Mindat Mineralogy Messageboard - Best Minerals G

Re: Galena

David Von Bargen — Mon, 24 Dec 2012 16:28:49 +0000

See the articles on the fakes
[www.mindat.org]
[www.mindat.org]

Re: Galena

Rock Currier — Sun, 23 Dec 2012 19:43:52 +0000

Yes, there probably should be. Can someone please upload a picture(s) of a know fake?

Re: Galena

Erik Vercammen — Sun, 23 Dec 2012 15:54:09 +0000

Shouldn't there be a mention of the "skeletal galena, that seems to be man-made? There has been quite some discussion about it.

Re: Galena

Rock Currier — Sun, 23 Dec 2012 12:12:55 +0000

A very fine addition!

Re: Galena

Rudolf Hasler — Sun, 23 Dec 2012 11:08:47 +0000

David and Rock,
I have added a very aesthetic Galena specimen to the Bleiberg area.
I hope you don't mind.

Rudolf

Re: Greenockite summit quarry vs summit

John A. Jaszczak — Mon, 10 Dec 2012 22:33:35 +0000

Beth,
It seems likely that Summit Quarry is your locality. Do you know your specimen's provenance? The Edwin Skidmore collected
many specimens, apparently. There are lovely crystals! Do you have access to the references?
John

Re: Greenockite

Everett Harrington — Fri, 30 Nov 2012 18:20:59 +0000

Fluorite with Greenockite xls and Greenockite included in smithsomite psudo casts after calcite Eureka Prospect Marion KY

if you like this photo I can upload to my gallery..

thanks
E

Re: Greenockite summit quarry vs summit

David Von Bargen — Fri, 30 Nov 2012 17:53:37 +0000

It is semiconducting and has been used in photovoltaics.

We don't list manufacturing/processing sites on mindat, only where minerals occur naturally.

Re: Greenockite

Beth Schaefer — Fri, 30 Nov 2012 16:25:53 +0000

I wanted to show the greenockite specimen I have from Bell Labs (NJ). I don't know when it was made (I assume the 1950s) or why Bell Labs was interested in greenockite. As far as I know, it isn't used for any lasers or semiconducting uses. If anyone has a guess what research Bell labs was doing in greenockite please reply!

But It is really pretty! It's about 1 cm long. I compressed it after using helicon software. There are two separate crystals - a flat plate with some thinner tabular crystals attached to it and another set of crystals.

Is there any way to put Bell labs on the location list - I have sapphire and ruby samples from them as well.

Re: Greenockite summit quarry vs summit

Beth Schaefer — Fri, 30 Nov 2012 16:03:55 +0000

I am very interesting in John Jaszczak's post - he wrote:

Transparent dark yellow crystals in prehnite-line vugs. Summit Quarry, Springfield, New Jersey.
Mineralogical Record 4 p. 234; Rocks & Minerals 62 p150; Rocks & Minerals 42 p483

I have a specimen I uploaded to mindat-its location was listed by Knoll as Summit, NJ. It occurred to me that when he wrote it, he meant Summit Quarry, and not just Summit. There isn't a listing of greenockite in plain old Summit, NJ. Does this make sense?

[www.mindat.org]

Re: Glaucophane-series

Olav Revheim — Wed, 27 Jun 2012 06:10:14 +0000

Thanks Erik,

You are right, Also handbook of mineralogy lists Ile d'Croix as a locality for ferroglaucophane, but glaucophane is the most common of the glaucophane minerals there. I have updated the article.

Olav

Re: Glaucophane-series

Erik Vercammen — Tue, 26 Jun 2012 18:36:51 +0000

Ile de Groix in France also has ferroglaucophane. I have 2 pieces, one bought from the late Dave Shannon, one from a friend who was a prof at the University of Leuven in mineral chemistry. I admit I have no analysis.

Re: Galena

Rock Currier — Tue, 05 Jun 2012 07:21:41 +0000

Miroslav and Kevin,
Thanks for bringing these to our attention, however you will have to upload them formerly to the Mindat Gallery before we will consider using them in any of the Best Minerals articles.

Re: Galena

Aleš Tomek — Mon, 04 Jun 2012 14:36:28 +0000

Hello all,

There is only one picture of galena from Czech republic, I would recommend to include also outstanding specimens already in mindat photobase:

[www.mindat.org]


©

[www.mindat.org]


© Weinrich Minerals, Inc.

