Hi all,

In explaining silicates and the Bowen reaction series in class and I have realized that I don’t understand how this works. I can sort of figure out melting behavior, but then I struggle with weathering behavior.

I think of melting temperature of a mineral as a function of the strength of the weakest bonds present in a mineral (what holds them together) and the repulsion between the cations occupying their sites (what pulls them apart). Because the bonds formed between Fe/Mg-O are shorter and can have lower coordination numbers than those formed between Na/K-O, it makes sense that nesosilicates have stronger “weakest” bonds than tectosilicates and melt at higher temperatures. In addition, with increasing polymerization, more silicons are forced into close proximity to each other causing repulsion between the tetrahedrally occupied sites. Therefore more polymerized silicates will have internal repulsion forces that will further facilitate bond breakage (again lower melting temperatures).

Here comes where I struggle. If the strength of the weakest bond is highest in nesosilicates, why do nesosilicates minerals weather easier than felsic ones (i.e., why are nesosilicates further from theromdynamic equilibrium at room temperatures than tectosilicates)? I know that we are now putting the minerals in a different solvent and that phase equilibrium predicts that forsterite is not stable at environmental conditions in the presence of water, the question is why is that more so for forsterite than for K-feldspars. Since dissolution into water also requires bond breakage intuitively it would make sense that olivine, which has stronger bonds and less internal repulsion forces, would survive better than feldspar. I can try to think about it in terms of the bonds formed by the elements in aqueous solution and whether the way in which sodium dissolves in pure water is more competitive than magnesium, but it is not clear to me that that would be the case.

So here is my question, why at the atomic structure level and not just from a descriptive perspective is forsterite more readily weathered than K-feldspar given that the bonds in olivine are significantly stronger and the internal repulsion forces between atoms weaker?

Thank you everyone,

Pilar

————————————————
Pilar Lecumberri-Sanchez
Associate Professor
Dept. of Earth & Atmospheric Sciences
1-26 Earth Sciences Building
University of Alberta
Edmonton, Alberta, T6G 2E3
Phone: 780-492-5071

https://cms.eas.ualberta.ca/lecumberri/
————————————————

When I teach this material to my students, I do not use bond strengths to explain the stabilities of the minerals, but rather the conditions at which they formed. Minerals like olivine that are stable in the mantle are less likely to be stable at the Earth's surface. This is due to differences in temperature, pressure, oxygen fugacity, and water activity/availability.  Minerals like K-feldspar that crystallize at the lowest temperatures in Bowen's reaction series can be stable in equilibrium with water, but only after the K-feldspar dissolves incongruently to form clays and the fluid composition evolves until it becomes saturated in K-feldspar. Hope this is helpful.

Professor John C. Ayers
Dept. of Earth & Environmental Sciences
Vanderbilt University
PMB 351805
2301 Vanderbilt Place
Nashville, TN 37235-1805
Tel 1-615-973-1879
https://my.vanderbilt.edu/johncayers

From: Pilar Lecumberri-Sanchez via MSA-talk msa-talk@minlists.org
Sent: Tuesday, January 30, 2024 10:32 PM
To: msa-talk@minlists.org
Subject: [MSA-talk] Polymerization, composition, melting point and weathering of silicates

Hi all,

In explaining silicates and the Bowen reaction series in class and I have realized that I don't understand how this works. I can sort of figure out melting behavior, but then I struggle with weathering behavior.

I think of melting temperature of a mineral as a function of the strength of the weakest bonds present in a mineral (what holds them together) and the repulsion between the cations occupying their sites (what pulls them apart). Because the bonds formed between Fe/Mg-O are shorter and can have lower coordination numbers than those formed between Na/K-O, it makes sense that nesosilicates have stronger "weakest" bonds than tectosilicates and melt at higher temperatures. In addition, with increasing polymerization, more silicons are forced into close proximity to each other causing repulsion between the tetrahedrally occupied sites. Therefore more polymerized silicates will have internal repulsion forces that will further facilitate bond breakage (again lower melting temperatures).

Here comes where I struggle. If the strength of the weakest bond is highest in nesosilicates, why do nesosilicates minerals weather easier than felsic ones (i.e., why are nesosilicates further from theromdynamic equilibrium at room temperatures than tectosilicates)? I know that we are now putting the minerals in a different solvent and that phase equilibrium predicts that forsterite is not stable at environmental conditions in the presence of water, the question is why is that more so for forsterite than for K-feldspars. Since dissolution into water also requires bond breakage intuitively it would make sense that olivine, which has stronger bonds and less internal repulsion forces, would survive better than feldspar. I can try to think about it in terms of the bonds formed by the elements in aqueous solution and whether the way in which sodium dissolves in pure water is more competitive than magnesium, but it is not clear to me that that would be the case.

So here is my question, why at the atomic structure level and not just from a descriptive perspective is forsterite more readily weathered than K-feldspar given that the bonds in olivine are significantly stronger and the internal repulsion forces between atoms weaker?

Thank you everyone,

Pilar

Pilar Lecumberri-Sanchez
Associate Professor
Dept. of Earth & Atmospheric Sciences
1-26 Earth Sciences Building
University of Alberta
Edmonton, Alberta, T6G 2E3
Phone: 780-492-5071

https://cms.eas.ualberta.ca/lecumberri/

Pilar and friends,

This is a fantastic question, and really gets one thinking about the underlying behavior of the bonds that hold together our favorite Earth materials. Here is my take:

One important difference is that olivine and other mafic minerals contain Fe2+, which is susceptible to oxidation in the high fO2 environment of Earth's surface. That difference basically boils down to electrochemistry. But for a moment, let's ignore the redox sensitivity of Fe, because I think there's an even more fundamental concept at work.... the unique chemistry of water and how it reacts to ionic vs. covalent bonds.

Olivine is composed of primarily ionic bonds between cations and anions, and the more covalent-natured Si-O bonds don't play a very large role because the silica component is unpolymerized (i.e., the SiO4 units just behave as their own "ions"). The highly polar water molecule is therefore able to rip cations and anions away from each other very effectively via electrostatic attraction. Whereas, in the absence of water, it takes a LOT of heat to overcome the attraction of the ionic bonds, because you're trying to do it by brute force (rather than with the help of such a highly effective molecular scalpel).

