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Re: Cordierite

Posted by Olav Revheim  
Re: Cordierite
October 20, 2009 08:07PM
    
This article has been prepared for the Mindat Best Minerals project. The aim of this project is to present information on important localities and specimens for each mineral specie. As new finds are made and new knowledge is made available the individual articles will be revised to include this information. Readers are encouraged to contribute by posting a response in this thread. All revisions will be stored, thus ensuring traceability and availability of previously included information. A complete list of articles can be found in the list of finished Best Minerals articles. To cite this version: Revheim, O. (2016) Cordierite. revision 2.0. Mindat Best Minerals Project, article "mesg-69-156971". Please be advised that the photos cannot be used without the consent of the copyright holder

Cordierite

(Mg,Fe)2Al4Si5O18

Orthorhombic






Cordierite is a relatively common rock forming mineral, listed from 929 localities (January 2016) on Mindat. It is most frequently found in high grade metamorphic rocks rich in Mg and Al. Cordierite is often found in metapelites associated with sillimanite, in cordierite/orthoamphibole rocks originating from basalts hydrothermally altered prior to metamorphosis and in high temperature contact metamorphic rocks such as hornfels. It is also found in some granites that are interpreted as recrystallized melted cordierite-bearing gneisses, or acid lavas chemically influenced by Al-rich sediments. Cordierite is an important mineral for petrologists, who use it to determine the geological history of rocks. In particular, the cordierite/anthophyllite rocks of Orijarvi, Finland have a special historical interest, and Eskola (1914) outlined the theory on metamorphic facies partly based upon studies on these rocks.

Cordierite generally has a low Fe content, but some of the Mg can be substituted with Fe2+, in particular when cordierite is found in igneous rocks. A Fe- analogue of cordierite, sekaninaite, is known but is a relatively rare mineral. The cordierite polymorph indialite is restricted to a few high temperature environments, and the Japanese sakura ishi is a metamorphosed intergrowth of cordierite and indialite found in a hornfels.

For those of us wanting to find or see attractive cordierite mineral specimens, it is easy to become disappointed. Like many minerals predominantly found in metamorphic rocks, large and well formed crystals and crystal clusters are quite rare. Cordierite is also easily altered to chlorites, micas and other minerals, and several varietal names have been used for various pseudomorphs after cordierite, with "pinite" the most commonly used today. Cordierite alters easily because its structure contains large open channels that readily accommodate larger ions and molecules such as water and alkali metals. Experiments have shown that, when exposed to K-bearing water at elevated temperatures, the alteration of cordierite to micas and chlorites is initiated in a matter of hours, explaining why even the best cordierite crystals often have a dull, greenish gray alteration skin hiding the glassy blue interior.

Well formed crystals can be found however, most frequently associated with quartz in coarse grained sections of cordierite-bearing rocks, most commonly available from the east coast of the USA and the southern coast of Norway. At many areas where the geology would appear right for cordierite specimens (such as east Africa, India and the Andes), none or very few crystallized specimens are available on the market. The reason seems to be the relatively unattractive appearance of cordierite crystals compared to other minerals, and because attractive specimens are more valuable as gem rough than as specimens. The localities described in this article are therefore unintentionally and unavoidably biased towards European and North American localities.

Even without crystal shape and with an alteration layer, cordierite can be of interest. It commonly provides clear blue gems, similar looking but less expensive than sapphire and tanzanite. As cordierite is harder than quartz, it is quite suitable as a gemstone, although it has a very distinct pleochroism that must be taken into consideration while cutting the stone. A variety of cordierite, called "blood shot eyes" in the gem trade, hosts inclusions of small reddish hematite crystals and is known from Sri Lanka and Norway. Due to its strong pleochroism, it has been claimed that cordierite was used by the Vikings as a polarizing compass, although there is little factual information supporting this idea.

Most museums hold several cordierite samples, but few have really good display specimens. Several American museum collections include well crystallized specimens from Richmond, New Hampshire. The Canadian province of Manitoba has also yielded some outstanding specimens. Some of the Norwegian museums have good specimens, amongst them the privately owned "Lom Steinsenter".

Well crystallized display-size cordierite specimens can reach retail values exceeding $1,000 USD, whereas cordierite used as a gemstone (often called iolite or water sapphire) is a more affordable alternative to sapphire and tanzanite.

Picking the best minerals and localities for cordierite has been a rather difficult task. It is a very common mineral, but does not often occur in good crystals. If you go for the largest, sharpest and high priced crystals, then Richmond, New Hampsire, USA seems to be high on the list together with a few Norwegian localities. Being a popular gemstone, it is difficult not to include the best gem localities, and the perfect, transparent microcrystals from the Eifel mountains in Germany simply cannot be ignored. It is equally difficult to ignore all the exciting pseudomorphs that can be found. Consequently, the “best mineral” section for cordierite will contain "best minerals" selected according to four different criteria: largest display size crystals, best gems, best microcrystals and best cordierite pseudomorphs.

