G. D. Hubbard— Colloids in Geologic Problems. 105 



between the surface of the one and the particles of the 

 other. 



Adsorption, then, probably explains some of the colors 

 of minerals, inasmuch as the particles of the mineral 

 crystal correspond to the surface of the wood or glass, 

 and the coloring is in such minute particles as to be in 

 the colloidal state. Examples of such intimate relation- 

 ships are found in the carbon and iron oxide which give 

 the smoky tint to cairngorm, and the carbon which is 

 believed to be responsible for the color of amethyst. 

 Rose quartz owes its color to the adsorption of colloidal 

 titanium on its particles as they come together to build 

 up the crystal, and chrysoprase is said to be colored much 

 in the same way by nickel oxide. Feldspars have long 

 been believed to owe their color to traces of iron oxide 

 which seems to be adsorbed by the molecules of the 

 feldspar. 



The beautiful blue color of some halite has been shown 

 to be due to metallic colloidal sodium, and probably the 

 bluish tint of sylvite may be traced to the same cause. 

 The colors of barite are thought by Patten to be due to 

 various oxides. He has shown that it adsorbs salts of 

 nickel, cobalt, chromium, iron and manganese. Calcite is 

 practically never a colloid, but it shows rather high 

 aclsorptive powers for certain colloids. The amber cal- 

 cite has been shown to be due to organic matter, and the 

 amber fluorite is probably due to hydrocarbons. Ruby 

 spinel seems to owe its beautiful tints to adsorbed chro- 

 mium oxide, and cerussite in blues and greens to colloidal 

 hydrous copper carbonate. More than likely, though this 

 is not demonstrated, the soft tints of the zinc salts, cal- 

 amine, and smithsonite, from Laurium, Greece, are due 

 to the adsorption also of copper carbonates. The colors 

 of many gems may probably be due to adsorption of some 

 finely divided substance which becomes so intimately 

 mixed with the gem materials, yet which occurs in such 

 minute quantities, as almost to defy detection. This 

 problem has only recently been taken up, and we may, as 

 the work goes on, find much more in it than has yet been 

 shown. 



Colloids and the Flotation Process. 1 * "While the flota- 



18 Moses, F. G., Colloids and Flotation, U. S. Bur. Mines, Tech. Paper 200 

 furnishes many data for this section. 



