The photo above, from near the Portside Towers in Jersey City, NJ is not intended to represent the entirety of "urban beaches", but rather to consider what makes up a particular sand and where does all that stuff come from?
The darkness of the sand suggests a significant heavy mineral percentage, however it is likely that as the tide was receding, the last waves moved the lighter quartz downslope, exposing a thin veneer of heavy minerals. Under the microscope, heavy minerals are not the dominant mineral-type. As in most sands, quartz is the dominant mineral.
To get a wider perspective of the location, the large building in the left foreground is the Goldman Sachs building. In the right background is a portion of downtown Manhattan, with the Hudson River between the two. To the left of the beach is a narrow spit of land adjacent to a portion of an abandoned, historic canal.
In an effort to prevent erosion of the narrow spit, a wide variety of materials have been piled in the upper beach area, as well as miscellaneous old tires as well as other items of human flotsam and jetsam.
The remainder of the photos are to illustrate different perspectives on the beach materials that contribute to the sand chemistry with the never-ending wave action.
Under the microscope a brief examination of the sand - without concern to relative percentages - the constituents of the sampled beach sands include: 1) Angular quartz; 2) Well-rounded quartz; 3) Brick; 4) Glass; 5) Coal fragments; 6) Greenish Fe-Mg minerals; 7) Garnet; 8) Rutile; 9) Magnetite; 10) Ilmenite; 11) Slag,...
Among the particles, there are some odd, spherical grains that seem slightly magnetic, a quick guess might be magnetite, slag, or (least likely) Fe-Ni micro-meteorites. There will be some more examinations of the sand and the spheres in particular.