Contrast this with K-spar and other network silicates, which are held together mostly by bonds with much more covalent nature, where electrons sit directly between the atoms. In the absence of water, these bonds will more easily get "mushy" (like a limp rope) when heated and soften the silica framework to the point where it can squish and slide, forming a melt (where bonds are weakened but still present). You don't even have to break all the bonds, you just need to break enough for bond angles to slide and twist around... just enough to soften the silica network. But at room temperature, even in the presence of water, the covalent bonds will keep Si and O atoms tethered to each other tightly because they don't care whether the water molecular is polar or not. There is not enough separation of charge for those bonds to be susceptible to the + or - end of a H2O molecule, so the Si-O framework just sits there, and at best you get a little bit of incongruent dissolution of components like K and Na, as pointed out by John Ayers. Both olivine and K-spar could probably sit happily in equilibrium with air for millions of years with very little change, but water will make a tremendous difference.

So in my view, we need to think beyond just "strong" vs. "weak" bonds, we also have to think about how different the nature of the bonding is at each end of Bowen's reaction series. Each style of bonding reacts very differently to heat vs. reactive molecules like H2O and O2, and I suspect that's the key to understanding these trends on a molecular level. Of course, there is sure to be much more nuanced thermodynamics at play than what I've outlined here, but hopefully this is a useful way to frame the issue from a broad perspective. Very interested to hear other thoughts on this.

Best,
Dan

From: Ayers, John C via MSA-talk msa-talk@minlists.org
Sent: Wednesday, January 31, 2024 9:16 AM
To: Pilar Lecumberri-Sanchez lecumber@ualberta.ca; msa-talk@minlists.org
Subject: [MSA-talk] Re: Polymerization, composition, melting point and weathering of silicates

[EXTERNAL EMAIL ALERT]: Verify sender before opening links or attachments.
When I teach this material to my students, I do not use bond strengths to explain the stabilities of the minerals, but rather the conditions at which they formed. Minerals like olivine that are stable in the mantle are less likely to be stable at the Earth's surface. This is due to differences in temperature, pressure, oxygen fugacity, and water activity/availability.  Minerals like K-feldspar that crystallize at the lowest temperatures in Bowen's reaction series can be stable in equilibrium with water, but only after the K-feldspar dissolves incongruently to form clays and the fluid composition evolves until it becomes saturated in K-feldspar. Hope this is helpful.

Professor John C. Ayers
Dept. of Earth & Environmental Sciences
Vanderbilt University
PMB 351805
2301 Vanderbilt Place
Nashville, TN 37235-1805
Tel 1-615-973-1879
https://my.vanderbilt.edu/johncayers

From: Pilar Lecumberri-Sanchez via MSA-talk <msa-talk@minlists.orgmailto:msa-talk@minlists.org>
Sent: Tuesday, January 30, 2024 10:32 PM
To: msa-talk@minlists.orgmailto:msa-talk@minlists.org
Subject: [MSA-talk] Polymerization, composition, melting point and weathering of silicates

Hi all,

In explaining silicates and the Bowen reaction series in class and I have realized that I don't understand how this works. I can sort of figure out melting behavior, but then I struggle with weathering behavior.

I think of melting temperature of a mineral as a function of the strength of the weakest bonds present in a mineral (what holds them together) and the repulsion between the cations occupying their sites (what pulls them apart). Because the bonds formed between Fe/Mg-O are shorter and can have lower coordination numbers than those formed between Na/K-O, it makes sense that nesosilicates have stronger "weakest" bonds than tectosilicates and melt at higher temperatures. In addition, with increasing polymerization, more silicons are forced into close proximity to each other causing repulsion between the tetrahedrally occupied sites. Therefore more polymerized silicates will have internal repulsion forces that will further facilitate bond breakage (again lower melting temperatures).

Here comes where I struggle. If the strength of the weakest bond is highest in nesosilicates, why do nesosilicates minerals weather easier than felsic ones (i.e., why are nesosilicates further from theromdynamic equilibrium at room temperatures than tectosilicates)? I know that we are now putting the minerals in a different solvent and that phase equilibrium predicts that forsterite is not stable at environmental conditions in the presence of water, the question is why is that more so for forsterite than for K-feldspars. Since dissolution into water also requires bond breakage intuitively it would make sense that olivine, which has stronger bonds and less internal repulsion forces, would survive better than feldspar. I can try to think about it in terms of the bonds formed by the elements in aqueous solution and whether the way in which sodium dissolves in pure water is more competitive than magnesium, but it is not clear to me that that would be the case.

So here is my question, why at the atomic structure level and not just from a descriptive perspective is forsterite more readily weathered than K-feldspar given that the bonds in olivine are significantly stronger and the internal repulsion forces between atoms weaker?

Thank you everyone,

Pilar

Pilar Lecumberri-Sanchez
Associate Professor
Dept. of Earth & Atmospheric Sciences
1-26 Earth Sciences Building
University of Alberta
Edmonton, Alberta, T6G 2E3
Phone: 780-492-5071

https://cms.eas.ualberta.ca/lecumberri/

Dear Pilar,

This is a broad, interesting, and important question, and I appreciated that it gave me a lot to think about.  One of the first things that should be said is that melting temperature is not as predictable as we might like to think.  But I don't think that it is unreasonable to try to find qualitative or semi-quantitative patterns, which, as you point out regarding geologic systems, are certainly there for all to see.

I think that a couple of potential sources of confusion in your analysis are:  1) a qualitative analysis of the relationship between coordination, bond length, and bond strength that overlooks important quantitative results, 2) a tacit assumption that melting and weathering are governed by the same attributes of crystals, and 3) certain conclusions that you've drawn about the relationship between polymerization and melting temperature.  I'll try to elaborate on each of these.

1. What I mean by a quantitative relationship between bond length, bond strength, and melting temperature might be better understood by considering these attributes in the context of anorthite and albite.  I think that these are useful examples because the ionic radii of Ca2+ and Na+ are so similar (1.00 versus 0.99 angstroms).  Consequently, differences in bond lengths are largely governed by Al and Si.  Al and Si are both in 4-coordination with O, but the ionic radius of Al is about 60% larger than that of Si (0.39 versus 0.26 angstroms).  The T-O bond length in feldspar ranges from about 1.74 angstroms for Al-O and 1.61 angstroms for Si-O in albite to about 1.75 for Al-O and 1.63 for Si-O in anorthite (acknowledging that these are reasonable mean values and that there is variation in both of these sets related to compositional variation and order-disorder).  In any case, we note that the percent difference between these values is small compared to the percent difference in melting temperature between albite and anorthite (1391 K versus 1826 K, respectively).

