ART. 23 DURANGO MINERALOGY AND GEOLOGY— "^^'CH AG 9 



MINERALOGY 



The mineralogy of the iron deposits of Cerro Mercado is conipara- 

 tivel}' simple. The iron minerals are in very large excess over the 

 other minerals and consist almost entirely of the pseudomorphous 

 variety of hematite known as martite. Other iron minerals are incon- 

 spicuous in the deposit; they may actually be ,said to be rare. Asso- 

 ciated minerals are not abundant. The iron ore forms a mass of con- 

 siderable ])urity and the accessory minerals are found onl}^ in the 

 vugs and cracks of this ore. 



In addition to the chief ore mineral, martite, there are also goethite 

 and limonite; and the accessory minerals include apatite, dahllite, an 

 unknown phosphate mineral, hedenbergite, sepiolite, quartz, chal- 

 cedony, opal (variety hyalite), titanite, calcite, and barite. Other 

 minerals have been listed from this locality but some of them have 

 certainly been erroneous determinations, while others, such as topaz, 

 have come from neighboring localities but not from Cerro Mercado 

 itself. 



Martite. — All of the ore tliat is found in well-defined crystalline 

 form belongs to the pseudomorphous variety of hematite called mar- 

 tite. This leads to the reasonable assumption that the massive mate- 

 rial is of the same nature. At Penascos de la Industria it is hard 

 and firm with numerous cavities lined with well-formed or flattened 

 octahedral crystals (92151). Martite crystals up to 2 and 3 inches 

 across have been found here. The octahedrons are frequently flat- 

 tened and then resemble stout rhombohedral crystals of hematite. 

 Twins among the flattened crystals are common; sometimes larger 

 crystals show several smaller individuals in twinned position along 

 the edges. In the finer grained ore of Picacho Socavon^ 1 (92160) 

 one can readily distinguish the small constituent octahedrons of 

 martite under the binocular microscope. A section of this under the 

 petrographic microscope reveals the martite crj'^stals arranged in 

 delicate fernlike groups, suggestive in their arrangement but lacking 

 the delicacy of the magnetite crystals that are found between lamellae 

 of mica. Some of the harder lenses of this material (92161) have 

 the same structure but owe their superior hardness to an impregna- 

 tion of quartz and chalcedonic silica. The comj)act ore of Picacho 

 Socavon 2 (92162) is also martite. It is firm and compact with a 

 semimetallic luster. In some of the cavities, now lined with crystals 

 of quartz, are to be found small brilliant modified octahedrons. 

 Since they show no magnetism and have a red streak they, too, are 

 martite. This ore on polished section appears to be homogeneous 



" The word socavon is the Spanisli term for adit. The various adits of Cerro Mercado 

 have lent their names to some of the individual hills. 



16738 28 2 



