QUARTZ-FELDSPAR-PORPHYRY 



(GRANIPHYRO LIPAROSE-ALASKOSE) 



FROM LLANO, TEXAS. 



There occurs In the vicinity of Llano, Tex., a porphyry which is 

 very interesting petrographically, and may prove equally so commer- 

 cially. It forms a large body whose shape and geological occurrence 

 have not yet been described. It is said to be quite uniform in char- 

 acter. The material submitted by Dr. William B. Phihips, Director of 

 the University of Texas Mineral Survey, for petrographical study is a 

 gray porphyry with abundant phenocrysts of red feldspar and blue 

 quartz, the matrix or groundmass being aphanitic to phanerocrystal- 

 line. It appears to have a crystalline texture, but the individual 

 grains are not distinctly visible without a microscope. The rock is 

 therefore mottled red and gray, with light blue spots of opalescent 

 quartz. 



The phenocrysts vary in size, the largest feldspars being lo'"'" in 

 diameter, the largest quartzes 5""". The quartzes exhibit a beautiful 

 blue color, which is light sky-blue in the central part of the crystal 

 and dark at the margin. The crystals are not all colored to the same 

 degree; some are lighter than others. The color does not change 

 perceptibly with a change in the angle of incidence, or in the position 

 of observation, except that in certain positions there is a brilliant 

 light blue luster. The feldspars are rather uniformly colored light 

 Indian-red, the larger crystals being mottled with gray. 



The proportion of phenocrysts and groundmass estimated from 

 the surface of the specimen and from three thin sections is : 



( quartz - - - - - 10.7 



Phenocrysts "i r u a - 



•^ ( feldspar - - - - - 20.5 



Groundmass - - - - - - -62.8 



Under the microscope the groundmass is seen to be holocrystal- 

 line and microcrystalline, and is composed of feldspar and quartz in 

 nearly equal proportions, together with a small amount of brownish- 

 green mica, and still less fluorite, magnetite, apatite, and zircon. 



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