If we consider bond strength, looking at the enthalpies associated with simple binary bonds, we find Si-O has a strength of 798 kJ/mol, Al-O has 512 kJ/mol, Na-O has 257 kJ/mol, and Ca-O has 464 kJ/mol.  If we were to do emulate Fermi and do a quick back-of-the-envelope estimate of the total bond energy contained in equal portions of albite and anorthite unit cells, we would (for some arbitrary but equal portions of respective unit cells) come up with:  albite, 15 Si-O, 5 Al-O, 5 Na-O, to yield about 15.8 MJ/mol versus anorthite, 10 Si-O, 10 Al-O, 8 Ca-O, to yield about 16.8 MJ/mol.  Again, the percent difference between these values is significantly smaller, though less so, than the percent difference between the absolute melting temperatures of the end-member species.

This suggests that we should look elsewhere for an explanation; that there is something more complex happening.  Ultimately, locating transitions between states relates to equilibrium phenomena, so just as we can establish stability fields for mineral species in p-T space by determining loci of thermodynamic points, we can do something similar looking in compositional X-T space.  If we look at the enthalpies of formation, which account for more than just bond energies of isolated atomic pairs, then we find approximately 3900 kJ/mol for albite and approximately 4200 kJ/mol for anorthite, both at STP.  But enthalpy of formation does not govern mineral equilibria on its own.  A fuller treatment requires looking at entropy, temperature, and pressure to develop a Gibbs Free Energy expression and its first- and second-order derivatives for the mineral, which allow for a more meaningful association of thermodynamic properties, extensive variables, and changes of state.

1. It's probably best to elucidate this by just focusing on weathering.  Consider your example of forsterite and K-feldspar.  The best qualitative answer to the question is that minerals are most stable when the surrounding conditions are most similar to the conditions of formation; so, olivine weathers relatively quickly at the surface, whereas feldspars are more persistent, and clays can just sit at the surface essentially in perpetuity.  But your question is why this is true.  Again, establishing Gibbs Free Energy expressions for minerals and their weathering products allows for equilibrium coordinates between the minerals in p-T space.  The implication of that answer is that there is not a single, simple reason but rather that there is a complex interaction between thermodynamic properties of both each individual mineral and also between those of the two phases.

There is another layer to weathering, however, which relates to redox conditions, the oxidation state of particular elements in the mineral, and the solubility both of particular mineral species but also of ions of a given element in different oxidation states.  I think a really good example of this can be seen in crystals of Fe-rich pyroxenes from alkaline igneous rocks.  Many such crystals have an augite-rich core and an aegirine-rich rim, and when they are found weathered out of their parent rock, they typically have an earthy, rusty center, surrounded by a hard, lustrous exterior.  In augite, iron is in a +2 state, and in aegirine, it is in a +3 state; that is, the iron in augite can still be oxidized to a higher oxidation state, whereas the iron in aegirine is as oxidized as it can be.  The consequence is that the augite preferentially weathers to silica, iron oxides, and soluble ions, while the aegirine remains largely unaltered.

1. I think that a good place to start is to consider quartz, which ideally has no other cations besides silicon in its structure and which in practice has an extremely low content of trace elements.  According to your analysis, bringing Si atoms closer together will increase repulsive forces between them.  If, however, we consider melting temperatures, "quartz", which reconfigures to cristobalite near its melting temperature, has a melting point of 1982 K, well above the melting temperature of albite, and just under the melting point of forsterite (2163 K).  I think it's beyond the scope of this reply to try to quantify a comparison between these three, but I think that our intuition might lead us to conclude that the intervening atoms in albite, by reducing repulsive forces between silicon atoms, should actually increase the melting temperature.  Similarly, enstatite, which is much more like forsterite than albite or quartz, only has a melting temperature of 1860 K.  So, I think that there is more going on than just the degree of polymerization.

I think that a great deal more can be said on all of these points, as well as on other points as yet unnamed here, but then we'd be starting to write another textbook.

I enjoyed reading the comments of previous respondents, and I agree that I look forward to further musings on this question.

Best,
Peter

From: Pilar Lecumberri-Sanchez via MSA-talk msa-talk@minlists.org
Sent: Tuesday, January 30, 2024 11:32 PM
To: msa-talk@minlists.org
Subject: [MSA-talk] Polymerization, composition, melting point and weathering of silicates

[*This email is from an outside sender. Please be careful clicking on links and attachments.]
Hi all,

In explaining silicates and the Bowen reaction series in class and I have realized that I don't understand how this works. I can sort of figure out melting behavior, but then I struggle with weathering behavior.

I think of melting temperature of a mineral as a function of the strength of the weakest bonds present in a mineral (what holds them together) and the repulsion between the cations occupying their sites (what pulls them apart). Because the bonds formed between Fe/Mg-O are shorter and can have lower coordination numbers than those formed between Na/K-O, it makes sense that nesosilicates have stronger "weakest" bonds than tectosilicates and melt at higher temperatures. In addition, with increasing polymerization, more silicons are forced into close proximity to each other causing repulsion between the tetrahedrally occupied sites. Therefore more polymerized silicates will have internal repulsion forces that will further facilitate bond breakage (again lower melting temperatures).

Here comes where I struggle. If the strength of the weakest bond is highest in nesosilicates, why do nesosilicates minerals weather easier than felsic ones (i.e., why are nesosilicates further from theromdynamic equilibrium at room temperatures than tectosilicates)? I know that we are now putting the minerals in a different solvent and that phase equilibrium predicts that forsterite is not stable at environmental conditions in the presence of water, the question is why is that more so for forsterite than for K-feldspars. Since dissolution into water also requires bond breakage intuitively it would make sense that olivine, which has stronger bonds and less internal repulsion forces, would survive better than feldspar. I can try to think about it in terms of the bonds formed by the elements in aqueous solution and whether the way in which sodium dissolves in pure water is more competitive than magnesium, but it is not clear to me that that would be the case.

So here is my question, why at the atomic structure level and not just from a descriptive perspective is forsterite more readily weathered than K-feldspar given that the bonds in olivine are significantly stronger and the internal repulsion forces between atoms weaker?

Thank you everyone,

Pilar

Pilar Lecumberri-Sanchez
Associate Professor
Dept. of Earth & Atmospheric Sciences
1-26 Earth Sciences Building
University of Alberta
Edmonton, Alberta, T6G 2E3
Phone: 780-492-5071

https://cms.eas.ualberta.ca/lecumberri/

Disclaimer: This email may contain information protected under the Family Educational Rights and Privacy Act and/or confidentiality requirements. Unauthorized use, disclosure, or copying is strictly prohibited and may be unlawful. If this email contains student information and you are not entitled to access such information under FERPA, federal regulations require that you destroy this email without reviewing it and you may not forward it to anyone. If you have received this communication in error, please notify sender immediately.

