Re: Is mercury a rock? After all, it is a metal.

To answer your question, let’s first talk about what a "rock" is.  
Technically, according to geologists, a rock has to pass three tests.  
First, the rock must occur naturally on the Earth.  The rock has to 
come into existence with no help from humans or animals.  So, when 
humans 
build roads everywhere that are solid and made up of concrete, the 
concrete doesn’t count as a rock, because humans made it.  Second, 
a rock is not alive.  This point gets a little tricky because living plants 
and animals often create hard substances, like shells and teeth, 
which may seem like rocks.  But, if a snail makes a shell and then 
dies and you find the shell on the beach, it's not a rock, it's just a 
shell.  Finally, a rock must be a solid material.  This seems like an 
easy requirement, but actually the reason that the element mercury is 
not a rock on Earth is because it is not a solid.  However, the element 
mercury does appear in rocks and minerals and, under certain 
conditions (maybe on another planet) mercury might exist as a rock.  
On Earth, it does not and let me try to explain why.

There are three phases of matter – three main forms that matter can 
take.  I’m going to use water throughout this example because water 
can be in all three different phases of matter on Earth.  Not many 
materials can exist in all three phases on Earth, so water is very 
special!  The three different phases of matter are gas, liquid and 
solid.  Molecules of water in the Earth’s atmosphere are in the gas 
phase.  When there is a lot of water molecules in the atmosphere it 
feels very humid outside.  Molecules of water in a lake or that come 
out of a fountain to drink are in the liquid phase.  Molecules of water 
can also be in the solid phase and then they form snow or ice.  

Water is special on Earth because we have the exact right kind of 
temperature and pressure requirements to allow water to exist in all 
three phases.  If you took a pot of water and heated it up on the stove, 
the water would get hotter and eventually start boiling.  The boiling 
water is actually water molecules that have changed from being a 
liquid to a gas and float upward toward the surface of the liquid water 
in the pot.  Those gas water molecules go into the atmosphere in 
your kitchen. If you heat up the water enough and keep boiling it long 
enough, all the liquid water will turn into a gas and there will be no 
liquid water in the pot anymore (although your kitchen will have extra 
water in the atmosphere).  Alternatively, when you put a tray of water 
in the freezer and cool the water down, the liquid water will become a 
solid and you end up with a tray of ice cubes.  Technically, water in its 
solid form is a rock!  So, the phase that water is in at any time 
depends on the temperature (and the pressure too, but I’m not going 
to talk about that part).  

If you start with a liquid and increase the temperature (heat it up), the 
liquid becomes a gas.  If you start with a liquid and decrease the 
temperature (cool it down), the liquid freezes and becomes a solid.  
This basic statement is true of all of the different materials that we 
have on Earth.  But, different materials have different temperatures at 
which they either boil (go from liquid to gas) or freeze (go from liquid 
to solid).  Water boils at 100 degrees Celsius (212 degrees 
Fahrenheit) and freezes at 0 degrees Celsius (32 degrees 
Fahrenheit).  In your home and on the surface of the Earth, this range 
of temperatures is common and so we get to see water in all three 
different phases.  

But the element mercury boils at 356 degrees Celsius (673 degrees 
Fahrenheit) and freezes at –39 degrees Celsius (-38 degrees 
Fahrenheit).  On Earth, it rarely gets cold enough to freeze mercury 
and it certainly never gets hot enough for mercury to boil away.  
Therefore, mercury is said to be "stable" at the surface of the Earth as 
a liquid.  Water can be "stable" in all three forms, but mercury is only 
"stable" as a liquid.  So, by definition, we never see rocks made up 
entirely of the element mercury because mercury cannot be a solid 
on Earth.  It might be, that on other planets that are much colder than 
Earth, you could find solid mercury, in which case it would be a rock.  
But geologists don’t have a name for a rock made up entirely of 
mercury, because that rock doesn’t exist on the Earth.  If you 
discovered that rock on another planet, you could name it whatever 
you wanted – like Vincentite!

But, even though mercury is a liquid, it is still a metal.  A metal is a 
special material whose electrons can move from atom to atom.  This 
is how metals can conduct electricity.  When mercury is a liquid, its 
electrons can move from atom to atom just like electrons can in solid 
copper.  So liquid mercury conducts electricity and is a metal even 
though it isn’t a solid.

When I was thinking about your question, I got to this point and then I 
started thinking to myself, "well, where do we get mercury from then?"  
I know there aren’t big lakes of mercury on Earth, yet humans use 
mercury for all sorts of things.  Well, the answer is that if the element 
mercury combines with other elements to produce a different type of 
molecule then that molecule might be stable as a rock at the Earth’s 
surface.  Then we humans just have to figure out how to get mercury 
out of that rock.

The rock that we get almost all of our mercury from is called 
"Cinnabar".  Its chemical formula is HgS – Mercury Sulfide (because 
it combines with sulfur).  Cinnabar is very rare, but it exists in places 
like China, Spain, and California and New Mexico in the United 
States.  Cinnabar is very distinctive because it has a purple to bright 
red color.  I’ve attached some links below to a lot of great photos of 
Cinnabar from the largest mineral database on the internet, 
MinDat.org.  Also, if you are interested, you can read about the history 
of how humans have used mercury and see a great photo of a red 
rock with droplets of silver mercury on it on the second link below.  
The red rock is Cinnabar, and if you heat it up, some of the solid 
mercury turns into a liquid and seeps out of the rock to collect on the 
surface.  It’s pretty cool.  But mercury is also very poisonous to 
humans and this same page talks about how you can help make 
sure your school and home are safe from the dangers of mercury.  
Actually, this page is designed for your teacher to help you use, so 
make sure to tell you teacher about this web page if you want to learn 
more about mercury.

MinDat’s gallery of Cinnabar photos http://www.mindat.org/gallery.php?min=1052

Here is more information about the history of Mercury.  It has a great 
photo of Cinnabar with droplets of liquid mercury on it.  http://mercuryinschools.uwex.edu/curriculum/hg_in_world.htm