10 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 74 



except for small areas within the ore and for thin bands lining the 

 cavities that have a yellow streak and are geothite. 



All the martite is in part magnetic, although visible magnetite is 

 absent. Analyses of the ore, however, show a content of ferrous 

 iron sufficient to account for its magnetism. Wliether this magnetic 

 portion is finely dispersed throughout the martite grains or whether 

 it represents residual masses of unaltered magnetite within the octa- 

 hedrons is unlaiown. An analysis of the magnetic portion of a 

 sample from the Penascos de la Industria made in the laboratory of 

 the United States National Museum yielded the following results : 



Magnetic portion of ore, Penascos de la Industria 



Forest A. Gonyer, analyst 



Insoluble 10. 84 



Fe^Os 80. 72 



FeO 2. 58 



H2O 1. 36 



Undeterminedi 4. 50 



100.00 



The ferrous-iron content of the above sample corresponds to 2.77 

 per cent of magnetite in the ore. Other analyses of the ores exam- 

 ined with reference to their ferrous-iron content show the amount of 

 ferrous iron to vary from 0.5 to 5 per cent. 



Goethite. — The hydrous oxide of iron goethite is widespread al- 

 though not abundant in the deposits. It forms thin velvety blooms 

 of a light chocolate-brown color on martite crystals in the vugs of 

 the more massive ore (92155), especially at Penascos de la Industria. 

 The lens reveals these velvety blooms to be made up of thin needle- 

 like crystals. Rarely larger crystals of a thin barrel shape and bril- 

 liant black color and up to 1 inch in length are found in the vugs. 

 In other places the goethite occurs as crusts, with drusy surface and 

 made up of radiating laths (92152). 



Limonite. — The amorphous hydrated oxide of iron, limonite, is 

 only sparingly present as thin secondary films due to hydration, the 

 result of ordinary surface weathering. 



Hematite and magnetite. — These minerals, other than in the form of 

 martite, are essentially unimportant. Martinez-Quintero ^° describes 

 specular hematite in the form of beautiful rhombohedral crystals 

 tabular to c and often twinned on the base. This description fits the 

 characters of the flattened twinned octahedrons of martite so well 

 as to appear likely that the two occurrences are the same mineral." 



»»Inst. Geol. Mexico, Bol. 44, p. 35, 1923. 



^The flattened fornr in twin crystals is very common especially in thie spinel twins of 

 magnetite and related minerals. (Note the flattening of quartz twins as well.) 



