Ch. 28] IRON OXIDES AND HYDROXIDES 513 



serpentine or limestone, or it may be relatively high, as in the banded 

 siliceous iron formations of the Lake Superior type. 



When serpentine is subjected to prolonged weathering in a warm, 

 humid climate, the more soluble constituents are removed from the 

 zone above the water table, particularly in plateau areas with active 

 ground-water drainage, and the less soluble constituents, such as iron 

 and alumina, remain to form a residual deposit termed laterite. The 

 laterite may be ferruginous or aluminous, depending on the relative 

 proportions of the two constituents in the original rock. 



Ferruginous laterites usually have a thin upper zone which consists 

 of earthy, oolitic, or pisolitic hematite with a minor amount of mag- 

 netite, whereas the main portion of the deposit is limonite. The con- 

 tact with the underlying serpentine is usually very sharp, but extremely 

 irregular. The irregularity is due to selective weathering of the parent 

 rock along joints or zones of higher permeability. The porosity of a 

 laterite may be as high as 75 percent, but it usually decreases near the 

 surface. The thickness and areal extent of laterite deposits is variable ; 

 many are thin and cover a limited area, but others such as the Cuban 

 and the Conakry deposits of French Guinea reach a maximum thick- 

 ness of 100 and 200 feet, respectively, and form a mantle over an area 

 of many square miles. 



The chemical composition of laterite is directly related to the com- 

 position of the parent rock. Insoluble constituents besides iron and 

 alumina tend to be concentrated and assume significant proportions in 

 the ore. Laterites derived from serpentine usually contain chromium, 

 nickel, and cobalt, whereas those formed on basaltic rocks contain an 

 appreciable amount of titanium. Gold, tin, copper, diamonds, and 

 other resistant minerals occur in some deposits as important con- 

 stituents. The iron content of the Cuban laterites, which are of Ter- 

 tiary age, ranges from 40 to 60 percent, alumina is usually high, and 

 phosphorus, sulphur, and silica are low. These deposits have been 

 described by Kemp (1915, pp. 129-154) and Leith and Mead (1912, 

 pp. 90-102 ; 1915, pp. 1377-1380) , but those of South America, India, 

 Africa, Borneo, the Celebes, and Mindanao in the Philippines are less 

 well known. 



Residual iron deposits are also formed on limestones and dolomites 

 under conditions of thorough weathering. Siderite, ankerite, or pyrite 

 disseminated in the parent rock oxidizes to goethite, and large quan- 

 tities of calcite or dolomite are removed in solution. Other insoluble 

 impurities such as clay and quartz sand accumulate with the goethite 

 and dilute the iron deposit. 



