ART. 23 DURANGO MINERALOGY AND GEOLOGY FOSHAG ' / 



latite (92143) colored mottled brown and blavk, with prominent 

 brownish feldspar phenocrysts up to 8 millimeters in size. The 

 gromidmass is a brown perlitic glass with very minute microlites. 

 Throughout the groundniass are scattered phenocrysts of both ortho- 

 clase and plagioclase. The orthoclase is variable in its optical axial 

 angle, in a few grains it is small but many have a large angle. The 

 plagioclase is andesine. Twinning is not very prominent in the 

 plagioclase but does occur in some grains as very fine striae. Augite 

 is present in irregular to subhedral grains with a pleochroism of c = 

 brownish green and a=brow"n. 



Tuff. — The tuff ranges from buff-colored fine-grained rocks with 

 a harsh feel to brick-red forms with fragments over 1 centimeter in 

 size (92146). The original fragments are all strongly altered. Under 

 the microscope the groundmass is amorphous except where it carries 

 abundant secondary chalcedony as radiating globules or masses. The 

 rock fragments are made up of a very fine birefracting substance 

 thought to be montmorillonite or a similar clay mineral. 



The finer grained tuff from the base of Picacho de la Cruz 

 (92145) show^s under the microscope a complete crystalline structure 

 of interlocking grains of quartz, some bright-green gi-ains of augite, 

 a few flakes of muscovite, and rarely some feldspar. This tuff has 

 undoubtedly been recrystallized and largely silicified. 



Rhyolite. — The rhyolite (92144) forms a vinaceous to cinnamon- 

 red rock with numerous glassy phenocrysts of quartz and smaller 

 ones of feldspar in an aphanitic groundmass. Small grass-green 

 augites occur in some of the more chalky streaks and are often ar- 

 ranged in small circular masses. The groundmass, as seen under the 

 microscope, is made up of interlocking grains and apparently has 

 been someAvhat recrystallized and silicified. The chief constituents 

 are quartz and orthoclase. In some parts the grass-green augite is 

 abundant in thin veinlets. About these veinlets the rock is some- 

 what recrystallized. Hematite in thin hexagonal scales occurs rarely, 



ROCK ALTERATIONS 



The common type of wall-rock alteration is a " kaolinitization " 

 and is often accompanied by the introduction of much diopside. This 

 hydration is strongly developed in Socavon 4, after this adit has 

 passed through the iron-ore body. The remainder of the adit is 

 entirely in a completely altered latite. The texture of the original 

 rock is well retained, the small feldspar phenocrysts are easily 

 observable, but the entire mass is now hydrated to a soft claylike 

 mineral. Under the microscope this mineral appears as matted small 

 shreds with a strong birefringence and with a mean index of 1.49. 

 A partial analysis of a sample of the pure mineral gave: Water 



