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Post by stephan on Dec 17, 2018 15:15:34 GMT -5
vegasjames , stephan : Limonite or goethite? I've done a lot of reading about fire agate over the years because I've cut a lot of it and have been curious about the cause of its iridescence. Much conflicting theorizing has been published about it. I rely on the GIA for authoritative information on gems because the organization has the resources, equipment and expertise to do reliable research. So here's the verdict from John Koivula, the GIA's top research gemologist: www.gia.edu/gems-gemology/winter-2015-microworld-dragons-eye-fire-agate Are you seriously telling me that something I read on the internet might not be true?
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Post by vegasjames on Dec 17, 2018 16:17:15 GMT -5
vegasjames , stephan : Limonite or goethite? I've done a lot of reading about fire agate over the years because I've cut a lot of it and have been curious about the cause of its iridescence. Much conflicting theorizing has been published about it. I rely on the GIA for authoritative information on gems because the organization has the resources, equipment and expertise to do reliable research. So here's the verdict from John Koivula, the GIA's top research gemologist: www.gia.edu/gems-gemology/winter-2015-microworld-dragons-eye-fire-agate Actually all are right. Keep in mind that as I mentioned before limonite is not a particular mineral. It is a mix of iron oxides and hydroxides, which includes goethite:
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Post by vegasjames on Dec 17, 2018 16:17:46 GMT -5
vegasjames , stephan : Limonite or goethite? I've done a lot of reading about fire agate over the years because I've cut a lot of it and have been curious about the cause of its iridescence. Much conflicting theorizing has been published about it. I rely on the GIA for authoritative information on gems because the organization has the resources, equipment and expertise to do reliable research. So here's the verdict from John Koivula, the GIA's top research gemologist: www.gia.edu/gems-gemology/winter-2015-microworld-dragons-eye-fire-agate Are you seriously telling me that something I read on the internet might not be true? Miracles do happen.
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gemfeller
Cave Dweller
Member since June 2011
Posts: 3,775
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Post by gemfeller on Dec 17, 2018 16:51:37 GMT -5
vegasjames , stephan : Limonite or goethite? I've done a lot of reading about fire agate over the years because I've cut a lot of it and have been curious about the cause of its iridescence. Much conflicting theorizing has been published about it. I rely on the GIA for authoritative information on gems because the organization has the resources, equipment and expertise to do reliable research. So here's the verdict from John Koivula, the GIA's top research gemologist: www.gia.edu/gems-gemology/winter-2015-microworld-dragons-eye-fire-agate Actually all are right. Keep in mind that as I mentioned before limonite is not a particular mineral. It is a mix of iron oxides and hydroxides, which includes goethite: I concede it's a question of how many angels can dance on the head of a pin, as Medieval scholars pondered. Mineralogist Dr. Frederick H. Pough says the difference between limonite and goethite is that limonite is amorphous while goethite has a fibrous crystal structure. It's mainly a question of terminology since they used to be considered to be the same mineral. But now there appears to be a consensus that the fibrous mineral in fire agate (a misnomer since it's chalcedony, not banded agate) is goethite.
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Post by vegasjames on Dec 17, 2018 17:59:43 GMT -5
Actually all are right. Keep in mind that as I mentioned before limonite is not a particular mineral. It is a mix of iron oxides and hydroxides, which includes goethite: I concede it's a question of how many angels can dance on the head of a pin, as Medieval scholars pondered. Mineralogist Dr. Frederick H. Pough says the difference between limonite and goethite is that limonite is amorphous while goethite has a fibrous crystal structure. It's mainly a question of terminology since they used to be considered to be the same mineral. But now there appears to be a consensus that the fibrous mineral in fire agate (a misnomer since it's chalcedony, not banded agate) is goethite.
" Limonite is crystaline to amorphous"
This all goes back to what I have been saying that limonite is not a singular mineral with a definite chemical composition. It is a mix of iron oxides and hydroxides. Some of those compounds can be crystalline and some amorphous.
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