Stilpnomelane at Blanchard Mountain, Skagit County

By Dave Tucker  March 3, 2010

Black rosettes of stilpnomelane in quartz. Click this or any image t0 enlarge.

A road cut on Blanchard Mountain allows easy access to an unusual exposure of the mineral stilpnomelane. This is a striking black mineral crystallized as shiny rosettes in or near white quartz veins, which in turn cut beautiful green chert. The chert is bedded between phyllite exposures to east and west. The entire outcrop is steeply dipping. The rocks are part of the Easton Metamorphic Suite, best known for the Darrington Phyllite and Shuksan Greenschist rock units in the North Cascades, which formed during subduction of latest Jurassic – earliest Cretaceous seafloor sediments. See the field trip to the nearby Oyster Dome and Bat Caves to learn more about the geology of this suite of rocks. This outcrop is popular with mineral collectors, and shows a fair amount of wear and tear. Before you smash the rock for a sample, look around in the plentiful talus at the base. BE VERY CAREFUL HAMMERING THE GREEN CHERT! This rock is very brittle and breaks up into tiny glassy shards. Similar rock has been used for tool points- “flint” is a dark variety of essentially the same stuff.

White quartz veins cut green chert on Blanchard Mountain

Stilpnomelane is an iron-rich  mineral; these minerals are structured as weakly bonded sheets of silica. Minerals with this structure are in the  phyllosilicate class, which you will of course remember if you took mineralogy. You may recognize the suffix phyllon, Greek for ‘leaf’. Chlorite and the mica minerals biotite and muscovite are more familiar minerals in the phyllosilicates. The mineral’s descriptive name is also derived from the classical Greek that all academic geologists use in their daily conversations when they don’t want their students to hear them: stilpnos , ‘shiny’, and melanos, ‘black’. For more on the mineralogy, including photomicrographs, see George Mustoe’s paper (available online) listed in the references.

Chert is a microcrystalline rock that is primarily silica (SiO4). Two origins for the chert on Blanchard Mountain have been proposed. Dragovich ( 1998)  suggests that since the sediments  that formed the rocks of the Easton Suite are derived from a volcanic island arc, at least some of the nearly pure silica that comprises the chert fell as fine volcanic ash on the seafloor. Gallagher and others (1988) holds to a classical origin for the siliceous sediment as a radiolarian ‘ooze’, deposited on a shallow ocean floor by the rain of radiolarian tests (‘shells’) when these planktonic protista died. Rapid burial by more tests or muddy sediment trapped the silica, which eventually lithified to form the chert. The phyllite that sandwiches the chert is derived from lithified mud.

Phyllite (curved foliation right of hammer) and the contact with the massive green chert, cut by white quartz and black stilpnomelane.

The seafloor sediments were subjected to high pressure but relatively low temperatures as they were  eventually subducted and metamorphosed to the rocks we see today on land. Mustoe (1998) speculates that the stilpnomelane crystalized during hydrothermal activity, which introduced iron into the surrounding chert in sufficient amounts to enter silicate structures and become stilpnomelane.

Getting there: The simplest route is via I-5. Exit at Bow Hill Road, exit #240. Head northwest on Lake Samish Road. Mileages are given from the end of the southbound off ramp. In 0.4 miles (0.6 km) turn left (west) onto Barrel Springs Road, which curves due south. 1.1 miles from I-5, turn right on gravel road B-1000, signed “Blanchard Trail”. Follow this curving gravel road past a major trail head (the ‘lower’ trailhead to the Blanchard Mountain Trail ; at 2.6 miles, pass 30-foot-high cliffs of mangy-looking Darrington phyllite on the right. Just beyond, turn left at 2.7 miles on a road with a yellow gate (usually open, and sometimes signed B-2000) to “Samish Overlook”. (If you continue straight rather than taking this left, arrive shortly at the upper Blanchard Mountain trailhead). It is 0.4 mile from the yellow gate to the stilpnomelane outcrop (3.1 miles from I-5), an obviously different looking 60-foot-long by 15-foot-high greenish road cut cliff on the right.

The stilpnomelane/chert outcrop. Bob Mooers looks for a nice sample in the talus.

If you want a really spectacular view, continue beyond the stilpnomelane outcrop 1.9 miles to the Samish Overlook, (5 miles, 8 km, from I-5). Here is a great view out over Samish Bay to the San Juan Islands, the Samish River delta, the Olympics on a fine day.  A trail goes north from here 0.3 miles to join with the Oyster Dome/Bat Caves geology field trip, which you’ll join a few hundred yards before the ‘stretched pebble conglomerate’ outcrop. The road to the overlook passes through phyllite and semischist of the Easton Suite.

The view soutwest from the end of the road. Plumes are refineries at Anacortes. Samish River flats at far left.

Essential reference:

Mustoe, G.E., 1998, Stilpnomelane at Blanchard Mountain, western Skagit County, Washington: Washington Geology, vol. 26, no. 4, p. 3-7. This article is available in full online here.

Also cited:

Gallagher, M.P., Brown, E.H., and Walker, W., 1988, A new structural interpretation of the western part of the Shuksan blueschist terrane, northwestern Washington: Geological Society of America Bulletin, vol. 100, no. 9, p. 1415-1422.

Dragovich, J.D., and others, 1998, Geologic map and interpreted geologic history of the Bow and Alger 7.5-minute quadrangles, western Skagit County, Washington: Washington Division of Geology and Earth Resources Open File Report 98-5, 80 p., 3 plates.