<p>'Fugacity' is effectively the equivalent of chemical activity, but for gases. Fugacity relates to the partial pressure of a particular volatile component of a system in the same way that chemical activity relates to the concentration of a given chemical phase.</p>
<p>Most often in geology, this will refer to 'fugacity of oxygen (<em>f</em>O<sub>2</sub>)' in a system - this is effectively a measure of how reactive oxygen is, and thus how oxidized a given system will be, usually with respect to the minerals that are stable in them.</p>
<p>At temperatures that most mantle processes operate on, aqueous fluid phases generally exist in a supercritical state, which we also apply the concept of fugacity to. So, if the text you are reading states something about 'fugacity of H<sub>2</sub>O', then it is likely referring to the reactivity of supercritical water vapor in the lunar mantle system.</p>

CC BY-SA 4.0
2022-08-07T17:09:44.720Z
2022-08-07T17:18:26.627Z
2022-08-07T17:18:26.627Z
desander
<p>I am reading about water in the Lunar mantle, however it mentions fugacity a lot. What does it mean in this context?</p>

CC BY-SA 4.0
2022-08-04T18:27:31.160Z
This is not my area of expertise, but do these help: [Fugacity: definitions, concepts, and geology](https://www.geol.umd.edu/facilities/lmdr/fugacity.html) & [Introduction to oxygen fugacity and its petrologic importance](https://pubs.geoscienceworld.org/msa/rimg/article/25/1/1/87287/Introduction-to-oxygen-fugacity-and-its-petrologic)?
Fred
CC BY-SA 4.0
2022-08-04T18:34:55.250Z
From various online dictionaries. Fugacious: ephemeral , temporary , transient , transitory, fleeting, tending to disappear, unenduring volatile, evanescent, momentary.
Fred
CC BY-SA 4.0
2022-08-04T18:42:52.757Z
It might help others if you could provide a link to the item you have been reading.
Fred
<geology><geochemistry><planetary-science><mantle><moon>
What does fugacity mean in the context of geology?
Andy
# What does fugacity mean in the context of geology?

(Asked by: username_0 on 2022-08-04)

I am reading about water in the Lunar mantle, however it mentions fugacity a lot. What does it mean in this context?

**Tags:** geology, geochemistry, planetary-science, mantle, moon

> **Comments:**
> **username_1**: This is not my area of expertise, but do these help: [Fugacity: definitions, concepts, and geology](https://www.geol.umd.edu/facilities/lmdr/fugacity.html) & [Introduction to oxygen fugacity and its petrologic importance](https://pubs.geoscienceworld.org/msa/rimg/article/25/1/1/87287/Introduction-to-oxygen-fugacity-and-its-petrologic)?  
> **username_1**: From various online dictionaries. Fugacious: ephemeral , temporary , transient , transitory, fleeting, tending to disappear, unenduring volatile, evanescent, momentary.  
> **username_1**: It might help others if you could provide a link to the item you have been reading.  

---

## 1 answers

### Answer by username_2 on 2022-08-07. Score: 1

'Fugacity' is effectively the equivalent of chemical activity, but for gases. Fugacity relates to the partial pressure of a particular volatile component of a system in the same way that chemical activity relates to the concentration of a given chemical phase.

Most often in geology, this will refer to 'fugacity of oxygen (*f*O2)' in a system - this is effectively a measure of how reactive oxygen is, and thus how oxidized a given system will be, usually with respect to the minerals that are stable in them.

At temperatures that most mantle processes operate on, aqueous fluid phases generally exist in a supercritical state, which we also apply the concept of fugacity to. So, if the text you are reading states something about 'fugacity of H2O', then it is likely referring to the reactivity of supercritical water vapor in the lunar mantle system.


---

