where did the nuggets come from?

[Shiraz (Ray)]

Almost everything around us originated in exploding stars - presumably the violence of a supernova would have produced clouds of matter separated at the atomic level.

If that is the case, how come we get nuggets of pure gold, mountains of iron, hills of uranium ore etc. - what mechanisms have caused materials to cluster and how does this relate to the second law of thermodynamics?

Not trying to make a point - am just curious - any geologists out there?

regards Ray

[renormalised (Carl)]

Originally Posted by Shiraz

Almost everything around us originated in exploding stars - presumably the violence of a supernova would have produced clouds of matter separated at the atomic level.

If that is the case, how come we get nuggets of pure gold, mountains of iron, hills of uranium ore etc. - what mechanisms have caused materials to cluster and how does this relate to the second law of thermodynamics?

Not trying to make a point - am just curious - any geologists out there?

regards Ray

Yes, I'm a geologist. But right now, my brain isn't in full science mode due to a bout of insomnia lately. Suffice to say, your gold nuggets form from minute particles of gold which are suspended in a colloidal solution within the soils they form in. Your iron forms from precipitates in ocean sediments and uranium has a number of formative processes...sometimes as massive vein hosted deposits in granites, unconformity hosted deposits in sandstones/mudstones, roll front precipitates in sediments, various conglomerate hosted deposits, organic activated (carbon catalysed precipitates) shale/coal and mudstone deposits. There's about 20 or so different types. I won't go into the details of the formation of the deposits etc. Right now I couldn't handle the brain strain

[Shiraz (Ray)]

Thanks very much Carl - did some reading on lines you suggested and now have a better understanding of mineral concentrating mechanisms. Appreciate your reply.

What I still have trouble with is the way that a very well organised solar system has condensed from a dispersed cloud of matter. And some of the minerals in the earth have even pulled themselves together into organised structures under the action of natural processes (and of course living systems are highly ordered). This goes against all that I thought I knew about time and order - what happened to entropy in this process? Is it possible that the order we now see was inherent in the cloud of matter and has been "expressed" somehow by the action of gravity - or has local entropy reduced - or what?

regards Ray

[sjastro]

What I still have trouble with is the way that a very well organised solar system has condensed from a dispersed cloud of matter. And some of the minerals in the earth have even pulled themselves together into organised structures under the action of natural processes (and of course living systems are highly ordered). This goes against all that I thought I knew about time and order - what happened to entropy in this process? Is it possible that the order we now see was inherent in the cloud of matter and has been "expressed" somehow by the action of gravity - or has local entropy reduced - or what?

"Local" entropy is reduced where gravity causes matter to condense. However when matter condenses it undergoes inelastic collisions where heat is produced. The heat is transferred to the cooler regions of the cloud and entropy increases there.
The net result is that entropy still increases.

Regards

Steven

[CraigS]

Hi Ray;
Its interesting …
As Steven said, huge volumes of gas, (with high entropy), pull together under gravitational attraction and form denser clumps. Further collapse then gives off large amounts of heat into the surrounds which, overall, offsets the resulting entropy of the well-ordered structures formed at the larger scales.

I think the classic example of this is a Black Hole, where as (almost) nothing can escape its event horizon, they are the icons of maximal entropy in our present universe (compared with anything else of the same size). They give out (virtually) no information about their state of disorder, thus they have the highest entropy. Gravity could thus be viewed as the most efficient generator of entropy in the known universe.

Extrapolating this further, the ultimate source of an ordered, low entropy state must therefore be the big bang itself. It must have been filled with a hot, low density, uniform gaseous mixture (of hydrogen and helium). Ever since everything we observe has been increasing in entropy (at the larger scales), but with gravity acting to form lower entropy structures at the smaller (local) scales. This then must have been what gave time its direction and has been continuing the same way ever since. (Increasing time => increasing disorder, increasing entropy).

I think the key to all this is the overwhelming proportion of observational evidence in favour of disorder at the larger scales. Disorder also is seen at the smaller scales too: ice cubes melt (they don't freeze), milk mixes with coffee (but doesn't unmix), we remember the past, but not the future and gold nuggets form. There's also an asymmetry in all of this which also adds weight to time's arrow.

Pretty logical stuff. Noteworthy for me, is that both disorder and order occur side-by-side, simultaneously, but at the largest scale, the overall effect is towards disorder (higher entropy).

Cheers

[Shiraz (Ray)]

Thanks Steven and Craig.

Yes it is interesting - the glib assertion that entropy can be seen as a decrease in order is pretty much full of holes.