[www.mindat.org]


© Jakub Jirásek

[www.mindat.org]


© A.Tomek

regards,

Aleš.

Re: Galena

Kevin Farrell — Mon, 04 Jun 2012 13:06:37 +0000

Mogul Mine, Silvermines District, Co. Tipperary, Ireland
2.9 x 2.4 x 1 cm

Re: Galena

Miroslav Gulika (2) — Mon, 04 Jun 2012 09:38:47 +0000

Stříbro, / Mies /, Plzeň Region, Bohemia, Czech Republic

Historical / 1872 - 1874 / specimen size 12 x 8 cm, crystal 8 x 7 cm, crystal cubo - octahedral / cubo 4cm /

Glaucophane-series

Olav Revheim — Thu, 24 May 2012 08:38:14 +0000

First Draft

Click here to view Best Minerals G , and here for Best Minerals A to Z and here for Fast Navigation for finished Best Minerals articles.

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 glaucophane 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:

Ferroglaucophane
[ ]Na₂(Fe₃Al₂)Si₈O₂₂(OH)₂

Glaucophane
[ ]Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂


Glaucophane 8mm crystals	© DSW 2010

Glaucophane and Ferroglaucophane are typical minerals of high pressure, low temperature metamorphosed rocks. Such rocks are typical of subduction zones where continental and oceanic crust are pushed deep into the earth's crust relatively quickly ( in geologic terms, that is). If the subducted rocks stay deep in the crust, the P/T conditions will normalize and the glaucophane bearing rocks will be further metamorphosed to greenschist/amphibolite facies rocks, in which calcic amphiboles ( compositions from actinolite to pargasite) will be stable. Therefore, glaucophane bearing rocks found on surface is indicative of a rapid subduction into the crust and then an equally rapid return to the surface (again, rapid in geological terms). Many of the localities described here show a gradual transformation from glaucophane schists to caclic amphiboles as the main amphiboles, and in some of these rocks exoctic intemediate ambiboles such as barroisites and winchites can be found.

There is a continuous series between glaucophane and ferroglaucophane, with glaucophane as the most common mineral. The glaucophane series often also has a riebeckite (sometimes also an arfvedsonite) component, thus forming a series towards riebeckite. Crossite is a discontinued name for an intermediate composition between glaucophane and riebeckite. There is also a series between glaucophane via sodic-calcic amphiboles towards calcic amphiboles of actinolitic to pargasitic composition.

Glaucphane does not form large, free standing crystals, and do not offer mineral collectors anyhing but a nice blue color.

Glaucophane
Ferroglaucophane
France
Brittany, Morbihan, Groix, Groix Island


Glaucophane 7 cm specimen	© Brhounds


Glaucophane 5 cm FOV (in situ)	© P Le Roc'h

The Ile de Groix is located 10 km from Lorient (southern coast of Brittany) and represents, one of the few occurences of
blueschists from the Variscan orogony. This locality has been known since the 19th century and has been studied extensively by petrologists. Glaucophane has formed from high pressure/low temperature metamorphosed basaltic rocks. The glaucophane bearing blueschists occurs as either small (1–5 m) lenses (boudinaged layers or isoclinal folds) or thick layers (up to~50 metres) embedded in metasedimentary micaschists. Interlayering of micaschists and blueschists is observed at varying scales, with minor layers no less than a few centimetres in thickness. Glaucophane can occure in almost monomineralic bands in these rocks, but according to Bonney(1887), the individul glaucophane fibres rarely exceeds 1 cm in length. Amongst the metabasites, blueschists are largely predominant in the eastern part of the island, whereas greenschists predominate in the western part of the island.

The glaucophane may contain sufficient Fe to qualify as ferroglaucophane, but it should be noted that glaucophane with Mg>Fe is more common.

Literature:

P T. G. Bonney (1887): Note on Some Specimens of Glaucophane-Rock from the Ile de Groix, Mineralogical Magazine volume 7 pp 150-154.

M. Ballevre, P. Pitra, M. Bohn (2003): Lawsonite growth in the epidote blueschists from the Ile de Groix (Armorican Massif, France): a potential geobarometer, Journal of Metamorphic Geology, Volume 21, Number 7

D. Proust (1985): Amphibole weathering in a glaucophane schist ( Ile the Groix, Morbihan, France), Clay Minerals vol 20,pp 161-170

France
Brittany, Morbihan, Groix, Groix Island, Porh Morvil creek


Glaucophane 11 cm specimen	© Arliguie M

This is one of the many glaucophane oputcrops on Ile de Groix.

