MadSci Network: Earth Sciences

Re: Why don't rocks burn?

Date: Tue Nov 23 21:54:32 1999
Posted By: Matthew Buynoski, Senior Member Technical Staff,Advanced Micro Devices
Area of science: Earth Sciences
ID: 942549156.Es

A few rocks do burn...oil shale being perhaps the most well known (but it 
is sort of cheating...the hydrocarbons are not part of the rock but held in 
pores within it).  Diamonds can be burned as they are pure carbon. Various 
sulfide minerals can be oxidized as well. The latter two are not going to 
burn if you hold a match to them, but they will burn at higher 
temperatures/pressures/oxygen concentrations. Do not attempt to burn a 
diamond because the owner is likely to be *highly* upset (diamonds tend to 
be expensive), and do not attempt to burn sulfides as the byproducts 
include really nasty fumes.

Most rocks do not burn because the elements that compose them are already
"burned" in the sense that they have already combined with oxygen included 
in the crystal structure. This includes all the oxides, of course, plus all 
the silicate rocks (which are the vast majority of all rocks in the Earth's
crust and mantle). For example, magnesium silicate (MgSiO3) forms a huge
percentage of the Earth's mantle. This compound can be considered to 
be "combined from" magnesium oxide (MgO) and silicon dioxide (SiO2). 

The solid core of the planet can be considered a rock, albeit a somewhat
unusual one of iron and nickel in metallic form. If this material could
be brought to the Earth's surface, it also could be burned.  

There are rare cases of reducing conditions that do create native iron (so-
called bog iron is one type, native iron is also found in  some mines). 
When such iron is exposed to the air, however, it immediately starts to 
rust (rusting is a very slow burning of iron).

Leaving aside all the oddball stuff, it is fair to say that of any rocks
that most of us are likely to encounter on the surface of the Earth, 
virtually all are silicates, and thus will not burn.  Moreover, the oxygen
in the atmosphere will generally oxidize (slowly perhaps) pretty much 
anything exposed to it (like the rusting iron above). So unless you go seek 
out special conditions that are in some way isolated from the air, you will 
not find rocks that will burn.

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