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Questions and queries: Definition of garnet group

Posted by Peter Seroka  
avatar Questions and queries: Definition of garnet group
May 29, 2011 04:17PM
As stated in mindat, the garnet group members are minerals of the groups 9.AC.25, 9.AD.25 and 4.CC.25. Furthermore, garnets are considered to be nesosilicates.

*1. Apart from the classic definition of the garnet group minerals (pyralspite, ugrandite) , which all are silicates, a number of newly approved garnets ( Berzeliite, Bitikleite, Elbursite, Schäferite, Yafsoanite) do not contain any Si at all . They are - however - , still called silicates. Is there a logic ?

*2. Does anyone know, which commission of the IMA has worked / is working on the garnet group ?

*3. Could anyone (please) let me have links / a link to any IMA / CNMN- publication, defining the actual garnet group ?

*4. According to what I can read , garnets (now) can be silicates, oxides and / or of other chemical compositions. What is the actual terminology / denomination / definition / name / of the new garnets ? (compared to the old definition "nesosilicates" ?)

Thanks for yr. help

Peter Seroka
Re: Questions and queries: Definition of garnet group
May 29, 2011 04:49PM
Hi Peter,

in answer to your questions:

1. The most natural way of defining a "garnet" is by crystal structure rather than chemistry. All of these minerals share a crystal chemistry in which three big cations (8-coordinated by anions), two middle-sized (6-coordinated) and 3 small (4-coordinated) link together in a distinctive structure which ideally has cubic symmetry (small distortions and ordered variants of lower symmetry also exist). The first few common, naturally occurring garnets happened all to be nesosilicates, but many natural and synthetic examples are now known of nonsilicate garnets.

1a. This "garnet" entity would be at least a supergroup rather than a group, now that the definition of mineralogical "group" is being defined, since it spans several groups in different anion classes (nesosilicates, oxides, tellurates, fluorides...). When different criteria are used for classification, they can intersect each other like this. There are many other examples (e.g. galena is a sulfide member of the "halite supergroup").

2 and 3. As far as I am aware, no IMA subcommittee has been set up to look at garnet classification recently, since there is no real need just yet. Note that the Strunz and Dana numbering have no rigorous status, while the species all have obvious ideal endmembers and the same distinctive structure.

4. Any statement that "garnets" in the broadest sense have to be nesosilicates is outdated and oversimplified, but has no formal status.

Hope this helps,

A/.
avatar Re: Questions and queries: Definition of garnet group
May 29, 2011 05:05PM
Oh dear... I think we're descending into madness here...

What does "garnet" mean? what does "garnet group" mean?

As far as I'm concerned, "garnet" is a historical term relating to a group of silicate minerals that crystallise in the same way, have obvious visual as well as structural similarities, and have some logical connection.

In mineralogical terms the "garnet group" used to mean the same thing, slightly expanded with other minerals that have the same structure but are not what we would regard as a traditional "garnet".

However, the IMA are now using the term "garnet group" to describe anything with the same structure - which is not uncommon amongst a whole range of different chemistries, and not all of these are silicates.


So, as has been suggested. "Garnet Supergroup" could refer to everything that has this structure, but the term "Garnet", and to say "mineral X is a garnet" can ONLY refer to one of the classic garnet-group minerals, which are all silicates.

Because terms such as "garnet" (as opposed to garnet group or garnet supergroup) will never be defined by the IMA, we are free to define them ourselves here based on history and common sense, which is why the term 'Garnet' on mindat will have to change! It's NOT a synonym of 'Garnet group'.

Jolyon
Re: Questions and queries: Definition of garnet group
May 29, 2011 06:57PM
ca    
Right on Jolyon!!smileys with beer
avatar Re: Questions and queries: Definition of garnet group
May 29, 2011 11:17PM
Changes made.

We now have "garnet" (which has the traditional meaning you all love) and

"garnet structural group" (which is our name for the new 'garnet supergroup' being discussed)

I dislike the term "supergroup" for these groupings based entirely on structural rather than chemical differences, luckily we are not run by the IMA so we are free to use whatever names for groups here we decide are best!

So, for now, until someone convinces me that we shouldn't, the "garnet structural group" is what it will be called.
avatar Re: Questions and queries: Definition of garnet group
May 30, 2011 02:09PM
It seems a bit odd I agree to have non-silicate garnets, and also galena and halite in the same group, etc. Apatite is a similar case, with silicate analogues in the apatite group. To define a species you need both chemistry and structure, but with a group you only need the structure. But when you look at other systems, eg the alunite group, which you may think at first to be a sulphate group, half the members are phosphates or arsenates and some are mixtures (eg. the beudantite subgroup). So when you get into it you find you cannot be too proscriptive about classifying minerals in one chemical group or another - it can get very messy.

Regards,
Ralph
avatar Re: Questions and queries: Definition of garnet group
May 30, 2011 02:51PM
I have no problems with groups having complex definitions, but the confusion was saying that "Berzeliite is a garnet"

Which is just completely wrong. It's not a garnet. It just happens to share the same structure.