Thanks Pilar, John, Dan - Fascinating discussion. In my world, it is the spinel family that is of great interest. 1) In CV chondrites e.g. Allende, the MgAl2O4 in CAIs retains its O-16 rich character but the melilite, "fassaites" and anorthite equililibrate with the parent body. This effect is subdued in the less-hydrated CV-reduced (e.g. Vigarano) class (e.g., MacPherson 2014). 2) In the K-Pg boundary clay, the surviving condensates from the impact plume are magnesio-wustite spinels with wt% levels of NiO, the rest of the spherules were Ca-rich glass which is long altered everywhere (Ebel & Grossman 2005). 3) In fossil meteorites in Swedish quarries ~480 Ma we see the L chondrite textures, but the only surviving minerals are... spinels! And we can then tell the L typology from Cr-Mn isotope systematics in spinel (Alwmark & Schmitz 2009; Heck et al. 2010).
Ideas about why spinels are so persistent?
Thanks
D
references:

MacPherson G.J. (2014) Calcium-aluminum-rich inclusions in chondritic meteorites. In Meteorites, Comets and Planets (A.M. Davis, ed.), pp. 201-246. Volume 1 of Treatise on Geochemistry, (H.D. Holland and K.K. Turekian, eds.), Elsevier. 10 volumes.

Ebel, D.S., and L. Grossman (2005) Spinel-bearing spherules condensed from the Chicxulub impact-vapor plume. Geology 33: 293-296.

Alwmark, C. and Schmitz, B. (2009) The Origin of the Brunflo Fossil Meteorite and Extraterrestrial Chromite in Mid-Ordovician Limestone from the Gärde Quarry (Jämtland, Central Sweden), Meteoritics and Planetary Science, v. 44, p. 95-106.

Heck, P. R., Ushikubo, T., Schmitz, B., Kita, N. T., Spicuzza, M. J., and Valley, J. W. (2010) A single asteroidal Source for extraterrestrial Ordovician chromite grains from Sweden and China: High-precision oxygen three-isotope SIMS analysis,Geochimica et Cosmochimica Acta, v. 74(2), p. 497-509.

Denton S. Ebel, Curator, Dept. of Earth and Planetary Sciences
Chair, Division of Physical Sciences, American Museum of Natural History
200 Central Park West, New York, NY  10024
(212) 769-5381    http://research.amnh.org/~debel
space show VI: https://www.amnh.org/exhibitions/permanent/hayden-planetarium/worlds-beyond-earth
“Historically, pandemics have forced humans to break with the past and imagine their world anew. This one is no different. It is a portal, a gateway between one world and the next. We can choose to walk through it, dragging the carcasses of our prejudice and hatred, our avarice, our data banks and dead ideas, our dead rivers and smoky skies behind us. Or we can walk through lightly, with little luggage, ready to imagine another world. And ready to fight for it.” -- Arundhati Roy

From: Hummer, Daniel R via MSA-talk msa-talk@minlists.org
Sent: Thursday, February 1, 2024 11:18 AM
To: Ayers, John C john.c.ayers@Vanderbilt.Edu; Pilar Lecumberri-Sanchez lecumber@ualberta.ca; MSA-talk@minlists.org msa-talk@minlists.org
Subject: [MSA-talk] Re: Polymerization, composition, melting point and weathering of silicates

EXTERNAL SENDER

Pilar and friends,

This is a fantastic question, and really gets one thinking about the underlying behavior of the bonds that hold together our favorite Earth materials. Here is my take:

One important difference is that olivine and other mafic minerals contain Fe2+, which is susceptible to oxidation in the high fO2 environment of Earth’s surface. That difference basically boils down to electrochemistry. But for a moment, let’s ignore the redox sensitivity of Fe, because I think there’s an even more fundamental concept at work…. the unique chemistry of water and how it reacts to ionic vs. covalent bonds.

Olivine is composed of primarily ionic bonds between cations and anions, and the more covalent-natured Si-O bonds don’t play a very large role because the silica component is unpolymerized (i.e., the SiO4 units just behave as their own “ions”). The highly polar water molecule is therefore able to rip cations and anions away from each other very effectively via electrostatic attraction. Whereas, in the absence of water, it takes a LOT of heat to overcome the attraction of the ionic bonds, because you’re trying to do it by brute force (rather than with the help of such a highly effective molecular scalpel).

Contrast this with K-spar and other network silicates, which are held together mostly by bonds with much more covalent nature, where electrons sit directly between the atoms. In the absence of water, these bonds will more easily get “mushy” (like a limp rope) when heated and soften the silica framework to the point where it can squish and slide, forming a melt (where bonds are weakened but still present). You don’t even have to break all the bonds, you just need to break enough for bond angles to slide and twist around… just enough to soften the silica network. But at room temperature, even in the presence of water, the covalent bonds will keep Si and O atoms tethered to each other tightly because they don’t care whether the water molecular is polar or not. There is not enough separation of charge for those bonds to be susceptible to the + or – end of a H2O molecule, so the Si-O framework just sits there, and at best you get a little bit of incongruent dissolution of components like K and Na, as pointed out by John Ayers. Both olivine and K-spar could probably sit happily in equilibrium with air for millions of years with very little change, but water will make a tremendous difference.

So in my view, we need to think beyond just “strong” vs. “weak” bonds, we also have to think about how different the nature of the bonding is at each end of Bowen’s reaction series. Each style of bonding reacts very differently to heat vs. reactive molecules like H2O and O2, and I suspect that’s the key to understanding these trends on a molecular level. Of course, there is sure to be much more nuanced thermodynamics at play than what I’ve outlined here, but hopefully this is a useful way to frame the issue from a broad perspective. Very interested to hear other thoughts on this.

Best,

Dan

From: Ayers, John C via MSA-talk msa-talk@minlists.org
Sent: Wednesday, January 31, 2024 9:16 AM
To: Pilar Lecumberri-Sanchez lecumber@ualberta.ca; msa-talk@minlists.org
Subject: [MSA-talk] Re: Polymerization, composition, melting point and weathering of silicates

[EXTERNAL EMAIL ALERT]: Verify sender before opening links or attachments.

When I teach this material to my students, I do not use bond strengths to explain the stabilities of the minerals, but rather the conditions at which they formed. Minerals like olivine that are stable in the mantle are less likely to be stable at the Earth’s surface. This is due to differences in temperature, pressure, oxygen fugacity, and water activity/availability.  Minerals like K-feldspar that crystallize at the lowest temperatures in Bowen’s reaction series can be stable in equilibrium with water, but only after the K-feldspar dissolves incongruently to form clays and the fluid composition evolves until it becomes saturated in K-feldspar. Hope this is helpful.

