mares tail [OT]
 

"Malcolm Ogilvie"
If gold dissolves in water, how come one can pan for it in streams?

There was a good prog on the other night about Irish gold in the Bronze
Age. In saying how the deposits got there in quite some quantity, the
geologist said that it was deposited in quartz from solutions of metals
welling-up from deep in the earth's crust. Once the gold has been
precipitated it's insoluble in water. It does dissolve in "aqua regia" (a
mixture of hydrochloric and nitric acids, IIRC) and cyanide, in solution,
is used to extract gold commercially from low-grade ores. Gold
tetrachloroaurate [AuCl4], gold cyanide [Au(CN)2] and gold thiocyanate
[Au(SCN)4] are all soluble in water.

I don't know the exact chemistry of the solubility/insolubility puzzle,
but it must be similar to the chemistry of the flints I have in abundance
in my garden (central Essex) which came originally from aqueous solutions
as well, although they're obviously completely insoluble now.


- Tom Bennett