Cordierite,
Australia,
Northern Territory, Central Desert Region, Harts Range (Harts Ranges; Hartz Range; Hartz Ranges), Entia Valley



Cordierite is found as porphyroblasts in lenses of mica schist in the otherwise quartzo-felspathic Entia gneiss. Dobos (1978) describes one, in his words, a typical 60 m wide and 110 m long mica schist lens near the Valley Bore. He found up to 22 cm wide and 35 cm long cordierite pods with patches of inclusion- and crack-free portions up to 120 cm in volume. Cordierite occurs with gedrite, mica and albite in quartz-rich areas of the mica schist, which also contains areas rich in kyanite and gedrite that yield excellent specimens. Thompson (1986) describes two collecting sites in the Entia valley, where gem grade cordierite could be retrieved by careful digging followed by dry sieving of the loose material. The geology of these deposits aligns well with the description given by Dobos. For further details on the geology, please see Ahmad and Munson (2013) and Arnold et al. (1995). Most of the specimens are cut.


Cordierite,
Austria,
Lower Austria, Waldviertel, Gföhl, Felling


Cordiertite is a rock forming mineral in some horizons of amphibolite-granulite facies metasedimentary gneisses (paragneisses) near Gföhl. These rocks belong to the Varied Groups of the Drosendorf Unit, containing numerous intercalations/small bodies of amphibolites, marbles, calc-silicate rocks, orthogneisses, quartzites, graphitic gneisses and mafic rocks.

The cordierite crystals are found in pegmatites formed by melted Al- and Mg-rich gneisses, and occur as well formed crystals primarily associated with quartz and plagioclase as well as other Al-Mg silicates like dumortierite, andalusite and mica.


Cordierite,
Austria,
Lower Austria, Waldviertel, Gföhl, Hohenstein


See entry above.

Cordierite,
Bolivia,
Potosí Department, Rafael Bustillo Province, Llallagua, Siglo Veinte Mine (Siglo XX Mine; Llallagua Mine; Catavi)

Cordierite, ~7 cm wide

Llallagua, once one of the world's richest tin deposits, is among the most important mineral localities in South America and deserves its own article in Mindat. Cordierite is a very rare mineral in the mine, and it is likely that the photographed specimen was part of a small, isolated and relatively old find.

The Llallagua deposit is hosted in a quartz porphyry emplaced in a vent of a felsic stratovolcano penetrating a series of sedimentary rocks (shale, graywacke and sandstone). This first event was followed by a series of hydrothermal events, including seritication, tourmalinization and silification of the quartz porphyry prior to the deposition of the ores. The cordierite must have been formed as a result of contact metamorphism between the hydrothermal solution and the sediments. Gordon (1944) reports evidence of hydrothermal alteration of the sediments up to 300 m away from the contact between the quartz porphyry and the sediments. Due to the active hydrothermal systems, the cordierite is probably metamorphosed (pinitized).



Cordierite,
Brazil,
Minas Gerais, Doce Valley, Coroaci


Cordierite is a relatively common mineral in metasedimentary rocks (paragneisses) and some granites belonging to the Araguai orogenic belt. Fonseca et al. (2004) provide a good overview of the various events of the Brazilian part of the Brasiliano-Pan-African Orogeny of which the Araguai belt is one part, and Gradim et al. (2014) give a detailed account on the development of what they call cordierite-rich G3 leucogranite, which seems a plausible host rock for these cordierites. The G3 granites occur as small veins and pods and are interpreted as melted and recrystallized peraluminuous paragneisses.

The etched crystals originate from altered pegmatites and surrounding rocks (many of the crystals are real floaters, i.e. no attachment points) and are found in both Coroaci and Virgolândia. The largest crystals can be 4 cm or larger, but most of the available crystals are 2-3 cm.


Cordierite,
Brazil,
Minas Gerais, Doce Valley, Coroaci, Virgolândia

Cordierite, 1.6 cm wide
Cordierite, etched floater 1.5 cm tall
Cordierite, 1.6 cm wide
Cordierite, etched floater 1.5 cm tall
Cordierite, 1.6 cm wide
Cordierite, etched floater 1.5 cm tall

Cordierite is found in the same environment and of similar quality from localities near Virgolândia as described from Coroaci above.


Cordierite,
Canada,
Ontario, Thunder Bay District, Manitouwadge, Geco Mine




The Geco Mine is located near Manitouwadge, north of Lake Superior on the USA - Canadian border. The mine has been run on a large sulphide ore body of vulcanic origin. 55 million tons of ore were produced during the period 1957-1995, mainly chalcopyrite. The host rocks for the ore are metamorphosed to amphibolite facies and have seen Al enrichment and metasomatic mobility of alkali elements, presumably in conjunction with the placement of the ores. Williams et al. (1991) describe the petrology of the rocks associated with the ore, and describes the following units: The Sericite Schist Group, hosting the ore, the Granite Gneiss Group, the Grey Gneiss Group and the Hornblende Schist. Cordierite occurred in the "erratic lenses and pods of medium to coarse grained biotite-anthophyllite-cordierite hornfels" and as a "medium to coarse grained...biotite-anthophyllite (gedrite)-cordierite gneiss".