Glaucophane
Greece
Aegean Islands (Aiyaíon) Department, Kykládes Prefecture, Cyclade Islands (Cyclades; Kikladhes; Nomos Kikladhon), Syros Island (Syra), Grammata Bay, Cape Marmari


Glaucophane 2 cm specimen	© Van King

Syros is one of the Cycladic islands. It contains high pressure metamorphic rocks, which represents the crustal root of the Cycladic orogenic belt formed during a Mesozoic Eurasia-Africa subduction. Glaucophane is commonly found in blueschists and some marbles on the island. The glaucophane is normally somewhat ferrous, but rarely, if at all, with a Fe/Mg ratio >1, in which case the amphibole would be a ferroglaucophane. The published analysis are all well within the glaucophane area, with only minor riebeckite or arfvedsonite components.

The blueschists is formed at pressures around 14kb and 450-500 deg C from gabbros and seabed basalts. Surprisingly, the grain size of the original igneous rocks has survived the high pressure and complete change of minerals, so that the largest individual glaucophane grains (>1cm) can be found in metagabbros. Marmari bay is one of many bluescist outcrops on the island, Lazarus (2004) provides excellent field descriptions.

The occurrence of glaucophane-bearing marbles on Syros is noteworthy because marbles that contain glaucophane are rare, but the less than 2mm long individual fibres will not make collectable mineral specimens.

Literature:

Gideon Rosenbaum, Dov Avigad, Mario Sanchez-Gomez (2002): Coaxial Flattening at deep levels of orogenic belts: evidence from blueschists and eclogites on Syros and Sifnos (Cyclades, Greece), Journal of Structural Geology, Volume 24, pp1451-1462

John C. Schumacher, John B. Brady, John T. Cheney, Robert R. Tonnsen (2006): Glaucophane-bearing marbles on Syros, Journal of Petrology, Volume 49, Number 9, pp 1667-1686.

John T. Cheney, John T. Brady, Michele J. Markley, John C. Schumacher (2000): Evolution of cycladic subduction zone rocks: Syros Blueschist-Eclogite terrane II, Keck Symposium Paper

Eli D. Lazarus (2004): Characterization of high-angle faults on the island of Syros, Greece, Thesis, Williams College, Williamstown Massachussetts.

Ferroglaucophane
Glaucophane
Italy
Aosta Valley, Champdepraz, Herin mine


Ferroglaucophane up to 2,6cm crystals	© G.Fraccaro

In Italy, glaucophane is predominantly found in the Western Alps, where high and ultra high pressure rocks are found in a semi-circular arc starting at Genoa and following the southern part of the Alps to the Monte Rosa Massif north-north-east of Torino. These rocks where metamorphosed to bluescist and eclogite facies during the subduction of continental and ocean bed crust prior to the onset of the Eurasia/Africa collision some 100-35 mill years ago. The rocks show a wide range of both seabed and continental origin. Some of these rocks carry glaucophane and there must hundreds of localities spread over the area.

The alpine landscape with mountain sides crosscutting large sections of crust and not much vegetative cover has attracted petrologists and prospectors for hundreds of years. There is consequently a large volumen of information available, and I have found Compagnoni(2003) and Agard et al (2009) most useful in presenting an overview of the area.

The Herin mine has probably been worked for chalkopyrite since the antique, and the earlies written documentation on mining activity is from around 700. The productivity from the mine has been low, partly because of poor extraction techniques and disagreement on ownerships. Industrial production did not start until the 20th century, and production ceased in 1957. The copper mineralization is interpreted to originate from hydrothermal seabed vents (black smokers) on the ancient seabed later metamotphosed to glaucophane bearing schists and eclogites.

The dumps are still available for collectors. I have not been able to find out how common glaucophane bearing rocks are at Herin, or how good the specimens get. It should be fair to assume that the pictured specimen is above average.

Herin is the type locality for ferroglaucophane, but the Mg/Fe ratio is variable, so glaucophane specimens from here should probably be labeled glaucophane-ferroglaucophane series unless an anlysis of the individual specimen shows a Mg/Fe ratio <1 for large portions of the specimen.

Literature:

www.geosearch.it

P. Agard, P. Yamato, L. Jolivet, E. Burov (2009): Exhumation of oceanic blueschists and eclogites in subduction zones:
Timing and mechanisms, Earth-Science Reviews 92 pp 53–79

COMPAGNONI, R. (2003): HP metamorphic belt of the western Alps. - EPISODES 26, 3, 200-204.

