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Riccardo Modanesi
Joined: 07 Nov 2011
Posts: 627
Location: Milano


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Posted: Jan 28, 2025 03:11 Post subject: Re: Limonitized garnet? |
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Hi Roger!
I think I agree with you: just label it as olivine or peridot and don't search strange adventures. Glossary and other manuals keep just extreme terms in mineral series. Plagioclases, for instance, are just called "albite" and "anorthite", the intermediate terms "oligoclase", "andesine", labradorite" and "bytownite" are cancelled, as well as the pyroxene intermediate terms "hypersthene" and "bronzite". If you want to have an opinion of mine, I don't agree with it. and I don't agree with this new terminology which calls some minerals with a chemical element in brackets. AWFUL! The trend shold be calling almandine "garnet (Fe,Ca)! BLEAH!!!
greetings from Italy by Riccardo.
_________________ Hi! I'm a collector of minerals since 1973 and a gemmologist. On Summer I always visit mines and quarries all over Europe looking for minerals! Ok, there is time to tell you much much more! Greetings from Italy by Riccardo. |
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Herwig
Joined: 04 Jan 2016
Posts: 24
Location: Hasselt


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Posted: Jan 28, 2025 03:31 Post subject: Re: Limonitized garnet? |
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Cher Roger,
It all depends on what you feel comfortable with, and what your goals are.
If you strive to use IMA accepted mineral names on your labels, then this specimen should get the name "forsterite".
If you want to be more specific, you could add the varietal name "peridot" to your label:
forsterite var. peridot
[peridot is a very old name for the green gem variety of forsterite]
Using "olivine" on your label makes things complicated, since there is an Olivine Group now. And that olivine group has quite a few more minerals than just forsterite and fayalite.
See https://www.mindat.org/min-29264.html#autoanchor8
I hope this helps! Bien cordialement,
Cheers, Herwig
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Roger Warin

Joined: 23 Jan 2013
Posts: 1215



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Posted: Jan 28, 2025 04:53 Post subject: Re: Limonitized garnet? |
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Hello Herwig,
Thank you for giving me your opinion, which is very respectable since it's the official standard.
By profession, I've always been on the frontiers of knowledge. But of course my opinions were controlled by referees.
I know the French language very well, without the help of ChatGPT5.
My specimen is Mg-rich olivine, not Fe-poor forsterite (fayalite).
The solid solution (binary) state diagram of this specimen is, for example, the one I present. This mineral is a solid solution that always bears the name olivine.
That's the name I'll use on the label.
It's not a question of personal intellectual comfort, but of rationality.
Mineral: | Olivine diagram |
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Herwig
Joined: 04 Jan 2016
Posts: 24
Location: Hasselt


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Posted: Jan 28, 2025 05:48 Post subject: Re: Limonitized garnet? |
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Cher Roger,
quoting mindat:
olivine : Commonly used as a synonym for a member of the Fayalite-Forsterite Series, but more generally and properly applied to any olivine group member. The vast majority is forsterite.
See https://www.mindat.org/min-2983.html
From a petrological point of view, your "olivine" label might still be acceptable, because olivine always has been the name for the intermediate solution.
From a mineralogical point of view, however, the only valid options are the ones I specified in my previous message.
Cheers, Herwig (in California now)
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Roger Warin

Joined: 23 Jan 2013
Posts: 1215



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Posted: Jan 30, 2025 15:05 Post subject: Re: Limonitized garnet? |
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Still on the subject of olivines and their solid solutions.
I say this because I know the world of amateurs, having run a club for over 50 years. We're an interphase between scientists and beginners.
Coincidentally, a similar discussion is taking place on this forum about the hermine.
I can complete the label on my specimen. It seems to me that in addition to rigor, there is common sense. For example: Olivine-(Mg,Fe).
For an iron-rich olivine, we'd write: Olivine-(Fe,Mg).
For an olivine very rich in Mg, we also speak of Forsteritic olivines.
I fully endorse the work of these authors, including Alex Strekeisen.
Ternary system Ca2SiO4 - Mg2SiO4 - Fe2SiO4 for olivine minerals.
Ternary diagram showing the chemical variability of the olivine group members.
Two main solid solutions occur: forsterite-fayalite and monticellite-kirschsteinite.
Common rock-forming olivines belong to the forsterite-fayalite series.
Ca-olivines are rare in nature.
Mineral: | Forsterite |
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Roger Warin

