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Hi everyone,

I recently collected some rocks from northern Minnesota that are much like
slate in form, but are darker (black) in color.  Multifaceted, very angular.
IMO they would provide a nice contrast in a plant tank.  Based on the
region, I guessed they were Taconite, but emailed the Minnesota Geological
Survey group to get an expert opinion.  The response is below.  I would
appreciate any knowledge or opinions on the suitability of Taconite in an
aquarium.  Thanks in advance.

Mike Grace
It sounds to me, from your description that the rock you have is taconite,
is a type of iron formation.  The tendency for the slabs to split apart
to me that the rock is specifically what we refer to as "slaty iron
which is commonly not magnetic.  The deep rust shade is probably from the 
oxidation of minor amounts of the mineral pyrite, which is a mineral made of

iron and sulfur, so on that basis perhaps the rock may not be suitable for
in a fish tank, although I do not know what consitutes acceptable vs. 
unacceptable material for that use.  Most of the iron in the rock would be
the form of magnetite, for which the chemical formula is Fe3O4, or hematite 
(Fe2O3).  The most common minerals in the rock are probably quartz and other

iron silicate minerals.

Terry Boerboom
Minnesota Geological Survey