MANGANESE OXIDES AND MANGANESE NODULES IN MARINE DEPOSITS. 723 



These silicates are in all probability the original source of all the carbonate and 



II. In the following rocks the manganese occurred only partly in a soluble condition : — 



contained from 



Mica-schist, 



Sandstone, 



Felstone, 



Basalt, „ 



Volcanic rocks, „ 



from Glenmorag, 

 ,, Lesmahagow, 

 „ Pentland Hills, 

 „ Clyde Area, 

 „ Castle Hill, Edinburgh, 

 Argyllshire, 



III. In the following rocks the manganese was in an insoluble condition, probably silicate 



Pumice, 



from Lipari, 



contained from 



(black) 



Pumice tuff, 

 Iceland ,, 

 Felstone, 



>) 

 Dolerite, 

 Greywacke, 

 Granite, 

 Melaphyr, 

 Serpentine, 

 Peridotite, 



Iceland, 



Ascension, 



Rhineland, 



Dannibora, 



Coldstream, 



Kilmalcolm, 



Kirk o' Shotts, 



Abington, 



Lanarkshire, 



Bowling, 



Glenfalloch, 



Ver cent. MnO. 

 0-1 to 0-5 

 0-5 to 1-0 

 0-5 to 1-0 

 0-1 to 0-5 



0-01 to 0-1 

 0-1 to 0-5 



Per cent. MnO. 

 0-5 to 1-0 

 0-5 to 1-0 

 0-1 to 0-5 

 0-1 to 0-5 

 0-1 to 5 

 0-1 to 0-5 

 0-5 to 1-0 

 0-1 to 0-5 

 0-1 to 0-5 

 0-1 to 0-5 



0-01 to 0-1 

 trace 



0-01 to 0-1 



Besides the above-named rocks, about fifty others were examined for manganese, which was found to be present 

 in amounts varying from traces to 05 per cent, of MnO. 



Fresh Blue Muds, from various places in the estuaries of the Clyde and Forth, were examined and found to contain 

 manganese partly as silicate and partly as carbonate. There was a slight trace of manganese found in some samples of 

 Globigerina Ooze, but none in corals, boiler deposits, or siliceous sinter (from Iceland). A piece of coral (Pleurocomlliimi 

 johnsoni, taken by the "Challenger," Station 3, 1525 fathoms), was coated on the outside with manganese dioxide, but 

 internally there was no trace of manganese. Minute traces were found in kelp and in sea-weed ash. 



The process adopted for the comparative estimation of manganese was the colour-test, obtained when the material 

 under examination was fluxed with potassium and sodium carbonates and a little pure potassium nitrate. The 

 fused mass, on cooling, gives the green colour characteristic of manganates — even when manganese is only present in 

 minute traces. Comparison with a standard series of coloured fluxes, each containing a known amount of manganese 

 varying from O'Ol to 1*0 per cent., was an easy, quick, and at the same time a comparatively accurate, method of 

 estimating the quantity of manganese present. The proportion of manganese existing in the rocks under the various 

 conditions of carbonate, silicate, and higher oxides might be roughly determined by thorough trituration and treatment 

 with carbonic acid in aqueous solution ; but the expenditure of time involved in the prosecution of this process renders 

 the substitution of dilute acetic acid for carbonic acid advisable, the dilute acetic acid attacking the carbonate alone. 

 The amount of peroxide present was determined as usual by the Bunsen process, i.e., by taking advantage of its 

 power of liberating chlorine from hydrochloric acid. Crystalline silicates, even when reduced to the finest state of 

 division, are only very gradually decomposed by carbonic acid. We found that all the manganese which is combined 

 with carbonic acid in the rocks is thus rapidly extracted by the use of dilute acetic acid, and can be determined by 

 evaporating the solution so obtained to dryness, and treating the dried residue with fluxing materials at a red heat, 

 or by actual precipitation by the general methods in use for manganese determination. We can thus determine the 

 proportion of manganese existing in a rock or mineral as carbonate. If, however, the manganese exists partly as 

 carbonate and partly as silicate, we obtain the portion present as carbonate in the acetic acid solution, and the portion 

 existing as silicate or the higher oxides in the insoluble residue. 



The process adopted by us for the determination of the quantity and state of combination of the manganese 

 in a rock sample was as follows : — 



(1) Exactly 1 gramme of the sample, reduced to a fine powder, was intimately mixed with 4 grammes of the fusion 

 mixture already referred to (20 parts Na 2 C0 3 , 26 parts K 2 C0 3 , 1 part KN0 3 ), and heated in a platinum crucible 

 over the blow-pipe until tranquil fusion supervened. The liquid mass was then poured out upon a porcelain slab and 

 allowed to cool. The quantity of manganese present in the fused magma was ascertained by comparing its colour with 

 that of the series of standard coloured fluxes. This gave the total percentage of manganese in the sample. 



(2) Another portion from the same pulverised sample, weighing 1 gramme, was exhausted with dilute acetic 

 acid (to extract the carbonates), and the amount of manganese in the residue determined colorimetrically after fusion 



