A. BraiiiTnall — Some Microcheinical Methods. 123 



Some Microchemical Methods. 



By Alfred Brammall, B.Sc. (Lond.), F.G.S., Imperial College of 

 Science and Technology, South Kensington 



rpHE well-known and highly sensitive tests afiorded by ferricyanide, 

 -*- ferrocyanide, and thiocyanate solutions for the detection of 

 iron may be adapted, at the cost of a little trouble, to the micro- 

 chemical investigation of certain rocks in thin section, as can also 

 the equally well-known fusion test for manganese. The following 

 methods have proved useful in special circumstances of the nature 

 indicated in each case. 



1. The necessity arose to determine the distribution of hydrated 

 ferric matter in sediments affected by low-grade metamorphism, 

 and to detect any marked tendency towards the accumulation of 

 this substance in and about foci of recrystallization. Method applied 

 to rock sHces : The cover-glass having been removed, the slice is 

 first cleansed of balsam, then polished, again cleansed, and warmed 

 to evaporate any spirit retained by capillarity. Meanwhile, reaction 

 papers are prepared : a piece of white drawing-paper, of area about 

 twice that of the rock slice, is soaked in strong HCl and pressed 

 between sheets of similar paper until it is no longer " wet " ; an 

 ammonium thiocyanate paper is prepared in a similar way. The 

 acid paper is carefully laid upon the slice and covered with a strip 

 of cardboard on which is laid a hot iron (2-3 lb. weight), care being 

 taken not to shear the slice. After two or three minutes the weight, 

 cardboard, and paper are carefully lifted from the slice, which is 

 allowed to dry ; the thiocyanate paper is applied (the same pre- 

 cautions being observed) and jDressed, in the cold, to ensure good 

 contact with the slice for a few minutes ; it is then removed and the 

 sHce is again thoroughly dried before being examined in transmitted 

 light. The distribution of ferric iron is revealed by a transparent 

 blood-red stain of Fe(CNS)3. 



Potassium ferrocyanide may be substituted for the thiocyanate, 

 but the blue coloration produced is due to an opaque pigment, 

 which must be examined in reflected light. The thiocyanate stain 

 has the advantage of transparency. 



[a) The method has proved of esjDecial value in the examination 

 of " spotted " rocks : a typical case is described : In a Knoten- 

 ■schiefer from Andlauthal, Vosges, the spots, conspicuous enough 

 in hand specimens, are differentiated from the matrix in thin 

 section only by the following features : (i) abundant inclusions of 

 opacite (carbon, largely) ; the inclusions in some spots traversed 

 by pigment-rich bedding planes appear to have been derived, in 

 part, from these planes, which are relatively free from this opaque 

 matter for a short distance on each side of the spot ; (ii) an irregular, 

 brownish, transparent stain forming a peripheral zone ; (iii) 

 polarization colours sensibly lower than those of the matrix ; some 

 «pots may be almost isotropic. On microchemical treatment the 



