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Identity HelpRe:Garnet

1st Jan 2006 19:42 UTCIlkka Mikkola Expert

What species?

Small dark-brown anhedral crystals from the same cavity as the Mn-rich "apatite".

Apatite is dark greenish-brown.

Microprobeanalysis of the garnet:

SiO2 39.10 Al2O3 21.00 FeO 6.95 MnO2 11.20 MgO 7.90 CaO 12.10 Tot 98.25(w%)

Formula CaO 0.99 MgO 0.90 MnO 0:72 FeO 0.44 SiO2 2.98 Al2O3 1.91

I think that this is unusual analysis for a pegmatite garnet.

The pegmatite contains lot of usual red-brown spessartine,brown tourmaline,"columbite",vivianite,albite,quarz,muscovite, beusite,beryl and some

other normal pegmatite minerals.


1st Jan 2006 22:09 UTCAlan Plante

Wouldn't that make it a grossular composition with an anomolously high Mg content? I don't think there are solid solution series between grossular and any of the Mg species - pyrope, majorite and knorringite. I think the series that grossular form are grossular-andradite (Al-Fe substitution), grossular-uvarovite (Al-Cr substitution), and the two nominal "hydrogarnets", katoite and hibschite (Si-OH substitution???). Dana 8th does note, though, regarding grossular: "However, solid solution to varying extent occurs with other garnets." So perhaps it is not all that anomolous? (Don't you just love it when authors make vague statements like that, not presenting any data to support them??? :~} )



3rd Jan 2006 06:58 UTCIlkka Mikkola Expert

Hello Alan

Thank you.

Best regards.

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