124 A. Brainmall — Some MicrocJiemical Methods. 



spots yield a pronounced stain, while the matrix takes only a feeble 

 stain on the whole ; the peripheral zone is somewhat less responsive 

 to treatment than the rest of the spot, but ultimately is stained 

 throughout. 



(6) The dark inclusions in chiastolite (andalusite) react very feebly, 

 but the narrow opaque zone of matrix immediately surrounding 

 the crystals usually reacts very strongly. The stains on the 

 thiocyanate paper itself are worth examination. 



(c) The alteration of chiastolite (andalusite), yielding structures 

 closely resembling those observed in olivine altering to serpentine, 

 often involves permeation of the mineral by iron-rich water impreg- 

 nating the matrix ; the channels of alteration and their dust-filled 

 dilations then react strongly for iron. 



{d) Lamination in shales is frequently emphasized by narrow 

 bands rich in opaque (carbonaceous) matter ; these layers are also 

 usually rich in some easily soluble compound of " ferric " iron — 

 a feature which may be, in part, at least, original, and possibly due 

 to the incompatibility of iron with colloidal organic matter in 

 solution, the two substances being precipitated together. 



[e] Magnetite being much more easily soluble in HCl than ilmenite 

 (or titanoferrite), it reacts much more readily by this test than 

 does the latter. The reaction papers in this case should be prepared 

 from blotting-paper, and should be slightly " wet ", so that the 

 thiocyanate stain may spread somewhat by capillarity ; the stain 

 on the thiocyanate j)aper itself should not be disregarded. 



The rare occurrence of grains of native iron in certain rocks is 

 more readily detected by applying a wash of slightly acidulated 

 CuSO^ solution, which coats the iron with a film of metallic copper, 

 recognizable in reflected light. 



(/) The method is also useful for demonstrating the distribution of 

 chalybite in oolitic ironstones and less ferruginous oolites. Although 

 chalybite is dissolved readily by HCl in the cold, the weight used 

 to press down the acid reaction paper upon the rock slice should be- 

 hot so that the hydrated ferrous chloride first formed may be the 

 more rapidly oxidized to the basic ferric chloride ; the acid is in 

 sufficient excess to convert some of this basic compound to the normal 

 ferric salt. As the products of reaction with HCl are hydrates 

 and very hygroscopic, the application of the thiocyanate paper 

 tends to produce smearing ; this is largely prevented by using 

 reaction papers which are not " wet ", by drying the preparation, 

 in a hot atmosphere or a desiccator after each reaction, and by 

 examining the preparation without delay. 



It is well to supplement the thiocyanate preparation by two 

 others, using potassium ferricyanide and potassium ferrocyanide 

 on separate slices ; the blue pigment produced is examined by 

 reflected light. The deliquescent CaClg may be removed by first 

 thoroughly drying the preparation (so as to " cake " the pigment) 

 and then carefully washing it — the pigment remaining fixed. 