The coriderite is mostly massive, but well developed, relatively large crystals are known (see photo in Sabina,1991), and faceting grade material is known from the Granite Gneiss. Faceted stones are among the gems in the collections of the National Museum of Canada and the Royal Ontario Museum. Rock Currier (2009) describes the following experiences with cordierites from Geco:

"Many years ago I drove through Manitouwadge after a tip by Charlie Key, who told me this copper mine produced good gem quality cordierite. I located the sample man at the mine and he sold me and later sent me some rather nice pieces of rough gemmy Cordierite, some of which I had cut into small gem stones. They were a little "sleepy" looking. Many years later I was in Elco, Nevada attending an MSHAW training course on mine safety so I could collect underground at Elmwood, Tennessee. That never came to pass, but in that class were an entire range of people who wanted to work in the various gold mines in northern Nevada. Even if you were going to drive a water truck on the surface at a mine site you had to have the course. One of the guys in the class never had much to say. Eventually I got to know him a bit and he had been an underground miner for 20 years but still had to take that dumb class. He got to know me a bit and of my interest in mineral specimens. One day he asked me if I knew a mineral that was white and blue at the same time. I told him that I did not, but did know one that was pale yellow blue. Cordierite. I told him that the best locality for the mineral in North America was the Geco mine. He seemed very surprised and then told me that he had been a miner at the Geco Mine for many years and that they used to get large pieces of the material in the scram. The scram is a bucket device that is pulled back and fourth by a cable to drag and move blasted rocks, in this case to a grizzly. The grizzly he was talking about was a hole covered by thick steel bars that lets fragments below a certain size fall through them into a chute or onto a conveyor belt. I ask him if this was the kind of stuff that would show blue when he put his mine lamp behind the piece and he confirmed that it was. I then ask him how big the pieces got. He told considered for a minute and then pointed to a blue plastic trash can in the corner of the room of about 30 gallon capacity. He told me that he had helped the mine manager build a fireplace in his home from the material. To this day I believe that in one of the modest homes in Manitouwadge is one with a fire place that is worth more in gem grade iolite than the house is worth. I just shook my head and told him that if he had collected a garage full of the stuff that he could have retired on the proceeds. All the material I ever saw from this locality was massive and I never saw or heard of a well formed crystal."


Cordierite,
Canada,
Manitoba, Snow Lake, Stall Lake Copper Mine



Coriderite is found at several localities in the Canadian Province of Manitoba, and Phillips (1978) provides an extensive overview, for which a brief summary is given here:

- Northeast of Sherridon, well formed crystals up to 3 inches were found with gem quality; massive material and lenticular quartz blebs coalesce into an 8-foot, roughly spherical pod.
- Near Utik Lake, up to inch-long crystals were found in an altered and metamorphosed basalt that locally can have up to 70% cordierite.
- Along Birchtree Brook, further north, well formed crystals up to 2 cm can be found associated with garnets in a paragneiss.
- Near Burntwood lake (Map P), cordierite is prominent as outstanding rectangular crystals (2 - 10 cm) on outcrops.of migmatitic greywacke-gneiss.
- Cordierite crystals of unusually good quality originated from a granitic contact on the west shore of Rat Lake. The best cordierite specimens, found within an area of 200 x 90 feet, were from a very coarse cordierite-feldspar rock in which the crystals were from 1/2 inch up to 3 - 4 inches across. These particular outcrops are now largely under water.

Cordierite crystals, up to 0.6 m long and 15 cm in diameter, are reported by Southern (2009) from the Stall mine and among the largest reported. The deep green crystals with chlorite reaction rims were found in the Alteration Zone, associated with the Cu-Zn ore during mining operations. They are from the #4 Orebody, 3341 Stope Alteration Zone of the now defunct Stall Lake Copper Mine.

The Stall Lake Copper Mine produced copper from 1964 to 1994 on a Volcanogenic Massive Sulphide (VMS) hosted in the Anderson meta-rhyolite. It is one of three mined-out Cu-rich deposits (Anderson, Stall and Rod) that originally contained a total of 11.0 Mt with an average grade of 4.27% Cu, 0.56% Zn and 1.5g/t Au.

In the footwall stratigraphy to the VMS deposits, alteration zones represent seawater-related hydrothermal alteration; the rest of these rocks are metamorphosed to the amphibolite facies. These zones are cut by a number of discordant, aluminosilicate-rich alteration zones that can be spatially related to overlying VMS deposits, and it is in these Al-rich alteration zones that the cordierite was found.


Cordierite,
Canada,
Manitoba, Ruttan Lake, Ruttan Mine


The cordierite from here was found in strongly chloritized rocks near the ore bearing formation. The area is metamorphosed to high greenschist, low amphibolite facies.


Cordierite,
France,
Auvergne, Puy-de-Dôme, Menat, Saint-Pardoux



Cordierite pseudomorphosed to pinite was known from the vicinity of Menat since 1805, when well formed crystals with a smooth and slightly shiny surface were described by Drappier. He noted that the crystals were normally regular hexahedral prisms, and more rarely twelve-sided prisms. The terminations were sometimes faceted on each of the base angles. The color was greenish or blackish brown; the blue color shown in the photos above are not representative, as the photo caption describes the color of these crystals as black.

The pinite occurs as phenocrysts in microgranites of Variscian origin. Pinite pseudomorphs after cordierite are also found as irregular patches of up to several cm in gneisses in the area.


