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Re: Petrified Wood - Inert Rock

On Tue, 12 Jan 1999, Tom Wood wrote:


> But I was most surprised at his recommendation to use petrified wood in
> a planted tank.  Is petrified wood always non-calcerous?  Is it always
> inert?  What other rocks are tank safe?

Petrified wood is almost always made up of microcrystalline quartz.  It's
very inert and safe to use in an aquarium.  That said, let me point out
some exceptions.  Calcite (calcium carbonate) is almost ubiquitous so
there's always a likelihood that a bit of petrified wood that you pick up
(or buy) somewhere is going to have some calcite in it or on it - the
calcite is not usually a large part of the petrified wood and usually just
washing the thing off will get rid of most of it.  On rare occasions there
are examples of petrified wood that's been petrified with something other
than quartz - I've seen or heard of wood petrified with carnotite (a
bright yellow uranium mineral), chalcopyrite (usually a metallic-looking
copper ore) and a few other things.  Those examples are real geologic
oddities and you'd probably have to go *way* out of your way to find them.

It's fairly difficult to generalize about what rocks are safe in an
aquarium.  Petrified wood is probably safest of all; all forms of quartz
are safe.  "Granite" will usually be safe, as well.  As generalizations
go, it's probably fairly safe to say that a stone (pebble, cobble,
whatever) that's been rounded and polished in a stream or on a beach is
probably going to be safe to use.

Just about any time you use a mined or quarried stone you are taking some
risk that it will contain chemically unstable minerals that will do
something unpleasant in your tank.

Some specifics you should avoid:

	marble, limestone, dolomite, shells, coral skeletons or anything
similar (the preceding all contain calcium and/or magnesium carbonates and
fiz in acid - the dolomite you might have to scratch before it fizzes

	sandstones (rocks that look like cemented or compacted sand - they
often are cemented with calcite and will fizz in acid),

	anything with a shining metallic luster like "fools gold"  (these
are usually sulfides that can go through acid-producing reactions and/or
release metals into solution),

	coal (pretty under water, but it's often treated with oil to
suppress dust and it often carries a high level of sulfur and metals that
can be a problem if released into the water).

I'm sure there's lots of others.

You can find a lot of beautiful stones at rock and mineral shops, and some
of those can be great aquarium decorations, but personally I prefer
petrified wood and the look of stream-rolled stones.  But some
probably safe examples that you might find at a rock shop are:

	Varieties of microcrystalline quartz (for example jasper,
chalcedony, agate, sard, carnelian),

	jade (attractive and inert but IMHO the green color of jade
clashes with the greens in plants),

	all forms of crystalline quartz (e.g. rock crystal, amethyst,
citrine, rose quartz, smokey quartz).

Fossils look very cool, but many contain calcite or pyrite and
shouldn't be used and some are varnished for preservation and shouldn't
be submersed.

Rock shop owners and/or sales people might be knowledgable enough
to help you figure out what will be safe in an aquarium.  Ask them.

Roger Miller