Professor John C. Ayers

Dept. of Earth & Environmental Sciences
Vanderbilt University
PMB 351805
2301 Vanderbilt Place
Nashville, TN 37235-1805
Tel 1-615-973-1879

https://my.vanderbilt.edu/johncayers

From: Pilar Lecumberri-Sanchez via MSA-talk <msa-talk@minlists.orgmailto:msa-talk@minlists.org>
Sent: Tuesday, January 30, 2024 10:32 PM
To: msa-talk@minlists.orgmailto:msa-talk@minlists.org
Subject: [MSA-talk] Polymerization, composition, melting point and weathering of silicates

Hi all,

In explaining silicates and the Bowen reaction series in class and I have realized that I don’t understand how this works. I can sort of figure out melting behavior, but then I struggle with weathering behavior.

I think of melting temperature of a mineral as a function of the strength of the weakest bonds present in a mineral (what holds them together) and the repulsion between the cations occupying their sites (what pulls them apart). Because the bonds formed between Fe/Mg-O are shorter and can have lower coordination numbers than those formed between Na/K-O, it makes sense that nesosilicates have stronger “weakest” bonds than tectosilicates and melt at higher temperatures. In addition, with increasing polymerization, more silicons are forced into close proximity to each other causing repulsion between the tetrahedrally occupied sites. Therefore more polymerized silicates will have internal repulsion forces that will further facilitate bond breakage (again lower melting temperatures).

Here comes where I struggle. If the strength of the weakest bond is highest in nesosilicates, why do nesosilicates minerals weather easier than felsic ones (i.e., why are nesosilicates further from theromdynamic equilibrium at room temperatures than tectosilicates)? I know that we are now putting the minerals in a different solvent and that phase equilibrium predicts that forsterite is not stable at environmental conditions in the presence of water, the question is why is that more so for forsterite than for K-feldspars. Since dissolution into water also requires bond breakage intuitively it would make sense that olivine, which has stronger bonds and less internal repulsion forces, would survive better than feldspar. I can try to think about it in terms of the bonds formed by the elements in aqueous solution and whether the way in which sodium dissolves in pure water is more competitive than magnesium, but it is not clear to me that that would be the case.

So here is my question, why at the atomic structure level and not just from a descriptive perspective is forsterite more readily weathered than K-feldspar given that the bonds in olivine are significantly stronger and the internal repulsion forces between atoms weaker?

Thank you everyone,

Pilar

————————————————
Pilar Lecumberri-Sanchez

Associate Professor

Dept. of Earth & Atmospheric Sciences
1-26 Earth Sciences Building
University of Alberta
Edmonton, Alberta, T6G 2E3

Phone: 780-492-5071

https://cms.eas.ualberta.ca/lecumberri/
————————————————

Hi,  the thermodynamic environment plays a huge role in all of this.  In my own lab, we expend a lot of effort to create a “closed high pressure and high temperature environment.” Closed is one of the keys there.  But in weathering the system is open.  We need to understand all the possible environments for the target materials and how they have affected the equilibrium…

Kurt

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From: Denton Ebel via MSA-talk msa-talk@minlists.org
Sent: Thursday, February 1, 2024 12:00:23 PM
To: Ayers, John C john.c.ayers@Vanderbilt.Edu; Pilar Lecumberri-Sanchez lecumber@ualberta.ca; MSA-talk@minlists.org msa-talk@minlists.org; Hummer, Daniel R daniel.hummer@siu.edu
Subject: [MSA-talk] Re: Polymerization, composition, melting point and weathering of silicates

Thanks Pilar, John, Dan - Fascinating discussion. In my world, it is the spinel family that is of great interest. 1) In CV chondrites e.g. Allende, the MgAl2O4 in CAIs retains its O-16 rich character but the melilite, "fassaites" and anorthite equililibrate with the parent body. This effect is subdued in the less-hydrated CV-reduced (e.g. Vigarano) class (e.g., MacPherson 2014). 2) In the K-Pg boundary clay, the surviving condensates from the impact plume are magnesio-wustite spinels with wt% levels of NiO, the rest of the spherules were Ca-rich glass which is long altered everywhere (Ebel & Grossman 2005). 3) In fossil meteorites in Swedish quarries ~480 Ma we see the L chondrite textures, but the only surviving minerals are... spinels! And we can then tell the L typology from Cr-Mn isotope systematics in spinel (Alwmark & Schmitz 2009; Heck et al. 2010).
Ideas about why spinels are so persistent?
Thanks
D
references:

MacPherson G.J. (2014) Calcium-aluminum-rich inclusions in chondritic meteorites. In Meteorites, Comets and Planets (A.M. Davis, ed.), pp. 201-246. Volume 1 of Treatise on Geochemistry, (H.D. Holland and K.K. Turekian, eds.), Elsevier. 10 volumes.

Ebel, D.S., and L. Grossman (2005) Spinel-bearing spherules condensed from the Chicxulub impact-vapor plume. Geology 33: 293-296.

Alwmark, C. and Schmitz, B. (2009) The Origin of the Brunflo Fossil Meteorite and Extraterrestrial Chromite in Mid-Ordovician Limestone from the Gärde Quarry (Jämtland, Central Sweden), Meteoritics and Planetary Science, v. 44, p. 95-106.

Heck, P. R., Ushikubo, T., Schmitz, B., Kita, N. T., Spicuzza, M. J., and Valley, J. W. (2010) A single asteroidal Source for extraterrestrial Ordovician chromite grains from Sweden and China: High-precision oxygen three-isotope SIMS analysis,Geochimica et Cosmochimica Acta, v. 74(2), p. 497-509.

Denton S. Ebel, Curator, Dept. of Earth and Planetary Sciences
Chair, Division of Physical Sciences, American Museum of Natural History
200 Central Park West, New York, NY  10024
(212) 769-5381    http://research.amnh.org/~debelhttps://urldefense.com/v3/__http://research.amnh.org/*debel__;fg!!IKRxdwAv5BmarQ!bfDl6N87RIWQ_UwoBYEwstXjUayvPK5E-mBoiu2lMYwKfJiH71GjnGVQM6EdnPkrK_WZ89kyR3AFsDdV$
space show VI: https://www.amnh.org/exhibitions/permanent/hayden-planetarium/worlds-beyond-earthhttps://urldefense.com/v3/__https://www.amnh.org/exhibitions/permanent/hayden-planetarium/worlds-beyond-earth__;!!IKRxdwAv5BmarQ!bfDl6N87RIWQ_UwoBYEwstXjUayvPK5E-mBoiu2lMYwKfJiH71GjnGVQM6EdnPkrK_WZ89kyR-D98epI$
“Historically, pandemics have forced humans to break with the past and imagine their world anew. This one is no different. It is a portal, a gateway between one world and the next. We can choose to walk through it, dragging the carcasses of our prejudice and hatred, our avarice, our data banks and dead ideas, our dead rivers and smoky skies behind us. Or we can walk through lightly, with little luggage, ready to imagine another world. And ready to fight for it.” -- Arundhati Roy