Cordierite,
Germany,
Baden-Württemberg Black Forest, Murg Valley, Wickartsmühle Quarry ,
Baden-Württemberg Black Forest, Zastlertal, Zeigerhalde ,

In the southern Black Forest, Baden-Württemberg, Germany, high temperature paragneisses (anatectic, with broad quartz-feldspar leucosomes) occur at several localities. In the Murg Valley (especially at Wickartsmühle Quarry near Rickenbach), at the Zeigerhalde, Zastlertal (Zastler Valley) and Oberried near Freiburg, fresh, blue-grey cordierite occurs in crystals of 2 - 4 cm (Zeigerhalde), being prismatic and showing pseudohexagonal forms. Most of the cordierite in the rock, making up up to 30% of it, is unfortunately weathered to grey clay minerals (Sebastian Möller, 2009).


Cordierite,
Germany,
Rhineland-Palatinate, Eifel Mts ,

The Vulkaneifel lies in western Germany, near the borders with Belgium and Luxembourg. It contains a wealth of different minerals and is one of the world's geological provinces that are richest in mineral species. 168 different localities in the area are recorded in Mindat, listing an astonishing 409 valid minerals, of which 43 were originally described from Eifel.

Volcanism in Eifel is part of a series of intra-plate volcanic fields in central Europe that have been active throughout the Tertiary and Quaternary, and is largely related to rifting of the Rhine Graben and broad uplift of pre-Tertiary basement in the Rhenish Shield. A large number of volcanoes exist in the area, with the Laacher See (13,000 years old) being the youngest. An annual landlift of 1-2 mm shows that the area is still active. Being in the middle of a continential crust, the lava from magma chambers underneath the Eifel penetrate a thick and diversified crust. The rich mineralization is found in rock fragments (xenoliths and ejectas) carried to the surface by the lavas. The contact between the different lavas and a variety of crustal rocks at different depths provide a wide diversity of chemical and PT environments for minerals to form. Most of the minerals occurs as crystals in cavities in such xenoliths.

Cordierite is identified in 22 of the localities, and Caspar Quarry also carries the closely related minerals indialite and ferroindialite. Cordierite occurs in thermo-metamorphic shale or clay xenolites fallen into the basanitic lava flows. It most commonly forms thick tabular or prismatic pseudo-hexagonal crystals with flat end terminations, although other forms are known. Crystals can be more than 0.5 cm and the color of the transparent crystals ranges from colorless to gray and yellow to blue, blue-violet and brown. The opaque brown to red cordierite is colored by inclusions and the interior of such crystals is mostly unaltered and glassy.

Cordierite usually occurs with sanidine, sillimanite and corundum, and other minerals including tridymite, cristobalite, biotite, mullite, osumilite, hematite and pseudobrookite.

Corderite,
Germany,
Rhineland-Palatinate, Eifel, Andernach, Nickenich, Nickenicher Sattel (Eicher Sattel)


This is one of the many Eifel cordierite localities.


Corderite,
Germany,
Rhineland-Palatinate, Eifel Mts, Andernach, Nickenich, Nickenicher Weinberg Mt. (Nickenicher Sattelberg Mt.)

Cordierite, FOV 4 mm
Cordierite, 1.2 mm w/ purple Mullite
Cordierite, FOV 4 mm
Cordierite, 1.2 mm w/ purple Mullite
Cordierite, FOV 4 mm
Cordierite, 1.2 mm w/ purple Mullite

This is one of the many Eifel cordierite localities.


Corderite,
Germany,
Rhineland-Palatinate, Eifel Mts, Mayen, Ettringen, Bellerberg volcano


This is one of the many Eifel cordierite localities.

Corderite,
Germany,
Rhineland-Palatinate, Eifel Mts, Mayen, Ettringen, Bellerberg volcano, Caspar quarry



This is one of the many Eifel cordierite localities.


Cordierite,
Italy,
Lombardy, Sondrio Province, Valchiavenna, Novate Mezzola, Codera Valley, Bresciadega Peak



The rocks of this locality belong to the Gruf complex of the Eastern Pennine Alps. This complex consists predominantly of migmatitic quartzo–feldspathic orthogneisses, paragneisses and biotite–sillimanite–garnet–(±cordierite)-bearing metapelitic amphibolite facies rocks. Cordierite has been found in many of these rocks, also as a retrograde mineral in a UHT granulite facies, sapphirine-bearing rocks. It is most commonly found in plagioclase and quartz-rich bands in metapelites associated with silimanite, but the largest crystals (up to 10 cm) are found associated with anthophyllite in metavolcanics.


Cordierite,
Italy,
Sardinia, Oristano Province, Marrubiu, Mt. Arci, Funtanafigu Quarry,


Cordierite and other minerals such as osumilite, mullite and pseudobrookite are found in cavities in rhyolitic, intermediate and alkaline acid lava flows associated with the volcanism of Mt Arci. The minerals have formed from gas bubbles trapped within the lavas. Cordierite occurs as small, well formed crystals in rhyolite cavities. Even though the crystals interiors are blue, they are often covered with a red alteration patina.

