substrate mineral composition

The studies I've read don't mention the relatively inert mineral composition
of soils so it would seem not to be very important. Some questions occur to
me however.

What difference will it make if Fe compounds in the substrate are iron oxides,
iron silicates, iron pyrites or other compounds? It seems to me that iron oxides
would be more chemically active but I have little to support this.

We've mentioned lead (Pb) and copper (Cu) metal excess. What other metals
or alkaline earth metals could commonly occur in various soil types in excess
and what would be the effects upon plants, invertebrates and fish? How
common is Pb or Cu in surface clays or clays which might be mined for use
in ceramics?

If we are interested in finding sources of iron-rich clays from pottery clay
suppliers, can they tell us the mineral composition of the clays? Are there
other information resources that we should be consulting when considering
a particular type of clay? or a local soil?

Suppose a particular soil or clay has a lot of magnesium or calcium compounds
such as from dolomite or limestone, is this a cause for concern as a substrate?
Folks have often mentioned problems with rising pH due to seashells or limestone
in their substrate. Is this really a concern for plants? or fish? Can this
be balanced by the use of peat or other sources of humic acids?

I've been soaking several egg shells in a jar of water for a couple weeks
to remove soluble proteins per Paul K's suggestion, however, there is a
distinct "bad egg" smell when I open the jar and replace the water. Is this
cause for concern? Before I put the egg shells in the jar, I washed them
using my finger and running water to remove the albumen while it was still
soft. A white membrane is found on the inside of the egg shell. Is it
important to discard this? It doesn't seem to dissolve. I hope to clean
these egg shells and turn them to powder in my blender and add them to
new substrates as I set them up. Is it preferable to compost the egg shell
powder aerobically first with soil?

Is there something about the fine texture of clays and silts which is
important for aquatic plant roots (leaving nutrients and minerals out
of the picture for the moment)?