So... entropy (as order) can go up or down at some local scale (local being on any scale from atoms to galaxy clusters?), but at the scale of the universe (whatever that means in an expanding universe), entropy always increases overall. If any force (eg gravity, nuclear) acts to increase order of any part of the universe, it does work, and in the process generates a nett increase in overall entropy, which shows up as heat. For example, a Toyota Corolla is a low entropy object, but the process of making it generates a lot of heat. Excess heat is spread to the rest of the universe by radiative transfer, changing entropy in other places. If the universe was not expanding it would eventually settle to a bland environment with no more entropy increase possible.

Seems to me that trying to explain entropy in terms of the amount of order is a bit too simplistic to get across the true nature of the concept. It also has obvious contradictions - for example, an iceblock melts, but a gold nugget self-assembles - coffee and milk stay mixed, but maganese nodules form by spontaneous separation from sea water etc. etc.

Anyway, will definitely continue to think on this - but very much appreciate the discussion guys. Regards Ray

[CraigS]

Hi Ray;

I kinda like the direction this conversation is heading … I'll be interested to see what you come up with … (I have a feeling you're aware of the many more facets of the concept).

Cheers

[CraigS]

Here's another definition … entropy is a measure of the number of arrangements that conform to some specific recognisable pattern, ultimately entropy is hidden information.
Cheers

PS: So what is order ?

[CraigS]

I think a good handle on Equilibrium is also handy to have in order to grapple with the concept of entropy. Equilibrium could be argued to be reached when every process acting in a system, is balanced in a globally averaged sense (not at a detailed level).

Random influences, which we rarely attempt to keep track of in detail, are said to achieve a balance with the non-random influences, (the latter of which we seem to track and analyse more often .. cause its easier). This balance is detected when the influences we do track, are no longer time-varying over the study period.

Overall, I think the concept of Entropy, in isolation, is very much in our own minds. The combination however of Equilibrium (the zeroth law), and then energy transfer as work and as heat (the 1st Law), very much sets the context for an objective interpretation of entropy in a physical sense.

My 2 cents worth, anyway. Comments welcome.

Cheers

[sjastro]

If one defines the order of a system as the number of possible microstates that make up the system then entropy is a function of the number of microstates. The less the number of microstates the more ordered the system.

For example water vapour has a large spectrum (microstates) of water molecules of differing kinetic energies. In the liquid and solid states the spectrum is correspondingly less. Hence entropy decreases as one goes from gas to liquid to solid.

Regards

Steven

[mishku]

For someone who knows so little about geology, this is fascinating. Feel smarter for having read it, look forward to see where the conversation goes next!

Thanks, guys

[Shiraz (Ray)]

Hi Guys. I was initially concerned that my understanding of entropy did not match what I observed - an underground mine that was tapping into a lode of ore that should not have been there if my understanding that "order decreases" was correct. Carl set the record straight on mechanisms for ore concentration, but obviously something was wrong with my view of entropy.

From the discussion here and much recent reading, it seems that the difficulty in explaining entropy in readily understood terms is widely recognised - I am definitely not alone in finding problems with the conventional explanation based on decreasing order. However, it is also clear that entropy really does represent the principal physical driver of "life the universe and everything" so it is probably worth looking for a succinct but meaningful statement of just what it is.

The image that I now have in my mind is of a stream flowing to a lake. The water forms relatively stable eddies in the rapids and pools along the way - but these reduce as the stream widens and eventually all of the water stops flowing in a calm lake. The eddies and patterns of flow are temporary foci of order that represent what we see as reality (atoms, galaxies, our own lives etc), where order can appear to increase for a time, but eventually the eddies break up and everything continues on the inexorable trip to the still lake. And the imperative to increase Entropy (get to the lake) is what is driving it. This image will do me for now, but it would be great to find a less lyrical way to express it.

The idea of microstates is appealing and the idea of a balance between random and non-random processes is also interesting, but both require a fair degree of prior knowledge. Maybe that is the root of the problem - maybe entropy is just too complex and wide-ranging a concept to be fully conveyed in a few words?

Regards Ray

[renormalised (Carl)]

Most of this post isn't even about geology....it's about thermodynamics. Which was only touched upon within the general geological question.

Actually, the question was rather poorly conceived (no offense, Ray). If it was a question about thermodynamics as pertaining to the chemical processes which drive ore deposition, then it should've addressed thermodynamics within aqueous solutions and chemical equilibrium reactions. In any case, the 2nd law pertains to isolated physical systems undergoing irreversible equilibration, which is something an ore deposit is not....isolated. There are always external inputs into ore formation systems. Given the right conditions, the reversal of the reactions which initially formed an ore deposit can occur. Meaning it's not entirely an irreversible process.