Cordierite,
Italy,
Tuscany, Grosseto Province, Roccastrada, Roccatederighi ,

Cordierite, crystal 3.6 mm
Cordierite crystal 0.5 cm
Cordierite, crystal 3.6 mm
Cordierite crystal 0.5 cm
Cordierite, crystal 3.6 mm
Cordierite crystal 0.5 cm


In the Roccastrada-Roccatederighi area there are occurrences of rhyolitic lava domes and small exogenous extrusions of these magmas. Barsanti (2010) considers the rhyolite domes occurring near (and under) the village of Roccatederighi as "the most famous”. The rock is usually deeply altered, but it is still possible to recognize and find quartz, sanidine (K-feldspar up to 10 cm in size!), biotite and some plagioclase. As accessory minerals in these lavas there are cordierite crystals, occurring as euhedral-subhedral to anhedral crystals, generally not greater than 15 - 20 mm and mostly altered more or less deeply, into pinite which gives to them a greenish - brownish coating. However I have seen specimens with small xls (not more than 4 mm) still “fresh” showing a pale violet color.

"These rhyolitic rocks are very interesting simply for the presence of cordierite, which is an index of a peraluminous character very similar to most monzogranites and the other rhyolites (of the so called "Tuscan Magmatic Province”) all highly Al-rich and suggesting an anatectic origin, for fusion of probably meta-sedimentary previous rocks. "


Cordierite,
Ivory Coast,
"South West towards the Liberia border"


Mouthon (2010) describes cordierite from different associatons from the western/southern part of the Ivory Coast near the border of Liberia:

"- brown, yellow and colorless in clay migmatite,
- deep blue in 'tardimigmatitic' granite, and always with some sillimanite and magnetite in the same area, but not in clay.

Crystals are rare, more often with the blue cordierite. I can find some columnar pieces of brown cordierite, but these are usually altered and disseminated."


The rocks in this area belong to the Man Shield, a sequence of Archean rocks (migmatized meta-arkoses injected by amphibolite-hypersthene-diorite gneisses, acid granulites and by amphibolites and pyroxenites) metamorphosed to amphibolite/granulite facies. Sillimanite, garnet, staurolite and cordierite are typical minerals of the high grade rocks.



Cordierite,
Japan,
Honshu Island, , Kinki Region, Kyoto Prefecture, Kameoka City


Kameoko is the best locality for sakura ishi (cherry blossom stones), which are muscovite pseudomorphs after cordierite. The name "sakura ishi" comes from the six-sided trapiche structure of the crystals, giving a flower-like appearance, some of which are partly or fully replaced by mica. The crystals can be found as porphyroblasts in a fine grained hornfels, and can often be found as complete barrel-shaped crystals weathered free from the host rock. The hornfels is found in the contact zone between mud and chert sediments penetrated by a granite melt 98 million years ago.

The trapiche pattern is formed by a complex intergrowth between cordierite and indialite crystals, where indialite forms the dark "flower core" , and cordierite the "flower leaves". Unaltered crystals can be found near Kyoto in unaltered hornfels.

The crystals can be easily cleaved perpendicular to the long axis to expose the internal hexagonal pattern. The hornfels can be very friable, as are the sakura ishi, which makes it easy to find nice crystal sections but difficult to collect matrix specimens Matrix specimens can be partly stabilized by glue impregnation. Many local collectors paint a solution of warm water and wood glue on specimens, which quickly penetrates the porous hornfels.

Cordierite,
Japan,
Honshu Island, Chubu Region, Yamanashi Prefecture





Cordierite,
Madagascar,
Antananarivo Province, Vakinankaratra Region, Antsirabé 2 District, Ibity Commune, Ibity massif, Ambatomanoana




Cordierite,
Madagascar,
Tuléar (Toliara) Province, Androy, Tsihombe


Although most of the mining for gem cordierite takes place north of Tsihombe, cordierite is a common rock-forming mineral in large parts of eastern Madagascar. This part of the island is a high grade metamorphic terrane, closely related to the Pan-African rocks of southern India, Sri Lanka and east Africa. Dissanayake and Chandrajith (1999) give a thorough overview of the Precambrian metamorphic rocks of Madagascar, and cordierite-sillimanite-garnet gneiss and partly cordierite-bearing granulites are the dominant rocks south of Ihosy.

Ackermand et al. (1991) describe meter-thick cordierite-sillimanite layers in gneisses in the 300 km-long and 20 km-wide Beraketa belt; they use cordieritite as a name for the more cordierite-rich rocks. The Berateka belt is a Pan-African deformation zone, consisting mostly of Mg-Al rich paragneisses (metasedimentary rocks) of amphibolite to (mostly) granulite facies. This belt runs roughly from Ihosy via Beraketa to Tsihombe.

Nicollet (1985) describes cordierite as an important constituent of banded gneiss belonging to the “Ihosy formation”, which covers a geographical area closely corresponding with the Berateka belt. He describes multiple centimeter-long, ultramarine to grey (couleur bleu marine - gris ) cordierite crystals as “abundant” in this gneiss. He describes the crystals as the best crystals on earth ("plus beaux specimens de cordierite du monde"), and states that they are frequently sold as gem rough. Unfortunately, very few of these crystals reach the specimen market.


