SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. lOI 



The unaided eye readily recognizes in addition to the muscovite, 

 nearly an equal amount of quartz and flakes of biotite. The microscope 

 shows the muscovite to be intergrown with the biotite in some cases, 

 and entirely distinct from it in others. That it may be in part an 

 alteration product from the biotite is indicated by the presence of second- 

 ary magnetite. Quartz occurs not only in grains between the mica 

 flakes, but is also enclosed in the muscovite, producing distinct micro- 

 poikilitic texture. The quartz contains liquid inclusions arranged in 

 rows parallel to the foliation, and apatite needles extending in the 

 same direction. Minute zircon crystals are enclosed in both muscovite 

 and quartz. 



SlLLIMANITE-SCHIST 



The best exposures of sillimanite- schist are in the metamorphic area 

 northeast of Sugarloaf Mountain at short intervals along the line of the 

 latite dike. The rock is dark colored and closely packed with small 

 bundles of sillimanite crystals which range from short, minute fibers to 

 prismatic crystals 15 mm. long and less than 1 mm. thick. The silli- 

 manite is light in color and becomes bleached on weathering, thus 

 accentuating the contrast with the remainder of the rock. The inter- 

 spaces are filled with biotite, and small flakes of muscovite are common 

 in the most weathered specimens. The schist is in bands alternating 

 with layers of an aggregate of quartz and feldspar. Lenticular masses 

 of pegmatite are also present, but of smaller dimensions than those 

 in the granitic gneiss. 



Under the microscope the sillimanite crystals are usually seen to be 

 broken with the fractures healed by a secondary colorless mineral. 

 A few minute zircons are enclosed in the sillimanite. The biotite has 

 formed later and forms a matrix for the sillimanite. Often a single 

 crystal of biotite will enclose a number of fibers of the earlier formed 

 mineral in poikilitic manner. In one slide examined the biotite encloses 

 slender microlites intersecting at an angle of 6o°. They thus have the 

 habit of rutile but are colorless and resemble the sillimanite. They 

 are too small, however, for certain identification. A small amount of 

 orthoclase is present, involving the sillimanite crystals in the same 

 manner as does the biotite. 



