Earth-Like Planets & Iron: Questions Answered

[dragoneyes001]

Molten what? Takes a long time to completely degas anything.

he mentioned volcanism bringing up the iron in silicates wouldn't that mean being within the magma and in a molten state?

 

[Bystander]

iron in silicates

There's a lot of chemistry going on in the crust. "Molten" does not necessarily mean elemental.

 

[dragoneyes001]

There's a lot of chemistry going on in the crust. "Molten" does not necessarily mean elemental.

I meant in the magma not the crust since it would need to be carried up during volcanic activity it'd need to be fluid plus the crust would have formed of the lightest elements in the original magma during the cooling of the outer surface. to be more precise i'd expect some of the silicates would have formed into the crust but not sure if the volume we see would be accountable by this including volcanic activity because I suspect a lot of the iron would be lost to the affect of being in a molten state for so long.

 

[Ken G]

Edit: Everyone talking about the late heavy bombardment as the source of iron in the crust since iron would have been dragged down into the core during differentiation - please remember the core contains metallic iron! Oxidised phases of iron in silicates are less dense and there'd be no problem with them remaining in the crust (or mantle and then being brought up to the crust during volcanism) from the Earth's formation. Take the ferromagnesian minerals that make up much of the oceanic crust, for example.

You make a lot of good and detailed points, but the article sited about the iron content in the crust was talking about iron ore veins that get mined for iron, so that is not elemental iron either-- and it did claim they would have been differentiated had they not appeared after the crust solidified. Perhaps they are not elemental, but they are iron oxides that are still denser than the average silicate. I don't know the role that vulcanism could have played, it does seem a bit odd to imagine this stuff has just sat there since late bombardment!

 

[Bystander]

I meant in the magma not the crust since it would need to be carried up during volcanic activity it'd need to be fluid plus the crust would have formed of the lightest elements in the original magma during the cooling of the outer surface.

Might be worth your while to go through Mason's Principles of Geochemistry, for an overview of what's suspected to have gone on based on what's suspected to have been the starting condition for planetary accretion and what are suspected to have been the operative differentiation processes.

 

[Amaterasu21]

wouldn't being in a molten state have allowed the oxygen to escape from the iron?

Most of the oxygen in rocks is combined with silicon to form silicate tetrahedra:

When a silicate rock partially melts, it's usually these silicate tetrahedra the crystal structure breaks into, and it's not a high-energy enough environment to separate the oxygen from the silicon.

Note that "oxidised" doesn't just mean "combined with oxygen" also! I was thinking more along the lines of iron-containing silicate minerals like olivine and pyroxene than pure iron oxide. I think iron oxide would decompose upon heating and release the oxygen (someone correct me if I'm wrong!) but the silicate minerals would decompose into tetrahedra like that one. Notice the oxygen ions give the structure an overall negative charge, so positive iron (and magnesium and other metal) ions can form ionically-bonded minerals like olivine and pyroxene out of these tetrahedra in different arrangements:

(Image source: http://www.earth.lsa.umich.edu/earth118/figure-03-11-2.jpg)

 

[dragoneyes001]

Most of the oxygen in rocks is combined with silicon to form silicate tetrahedra:

When a silicate rock partially melts, it's usually these silicate tetrahedra the crystal structure breaks into, and it's not a high-energy enough environment to separate the oxygen from the silicon.

Note that "oxidised" doesn't just mean "combined with oxygen" also! I was thinking more along the lines of iron-containing silicate minerals like olivine and pyroxene than pure iron oxide. I think iron oxide would decompose upon heating and release the oxygen (someone correct me if I'm wrong!) but the silicate minerals would decompose into tetrahedra like that one. Notice the oxygen ions give the structure an overall negative charge, so positive iron (and magnesium and other metal) ions can form ionically-bonded minerals like olivine and pyroxene out of these tetrahedra in different arrangements:

(Image source: http://www.earth.lsa.umich.edu/earth118/figure-03-11-2.jpg)

this helps explain why so much gold is found around quarts

 

[D H]

Most of the iron in the crust is thought to originate from bombardment. Why?

