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MineralsTounkite

11th Aug 2013 07:22 UTCMichael Scheven

Hi,

15 years ago I have purchased bluish sceletal crystals of spinel associated with nearly colorless Diopside xls and white Tremolite from "Sludyanka". I have confirmed the diagnosis "Spinel" by testing the Mohs hardness. The specimen is acid-leached because of abundance of minor calcite. I assume, the exact finding locality could be Tultui. Associated with the Spinel there occurs a acid-resistant bluish-green translucent grain. It seems neither to be Tounkite nor a Copper silicate, because these minerals are acid-soluble.

What could it be? - Chromium diopside?

Sincerely,

Michael Scheven

11th Aug 2013 18:26 UTCRob Woodside 🌟 Manager

How acid resistant? It could be a cancrinite/sodalite group mineral. They will slowly turn to jelly if left in acid long enough. Photo?

12th Aug 2013 16:58 UTCMichael Scheven

Thank you very much for your hint, Rob. The specimen has been etched, and has been cracked after etching into several fragments before I have purchased it. The mineral grain is very small (0,2 mm). Since I have no equipment für microfotography I can't provide you a foto.

Thanks again,

Michael Scheven

12th Apr 2024 08:09 UTCŁukasz Kruszewski Expert

I've been trying to input the correct Mindat formula, but without success: the system continuously suggests there is an ASCII-related issue, even though I've removed all the "x", y", "2-x", multiplification dot for water, "n", etc. Please add the correct formula (it is given in Formula Notes currently).

Thanks!

Luke

12th Apr 2024 09:30 UTCFrank K. Mazdab 🌟 Manager

The formula listed in the formula notes with the compositional ranges is NOT the appropriate formula that should be presented for this mineral!

You wouldn't use the formula of forsterite reported from some specific locality and given as the analytical range "(Mg_0.89-0.93Fe²⁺_0.07-0.10Ni_x)₂SiO₄ (where x < 0.01)" as the appropriate formula we should head the "forsterite" page with. We would give the end-member formula, Mg₂SiO₄, or in the absence of a known end-member formula (and this is problematic in its own right... then is it really a unique species?), then (Mg,Fe²⁺,Ni)₂SiO₄.

I see these awful non-end-member formulas very commonly presented in Russian new minerals papers from the 1990s and before. Did they not teach 70's/80's era Russian mineralogy students what an end-member formula was?

I've replaced the blank mindat formula with the current IMA formula, which is not an end-member formula but is the only useable formula we have at the moment. What's interesting is that the IMA has been moving recently to converting non-end-member formulas (with parentheses and commas) to proper end-member formulas, so either they haven't gotten around to this one yet, or they don't know what to do with this compositional mess.

12th Apr 2024 12:09 UTCKeith Compton 🌟 Manager

Hmm

Me thinks the later ))-:)

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