Hematite: a Rock Made by Bacteria and Ocean Waves

The ore that made Red Mountain famous can be found along the trails at Red Mountain Park. The rock is called hematite, or iron ore, and it’s unusual in many ways. To begin with, it’s far richer in iron than other rocks in the Birmingham area. Also, some varieties of it are weak and readily crumble when crushed (next time you find some along the trail, try smashing a chunk of it with another rock). Hematite is also unusual because of the way it formed.

The typical sedimentary rocks of the Birmingham region, like sandstone and shale, formed from the cementation (bonding) of small sediment particles after they were washed to the coast by rivers and deposited offshore. Limestone, an abundant rock in the area, formed from the cementation of rubble from coral reefs and other marine ecosystems that were home to creatures that protected themselves with shells, skeletons, and armaments made from calcium carbonate. But unlike these other rocks, hematite formed when iron that had previously dissolved in seawater changed into a solid form and coated materials on the seafloor.

It all began in the Silurian Period (444–416 million years ago), when the earth’s crust where Red Mountain is now was an ancient tropical shoreline. Based on the banding pattern in the other rocks of the Red Mountain Formation, geologists believe this coast had numerous barrier islands and tidal channels. Just to the east (relative to our current position) were the Taconic Mountains, a mountain chain that preceded the formation of the Appalachians, including Red Mountain, by millions of years. Erosion of these mountains occurred rapidly since land plants hadn’t yet evolved and there were no root systems to hold soil in place. The volcanic and metamorphic rocks of the Taconic Mountains contained lots of iron. Some wound up in iron-rich clays deposited along the coastline that became Red Mountain. Some of it was dissolved into the seawater of the region.

Sample from Red Mountain Formation

Through a process that is not fully understood by geologists, the iron dissolved in the seawater began to precipitate (change from a dissolved form to a solid form) and coat the clay particles. It also coated shell fragments and other sediments along the shoreline. This iron coating was hematite. Some experts suspect that a bacterium was responsible for triggering hematite’s formation. They believe the bacteria were extracting energy from the dissolved iron, and the iron that precipitated was the by-product of this process.

Much of the hematite in the Red Mountain Formation is in the form of small rounded pellets cemented together. Geologists call these structures ooids, and iron ore with this composition is known as oolitic hematite. They suspect that the ooids began as sediment particles on the seafloor accumulated thin layers of lime. The pellets obtained their rounded shape due to being gently rolled back and forth by the waves at the surface. Later, and probably due to that bacteria, hematite began coating the surface of the ooids, in some cases replacing much of the lime.

Purple-brown soil on  Red Mountain Formation

Next time you’re near one of the mines at Red Mountain Park or Ruffner Mountain Nature Center, look for a piece of hematite and examine it closely. If they’re there, the small round ooids are easy to see. Look around a little harder, and you should be able to spot fossilized shell fragments coated with hematite.

-R.Scot Duncan