The halo effect scientists takes about, and the ones detectorists talk about, are two different animals.
All of us know that late-minted pennies are mostly zinc, and heavily plated with copper. Imperfections in the plating expose the zinc to chemicals in the soil, which are primarily alkalis, but can be acidic. As a result, the zinc oxidizes (ZnO). That's the whitish stuff on the surface, usually in the form of nodules. ZnO is almost insoluble in water, but it nonetheless leaches to the surrounding soil. This causes the apparent target size to appear larger; haloing as it were.
Silver is a lot less chemically active than zinc. It doesn't oxidize easily, even in the presents of most acids and alkalis found in soil. Silver oxide, Ag2O, is rather dark brown in color. This is why silver coins which have been buried for a long time, have a dark brown (almost black) hue to them. In any case, any halo effect they produce, is a lot less than that of zinc.
Alan Applegate