Corderite,
Norway,
Aust Agder, Akland


Cordierite has been known from Akland a long time. During road construction work in 2002-2003, two cordierite-bearing sections were exposed for a while. In one section, cordierite occurred as large (up to 50 cm) nodules of sky blue cordierite in white quartz. Rounded pseudohexagonal crystals, sometimes fresh and sometimes with a thin light green alteration rim, occasionally could be found. In this section, gem grade material was rare. The other section was a sillimanite-bearing, mica-rich gneiss/micaschist also bearing cordierite nodules. Here the color was more violet blue, and the nodules were frequently altered to a chlorite/mica mixture, but gem sections were not uncommon. Occasionally, small hematite inclusions colored the cordierite brownish red and created a nice "sunstone" effect similar to that observed in feldspar. Large sections of this material combining the brownish red "cordierite sunstone", violet blue cordierite and light green "pinite" have been polished. Setesdalen Mineralpark has a large slap exceeding one square meter on display.

Corderite,
Norway,
Aust-Agder, Risør, Søndeled


Cordierite, 5 cm crystals picture shows from "in situ" to specimen
Cordierite, 5 cm crystals picture shows from "in situ" to specimen
Cordierite, 5 cm crystals picture shows from "in situ" to specimen
Cordierite, 5 cm crystal
Cordierite, 5 cm crystal
Cordierite, 5 cm crystal
Cordierite, 5 cm crystal
Cordierite, 5 cm crystal
Cordierite, 5 cm crystal
Cordierite, 5 cm crystal
Cordierite, 5 cm crystal
Cordierite, 5 cm crystal

The Bamble Sector is a Precambrian high-grade gneissic terrane in a 30 km-wide strip along the coast of southern Norway. The central part, including the Arendal region and off-shore islands, forms a well developed, continuous transition zone from amphibolite to granulite-facies metamorphic grade, which has attracted both intense geological interest as well as historical interest from mining and exploration.

This area is long famous for its beautiful cordierite crystals, often large, idiomorphic and beautiful blue, rarely colorless, violet or black, crystals. The most prominent localities are around Arendal, Tvedestrand, Kragerø, Akland and Brevig. The mineral has been reported under a variety of names like iolite, dichroite, aspasiolite and polychroite. Nijland et al. (1998) list Scheerer (1848), Lacroix (1889) and Hintze (1897a) as examples of early cordierite references, but Holm may have given the first written account as early as 1824.

Cordierite occurs in cordierite-orthoamphibole rocks, in biotite/orthopyroxene-rich rocks which often contain sapphirine and (in some) metapelites. Cordierite-orthoamphibole rocks occur throughout the area as small lenses and boudins. They are a minor lithology by volume, but have been important with respect to unravelling the metamorphic history and sedimentary environment. Different occurrences have been interpreted as either meta-evaporites or as metamorphosed volcanics which underwent alteration prior to metamorphism.

The photos above originate from a discrete find of partly unaltered, sharp euhedral, aquamarine blue-colored cordierite crystals up to 10 cm, discovered during autumn 2012 in the Bamble sector, Norway. The crystals was found embedded in quartz with several other Mg/Al minerals. These specimens are about as good as anyone can imagine a cordierite specimen.

Kihle (1993) describes a single crystal reaching 25 x 10 cm that originated from near Søndeled.


Cordierite,
Norway,
Telemark, Kragerø

Cordierite, 6.0 x 4.5 x 3.6 cm
Cordierite, "Star of Kragero" (10.43 ct)
Cordierite, 6.0 x 4.5 x 3.6 cm
Cordierite, "Star of Kragero" (10.43 ct)
Cordierite, 6.0 x 4.5 x 3.6 cm
Cordierite, "Star of Kragero" (10.43 ct)


Head-sized nodules of nicely colored cordierite are known from outside Kragerø. The crystals sometimes a have very sharp outline, but normally are partly altered to chlorite, muscovite or other minerals. Several good specimens can be found in museum collections and probably also in private collections.

Spencer Ivan Mather (2009) provides the following information: "The cordierite with hematite inclusions can be cut into beautiful gems, these are known as "blood-shot eyes" to a gemmologist and in the gem trade. I have cut many myself, which I found in the Kragero area in the late 1970s." This color effect is due to small copper colored hematite crystals that occur as exclusion lamellae originating from cordierite over-saturated with Fe3+.


Cordierite,
Spain,
Catalonia, Girona (Gerona), Baix Empordà, Mont-ras, Coll de la Boquera



All cordierites from this locality are pseudomorposed. The end product has been called pinite, gigantolite and lately chlorite, and is most likely a mixture of different minerals. The crystal form is well preserved, and the predominantly hexagonal crystals and crystal groups can exceed 5 cm in size.

The cordierite has formed in the high temperature contact between granites and shales together with andalusite, and the largest crystals are found in a pegmatite. It is quite possible that the cordierite and the pseudomorphs formed during the same event.


Cordierite,
Tanzania,
Manyara Region, Babati District, Babati


Gem cordierite is extracted from granulite facies rocks near Babati. The estimated annual production ranges from 145 to 310 kgs. The deposits are not well studied, and cordierite is not a widespread rock-forming mineral as it is in equivalent Pan-African rocks in Madagascar, Sri Lanka and India.

