<p>First off, we should also consider the bonding between the metals and oxygen, which also occur in common igneous-rock minerals. These bonds are predominantly ionic, as the metals have lower electronegativity and less ionization energy (to their normal ionic charges) than silicon (to its &quot;normal ionic&quot; charge of +4).</p>
<p><a href="https://doi.org/10.1016/0022-3697(82)90139-1" rel="nofollow noreferrer">Catti[1]</a> fitted the charge distribution to observed structural characteristics of the mafic mineral forsterite (<span class="math-container">$\ce{Mg2SiO4}$</span>), identifying the following results:</p>
<p><span class="math-container">$\ce{Mg}: +1.38$</span></p>
<p><span class="math-container">$\ce{Si}: +1.44$</span></p>
<p><span class="math-container">$\ce{O}: -1.05$</span></p>
<p>Since the theoretical ionic charges would include -2 for the electronegative oxygen atom, this represents about half the fully ionic charge separation (magnesium is 69% ionized to its noble gas core, silicon 36% showing the greater ionic character of the metal-oxygen bond). We would regard this as towards the ionic end of the bonding scale, because we would not expect 100% charge separation between individual atoms even in &quot;clearly ionic&quot; compounds like sodium chloride.</p>
<p><strong>Reference</strong></p>
<p>Michele Catti (1982),
&quot;Atomic charges in Mg2SiO4 (forsterite), fitted to thermoelastic and structural properties&quot;, <em>Journal of Physics and Chemistry of Solids</em>,
Volume 43, Issue 12, Pages 1111-1118,
ISSN 0022-3697,
<a href="https://doi.org/10.1016/0022-3697(82)90139-1" rel="nofollow noreferrer">https://doi.org/10.1016/0022-3697(82)90139-1</a>.</p>

CC BY-SA 4.0
2023-11-27T18:29:06.113Z
I very much appreciate the reply, but I don't quite understand how it relates. My chemistry background is quite deficient. Is your example showing that forsterite, despite being silica-based, is essentially ionic? What I'm gleaning is that the distinction between ionic and covalent is actually more of a continuum ...
vortek
CC BY-SA 4.0
2023-11-27T19:01:54.980Z
Yes, the "ionic/covalent" bonding is a continuum. If you try to construct molecular orbitals for a material such as sodium chloride, the occupied "covalent" orbitals come close to just having all the valence electrons on chlorine; what you attempted to render as covalent bonds end up being close to what fully ionic bonds would be. In the case of forsterite I am saying the valence electron distribution is more like that in sodium chloride than that in species showing no ionic properties such as diatomic element molecules or hydrocarbons.
Oscar Lanzi
CC BY-SA 4.0
2023-11-24T13:22:33.307Z
2023-11-25T05:55:14.090Z
2023-11-25T05:55:14.090Z
Oscar Lanzi
<p>Consider pure solid Ga: all the atoms have the same value of electronegativity and hence the maximum density of electronic charge will be found exactly at the center of the Ga-Ga bond.</p>
<p>Now consider GaAs: As has a stronger tendency to attract electrons (higher electronegativity) and so the maximum density of electronic charge will be found displaced towards As</p>
<p>Now consider ZnSe: the difference in the electronegativity between Zn and Se is larger than that calculated in GaAs for Ga and As. Hence, the maximum density of electronic charge will be found further nearer the electronegative element, that is Se.</p>
<p>It is clear that in this way you can move almost continuously from a purely covalent bond (for example that found in Ga, C,...) to a situation where the percentage of the ionic bond increases. Nonetheless, even in CsF, the compound with the largest ionic character, a fraction of the bonding is covalent. So ionic and covalent bonding are aspects of the same kind of bonding, namely a bonding where bonding electrons are not free as in metals.</p>
<p>In any case, you don't have to confuse the ionic charge, which is the charge that the atoms gain by transferring electrons for the bond formation, with the formal charge, that is used to describe the external electronic configuration of the atoms.</p>

CC BY-SA 4.0
2023-11-30T12:54:48.267Z
2023-12-01T12:56:42.490Z
2023-12-01T12:56:42.490Z
gryphys
<p>I'm just starting out in geology and I keep reading that ionic bonds predominate among minerals. However, the majority of minerals are silicates and the silicate ion has covalent bonds, so they obviously are everywhere, too. (Not to mention carbonates, sulfates, etc.).</p>
<p>So why do geologists think ionic bonds are dominant?</p>

<geochemistry>
Why are ionic bonds considered the "dominant" type in minerals?
vortek
# Why are ionic bonds considered the "dominant" type in minerals?

