QUARTZ-FELDSPAR-PORPHYRY FROM LLANO, TEX. 227 



The feldspar phenocrysts are microcline, with extremely minute 

 and regular multiple twinning in two directions. The delicacy of 

 the twinning suggests a possible soda content in the potash feldspar 

 approaching soda microcline. There is also a perthitic inclusion of 

 albite in irregularly shaped shreds, and also a slight clouding due to 

 alteration, which is probably kaolin with hydrous oxide of iron which 

 gives color to the feldspar. 



The quartz phenocrysts contain multitudes of minute inclusions, 

 rather evenly distributed through each crystal, except for a margin of 

 nearly pure quartz in some cases. The inclusions are of two kinds, 

 generally intermingled: one consists of extremely thin, colorless 

 prisms, sometimes passing into lines of minute grains, like broken 

 prisms; the other kind is in equally thin tabular crystals with six 

 sides and trigonal shapes, and a light brown color. The colorless 

 prisms have higher refraction than quartz, but the double refraction 

 is not recognizable. They resemble apatite rather than rutile, having 

 lower refraction than rutile and not being so long as rutile needles 

 often are. The width of these prisms varies from o.oooSoo"'™ to 

 much less; that is, it is mostly a fraction of a wave-length of light. 

 The brownish tabular crystals are equally thin, and range in diame- 

 ter from 0.004"^"^ to much less. Studied by incident sunhght, they 

 exhibit metalHc reflections of a bluish- white and also of other colors. 

 They have the crystal form and color of ilmenite. 



These inclusions lie at all angles within the quartz crystals, but 

 there appear to be sets of parallel directions intersecting at various 

 angles, so that in some positions many tabular microlites reflect light 

 in one direction. The same is true of the colorless needles. They 

 lie in parallel lines crossing at various angles, whose orientation with 

 respect to the inclosing quartz does not appear to be definite. 



The sky-blue opalescent color of the quartz phenocrysts is undoubt- 

 edly due to reflection of blue light-waves from the minute colorless 

 prisms, whose width is a fraction of the length of light- waves. It is 

 similar to the blue color of the sky. It is probable, however, that there 

 is also blue light produced by interference of the light reflected from 

 both sides of the minute tabular crystals, whose thickness is also of 

 the order of a fraction of a light w^ave-length ; so that both kinds of 

 phenomena occur within these quartzes. 



