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Question:

Which one came first?

Which one was first to form in our planet - the iron or the stone?I watched one engineering show on TV about making a tunnel under water (no under the Lamansh one) and they said that, the stainless steel claws of the excavation machine broke several times before they could dig through the lime stone.So I thought If the stone is harder than metal, then is it just a question of atomic density? and which was first to form in the beginning?

Answer:

Stainless steel is soft. If they use tungsten or carbide claws, may have better outcome. But some materials are very hard - like ruby, diamond. It's not about atomic density, it's about molecular bonding and structure. Titanium and liquid-metal are very good in structure.
the answer is probably yes the reason for the steel claws to continue breaking is their attempt to penetrate a layer that was harder than the steel.
Difficult to say. If we take the structure of Earth it consists of three parts. Nickel-Iron core called NiFe. The next one is SiMa- this is the silica magnesium layer. The final coming closer to surface is SiAl- The silica aluminum crust. I feel it must be the iron that formed first. Stone is nothing but silica mixed with other minerals. As for hardness it can be both ways. Hardest material is diamond which is primarily silicon based.Unalloyed Gold is soft. There could be other metals which are harder. I suppose it must be atomic density and how well they are packed/arranged at the level of crystal lattice- Body centered cubic, face centered cubic. Hexagonal. Orthorhombic etc.
If you HAVE to pick one, I would say the iron formed first, in the core and explosions of the stars that created the dust cloud which our Sun and Earth condensed out of. The iron atoms are billions and billions of years old. The stone only became stone after the Earth cooled off. It also has has four and a half billion years of metamorphosis, so it was probably made into its current form much later. This is certainly the case with limestone, which is a sedimentary rock formed by the precipitation of calcium carbonate. On the other hand, the iron only became steel when it was mined and refined, so in that sense the stone is much older.
Although the excavator claws broke, that does not mean the stone is harder, just that the repeated impacts caused metal fatigue and cracks in the claws. Cosmologists currently believe that all the elements and compounds that make up the earth were uniformly present in the dust cloud that it condensed from. Worked iron and steel are newer than the iron compounds they are refined from, and consolidated limestone and sandstone are newer than the particles they are aggregated from, so it's not clear that either of them could be said to have formed before the other.

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