(Asked by: username_0 on 2023-11-22)

I'm just starting out in geology and I keep reading that ionic bonds predominate among minerals. However, the majority of minerals are silicates and the silicate ion has covalent bonds, so they obviously are everywhere, too. (Not to mention carbonates, sulfates, etc.).

So why do geologists think ionic bonds are dominant?

**Tags:** geochemistry


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## 2 answers

### Answer by username_1 on 2023-11-24. Score: 1 | Accepted answer

First off, we should also consider the bonding between the metals and oxygen, which also occur in common igneous-rock minerals. These bonds are predominantly ionic, as the metals have lower electronegativity and less ionization energy (to their normal ionic charges) than silicon (to its "normal ionic" charge of +4).

[Catti[1]](https://doi.org/10.1016/0022-3697(82)90139-1) fitted the charge distribution to observed structural characteristics of the mafic mineral forsterite ($\ce{Mg2SiO4}$), identifying the following results:

$\ce{Mg}: +1.38$

$\ce{Si}: +1.44$

$\ce{O}: -1.05$

Since the theoretical ionic charges would include -2 for the electronegative oxygen atom, this represents about half the fully ionic charge separation (magnesium is 69% ionized to its noble gas core, silicon 36% showing the greater ionic character of the metal-oxygen bond). We would regard this as towards the ionic end of the bonding scale, because we would not expect 100% charge separation between individual atoms even in "clearly ionic" compounds like sodium chloride.

**Reference**

Michele Catti (1982),
"Atomic charges in Mg2SiO4 (forsterite), fitted to thermoelastic and structural properties", *Journal of Physics and Chemistry of Solids*,
Volume 43, Issue 12, Pages 1111-1118,
ISSN 0022-3697,
<https://doi.org/10.1016/0022-3697(82)90139-1>.

> **Comments:**
> **username_0**: I very much appreciate the reply, but I don't quite understand how it relates. My chemistry background is quite deficient. Is your example showing that forsterite, despite being silica-based, is essentially ionic? What I'm gleaning is that the distinction between ionic and covalent is actually more of a continuum ...  
> **username_1**: Yes, the "ionic/covalent" bonding is a continuum. If you try to construct molecular orbitals for a material such as sodium chloride, the occupied "covalent" orbitals come close to just having all the valence electrons on chlorine; what you attempted to render as covalent bonds end up being close to what fully ionic bonds would be. In the case of forsterite I am saying the valence electron distribution is more like that in sodium chloride than that in species showing no ionic properties such as diatomic element molecules or hydrocarbons.  

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### Answer by username_2 on 2023-11-30. Score: 2

Consider pure solid Ga: all the atoms have the same value of electronegativity and hence the maximum density of electronic charge will be found exactly at the center of the Ga-Ga bond.

Now consider GaAs: As has a stronger tendency to attract electrons (higher electronegativity) and so the maximum density of electronic charge will be found displaced towards As

Now consider ZnSe: the difference in the electronegativity between Zn and Se is larger than that calculated in GaAs for Ga and As. Hence, the maximum density of electronic charge will be found further nearer the electronegative element, that is Se.

It is clear that in this way you can move almost continuously from a purely covalent bond (for example that found in Ga, C,...) to a situation where the percentage of the ionic bond increases. Nonetheless, even in CsF, the compound with the largest ionic character, a fraction of the bonding is covalent. So ionic and covalent bonding are aspects of the same kind of bonding, namely a bonding where bonding electrons are not free as in metals.

In any case, you don't have to confuse the ionic charge, which is the charge that the atoms gain by transferring electrons for the bond formation, with the formal charge, that is used to describe the external electronic configuration of the atoms.


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