If it was a general thermodynamics question, then it should've been addressed in such a fashion.

[Shiraz (Ray)]

Sorry you feel that way Carl. I have really enjoyed the discussion and both of my questions - what are the mechanisms for ore body formation and what does this mean in terms of the second law, have been answered - or at least the discussion has forced me to read and think about things to the extent that I now understand a lot more than I did. Anyway, thank you for your contribution. Regards Ray

[renormalised (Carl)]

Originally Posted by Shiraz

Sorry you feel that way Carl. I have really enjoyed the discussion and both of my questions - what are the mechanisms for ore body formation and what does this mean in terms of the second law, have been answered - or at least the discussion has forced me to read and think about things to the extent that I now understand a lot more than I did. Anyway, thank you for your contribution. Regards Ray

You just needed to be a bit more specific with how you worded your question. Most of the mechanisms behind ore deposit formation can be explained without having to revert to thermodynamics to answer the question. But if you want to understand how fluid-rock and chemical equilibrium interactions within ore deposits occur, then a reasonable understanding of thermodynamics (when it comes to mathematically modelling those processes) is advantageous.

[CraigS]

Its interesting that Wiki says for an Irreversible Process:

All complex natural processes are irreversible. The phenomenon of irreversibility results from the fact that if a thermodynamic system, which is any system of sufficient complexity, of interacting molecules is brought from one thermodynamic state to another, the configuration or arrangement of the atoms and molecules in the system will change in a way that is not easily predictable. A certain amount of "transformation energy" will be used as the molecules of the "working body" do work on each other when they change from one state to another. During this transformation, there will be a certain amount of heat energy loss or dissipation due to intermolecular friction and collisions; energy that will not be recoverable if the process is reversed.

I guess if a non-isolated system can somehow transfer this heat energy loss to its environment and back again, it could be reversible ?

Cheers

[renormalised (Carl)]

Your quote from wiki is essentially correct and in ore forming processes there is loss of energy from the system to its surroundings. However, ore forming processes don't occur in isolated physical systems. Whilst thermodynamics is important in modelling heat flows, chemical equilibrium processes and the like, the system can be reduced back to its "original" state if an external input occurs (dissolution of the ore body). However, if you look at it in the longer term, then the process is ultimately irreversible. Simply because you can't have perpetual motion (or in this case an infinite external input) and the process cannot be brought back to its initial "initial" state.

[CraigS]

Thanks Carl;

I guess a lot of it depends on how you define "ore deposit".
Presumably, an ore deposit could be huge .. or say one tiny nugget.
I guess ore could form, the external environmental conditions could reverse the process, and then deposition could happen again … the net result might still be said to be an "ore deposit" but microscopically/sub atomically, not exactly identical to the first one.

All sounds like scale/word definition stuff.

I think the scale limits of what one defines the system to be acting over is key, also. (As are the time limit parameters)

Cheers

[renormalised (Carl)]

Ore deposits range in all sorts of sizes. If an incipient deposit was remobilised and then redeposited, the new deposit most certainly wouldn't be the same as the previous one. However, the processes that remobilised and redeposited the ore could very well be the same as those that deposited it in the first place. So long as the conditions for deposition remain similar (P-T-t considerations, host rock-fluid chemistry etc).

In terms of thermodynamics, the hot ore carrying fluids are in a state of mixed entropy and it is the reactions of these fluids with the host rocks of the deposit (under the changes in P-T-t etc), from which derive the highly ordered state of the ore deposit. However, it is also true that in relation to the energy (as heat) being carried within the fluids, the depositional phase of the ore sees an increase in heat entropy with the cooling of the fluids....hot flows to cold, therefore a loss of energy. You have two different types of entropy occurring within the system....1. disordered mineral bearing fluid to ordered mineralisation/crystallisation of the ore, and, 2. the cooling of the fluids from a high energy to a low energy state.

So, in 1....disorder to order, and in 2....order to disorder. Both an entropy decrease and increase within the same system, depending on what mechanism/process you look at.

[renormalised (Carl)]

The same occurred at the time of the BB, depending on what process you look at. But overall, with the expansion of the Universe, entropy will increase until everything comes to a dull, boring, lifeless equilibrium (given current theory holds correct). If the expansion is flat.

Funnily enough, if the Universal expansion accelerates to infinity (the Big Rip) and the cosmological constant (vacuum energy) also reaches this point, we'll be back to the original BB state of the Universe. A mixed (you could almost say indeterminate) entropic bag.