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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?

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Olivine is a 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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