Glaucophane
Italy
Aosta Valley, Montjovet


Glaucophane up to 7,6cm specimen	© Jasun McAvoy


Glaucophane	© Jasun McAvoy

Montjovet is not mentioned specifically in the literature as a glaucophane locality, but it is very likely that glaucophane bearing rocks belong to the Sesia zone are found in the vicinity of Montjovet as the Aosta Valley has been a source for glaucophane for a long a time, both from eclogites, blueschists and "glaucophanites". The following description by TG Bonney (1886) is characterisitic for the occurance of glaucophane in eclogites:

"...my eye fell upon a rock, which I at once recognised as a variety of eclogite, composed chiefly of small reddish garnets and dull-green hornblende. We secured specimens, though with considerable difficulty, for the rock was very totlgh ; made a few observations and passed on.Next morning, as I was preparing to pack up my specimen, I noticed a
peculiar violet-blue tint on its surface, which in certain positions became very marked....I at once felt certain that the tint indicated the presence of glaucophane, and on my return to England lost no time in having slices prepared for microscopic examination, which showed that mineral to be present in no small quantity."

The richest occurances of glaucophane are found in "glaucophanites", which are metamorphosed amphibolites. In these rocks, deep blue glaucophane is the dominant mineral, often with embedded crystals of garnet (up to 5mm) and lawsonite pseudomorphs ( up to 2cm). In the contact zone between glaucophanites and talcschists, glaucophane occurs "in abundance". Although abundant, the individual galucophane crystals does not reach any large size, typically the individual fibres are less than 1 cm.

The chemistry of the glaucophanes from the Western italian alps varies between and within the rock it occurs in. There are two compositional series that dominates; the glaucophane -ferroglaucophane series and the glaucophane-riebeckite series. For both these series, the glaucophane component dominates, making glaucophane proper the most common mineral, even for the crossites of older literature.

Literature:

T. G. BONNEY (1886): On a Glaucophane.eclogite from the Val d'Aoste.The Mineralogical magazine and Journal of the Mineralogical Society.

S. Martin, G. Godard, G. Rebay (2004): The subducted Tethys in the Aosta Valley (Italian western Alps) The 32nd international geological Congress, Field trip Guide Book B-02

Roberto Compangnoni (1977): The Sesia-Lanzo zone. High pressure low temperature metamorphism in the austroalpin continental margin. Societa Italiana di mineralogia e petrologia, Vol 33 (1) pp 335-374

W. G. Ernst (1981): Petrogenesis of eclogites and peridotites from the Western and Ligurian Alps, American Mineralogist, Volume 66, pages 443-472, 1981.

Glaucophane
Italy
Piedmont, Biella Province, Oropa


Glaucophane 18mm FOV	© Enrico Bonacina

Several outcrops of glaucophane bearing rocks belonging to the Sesia Zone can be found in the slopes of Monte Mucrone.

Literature:

Roberto Compacnoni, Giorgio V. Dal Piaz, Laura Fiora, Guido Gosso, Bruno Lombardo, Brunello Maffeo, Paul F. Willams (1977), EXCURSION TO THE SESIA-LANZO ZONE AND VALTOURNANCHE METAMORPHIC OPHIOLlTES
GUIDE BOOK, Societa Italiana di Mineralogia et Petrologia, vol 33 (1) pp 473-491

Glaucophane
Italy
Piedmont, Biella Province, Pollone, Chiavolino, Rio Oremo


Glaucophane 2mm FOV	© C.Boutry


Glaucophane 5mm crystals	© Antonio Borrelli


Glaucophane 7cm specimen	© Arliguie M


Glaucophane 7,3 cm specimen	© David J. Eicher

Glaucophane is found in a small outcrop near the path from an abandoned quarry along the Rio Oremo. Glaucophane can be found in up to 1 cm crystals with green fengite mica. carbonates and pyrite occurs as accessory minerals.

Literature:

Gabriella Porta and Massimo Biasetti (2003): Relazione geologica, Plano regulatore generale, Comune di Pollone

Italy
Piedmont, Torino Province, Canavese District, Quincinetto, Bric Vert


Glaucophane 1 cm crystals	© GiovanniFraccaro

Glaucophane is found in phengite, quartz, glaucophane and garnet rocks metamorphosed to eclogite facies belonging to the Sesia zone.