From: Hummer, Daniel R via MSA-talk msa-talk@minlists.org
Sent: Thursday, February 1, 2024 11:18 AM
To: Ayers, John C john.c.ayers@Vanderbilt.Edu; Pilar Lecumberri-Sanchez lecumber@ualberta.ca; MSA-talk@minlists.org msa-talk@minlists.org
Subject: [MSA-talk] Re: Polymerization, composition, melting point and weathering of silicates

EXTERNAL SENDER

Pilar and friends,

This is a fantastic question, and really gets one thinking about the underlying behavior of the bonds that hold together our favorite Earth materials. Here is my take:

One important difference is that olivine and other mafic minerals contain Fe2+, which is susceptible to oxidation in the high fO2 environment of Earth’s surface. That difference basically boils down to electrochemistry. But for a moment, let’s ignore the redox sensitivity of Fe, because I think there’s an even more fundamental concept at work…. the unique chemistry of water and how it reacts to ionic vs. covalent bonds.

Olivine is composed of primarily ionic bonds between cations and anions, and the more covalent-natured Si-O bonds don’t play a very large role because the silica component is unpolymerized (i.e., the SiO4 units just behave as their own “ions”). The highly polar water molecule is therefore able to rip cations and anions away from each other very effectively via electrostatic attraction. Whereas, in the absence of water, it takes a LOT of heat to overcome the attraction of the ionic bonds, because you’re trying to do it by brute force (rather than with the help of such a highly effective molecular scalpel).

Contrast this with K-spar and other network silicates, which are held together mostly by bonds with much more covalent nature, where electrons sit directly between the atoms. In the absence of water, these bonds will more easily get “mushy” (like a limp rope) when heated and soften the silica framework to the point where it can squish and slide, forming a melt (where bonds are weakened but still present). You don’t even have to break all the bonds, you just need to break enough for bond angles to slide and twist around… just enough to soften the silica network. But at room temperature, even in the presence of water, the covalent bonds will keep Si and O atoms tethered to each other tightly because they don’t care whether the water molecular is polar or not. There is not enough separation of charge for those bonds to be susceptible to the + or – end of a H2O molecule, so the Si-O framework just sits there, and at best you get a little bit of incongruent dissolution of components like K and Na, as pointed out by John Ayers. Both olivine and K-spar could probably sit happily in equilibrium with air for millions of years with very little change, but water will make a tremendous difference.

So in my view, we need to think beyond just “strong” vs. “weak” bonds, we also have to think about how different the nature of the bonding is at each end of Bowen’s reaction series. Each style of bonding reacts very differently to heat vs. reactive molecules like H2O and O2, and I suspect that’s the key to understanding these trends on a molecular level. Of course, there is sure to be much more nuanced thermodynamics at play than what I’ve outlined here, but hopefully this is a useful way to frame the issue from a broad perspective. Very interested to hear other thoughts on this.

Best,

Dan

From: Ayers, John C via MSA-talk msa-talk@minlists.org
Sent: Wednesday, January 31, 2024 9:16 AM
To: Pilar Lecumberri-Sanchez lecumber@ualberta.ca; msa-talk@minlists.org
Subject: [MSA-talk] Re: Polymerization, composition, melting point and weathering of silicates

[EXTERNAL EMAIL ALERT]: Verify sender before opening links or attachments.

When I teach this material to my students, I do not use bond strengths to explain the stabilities of the minerals, but rather the conditions at which they formed. Minerals like olivine that are stable in the mantle are less likely to be stable at the Earth’s surface. This is due to differences in temperature, pressure, oxygen fugacity, and water activity/availability.  Minerals like K-feldspar that crystallize at the lowest temperatures in Bowen’s reaction series can be stable in equilibrium with water, but only after the K-feldspar dissolves incongruently to form clays and the fluid composition evolves until it becomes saturated in K-feldspar. Hope this is helpful.

Professor John C. Ayers

Dept. of Earth & Environmental Sciences
Vanderbilt University
PMB 351805
2301 Vanderbilt Place
Nashville, TN 37235-1805
Tel 1-615-973-1879

https://my.vanderbilt.edu/johncayershttps://urldefense.com/v3/__https://my.vanderbilt.edu/johncayers__;!!IKRxdwAv5BmarQ!bfDl6N87RIWQ_UwoBYEwstXjUayvPK5E-mBoiu2lMYwKfJiH71GjnGVQM6EdnPkrK_WZ89kyRzjcHyp5$

From: Pilar Lecumberri-Sanchez via MSA-talk <msa-talk@minlists.orgmailto:msa-talk@minlists.org>
Sent: Tuesday, January 30, 2024 10:32 PM
To: msa-talk@minlists.orgmailto:msa-talk@minlists.org
Subject: [MSA-talk] Polymerization, composition, melting point and weathering of silicates

Hi all,

In explaining silicates and the Bowen reaction series in class and I have realized that I don’t understand how this works. I can sort of figure out melting behavior, but then I struggle with weathering behavior.

I think of melting temperature of a mineral as a function of the strength of the weakest bonds present in a mineral (what holds them together) and the repulsion between the cations occupying their sites (what pulls them apart). Because the bonds formed between Fe/Mg-O are shorter and can have lower coordination numbers than those formed between Na/K-O, it makes sense that nesosilicates have stronger “weakest” bonds than tectosilicates and melt at higher temperatures. In addition, with increasing polymerization, more silicons are forced into close proximity to each other causing repulsion between the tetrahedrally occupied sites. Therefore more polymerized silicates will have internal repulsion forces that will further facilitate bond breakage (again lower melting temperatures).

Here comes where I struggle. If the strength of the weakest bond is highest in nesosilicates, why do nesosilicates minerals weather easier than felsic ones (i.e., why are nesosilicates further from theromdynamic equilibrium at room temperatures than tectosilicates)? I know that we are now putting the minerals in a different solvent and that phase equilibrium predicts that forsterite is not stable at environmental conditions in the presence of water, the question is why is that more so for forsterite than for K-feldspars. Since dissolution into water also requires bond breakage intuitively it would make sense that olivine, which has stronger bonds and less internal repulsion forces, would survive better than feldspar. I can try to think about it in terms of the bonds formed by the elements in aqueous solution and whether the way in which sodium dissolves in pure water is more competitive than magnesium, but it is not clear to me that that would be the case.

