On another writing site, my expertise was requested. A reader posted about a specimen of natural iron ore that he could not identify which exhibited a curious transformation when super-heated and allowed to cool.
Natural Iron from the Earth
Iron ore from the earth is not uncommon. Heavy rocks with visible oxidation clearly have oxides of iron within them. Iron nodes form in some sandstone formations, leaving the ubiquitous ‘moqui marbles‘ that have mystified both native peoples and modern man alike. Recently, these same types of nodes were discovered on Mars, and named ‘blueberries.’
But the person whomposted this question stated something curious about his sample. It changed properties when he super-heated it with a torch. Whereas before it was dark and rusty-colored and not attracted by a magnet, now it changed to a dull silvery color and was highly attracted to a magnet. He asked if I knew what this was.
I Think I know What This Mystery Iron Node Is!
Something tickled the back of my skull and made me recall a little factoid I had learned about some forms of naturally-occurring iron. What this postee cited was that he had tested a strong magnet on it and there was no discernible attraction between the magnet and the iron specimen.
He further stated that after had heated the iron node to nearly white-hot using a (unspecified) type bottled cylinder of gas and allowed the metal to cool naturally. He did not state whether this was bottled propane, MAPP gas (methylacetylene or propyne) or if he had used an acetylene torch with an oxygen accelerant. Either way, his reported observations were interesting.
The Iron Node turned Silvery
It was here that I had my ‘ah-HA!’ revelation! Super-heating any rusty metal or iron ore will cause oxidation to become liberated, leaving just the iron after releasing the oxygen. This explained the color change although usually the rusty metal turns black, not silver. The metal had also become attractive to a magnet AFTER it was super-heated and allowed to cool. This was a big clue about what had occurred! Several physical changes to the sample had occurred.
An Allotrope of Iron
An allotrope, or the property of allotropism, is when a chemical element (not a compound) can exist in multiple (two or more) forms. Structural arrangements of atoms thereby can be vastly different, causing the element to be bonded together differently. Notably and most easily understood of these would be the allotropic nature of the element carbon. Carbon has three common main allotropic states:
- Diamond (Carbon atoms are bonded in a tetrahedral lattices)
- Graphite (Carbon atoms bonded in hexagonal lattice,) and
- Fullerenes (Carbon atoms bonded in either spherical, tubular or ellipsoidal formations)
These are all macro-molecules of carbon but in a different state of atomic arrangement, or differing allotropic form.
Just as how when carbon is superheated and super-compressed over millions of years forms a diamond, I was fairly certain that the mystery iron node having undergone a super-heating processas described was also some form of allotrope.
Iron Provides Best-Known Examples of Allotropy
While metals and metalloids tend to have fewer allotropies, the inert noble gases and non-metal elements tend to offer many more allotropic forms. These elements which have multiple allotropies in addition to carbon include oxygen, phosphorus, nitrogen, sulfur, silicon, boron, arsenic, antimony and several others. Allotropes of iron are fewer in number, but quite widely known.
Mystery of Life, The Universe, and Everything is …Google!
There are just three allotropes of iron, they are alpha, gamma, and delta
In short, when a natural iron is super-heated to 1418 °F (the Curie point, Tc) the iron becomes magnetic as alpha-iron, …which answers the postee’s question.
He had heated either a delta or gamma-allotropic form of natural iron ore into the ‘magnetic’ alpha state. In layman’s terms, the super-heating and allowing to slowly cool aligns all electron spins of the atoms in one domain to be in the same direction, making the material attracted to a magnet.
In the other two allotropes of iron, the electron spins are more random and tend to cancel each other out, which explains why the iron node he found in the native state was not attracted (or very weakly attracted) to a magnet.
A quick check with Google again to confirm the maximum temperature that a typical hand-held propane torch can reach is confirmed to be 1995 °C (3623 °F) which exceeds the required temperature to create alpha-state iron. This sort of proves that the postee had at least the common and necessary tools (e.g., -handheld propane torch) to convert the ore to higher allotropy state.
Iron ore (such as the specimen the postee possessed) when melted and blended with carbon, would create steel. Understanding the allotropes of iron is essential to the manufacture of good, high-quality steel.