Any idea on what these may be and how they could have formed?

They were found in deeply buried silty alluvial sediment samples from the Roanoke River in Virginia.

Based on EDS measurements, the bright areas that are shaped in various crystal or lattice configurations are very oxygen depleted and dominated by Fe (e.g., 4 to 5 % Oxygen and 90% or more Fe).

The magnetic fraction is full of orange oxidized fragments that have oxygen depleted Fe material imbedded on them.