Citation STRONGLY needed. That statement, as far as I can tell, is absolute nonsense.

Just because iron is strongly depleted in the crust and mantle does not mean it is absent. By mass, about 5% of the Earth's crust and about 6% of the Earth's mantle is iron. That mass is many order of magnitude greater then even the most extreme of estimates of the amount of mass gained from the late heavy bombardment. The consensus opinion is that the mantle was the source of the iron for the banded iron formations, not the late heavy bombardment.

Perhaps you are confusing the massive amounts of iron in the form of rust in the banded iron formations with the very tiny amounts of iron used to make iron trinkets prior to the iron age. Those pre-iron age iron trinkets are indeed thought to have a meteoric origin. Melting pure iron is relatively easy compared to smelting iron ore.

 

[jim mcnamara]

http://www.space.com/15172-early-asteroids-bombardment-impacts.html
Paragraph 5 -> 8 - this is NOT the original - I cannot get it - author Christopher Dale, can you, DH?

This states that iron and its friends (siderophiles) went to the core early on, and subsequent bombardments were responsible for later additions of iron and friends to the crust.

Unless the quote by Denise Chow is full of baloney, which is completely possible, I think this matches what I tried to say. The LHB is the most recent one.

 

[Bystander]

This states that iron and its friends (siderophiles) went to the core early on,

Equilibri(um/a) among iron oxides, metallic iron, and oxygen yield at least three phases at the temperature(s) proposed/assumed at the stage of aggregation and differentiation being discussed. The partitioning of elemental iron, oxidized iron, and oxygen among those phases is NOT well established, meaning SOME of the elemental melt "fell," some was no doubt reoxidized, some was entrained in other silicate melt phases. It was not a nice, clean, laboratory separation of all starting materials into separate phases nicely stratified in a beaker or separatory funnel.

 

[dragoneyes001]

Equilibri(um/a) among iron oxides, metallic iron, and oxygen yield at least three phases at the temperature(s) proposed/assumed at the stage of aggregation and differentiation being discussed. The partitioning of elemental iron, oxidized iron, and oxygen among those phases is NOT well established, meaning SOME of the elemental melt "fell," some was no doubt reoxidized, some was entrained in other silicate melt phases. It was not a nice, clean, laboratory separation of all starting materials into separate phases nicely stratified in a beaker or separatory funnel.

Curious has it been published what the effects of the massive energy impacts were creating during the bombardment? the impact that is attributed to killing off the dino's was mitigated into a solid crust but at the time of the bombardment the crust was anything but a (relatively) thick solid. wouldn't the impacts have allowed for more out-gassing because of all the impact energy spreading through the mantle and outer core?

 

[Bystander]

(0.1 → 2 cm²/d , at 300 K) translates to 10^-10±0.5 m²/s for magnitudes of diffusion coefficients in the melts. Concentration gradients? You're talking a path O(10⁶ m), and concentrations O(0.01 → 0.03). Not conditions for fast, clean, differentiations of chemical species.

 

[dragoneyes001]

thank you

 

[D H]

http://www.space.com/15172-early-asteroids-bombardment-impacts.html
Paragraph 5 -> 8 - this is NOT the original - I cannot get it - author Christopher Dale, can you, DH?

That would be Dale, et al. (2012), "Late accretion on the earliest planetesimals revealed by the highly siderophile elements," Science 336.6077:72-75. Official link: http://www.sciencemag.org/content/336/6077/72.short ; preprint currently available at http://dro.dur.ac.uk/10717/1/10717.pdf .

Rant on, and please note, Jim, I'm not griping at you. I'm griping at the popular media. Why the bleep don't those pop-sci articles cite the scientific article they are writing about? Drives me nuts!The underlying article looks at the highly siderophilic elements, substances such as gold, platinum, and iridium. Paradoxically, iron is not in the list of highly siderophilic elements. Iron readily binds with oxygen, which makes iron somewhat lithophilic. It also readily binds with sulfur, which makes iron somewhat chalcophilic. Iron isn't nearly as inert as those highly siderophilic elements. The highly siderophilic elements are extremely inert. They don't oxidize readily, they don't form sulfur compounds readily. They do however readily dissolve in pure iron.