Literature:

Mark R. Handy, Matthias Konrad‐Schmolke, Roberto Companogni, Marco Beltrando (2009): AlpShop09 Field Trip to the Sesia Zone.

USA
California, Contra Costa Co, Diablo Range, Clayton, Mount Diablo State Park, Mount Diablo (Mt Diablo)


Glaucophane 5 cm specimen	©

All the Californian glaucophane localities are found in the Franciscan Complex, a late Jurassic to Cretaceous subducion block metamorphiosed under high pressure and low temperature. The Franciscan terrain is typified by a chaotic mélange of sedimentary and other crustal rocks as well as rocks originating from an oceanic crust. The terrain are much studied by petrologists, and a wide array of publications are available.

Glaucophane are present in both metagraywackes, metabastaltic and also other rocks. In some rocks it is the dominant mineral.This is also the case for the so called “exotic blocks” of glaucophane schists that can be found here. They are metamorphsed basalts dominated by fine grained glaucophane, giving the rock a typical blue color.

Literature:

Lawson, A.C., 1914, Description of the San Francisco district: Tamalpais, San Francisco, Concord, San Mateo, and Hayward quadrangles: U.S. Geological Survey Geological Atlas, Folio 193,
scale 1:62,500.

Mount Diablo Interpretive Association, Guide to the geology of Mount Diablo State Paerk www.mdia.org

USA
California, Marin Co, Tiburon Peninsula


Glaucophane 11,4 cm specimen	© Rob Lavinsky


Glaucophane 11,4 cm specimen	© Rob Lavinsky

The glaucophane schist area on the Tiburon peninsula is about two hundred yards wide, and extends more or less
continuously from Tiburon to north of Reed's station. Northeast of the schist is serpentine which caps the hills; and southwest are sandstones and shales. The glaucophane occurs with omphacite in eclogite facies rocks, in which glaucophane locally is the dominant mineral.

Literature:

Ruliff S. Holway(1904): Eclogites in California, The Journal of Geology, Vol. 12, No. 4 pp. 344-358

Edward Hoit Nutter and William Burton Barber(1902): On Some Glaucophane and Associated Schists in the Coast Ranges of California, The Journal of Geology, Vol. 10, No. 7 pp. 738-744

USA
California, Sonoma Co, Coast Range , Cazadero , Ward Creek (1 )


Glaucophane 2,5mm FOV	© 2008 Jesse Crawford

Erickson (1995) gives a detailed summary of field observations, petrology, metamorphic history and mineral isogrades in the Ward Creek area. Glaucophane is present in the type II, III, and IV metabasalts which are divided based on metamorphic grade, with the eclogite-facies type 4 as the highest grade. It is the dominant mineral in "coarse" grained (>1mm) phyllitic or schistose rocks of a characterisitic blue color, but the "largest" crystal probably occur in the type IV rocks.

Literature:

Rolfe C. Erickson (1995): The geology of the Franciscan Complex in the Ward Creek-Cazadero area, Sonoma County, California. California Geology, 1995-11

USA
California, Sonoma Co, Valley Ford


Glaucophane 8mm crystals	© DSW 2010

Blocks of blueschist occur in a random fashion along Ebabias Creak near Valley Ford. These blocks of coarsely crystalline schist resemble the Type IV of Ward Creek, but deviates from these rocks in that it lacks the typical foliation. The rock is dominated by pale green jadeite and blue glaucophane. The glaucophane occurs as groups of radiating crystals, with each individual crystal rarely longer than 5mm.

Literature:

Terry E.C.Keith, Robert G. Coleman (1968): Albite-pyroxene-glaucophane schist from Vally Ford, California, US Geological Survey Professional Paper 600C, Pages C13-C17

USA
Washington, Skagit Co. Rockport,Skagit River


Glaucophane 13 cm specimen	©

This specific specimen was found in Skagit River alluvials, and consequently it is hard to pin down the exact origin of the specimen, Glaucophane bearing schists have been found newr the contact of a serpentinite bodies belonging to the Shuksan suite. These rocks are methamorposed up to eclogite facies. The green mineral associating the glaucophane is probably omphacite.

Literature: E.H.Brown(1989): Geology of the Skagit Crystalline Core and the Northwest Cascades System, Marblemount Area, Northwest Geological Society Field Trip.

Olav Revheim June 2012

Click here to view Best Minerals G , and here for Best Minerals A to Z and here for Fast Navigation for finished Best Minerals articles.

