.TOURING

COLORADO GEOLOGY

Sillimanite Roadcut

Along the north side of Highway 285 between mile markers 247 and 248 south of the Morrison exit, an interesting exposure can be seen in the roadcut.  The very light color, almost white, of the rocks are a contrast to the dark gray schists/gneisses that predominate this section of the road.  The mineral responsible for this whitish color is sillimanite, an alumino-silicate (Al₂SiO₅).  Also of interest is another mineral which occurs along the top of the exposure, a bright green mica called fuchsite, a variety of muscovite, which owes its green color to the presence of chromium.

Sillimanite

Sillimanite is an interesting mineral occurring in metamorphic rocks.  It is one of three polymorphs with a composition of Al₂SiO₅, the other two being kyanite and andalusite.  The presence of sillimanite indicates that these rocks were subjected to a temperature of at least 500º C and generally high pressures (regionally metamorposed rocks).  Andalusite forms in high temperature, relatively lower pressures, often found in contact metamorphic settings.  Kyanite is a high pressure, but lower temperature environment compared to sillimanite.

Sillimanite is orthorhombic, in long slender crystals without distinct terminations, in parallel groups, and whitish in color at this exposure.    . 

Fuchsite

A dark green mica also occurs at this outcrop.  As mentioned above, the green color is due to the presence of chromium.  Fuchsite is, however, not considered a unique mineral, only a variety of the common form of mica, muscovite [KAl₂(AlSi₃O₁₀)(OH)₂]