<p>Olivine is a <a href="https://en.wikipedia.org/wiki/Solid_solution" rel="nofollow noreferrer">solid solution</a> between forsterite and fayalite, so it is generally expressed as molar percentages of these end-members, for instance Fo<sub>80</sub>Fa<sub>20</sub>. Often only one end-member is mentioned, as it is sufficient to infer the other (their sum is always 100 %).</p>

<p>It is the same with plagioclase, which is a solid solution between albite and anorthite and is often expressed as "Ab<sub>xx</sub>" or "An<sub>xx</sub>", where xx is the molar percentage of considered end-member.</p>

<p>Edit: As for the range, e.g. 95-90, I'd say they analyzed several olivine grains in the same sample, and the range represents the variability of compositions they found. It could also be the analytical error on a single composition, but I doubt it would be that large, although it could be in some old papers when the precision of chemical analyses was not as good as today.  </p>

CC BY-SA 4.0
2019-12-26T08:25:00.287Z
2019-12-26T19:09:00.323Z
2019-12-26T19:09:00.323Z
Jean-Marie Prival
<p>I have noticed that in many articles on minerals that I have read the abbreviation of some mineral has a numerical subscript, for example, occurs with the forsterite (Fo<sub>95-90</sub>, Fo<sub>92-84</sub>, Fo<sub>94</sub>, etc.). What is the meaning of this subscript?</p>

<mineralogy><terminology>
What is the meaning of the subscript in the abbreviations of some minerals?
URIZEN
# What is the meaning of the subscript in the abbreviations of some minerals?

(Asked by: username_0 on 2019-12-26)

I have noticed that in many articles on minerals that I have read the abbreviation of some mineral has a numerical subscript, for example, occurs with the forsterite (Fo95-90, Fo92-84, Fo94, etc.). What is the meaning of this subscript?

**Tags:** mineralogy, terminology


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## 1 answers

### Answer by username_1 on 2019-12-26. Score: 4 | Accepted answer

Olivine is a [solid solution](https://en.wikipedia.org/wiki/Solid_solution) between forsterite and fayalite, so it is generally expressed as molar percentages of these end-members, for instance Fo80Fa20. Often only one end-member is mentioned, as it is sufficient to infer the other (their sum is always 100 %).

It is the same with plagioclase, which is a solid solution between albite and anorthite and is often expressed as "Abxx" or "Anxx", where xx is the molar percentage of considered end-member.

Edit: As for the range, e.g. 95-90, I'd say they analyzed several olivine grains in the same sample, and the range represents the variability of compositions they found. It could also be the analytical error on a single composition, but I doubt it would be that large, although it could be in some old papers when the precision of chemical analyses was not as good as today.


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