Beach Nourishment - Why is the sand on the newly dredged beach black?
Why is the sand on the newly dredged beach black? There have also been questions regarding the black residue that is present on the beaches. The black residue is not oil, it is a concentration of heavy minerals. When you examine the grains, you can see specs of magnetite, ilmenite and even purple tinted garnet sand size grains. These are trace minerals that are found mixed into the sand from the borrow area. The fact that these minerals are heavier than quartz (the mineral that makes up most of the Dare County beach sand grains) allow them to concentrate near the water line. The concentrated heavy minerals are likely to mix in with the native beach over time, but pose no health risk to visitors or wildlife (Julien Devisse, P.E. Coastal Planning and Engineering, excerpt from memo dated September 1, 2017).
The most common heavy minerals in beach sands, which give the black color to the heavy-mineral fraction, are the opaque minerals, commonly magnetite (iron oxide) and ilmenite (iron titanium oxide). The opaque minerals are responsible for the patches of black sand on beaches (sometimes mistaken for oil pollution)—concentrations of heavy minerals that are examples of natural placer deposits. The heavy minerals are dispersed through the entire beach, but under the right conditions, placer deposits form. For example, when a wave washes up on the beach, all of the sand is carried forward, but the back wash has much less energy. So the heavy mineral grains are more likely to be left behind because of their higher specific gravity (especially those with the very high specific gravity), while the lighter quartz, feldspar, mica and shelly sands are carried back seaward. Commonly these placers are found on upper beaches, especially after storms. Various combinations of wind and water separate the heavy minerals from the light minerals or other heavy minerals and concentrate them in patches of colored sand, dependent on the dominant mineral Such concentrates are sometimes seen in the wind-generated ripple marks on the back of the beach or where backwash creates crescent patterns around obstacles on the beach (excerpt from What the Sands Tell Us: a Look Back at Southeastern US Beaches; By Orrin H. Pilkey & William J. Neal).
