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Gedrite series
December 11, 2011 08:02PM
    
First Draft


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Can you help make this a better article? What good localities have we missed? Can you supply pictures of better specimens than those we show here? Can you give us more and better information about the specimens from these localities? Can you supply better geological or historical information on these localities?

The gedrite series minerals are minerals in the amphibole group, see Amphibole Group main article for an overview of the group. The series contains the following minerals:

Ferrogedrite
☐Fe22(Fe23Al2)(Si6Al2)22OH2

Ferrogedrite
☐Fe22(Fe23Al2)(Si6Al2)22OH2

Gedrite
☐Mg2(Mg3Al2)(Si6Al2)22OH2

Sodic-ferrogedrite
NaFe22(Fe24A)(Si6Al2)22OH2

Sodicgedrite
NaMg2(Mg4A)(Si6Al2)22OH2


Gedrite 95mm specimen, Schisshyttan , Sweden© Michael C. Roarke


The gedrite series minerals are part of the amphibole subgroup 1, the Mg–Fe–Mn–Li group. This group contains both monoclinic and orthorhombic amphiboles, the gedrite series minerals being orthorhombic. The gedrite series minerals forms a continuous series with the anthophyllite series of minerals with two chemical substitutions dominating:

1: (Mg,Fe)Si -- AlAl and
2: Si -- NaAl

There is a full solid solution series along these substitutions, and the divide between the two series are arbitrarily set to AlSi7 in the T position. Minerals in these series is therefore often refered to as members of the "anthophyllite-gedrite" series, not taking into consideration the Mg/Fe ratio or Na content.

The gedrite-series minerals can be considered the Al-rich end-members in this series, although silica deficient gedrites (Si<6,0 apfu) are known from nature. Gedrite is by far the most common of the gedrite series minerals ( 122 localities in mindat pr. feb-2012), while ferrogedrite are listed from 10 localities and sodic-ferrogedrite and sodic-gedrite from 2 and 1 locality respectively.

Gedrite occurs in metamorphic rocks, most commonly in amphibolites and gneisses together with other Mg/Al minerals such as other amphiboles, cordierite, phlogopite and plagioclase feldspars or with kyanite. It can also occur in more Fe-rich environments together with biotite, staurolite and almandine. Quartz is a rare associate with gedrite series minerals.

It is virtually impossible to distinguish from the more common anthophyllite which also occurs in the same type of environments, and it is not made easier when chemical analysis of a arge number of samples “shows that orthoamphiboles termed anthophyllites by one author overlap with gedrites of other authors and vice-versa.” Beeson 1978. That gedrite also can be mistaken for other amphiboles goes without saying.

Gedrite series minerals are consequently rarely identified or collected, and it is difficult to estimate how good specimens they may form. The star formed fans from the Bergslagen ore-district in Sweden are about as attractive an amphibole gets in a metamorphic rock. Also some of the orthoamphiboles (anthophyllite-gedrite series) from the Bamble formation can be quite attractive, for an amphibole that is.

Te pictured specimen and selected localities should be considered as typical rather than “best”.


Ferrogedrite
Czech Republic
Moravia (Mähren; Maehren), Vysočina Region, Žďár nad Sázavou, Dolní Bory

Ferrogedrite 4cm crystals© Jakub Jirásek


This pegmatite district is located within the area of granulitic rocks of the Bory granulit massif surrounded by cordierite migmatites and biotite-sillimanite migmatitic gneisses and Ferrogedrite occurs exclusively as dark fans on parting/cleavage planes in sekaninaite crystals. The sekaninaite crystals can form large crystals, and the individual ferrogedrite crystals can be multiple cm long.

The bulk chemistry of ferrogedrite and sekinanaite is similar. According to Schreyer (1965), ferrogedrite is stable at higher pressures and similar temperatures as sekaninaite. Ferrogedrite does therefore seem to be a primary mineral in the pegmaties, unlike chlorite and other common alteration products.

Literature
Josef Staněk (1997-2008): Minerály z Borů a Cyrilova ,oficialni stranky obce Bory- website

WERNER SCHREYER(1965): Zur Stabilit~it des Ferroeordierits, Beitr~ge zur Mineralogie und Petrographie Ii, 297--322 (1965)



Gedrite
France
Midi-Pyrénées , Hautes-Pyrénées, Luz-Saint-Sauveur , Héas valley , Gèdres

Gedrite 8cm specimen© Alain TUEL


Gedrite from the type locality forms large foliated( multiple cm crystalline masses of clove brown color. The pictured specimen is typical in that respect. I have not found any reference on neither associated minerals nor local geology.

