Re the discussion about "alluvial" lead (galena) deposits versus
tin (cassiterite) deposits.
I guess one factor giving rise to the fact that there are many
alluvial tin deposits, and few for lead, is the relative hardness
of their respective minerals.
According to "Rutley's Minerallogy", cassiterite's hardness is
between 6 and 7 on Moh's scale, while galena is a relatively
soft 2.5.
Under stream conditions, cassiterite grains would persist
further down stream than would grains of galena. Added to
which the latter probably oxidises and disintegrates further,
while cassierite, being already an oxide, is stable.
Tony Brewis
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