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hydrougrandite

Posted by Bill Shelton  
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Bill Shelton June 29, 2003 11:46AM
Why is this listed as not a valid species ? Does it go in the hydrogarnet series of the new Dana classification? Why ? Thanks, Bill
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David Von Bargen June 29, 2003 02:47PM
Hydrougrandite is listed in de Fourestier as between hibschite and katoite in composition. Bayliss as (OH) rich andradite (Am Min 50, 2100 (1965) and 51, 1825 (1966)). Hey lists as hydrogarnet (this was from a Chinese publication in 1964 - probably wasn't sent to CNMMN for approval). There is a good likelyhood there is complete solid solution between andradite, grossular and the metal hydroxides Ca2Al2(OH)12 - so it fails the 50% rule.
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alfredo July 02, 2003 10:46AM
In that case, why is hibschite a valid species? Shouldn't there be a 50:50 division between grossular and katoite?
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David Von Bargen July 02, 2003 03:05PM
If they had followed their normal rules hibschite shouldn't be a valid species, but this is apparently one of those exceptions (and one that they apparently did follow on the andradite/hydrougrandite side - if someone found an OH dominant andradite they could probably make the same argument).

From the abstract for katolite in the Am Min (1985 p 873)
".. the name grossular should be retained for the anhydrous end-member (100% Gr). Members in the series with 50% or more Gr ( ie. 50<=Gr%<100) should be designated hibschite, the name which has precedence over alternate terms.... Katoite should be adopted for members with less than 50% Gr, including the Ca3Al2(OH)12 end member."

I have a feeling that since end member grossular (at least in regard to hydroxyl substitution for silicate) is a relatively common mineral, it was decided to keep the name and to use hibschite (hydrogrossular) to refer to those unusual cases in which there is some substitution.
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alfredo July 02, 2003 03:34PM
Thanks for your input, David.

For the sake of consistency, we should do exactly the same for "hydrougrandite" as was done for hibschite. The same pros and cons would seem to apply in both cases.

And whereas we should all be grateful for the tremendous progress that the IMA-CNMMN has achieved in bringing some order to the chaos that used to reign in mineral naming, I think MINDAT should aim for an even higher standard of logic in its listing of the validity of species.

Cheers, Alfredo
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