So here is my question, why at the atomic structure level and not just from a descriptive perspective is forsterite more readily weathered than K-feldspar given that the bonds in olivine are significantly stronger and the internal repulsion forces between atoms weaker?

Thank you everyone,

Pilar

————————————————
Pilar Lecumberri-Sanchez

Associate Professor

Dept. of Earth & Atmospheric Sciences
1-26 Earth Sciences Building
University of Alberta
Edmonton, Alberta, T6G 2E3

Phone: 780-492-5071

https://cms.eas.ualberta.ca/lecumberri/https://urldefense.com/v3/__https://cms.eas.ualberta.ca/lecumberri/__;!!IKRxdwAv5BmarQ!bfDl6N87RIWQ_UwoBYEwstXjUayvPK5E-mBoiu2lMYwKfJiH71GjnGVQM6EdnPkrK_WZ89kyR20yQgvA$
————————————————

I think Barry is right on the nose - going back to Pauling's rules, we can even predict that ionic bonds involving more highly charged ions in lower coordination numbers are going to be particularly strong and resistant to cleaving, and this is what we exclusively find in the spinel structure (and also minerals like quartz, rutile, corundum, Al-oxyhdroxides, etc.).

Dan

From: Barry Bickmore bbickmore1970@gmail.com
Sent: Thursday, February 1, 2024 2:06 PM
To: Denton Ebel debel@amnh.org
Cc: Ayers, John C john.c.ayers@Vanderbilt.Edu; Pilar Lecumberri-Sanchez lecumber@ualberta.ca; MSA-talk@minlists.org; Hummer, Daniel R daniel.hummer@siu.edu
Subject: Re: [MSA-talk] Polymerization, composition, melting point and weathering of silicates

[EXTERNAL EMAIL ALERT]: Verify sender before opening links or attachments.
Hi Denton,

I will take a stab at this.  Spinel has a sort of framework structure of Al octahedra. The Al-O bonds would be relatively strong, and Al-oxyhydroxides tend to be a lot of what's left over after really intense weathering of soils.  In the gaps of the octahedral framework you have Mg in tetrahedral coordination, so the Mg-O bonds would be stronger than those in phases where Mg is 6-coordinated.

Barry Bickmore
Professor of Geological Sciences
Brigham Young University

On Feb 1, 2024, at 12:00 PM, Denton Ebel via MSA-talk <msa-talk@minlists.orgmailto:msa-talk@minlists.org> wrote:

Thanks Pilar, John, Dan - Fascinating discussion. In my world, it is the spinel family that is of great interest. 1) In CV chondrites e.g. Allende, the MgAl2O4 in CAIs retains its O-16 rich character but the melilite, "fassaites" and anorthite equililibrate with the parent body. This effect is subdued in the less-hydrated CV-reduced (e.g. Vigarano) class (e.g., MacPherson 2014). 2) In the K-Pg boundary clay, the surviving condensates from the impact plume are magnesio-wustite spinels with wt% levels of NiO, the rest of the spherules were Ca-rich glass which is long altered everywhere (Ebel & Grossman 2005). 3) In fossil meteorites in Swedish quarries ~480 Ma we see the L chondrite textures, but the only surviving minerals are... spinels! And we can then tell the L typology from Cr-Mn isotope systematics in spinel (Alwmark & Schmitz 2009; Heck et al. 2010).
Ideas about why spinels are so persistent?
Thanks
D
references:
MacPherson G.J. (2014) Calcium-aluminum-rich inclusions in chondritic meteorites. In Meteorites, Comets and Planets (A.M. Davis, ed.), pp. 201-246. Volume 1 of Treatise on Geochemistry, (H.D. Holland and K.K. Turekian, eds.), Elsevier. 10 volumes.
Ebel, D.S., and L. Grossman (2005) Spinel-bearing spherules condensed from the Chicxulub impact-vapor plume. Geology 33: 293-296.
Alwmark, C. and Schmitz, B. (2009) The Origin of the Brunflo Fossil Meteorite and Extraterrestrial Chromite in Mid-Ordovician Limestone from the Gärde Quarry (Jämtland, Central Sweden), Meteoritics and Planetary Science, v. 44, p. 95-106.
Heck, P. R., Ushikubo, T., Schmitz, B., Kita, N. T., Spicuzza, M. J., and Valley, J. W. (2010) A single asteroidal Source for extraterrestrial Ordovician chromite grains from Sweden and China: High-precision oxygen three-isotope SIMS analysis,Geochimica et Cosmochimica Acta, v. 74(2), p. 497-509.

Denton S. Ebel, Curator, Dept. of Earth and Planetary Sciences
Chair, Division of Physical Sciences, American Museum of Natural History
200 Central Park West, New York, NY  10024
(212) 769-5381    http://research.amnh.org/~debel
space show VI: https://www.amnh.org/exhibitions/permanent/hayden-planetarium/worlds-beyond-earth
"Historically, pandemics have forced humans to break with the past and imagine their world anew. This one is no different. It is a portal, a gateway between one world and the next. We can choose to walk through it, dragging the carcasses of our prejudice and hatred, our avarice, our data banks and dead ideas, our dead rivers and smoky skies behind us. Or we can walk through lightly, with little luggage, ready to imagine another world. And ready to fight for it." -- Arundhati Roy

From: Hummer, Daniel R via MSA-talk <msa-talk@minlists.orgmailto:msa-talk@minlists.org>
Sent: Thursday, February 1, 2024 11:18 AM
To: Ayers, John C <john.c.ayers@Vanderbilt.Edumailto:john.c.ayers@Vanderbilt.Edu>; Pilar Lecumberri-Sanchez <lecumber@ualberta.camailto:lecumber@ualberta.ca>; MSA-talk@minlists.orgmailto:MSA-talk@minlists.org <msa-talk@minlists.orgmailto:msa-talk@minlists.org>
Subject: [MSA-talk] Re: Polymerization, composition, melting point and weathering of silicates

EXTERNAL SENDER

Pilar and friends,

This is a fantastic question, and really gets one thinking about the underlying behavior of the bonds that hold together our favorite Earth materials. Here is my take:

One important difference is that olivine and other mafic minerals contain Fe2+, which is susceptible to oxidation in the high fO2 environment of Earth's surface. That difference basically boils down to electrochemistry. But for a moment, let's ignore the redox sensitivity of Fe, because I think there's an even more fundamental concept at work.... the unique chemistry of water and how it reacts to ionic vs. covalent bonds.