Literature
E. Schweizerbart, 1901 Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Volum 2;Volum 1901


Ferrogedrite
Japan
Honshu Island, Chubu Region, Gifu Prefecture, Ena City, Kawai mine

Ferrogedrite FOV 4mm©


The Kawai Mine is an abandoned mine that was operated during WWII, presumeably for lead and zinc Ferrogedrite hs been found and identified from rocks found in the mine dump. Ferrogedrite occurs as greenish black. sub-parallell aggregates up to 10mm long. It occurs with almandine, annite and quartz.

Finding an Al-enriched amphibole with quartz is highly unusual, and since the material is only found at the mine dump, it has not been possible to determine the geochemical environment enabling this mineral assemblage, although a metasomatic origin is suggested by Matsubara,Kato and Nomura.

Literature
Satoshi Matsubara, Akira Kato, Matsumitsu Nomura (1980): The occurance of Ferro-gedrite from the Kawai Mine, Ena, Gifu Prefecture, Japan. Bulletin for Natural Sciences Museum, ser C (Geology) 8 (4)


Gedrite
Norway
Aust-Agder, Froland,, Bøylefossbru

Gedrite 7 cm specimen© Leon Hupperichs


Bøylefossbru is one of many locations in the Bamble formation for ortho-amphiboles ( anthophyllite and gedrite series). These amphiboles are of various shades of brown and may be found in individual crystals and aggregates up to minimum 10 cm. The orthoamphiboles occurs with cordierite, phlogopite and plagioclase in gneisses and amphibolites from the Sveco-Norwegian orogeny (1,1-1,5GA), and are found to be nearest the Mg-rich end members in composition, but heir Al content varies between and within each location. It is consequently hard to distinguish between anthophyllite and gedrite without a chemical analysis.

Literature:
R. Beeson(1978): The Geochemistry of Orthoamphiboles and Coexisting Cordierites and Phlogopites From South Norway. Contributions to
Mineralogy and Petrology 66, 5-14

Nijland, T.G., J.C. Zwaan & L. Touret. Topographical mineralogy of the Bamble sector, south Norway.Scripta Geol., 118: 1-46,


Gedrite
Sweden
Dalarna, Ludvika, Väster-Silvberg, Stollberg Mines

Gedrite, 15cm FOV© B. Otter


The Paleoproterozoic Fe-Pb-Zn-Mn(-Ag) Stollberg deposit is situated in the Bergslagen region of south-central Sweden. 6.65 Mt of sphalerite, galena and manganiferous skarn magnetite ore occur as disseminations and semi-massive to massive ore bodies hosted by volcanogenic sediments and carbonate rocks.

Gedrite occurs as brown (dark) radiating fans (stars) with the individual crystals reaching multiple cm length, found in a metarhyolite in the footwall of the ore body. It is believed that the gedrite is formed by metamorphosis to amphibolite facies during the formation of the orebody. Later, retrograde metamorphosis has altered the gedrite to chlorite and lizardite, and a hardness test should be used to verify whether a specimen contains gedrite or it’s alteration products.

Literature:
Rodney Allen, Magnus Ripa, Nils Jansson(2008), Palaeoproterozoic volcanic- and limestone hosted Zn-Pb-Ag-(Cu-Au) massive sulphide deposits and Fe oxide deposits in Bergslagen,Sweden, Excursion No. 12 Bergslagen, IGCP Project 502- Global comparison of volcanic hosted, massive sulphide deposits

Ripa, Magnus(1996), The Stollberg ore field - petrography, lithogeochemistry, mineral chemistry, and ore formation, Ph.D. thesis, Institute of Geology, Department of Mineralogy and Geology, Lund University SI: 1-23 Abstract


Gedrite
Sweden
Dalarna, Ludvika, Väster-Silvberg, Schisshyttan

Gedrite 11 cm specimen© Jorge M. Alves



The Schisshyttan locality belongs to the same geological system as the Stollberg mines and gedrite has formed, and occurs as described above in several locations in the general area.


