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How do rocks get named?

Posted by Jason Evans  
Jason Evans March 04, 2012 02:09PM
I am not sure if i understand this correctly but I think the IMA is the organization that approved mineral names, but what about rock names, is there the IMA equivalent for rocks? I would like to find out if there is an official term for the rock consisting of greyish purple opaque zoisite , Tsavorite garnet, pyrite, graphite and possibly a few more that i have not noticed. and also is there a name for the rock from Turkey that's sold as Lavender Jade?
David Von Bargen March 04, 2012 03:34PM
"is there the IMA equivalent for rocks?" - No.
Michael Kieron March 04, 2012 05:14PM
For igneous rocks refer to The IUGS systematics of igneous rocks, 1991, by M. J . Le Bas & A. L. Streckeisen and you'll see the ways, means, and difficulties that go behind all these.

Anyway, rock classification is based on some systematics based on mineral content and there are ternary diagrams to give a reasonable (to a geologist) name to a rock. For example cumberlandite is technically just another magnetite melatroctolite based on the ratios of olivine, plagioclase, and pyroxene along with a large amount of magnetite.

Tanzanite and tsavorite may be mineral assemblages within the rock but I would not think of them as a rock type. I have rocks from an old copper mine with chalcopyrite, bornite, molybdenite, epidote and grossular but I would still call the rock a greenschist, maybe even an epidote greenschist.

From what I know, the rocks at Merelani are mostly dolomitic marbles and graphite gneisses and I would consider tanzanite/tsavorite as minerals hosted as veins and pockets within.

Jason Evans March 04, 2012 05:51PM
Thanks, OK so what defines a rock from a mineral assemblage? I though that's what a rock technically is? As the ruby/zoisite/.tschermakite mix gets its own name (Anyolite) i wonder why the purple zoisite/tsavorite mix also doesn't have its own name.

Edited 1 time(s). Last edit at 03/04/2012 05:58PM by Jason Evans.
David Von Bargen March 04, 2012 06:57PM
"i wonder why the purple zoisite/tsavorite mix also doesn't have its own name." - the people selling it have a bit more sense / don't need to hype it.

There are rock names such as granite, basalt, sandstone, schist, that have been used by geologists/petrographers for a long time. With a rock, you really need to have a lot of it (enough to qualify for mapping). Gemstone names tend to be coined to help sell something, either by adding adjectives to real minerals/gemstones (smoky topaz - really just smoky quartz) or trying to give it some cachet or suck up to someone (kunzite).
Jason Evans March 04, 2012 10:57PM
well i think the people who do sell it are hyping it by calling the purple zoisite, Tanzanite! If it occured as gemmy well formed crystals then maybe they could get away with using Tanzanite but all the specimens i have seen just have a very ugly grey purple opaque material, and I have also notice they must be photographing it with a flash or something because that seems to make the purple colour stand out more, in both of my specimens the actual colour is nothing like how it appears in all the photo's i have seen. It must be quite rare as I have not seen many places which have it, 1 or 2 online sites and occasionally a bit appears on eBay. but with so many on line mineral sites i'm surprised there is not more available. 1 of those sites gives the locality of their specimens as near the famous scorpion mine in the taita hills in kenya, if this is true then the locality info for Tanzanite needs to be changed.
I am still curuious about what i can call my lavender jade from turkey, i have found out that the jade part of the rock is actually jadeite but i cannot call it jadeite as its only 40-50% jadeite so should i just call it a jadiete containing metamorphic rock? I'm sorry if i seem pedantic about these things but i just like to have things in my collection labeled correctly.
Don Saathoff March 05, 2012 01:02AM
Jason, if your "lavender jade" were in my collection I'd label it as "jadeite/(whatever the metamorphic rock is), "jadeite/schist". As far as the zoisite is concerned, regardless of what the seller calls it, it is in your collection and you can correct incorrect usages for your satisfaction but there is no way to change marketing hype but through educated buyers.....

Ralph Bottrill April 14, 2012 09:19AM
Its good to see people conciencious about naming rocks! Metamorphic rocks are usually described with a base name like schist, hornfels etc prefixed by mineral modifiers, in the case of this zoisite rock, probably zoisite skarn. Re the purple jadeite, it depends what else is in it and how it occurs: possibly a jadeite granulite? Or jadeite blueschist? But as Michael indicated they may just occur as veins, pods or nodules etc and thus not merit formal names.


Edited 1 time(s). Last edit at 04/14/2012 09:24AM by Ralph Bottrill.
Marco Jamer April 23, 2012 03:43AM
It depends on how they were discovered. There are some which are named from the dominant material found in them. And there are usually some which are named based on the word that describes them the most. Usually, they are quite descriptive about it and in some rare instances, the names of the those who discovered them are put to consideration. Or even where and when they are found.
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