Joined: 23 Jan 2013
Posts: 1215



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Posted: Jan 30, 2025 15:10 Post subject: Re: Limonitized garnet? |
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The name forsterite is restricted to those species with no more than 10 percent magnesium substituting for iron. Compositions intermediate to these series end-members are identified which is an expression of the molar percentage of each compound.
For example, Fo70Fa30 denotes a composition of olivine that is 70 percent forsterite. The notation is shortened to Fo70. In addition to the forsterite-fayalite series, other complete solid-solution series exist among the various olivine minerals. The monticellite-kirschteinite series has larger unit cell than forsterite-fayalite series, accommodating the larger Ca cation.
My aka on this FMF is a barred olivine-(Mg,Fe), the main constituent of a chondrule.
The chondrite is DAG 1040 CV3, the same type as Allende.
The olivine bars are built on a single crystal lattice. These bars are the result of very rapid skeletal crystallization, with exsolution of other minerals in small proportions between the bars. Several unknown twins have been identified by Professor Hatert, ULiege.
This crystal habit cannot exist on Earth, only in space.
These bars are the result of very rapid skeletal crystallization.
If anyone wants an explanation, please contact me by private message.
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Herwig
Joined: 04 Jan 2016
Posts: 24
Location: Hasselt


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Posted: Jan 31, 2025 07:03 Post subject: Re: Limonitized garnet? |
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Dear Roger,
Despite the fact that I am traveling in the USA at the moment, I take the time to answer your message, so the other readers can learn about the mineral names that are currently valid for this diagram, and why some mineral names are valid and others are not.
Monticellite and kirschsteinite are two valid mineral names, and also the end-members of a solid solution series. Theoretically it would also be possible to give names to intermediates of this series, but fortunately this has never been done, and so there are only the names of those two end members, and it will be the percentage of iron (Fe) versus the percentage of magnesium (Mg) that will tell us if a specimen is monticellite (more Mg than Fe) or kirschsteinite (more Fe than Mg).
Forsterite and fayalite are two valid mineral names, and also the end-members of a solid solution series, and the same rules as above also apply to these minerals.
All the "mineral names" of all the "in betweens" of forsterite and fayalite, are nowadays varietal names, and thus those names are no longer valid mineral names.
The reason why, is that there is no difference in structure by going from one end member to the other. And there is only one place in the structure where Fe or Mg can go.
Consequently, nowadays, the only thing that matters is the composition. To use your abbreviations: anything that is Fo50 or more has the name forsterite, while anything less than Fo50 has the name fayalite.
It's always nice to know the actual composition, and it's good to put that on the label, but it is not recommended to keep using varietal names, unless they are mentioned AFTER the valid mineral name. I hope this makes things clear for all.
Of course, if you don't know the percentages of Fe and Mg, then you should put "fayalite-forsterite series" on your label, if you stick to the valid mineral names.
Most collectors will use "olivine", because that's a lot shorter, and since the name has been used for ages, that is acceptable, but again, it is not the name of a valid mineral species.
FYI: I will be driving most of the day (here) today, from Reno (Nevada) to Fallbrook (California), where the Pacific MicroMineral Symposium will take place. See for more info: https://www.mindat.org/mesg-679588.html
I hope some of our readers will join us at the symposium. It's a great event, with lots of minerals, good spirits, friendly people, and very educational.
Make sure to say hi to me when you get there; it's always a pleasure to meet other mineral collectors (that I haven't met before)!
Bien cordialement, mon cher Roger!
Cheers, Herwig
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