Cordierite,
USA,
Connecticut, Middlesex Co., Portland, Case Quarries



Three Case Quarries were worked for feldspar from pegmatites in the 1930s. They were later prospected for mica (1942) and beryl (1949), but neither of these commodities were produced commercially. The beryl occurs in attractive crystals and is much sought after by collectors. Cordierite, on the other hand, does not belong to the pegmatite paragenesis, but has been found as a curiosity in the the metamorphic rock surrounding the pegmatites . There it was found as dark purple to gray (altered to gray-green on the surface), elongated prisms in a coarse-grained phase (albite, smoky quartz, cordierite, biotite). Crystals to a few cm long have been recovered. Cordierite formation is probably not related to the pegmatite formation, but to the same regional metamorphic processes as the other New Engalnd cordierite localities.


Cordierite,
USA,
Connecticut, New Haven Co., Guilford, Hungry Hill



Cordierite was found as crystals to several cm in a biotite, quartz and plagioclase gneiss; the best ones formed in quartz-rich zones. Some gems have been cut from this material. The locality was first described by Hovey (1888) and crystals from this locality are represented in many museums and university collections on the USA east coast. The locality appears to be lost today.


Cordierite,
USA,
New Hampshire, Cheshire Co, Richmond


Cordierite from the vicinity of the Richmond Soapstone Quarry was first described by Charles T. Jackson (1844): "Antrophyllite [sic] and iolite of great beauty are found. Iolite is a rare mineral, and no other locality in this country furnishes so finely colored specimens as this spot.". He describes the soapstone occurrence as a 42 foot-thick bed of poor quality, and the production did not last long. Jackson lists "pinite" as an accessory mineral from the soapstone horizon. The best cordierite was not found in the Soapstone Quarry itself, but in a coarse quartz cordierite vein a small distance from the quarry. This vein was also known in the 19th century, and Robinson et al. (1986) give a detailed account of this outcrop:

"Outcrop W95 is an area where mineral collectors over the years have done rather extensive excavating and blasting in the search for fresh, blue cordierite which has escaped pinitization. This cordierite occurs as small to large euhedral crystals and continuous reaction rims on quartz veins cutting through coarse gedrite - cordierite - biotite gneiss. One outcrop shows the surface of such a vein for several square meters. This cordierite is generally more Fe-rich than the less obvious Mg-rich cordierite in the matrix of the rocks "

Robinson et al. (1986) also describe other cordierite outcrops from this locality, which they call "iolite hill". Robinson and Jaffe (1969) describe these cordierite bearing rocks as aluminous enclaves. They describe an interesting high-Al mineral assemblage including corundum, dumortierite and other minerals.

The cordierites from here are generally considered the best in the USA, even though the majority of them are "pinitized". Groups of crystals over 30 x 30 cm have been collected. Individual well formed, doubly terminated crystals to 20 x 10 x 10 cm are known. During the heyday of collecting, specimen size was dictated by how large a specimen could be extracted. Exceptional specimens were found by Dr. Warren Johanssen, and Charles Palache found many excellent specimens in the 1940s, as have collectors subsequently. The locality is still viable, although work now would be needed to clear the area. Specimens are difficult to obtain today and development work would be required for success. Most of the photos uploaded to Mindat appear to be of material found in the 1970s.

The Richmond occurrence represents one of many similar cordierite-bearing outcrops in rocks metamorphosed in the Acadian orogeny in New England. Cordierite is a diagnostic mineral in Zone VI of Lenz and Moreira (1999). This zone occurs as relatively small isolated patches over a relatively large area in high grade, sometimes melted and recrystallized, metamorphic rocks (sillimanite facies). They have either a pelite (clay) or altered basalt protolith, the latter with the characteristic presence of orthoamphiboles (anthophyllite/gedrite). A later low grade, retrograde overprint has transformed a cordierite-orthoamphibole rock to "soapstone", and is also the dominant cause for the "pinitization" of cordierite at the Richmond Soapstone quarry.


Cordierite,
USA,
New Hampshire, Grafton Co., Easton, Route 112, Wildwood Campground




A locality where cordierite has been found in similar environments as the other New England localities.



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Hyrsl, J. and Petrov, A. (2006): Famous Mineral Localities: Llallagua, Bolivia. Mineralogical Record, Vol. 37, pp 117-162.

Incani, Stefano. http://www.geomuseomontearci.it/

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Kaindl, R., Többens, D.M. and Haefeker, U. (2011): Quantum-mechanical calculations of the Raman spectra of Mg-and Fe-cordierite. American Mineralogist, Vol. 96, pp 1568-1574.

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RevisionHistory

Revision no date description newest photo no editor
2.0 Jan 2016 Reformatted and rewritten 697019 Olav Revheim
1.0 2010 First Draft Olav Revheim
Reviewed jan 2016 by Becky Coulson

Click here to view Best Minerals C and here for Best Minerals A to Z and here for Fast Navigation of completed Best Minerals articles.