Saying "Berzeliite is a member of the garnet structural group" is much clearer.
Re: Questions and queries: Definition of garnet group
May 30, 2011 06:54PM
At the risk of promulgating madness smiling smiley

The ceramicists, gemologists and solid-state chemists have been generalising terms like "garnet" on a structural basis for synthetic compounds for far longer than mineralogists have tended to for the rare nonsilicate structural analogues, and the usage abounds in the broader literature. "Yttrium aluminium garnet" = "YAG" for instance, the common synthetic gemstone,and its laser material relatives, are oxides.
Re: Questions and queries: Definition of garnet group
May 31, 2011 12:19AM
us    
At the risk of adding to the confusion, particularily because Joylon's ''garnet structural group" (which is our name for the new 'garnet supergroup') IS the same as the IMA's "supergroup" since according to "The standardisation of mineral group hierarchies: application to recent nomenclature proposals" Eur. J. Mineral.21, 1073–1080 (2009)

"A mineral group consists of two or more minerals with the same or essentially the same structure, and
composed of chemically similar elements."

"A mineral supergroup consists of two or more groups which have essentially the same structure and composed of chemically similar elements. Generally, a supergroup will contain members from the same mineral class (e.g. the epidote supergroup), however in rare cases a supergroup may also contain groups belonging to different classes, as for example in the alunite supergroup A supergroup may also contain isolated mineral species which do not belong to any mineral group, as for example vanadinite, which is the only vanadate in the apatite supergroup."

Mindat's nomenclature should reflect the IMA's, with additional explanations where needed.
avatar Re: Questions and queries: Definition of garnet group
May 31, 2011 08:27AM
What groups make up the Garnet supergroup?

As defined on mindat previously, it was NOT a supergroup, but simply a normal "group" with a list of minerals.

This is why it was changed. If I have misunderstood and there are other group levels we did not put into the system, then we can change it.

Jolyon



Edited 1 time(s). Last edit at 05/31/2011 08:29AM by Jolyon Ralph.
avatar Re: Questions and queries: Definition of garnet group
May 31, 2011 11:33PM
au    
There is a garnet subcommittee working on this at the moment and has been doing so for several months. So in half a year or so you shall get your answers :)
avatar Re: Questions and queries: Definition of garnet group
February 19, 2012 09:51AM
more than half a year (or so) has passed - are there any news regarding the definition of garnet group and supergroup ?
Re: Questions and queries: Definition of garnet group
September 21, 2012 07:07PM
at    
[meetingorganizer.copernicus.org]

"Counting Garnets: A New Nomenclature of the Garnet Supergroup"
avatar Re: Questions and queries: Definition of garnet group
April 01, 2013 10:29PM
au    
Edward S. Grew, Andrew J. Locock, Stuart J. Mills, Irina O. Galuskina, Evgeny V. Galuskin and Ulf Hålenius (2013) Nomenclature of the garnet supergroup. American Mineralogist, 98(4), 785-811.

Abstract

The garnet supergroup includes all minerals isostructural with garnet regardless of what elements occupy the four atomic sites, i.e., the supergroup includes several chemical classes. There are presently 32 approved species, with an additional 5 possible species needing further study to be approved. The general formula for the garnet supergroup minerals is {X3}[Y2](Z3)ϕ12, where X, Y, and Z refer to dodecahedral, octahedral, and tetrahedral sites, respectively, and ϕ is O, OH, or F. Most garnets are cubic, space group Ia3̄d (no. 230), but two OH-bearing species (henritermierite and holtstamite) have tetragonal symmetry, space group, I41/acd (no. 142), and their X, Z, and ϕ sites are split into more symmetrically unique atomic positions. Total charge at the Z site and symmetry are criteria for distinguishing groups, whereas the dominant-constituent and dominant-valency rules are critical in identifying species. Twenty-nine species belong to one of five groups: the tetragonal henritermierite group and the isometric bitikleite, schorlomite, garnet, and berzeliite groups with a total charge at Z of 8 (silicate), 9 (oxide), 10 (silicate), 12 (silicate), and 15 (vanadate, arsenate), respectively. Three species are single representatives of potential groups in which Z is vacant or occupied by monovalent (halide, hydroxide) or divalent cations (oxide). We recommend that suffixes (other than Levinson modifiers) not be used in naming minerals in the garnet supergroup. Existing names with suffixes have been replaced with new root names where necessary: bitikleite-(SnAl) to bitikleite, bitikleite-(SnFe) to dzhuluite, bitikleite-(ZrFe) to usturite, and elbrusite-(Zr) to elbrusite. The name hibschite has been discredited in favor of grossular as Si is the dominant cation at the Z site. Twenty-one end-members have been reported as subordinate components in minerals of the garnet supergroup of which six have been reported in amounts up to 20 mol% or more, and, thus, there is potential for more species to be discovered in the garnet supergroup. The nomenclature outlined in this report has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (Voting Proposal 11-D).



The paper is open source at [ammin.geoscienceworld.org]
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