Re: Galena

Rudolf Hasler — Mon, 14 May 2012 11:22:04 +0000

David,

I have posted 11 images of Galena specimens of the Bleiberg district in Carinthia, Austria. If you have a look at them in my home page you might find some useful ones for your article. Of course none of them can compare with Simone's.

Rudolf

Re: Goethite

Rock Currier — Sat, 04 Feb 2012 07:31:32 +0000

George,
You question about the cause of color banding in massive goethite specimens would probably draw more attention in one of the other forums rather than here in the Best Minerals section, but you may get lucky and someone will respond to with the information you want. I don't know the answer, but I would expect that the difference would be found in the variable mineralogy of fine grained iron minerals that undoubtedly comprise the bulk of these specimens. Perhaps some grad student has made a study of them.

Re: Goethite

George Balogh — Tue, 31 Jan 2012 03:54:57 +0000

Interesting selection of specimens. I love the iron oxide minerals but
often find that sellers do not check their material even with simple streak
test to distinguish what is likely hematite from goethite. For that matter the
manganese oxides with their gray streak also come into play here. But then
as I think Alfredo points out it may take formal testing to be certain
regarding some specimens. There are also pieces that vary in
streak depending on what part of the rock you check.

Shifting topics here, does anyone know the explanation for the color bands in some
massive goethite specimens? It is common in the Great Lakes Iron Ranges of North
America. In the posted images under "Best Minerals" there are examples from Marquette Co,
Michigan and Wisconsin as well as the Minnesota specimen. Sometimes the
banding is very regular, almost like tree rings and assume the solution from which
crystals form must oscillate in composition. Has anyone seen fine banding in micro
xls of goethite such as occurs in some linear malachite xls?

Thanks for your thoughts,

George

Re: Gedrite series

Rock Currier — Sun, 18 Dec 2011 21:16:35 +0000

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.

Re: Gedrite series

D Mike Reinke — Wed, 14 Dec 2011 21:50:45 +0000

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

Re: Gedrite series

Olav Revheim — Wed, 14 Dec 2011 20:00:58 +0000

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 :-)

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.

:-)

Olav

Re: Gedrite series

Rock Currier — Wed, 14 Dec 2011 06:08:02 +0000

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?

Gedrite series

Olav Revheim — Sun, 11 Dec 2011 20:02:53 +0000

First Draft

Click here to view Best Minerals G , and here for Best Minerals A to Z and here for Fast Navigation for finished Best Minerals articles.

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

Click here to view Best Minerals G , and here for Best Minerals A to Z and here for Fast Navigation for finished Best Minerals articles.

Re: Grunerite-series

Rock Currier — Thu, 13 Oct 2011 12:57:11 +0000

Olav,
I agree, doing the best minerals articles is a great learning experience.

Re: Grunerite-series

Olav Revheim — Wed, 12 Oct 2011 19:03:16 +0000

Rock,

When I started to do the amphibole group just over a year ago, I didn't think it was going to be pretty. >:D<

On the other hand I must admit that there are more nice looking amphiboles out there than I had ever believed, and a year into my amphibole project I think that I am slowly getting to understand and appreciate the diversity and complexity of this particular group of minerals.

In doing so I am, one small step at the time, achieving my learning objective for doing this.

(:P)

Olav

Re: Grunerite-series

Maggie Wilson — Wed, 12 Oct 2011 18:57:56 +0000

Hi Olav - glad I could help.

Thanks for the reference lead. We live about 10 - 12 hours away by car (with several comfort breaks on the way!) - hope to make a trip to the region next summer!

Maggie

Re: Grunerite-series

Olav Revheim — Wed, 12 Oct 2011 18:44:50 +0000

Maggie,

Thank you very much for your input. Knowing how these amphibole entries are identified are very importnt to me, as they are so horribly difficult to distinguish from each other. The combination of your input and the literature references should, I think, ID these specimens "beyond reasonable doubt".

The literature references might, by the way, be worthwhile to download, especially the 756 pages volume: "The geology of the Lake Superior region" should be interesting to browse through if you do not live too far away from the region.

(:P)

Olav

Re: Grunerite-series

Maggie Wilson — Wed, 12 Oct 2011 11:20:07 +0000

Hello Olav - the material from Wawa Ontario was acquired from an Ontario dealer in 2010 - at the time he advised that the grunerite had been ID'd by EDS and that it was from "a one-time find".

Hope this helps
Maggie