Particles much larger than 0.1 // absorb 

 nearly ail the electrons incident on them; those not 

 absorbed lose coherence through multiple scattering 

 and can only add to the diffuse background of the 

 diffraction pattern. Thus the effect is due pre- 

 eminently to the particles smaller than 0.1 //. Once 

 the various constituents of the sample have been 

 identified it is necessary to determine their relative 

 proportions and the particle sizes in which they are 

 present. A method of doing this is described. 



Preparation and examination of samples. — Samples 



