Stop 9: DISTRICT COURT BUILDING - 501 East Fayette Street

Stop 9
Figure 9

Cross Fayette Street, turn left and walk east along Fayette to the District Court Building. Observe the nearly black stone at the base of the District Court Building. It was quarried in Minnesota and is Precambrian in age (Figure 9). Its commercial trade name is “Veined Ebony Black Granite.” This is not a true granite, but rather an anorthosite, composed mostly of long, lath-shaped crystals of dark feldspar and some black biotite mica. The rock contains almost no quartz (a significant component of granite). Anorthosites range in color from light gray to black. Often anorthosite is largely composed of a particular feldspar mineral called labradorite, which is commonly dark blue, green, gray or brown, with an iridescence (frequently vivid blues or greens). At the District Court Building the feldspar crystals are mainly dark greenish-gray, dark blueish-gray and nearly black. Notice the different shapes and sizes of the minerals compared to other rocks seen previously. The large crystal size indicates that this anorthosite, like granites seen earlier in this tour, formed deep inside the earth where cooling took place slowly.

The rounded columns at the entry to 501 Fayette Street are also anorthosite. There is a red granite trim around the metal doors.