[Rockhounds] Geology in Progress

[jb]

Nathan Martin wrote:
> Larry,
> Seems as if you made good use of one of the few mild days we have had.  Lets
> hope that we get more of that "beach weather" soon!
>
> One of the things that I find interesting about your observations is that
> the magnetite was on top of the garnet. Based on the relative densities of
> magnetite (~5.15 gm/cm^3) and typical New England garnets like almandine
> (~4.2 gm/cm^3) or grossular (~3.6 gm/cm^3) , I would have expected just the
> opposite.  I wonder if it is possible that strong wave action could turn
> over a density-stratified layer of sand, garnet and magnetite to produce the
> layering that you saw?  I know that bulk granular materials can have some
> pretty unusual properties but I don't really know if what you observed is
> common.  It may be a really unusual occurrence.  Perhaps someone else on the
> list can comment.  Any sand collectors out there care to venture an opinion?
>
> best regards,
> Nate Martin
> Lexington, MA
>
>   
It's likely the the black sand is not 100% magnetite, but has a
considerable quantity of ilmenite mixed in. The lesser density of
ilmenite often makes "black sands" somewhat lighter in wave action.
Also, grain size has a lot to do with distribution. Smaller grain sizes
will end up on top of quartz sands even though they may be of a little
greater density. I should think that the only true density distribution
in a column would be settling in a calm water ignoring surface tension.


john