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Posted by Olav Revheim
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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 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:
Glaucophane < >Na2(Mg3Al2)Si8O22(OH)2
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.
Ferroglaucophane France Brittany, Morbihan, Groix, Groix Island
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.
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
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
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.
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.
Glaucophane Italy Aosta Valley, Champdepraz, Herin mine
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.
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
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.
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
Several outcrops of glaucophane bearing rocks belonging to the Sesia Zone can be found in the slopes of Monte Mucrone.
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 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.
Gabriella Porta and Massimo Biasetti (2003): Relazione geologica, Plano regulatore generale, Comune di Pollone
Italy Piedmont, Torino Province, Canavese District, Quincinetto, Bric Vert
Glaucophane is found in phengite, quartz, glaucophane and garnet rocks metamorphosed to eclogite facies belonging to the Sesia zone.
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)
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.
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,
Mount Diablo Interpretive Association, Guide to the geology of Mount Diablo State Paerk www.mdia.org
USA California, Marin Co, Tiburon Peninsula
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.
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 )
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.
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
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.
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
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.
Edited 13 time(s). Last edit at 06/27/2012 08:35AM by Olav Revheim.