Almost all of the pre-bombardment gold is indeed thought to have sunk to the Earth's core along with a good fraction of the pre-bombardment iron. But not all the iron. Some of it stayed in the primitive mantle.

 

[TEFLing]

So you are saying, that in the pressureless vacuum of space...

Matter basically has two phases...

Solid = condensed and stuck together

Gas = unbound and flying apart

(and plasma of course)

Only under pressure can the gas phase be compressed back down into a liquid?

No, it is still boiling point that matters.
There are materials whose triple point is higher than even the high pressure of Earth atmosphere. Notably carbon dioxide. It does not melt on Earth - it sublimates at -78 degrees, remaining completely dry. In order to melt it, at temperature -57 degrees, a pressure of 5,2 bar is needed.

Of course there are many materials which do melt under Earth atmospheric pressure, yet evaporate rapidly under their melting point. Water freezes at 0 degrees by definition, and at 1,01325 bar boils at 100 degrees by definition, but ice and snow readily evaporate in dry air. In fact, ice cannot melt under pressure of 611 Pa (6 mbar), like on Mars, any more than carbon dioxide can melt on Earth.

Compare quicksilver. It is liquid always when water is. But it is much harder to make into anything else. At 1 bar, water boils at 100 degrees, but quicksilver can be heated to 358 degrees before it boils. Also, quicksilver is resistant to frost - water freezes at 0 degrees, but quicksilver at -39 degrees.

Quite naturally, as water and ice evaporate appreciably below their boiling point, quicksilver also evaporates below its boiling point - but since it is the boiling point that is so high, the vapour pressure of quicksilver at the same temperature is much lower than vapour pressure of water or ice.

The triple point vapour pressure of quicksilver at -39 degrees is 0,165 mPa, compared to the 611 Pa of ice. Ice even at -40 degrees has vapour pressure of about 13 Pa.

The pressure in space is so low that even quicksilver cannot melt. But at same pressure and temperature, ice should evaporate much faster than solid quicksilver - although quicksilver has a lower melting point, and because it is the boiling point of quicksilver which is higher than that of ice.

 

[Bystander]

Only under pressure can the gas phase be compressed back down into a liquid?

Or, adsorbed on available condensed (solid) phases. Phase of adsorbed layers might be subject to some discussion.

 

[TEFLing]

Or, adsorbed on available condensed (solid) phases. Phase of adsorbed layers might be subject to some discussion.

Because the surficial atoms could be mobile and migrate around on the surface?
Forming a series of short lived bonds with a sequence of different atoms
Something like a liquid phase?

 

[Bystander]

Something like a liquid phase?

Not part of the actual substrate, but no longer part of a vapor phase, and sometimes more mobile, depending on whether it's in the first or nth absorption layer --- like I say, open to discussion.

 

[TEFLing]

Not part of the actual substrate, but no longer part of a vapor phase, and sometimes more mobile, depending on whether it's in the first or nth absorption layer --- like I say, open to discussion.

You seem to be differentiating

The uniform crystal structure of the interior

From partially formed surface layers

Having many unoccupied crystal sites

 

[Bystander]

uniform crystal structure of the interior
From partially formed surface layers Having many unoccupied crystal sites

I'm not about to speculate on the variety or properties of the adsorbing surfaces, just on the behavior of adsorbate(s) in the various layers that form.

 

[TEFLing]

I'm not about to speculate on the variety or properties of the adsorbing surfaces, just on the behavior of adsorbate(s) in the various layers that form.

And that behavior is hard to define

Because of the semi liquid like mobility of those adsorbates

Yes?

And that mobility is due to vacancies in the partially formed crystal lattice structure

Of those first few surface layers?

Why else would you refrain from classifying those surface layers as simply solid

Due to site hopping mobility?

 

[Bystander]

I am NOT recommending that you run right out and buy a copy. See if you can find this, or previous editions in whatever libraries you can access. It's been years since I messed around with it, so the "horse's mouth" is probably a much better source.
http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471148733.html