Olivine is composed of primarily ionic bonds between cations and anions, and the more covalent-natured Si-O bonds don't play a very large role because the silica component is unpolymerized (i.e., the SiO4 units just behave as their own "ions"). The highly polar water molecule is therefore able to rip cations and anions away from each other very effectively via electrostatic attraction. Whereas, in the absence of water, it takes a LOT of heat to overcome the attraction of the ionic bonds, because you're trying to do it by brute force (rather than with the help of such a highly effective molecular scalpel).

Contrast this with K-spar and other network silicates, which are held together mostly by bonds with much more covalent nature, where electrons sit directly between the atoms. In the absence of water, these bonds will more easily get "mushy" (like a limp rope) when heated and soften the silica framework to the point where it can squish and slide, forming a melt (where bonds are weakened but still present). You don't even have to break all the bonds, you just need to break enough for bond angles to slide and twist around... just enough to soften the silica network. But at room temperature, even in the presence of water, the covalent bonds will keep Si and O atoms tethered to each other tightly because they don't care whether the water molecular is polar or not. There is not enough separation of charge for those bonds to be susceptible to the + or - end of a H2O molecule, so the Si-O framework just sits there, and at best you get a little bit of incongruent dissolution of components like K and Na, as pointed out by John Ayers. Both olivine and K-spar could probably sit happily in equilibrium with air for millions of years with very little change, but water will make a tremendous difference.

So in my view, we need to think beyond just "strong" vs. "weak" bonds, we also have to think about how different the nature of the bonding is at each end of Bowen's reaction series. Each style of bonding reacts very differently to heat vs. reactive molecules like H2O and O2, and I suspect that's the key to understanding these trends on a molecular level. Of course, there is sure to be much more nuanced thermodynamics at play than what I've outlined here, but hopefully this is a useful way to frame the issue from a broad perspective. Very interested to hear other thoughts on this.

Best,
Dan

From: Ayers, John C via MSA-talk <msa-talk@minlists.orgmailto:msa-talk@minlists.org>
Sent: Wednesday, January 31, 2024 9:16 AM
To: Pilar Lecumberri-Sanchez <lecumber@ualberta.camailto:lecumber@ualberta.ca>; msa-talk@minlists.orgmailto:msa-talk@minlists.org
Subject: [MSA-talk] Re: Polymerization, composition, melting point and weathering of silicates

[EXTERNAL EMAIL ALERT]: Verify sender before opening links or attachments.
When I teach this material to my students, I do not use bond strengths to explain the stabilities of the minerals, but rather the conditions at which they formed. Minerals like olivine that are stable in the mantle are less likely to be stable at the Earth's surface. This is due to differences in temperature, pressure, oxygen fugacity, and water activity/availability.  Minerals like K-feldspar that crystallize at the lowest temperatures in Bowen's reaction series can be stable in equilibrium with water, but only after the K-feldspar dissolves incongruently to form clays and the fluid composition evolves until it becomes saturated in K-feldspar. Hope this is helpful.

Professor John C. Ayers
Dept. of Earth & Environmental Sciences
Vanderbilt University
PMB 351805
2301 Vanderbilt Place
Nashville, TN 37235-1805
Tel 1-615-973-1879
https://my.vanderbilt.edu/johncayers

From: Pilar Lecumberri-Sanchez via MSA-talk <msa-talk@minlists.orgmailto:msa-talk@minlists.org>
Sent: Tuesday, January 30, 2024 10:32 PM
To: msa-talk@minlists.orgmailto:msa-talk@minlists.org
Subject: [MSA-talk] Polymerization, composition, melting point and weathering of silicates

Hi all,

In explaining silicates and the Bowen reaction series in class and I have realized that I don't understand how this works. I can sort of figure out melting behavior, but then I struggle with weathering behavior.

I think of melting temperature of a mineral as a function of the strength of the weakest bonds present in a mineral (what holds them together) and the repulsion between the cations occupying their sites (what pulls them apart). Because the bonds formed between Fe/Mg-O are shorter and can have lower coordination numbers than those formed between Na/K-O, it makes sense that nesosilicates have stronger "weakest" bonds than tectosilicates and melt at higher temperatures. In addition, with increasing polymerization, more silicons are forced into close proximity to each other causing repulsion between the tetrahedrally occupied sites. Therefore more polymerized silicates will have internal repulsion forces that will further facilitate bond breakage (again lower melting temperatures).

Here comes where I struggle. If the strength of the weakest bond is highest in nesosilicates, why do nesosilicates minerals weather easier than felsic ones (i.e., why are nesosilicates further from theromdynamic equilibrium at room temperatures than tectosilicates)? I know that we are now putting the minerals in a different solvent and that phase equilibrium predicts that forsterite is not stable at environmental conditions in the presence of water, the question is why is that more so for forsterite than for K-feldspars. Since dissolution into water also requires bond breakage intuitively it would make sense that olivine, which has stronger bonds and less internal repulsion forces, would survive better than feldspar. I can try to think about it in terms of the bonds formed by the elements in aqueous solution and whether the way in which sodium dissolves in pure water is more competitive than magnesium, but it is not clear to me that that would be the case.

So here is my question, why at the atomic structure level and not just from a descriptive perspective is forsterite more readily weathered than K-feldspar given that the bonds in olivine are significantly stronger and the internal repulsion forces between atoms weaker?

Thank you everyone,

Pilar

Pilar Lecumberri-Sanchez
Associate Professor
Dept. of Earth & Atmospheric Sciences
1-26 Earth Sciences Building
University of Alberta
Edmonton, Alberta, T6G 2E3
Phone: 780-492-5071

https://cms.eas.ualberta.ca/lecumberri/

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I don't particularly like Bowen's reaction series as a teaching tool
because many students get the mistaken impression that the minerals are
arranged according to their melting points. They are not - mantle olivine
and quartz (cristobalite) have similarly high melting points (~1700C). The
order is a generalized crystallization sequence for silicate magmas. Worse,
it conveys the insidious idea that rocks melt (crystallize) one mineral at
a time according to which has the lowest (highest) melting point. Partial
melting is described this way in many introductory textbooks - quartz melts
first because it has the lowest m.p.! Thank goodness rocks melt at
temperatures (far) below the melting points of their constituents minerals
otherwise many would be out of a job.

Cheers,
Mike

Dr J. Michael Palin (Mike)
geologist
38 Rosebery Street
Belleknowes, Dunedin 9011
New Zealand
+64-3-453-1083 (answering machine)
jmpalin@gmail.com
ORCID 0000-0002-4972-7302

Rocks are better than people.