Gedrite
USA
Connecticut Middlesex Co ,Haddam, Beaver Meadow Road - Route 9 Interchange

Gedrite 21 cm specimen© 2010 Harold Moritz


The following is quoted from the mindat locality page:

“Extensive and deep roadcuts along both sides of the divided highway, the 4 on and off-ramps, and nearby Hubbard Road have exposed rock units of the Middletown Formation, a metamorphic sequence consisting mostly of amphibolite, orthoamphibole gneiss, and biotite gneiss. Lundgren (1979) provides a detailed map and description of the rock units and their mineralogy at this interchange. Most of the minerals mentioned are rock-forming grains, but he notes one distinctive unit,

“A gneiss in which gedrite occurs in well aligned prisms and in conspicuous rosettes of prisms. This unit also contains cordierite. It is well displayed in the cut on Hubbard Road at Beaver Meadow Road, the entrance to the northbound lane of Route 9, and in the exit from the southbound lane of Route 9 at Beaver Meadow Road”.


Olav Revheim Februar 2012

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Edited 1 time(s). Last edit at 02/09/2012 07:55PM by Olav Revheim.
avatar Re: Gedrite series
December 14, 2011 06:08AM
Olav,
Good effort. Sometimes it seems our work just drops off he edge of the earth. I wonder if you might say a few words about the significance of the minerals in the group currently? How many tons or grams o them are there thought to be and how many localities do we currently have for east species. Largest crystals approximately? I just don't have any internal clock for these things at all. Has the not long ago reworking of the amphaboles done much to obscure the specimens of this amphabole sub group? What I mean, are there some common types of specimens from this group that are likely to be found on specimens in many collections that should now have their labels changed?

Rock Currier
Crystals not pistols.
Re: Gedrite series
December 14, 2011 08:00PM
    
Rock,

I'll gradually expand the text to be in line with the other articles, such as the latest one: Cummingtonite series. There will always be things that I have missed, and if there is anything at all in any of the articles I have done do you feel are missing or incorrect, please feel free to ask or comment, and I or somebody else may or may not be able to address the issues smiling smiley

I can easily add the number of entries in Mindat to give some feel for how common each of these minerals are. I think I should do that for each of the articles.

I'll try to address as many of the issues you raise as I can as I move along with this article.

---

Generally, there was a paradigm shift in 1978 when B.Leake published a new classification system for the amphiboles. For me it seems that he did three drastic changes:
1) He applied the 50% rule and distinguished all amphiboles based on chemistry ( and cystal system) alone, and also set a standard for how to present the chemical formula.
2) He set subgroups and made charts for how the various amphiboles should relate to each other
3) He obsoleted over 200 amphibole names, and (re)defined a lot of the others.

The benefits of this is obviously a structured system with given boundaries between the individual minerals where these boundaries has been set according to predefined rules. As a result it became possible to assign one and only one name to an amphibole of a given composition.

The drawback is and will always be that the amphiboles are a complex group of minerals that isn't easily classified. In addition, obsoleted and (re)defined mineral names is not nessecarily synonymous with only one of the then "new" amphibole names but often two or three. This makes it difficult to correctly name amphiboles described before 1978. To add to this, the identification process is complex and expensive. There is ongoing work to revice Leake's classification, but these issues will probably not disappear entirely regardless how this group is classified.

Consequently, I think many amphiboles are mislabeled, both in private and museum collections, but this complexity doesn't justify giving up on the group either, as many seems to have done. To me it seems possible to make an approximate/probable identification inbetween "amphibole group" and the accurately defined mineral species based on associated minerals, geological environment and the relative commonness of the various minerals/series/groups, but I am not quite there yet.

I think there are quite a lot of work to address errors and inaccurate information in Mindat as well, but in many cases 100% accuracy is not possible, and it sometimes seems to be a reluctance to make changes to entries that are almost certainly wrong for this reason.

smiling smiley

Olav
Re: Gedrite series
December 14, 2011 09:50PM
Olav,
Nice job, appreciate your efforts.
Helps make mindat the superb site it is.
With this hobby of minerals, you wade into deep waters. Amphiboles, that's the edge of the abyss.
Keep swimming!

mike
avatar Re: Gedrite series
December 18, 2011 09:16PM
Olav,
Your general comments and observations above are just the kind of thing I think are very desirable for inclusion in the amphabole articles. I think that many collectors, curators and dealers have "given up" on them and I think I am guilty of that as well and that is probably why I am so glad that someone like yourself has stepped up to try and make the situation better even though they may fee somewhat inadequate for the job. Well, I know I am inadequate for the job. The questions I asked reflect, I asked because I would find the answers instructive to myself in trying to fit this vexing group of minerals into the real world. When I think I have started to learn some things about minerals, specimens and the real world, I run into a group of minerals like this one that I have never paid much attention to and am forced yet again that there appear to be more that I don't know about that I do. What ever I can do to encourage you to keep going, I will do.

Rock Currier
Crystals not pistols.
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