Edited 72 time(s). Last edit at 01/11/2016 01:40PM by Olav Revheim.
avatar Re: Cordierite
December 05, 2009 11:58PM
Bob Southern

Gents
I've attached three photos of some Cordierite. They are from the # 4 Orebody, 3341 Stope Alteration Zone of the now defunct Stall Lake Copper Mine,
in Snow Lake, Manitoba, Canada.
The orebody here was a Volcanic rock associated, Stratabound Massive Sulphide Type Deposit. The mines depth went to over 4000 feet. Cordierite crystals up to 0.6 m long
and 15 cm in diameter had been seen with Chlorite reaction rims in the Alteration Zone during mining operations.
Photo Cordierite 3 - Massive Cordierite with Pyrrhotite, Pyrite, Chalcopyrite rinds. Shows slight crystal form at bottom right & upper middle left. 22 cm x 23 cm x 9 cm
Photo Cordierite 2 - Opposite side of previous.
Photo Cordierite 4 - 4.5 cm x 4.5 cm x 3.5 cm crystal protruding from Pyrite, Pyrrhotite, Chalcopyrite matrix. Specimen size - 13 cm.

Thought I'd share these
Bob

Olav,
I'll look into getting as much info and photos as i can for the Manitoba, Canada occurrences. The Ruttan Mine is close by, and I know some people that worked there also.
I'll set up to upload photos this week.
Cheers
Bob

"If it can't be grown it's gotta be mined. "

Rock Currier
Crystals not pistols.



Edited 1 time(s). Last edit at 12/06/2009 12:00AM by Rock Currier.
avatar Re: Cordierite
December 06, 2009 12:01AM
Philippe M. Belley

Manitoba has some insanely nice/large cordierite crystals. Geco mine, Ontario http://www.mindat.org/loc-6771.html has produced very fine gem material.

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
December 06, 2009 12:02AM
Hello,

As to the Eifel Mts., the Bellerberg Locality is a very good choice. There have been at least two articles in Lapis (in German language), one is about pleochroitic, perfect, but small xls, the other about the locality in general.

The mineral occurs in thermometamorphic shale or clay xenolites fallen into the basanitic lava flows, usually together with sanidine. Other minerals can be sillimanite or corundum.


In the southern Black Forest, Baden-Württemberg, Germany high-temperature paragneisses (anatectic, with broad quartz-feldspar leucosomes) do occur at several localities. In the Murg Valley (especially at Wickartsmühle Quarry near Rickenbach) and at the Zeigerhalde, Zastlertal (Zastler Valley), Oberried near Freiburg blueish grey, fresh cordierite does occur, xls of 2 cm to 4 cm (Zeigerhalde) are known, being prismatic and showing pseudohexagonal forms. Most of the cordierite in the rock, making up up to 30 % of it, is weathered to grey clay minerals unfortunately.

Another German locality is Silberberg Mt., Bodenmais, Bavarian Forest, Bavaria. There metamorphic rocks with pyrrhotite ore bodies do occur, containing cordierite. Some had gem grade iolite.

Regards,
Sebastian Möller

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
December 06, 2009 12:04AM
Olav Revheim

Bob,

This information, as well as the photos, and of course the specimens are absolutely great. Just what we're after. Not only the cordierite, but also the association with the sulfides is amazing. I found a few articles on the general geology and mining history of the area, and I'll fill in a little bit more on the geology in due time.

I have added cordierite to the mineral list for Stall Lake Copper mine, and used your message here as reference for the entry.

I would really appreciate if you can elaborate a little bit on the cordierite finds in this mine.

- Do you if many specimens were saved, or just a few?
- Was cordierite frequently found ? or was this a one off?
- Are these specimens represented in any museums or public displays?


Also I would like to ask for another favor:

Can you upload the photos onto the webpage?

The procedure is as follows:
- Find the location page
- Press the add photo "button"
- Upload the photo from your computer
- Fill in the requested information.

I cannot ( I think) use photos that are attached to messages, only those uploaded to the photo gallery. Also it would be fantastic if you can take a close-up of the crystal in the lower right corner of the specimen pictured in your image "cordierite-3.jpg".



Thanks & regards

Olav

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
December 06, 2009 12:05AM
Bob Southern < PM >
Re: Cordierite
October 25, 2009 10:02PM IP/Host: thsnmb01dc1-200-92.dynamic.mts.net
Registered: 3 months ago
Posts: 9
Olav,
I'll look into getting as much info and photos as i can for the Manitoba, Canada occurrences. The Ruttan Mine is close by, and I know some people that worked there also.
I'll set up to upload photos this week.
Cheers
Bob

"If it can't be grown it's gotta be mined. "

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
December 06, 2009 12:06AM
Olav Revheim < PM >
Re: Cordierite
October 31, 2009 03:13PM IP/Host: cC8264BC1.dhcp.bluecom.no
Registered: 3 years ago
Posts: 115
I am slowly working my way through the cordierite locations in the "best minerals" article and have put together a preliminary draft for the Eifel region in Germany. I'd appreciate any suggestions/corrections to the text as it is now.

Any help will be appreciated.

Thanks

Olav

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
December 06, 2009 12:07AM
Sebastian Möller < PM >
Re: Cordierite
November 14, 2009 04:13PM IP/Host: Q12e2.q.pppool.de
Registered: 1 year ago
Posts: 502
Hello,

As to the Eifel Mts., the Bellerberg Locality is a very good choice. There have been at least two articles in Lapis (in German language), one is about pleochroitic, perfect, but small xls, the other about the locality in general.

The mineral occurs in thermometamorphic shale or clay xenolites fallen into the basanitic lava flows, usually together with sanidine. Other minerals can be sillimanite or corundum.


In the southern Black Forest, Baden-Württemberg, Germany high-temperature paragneisses (anatectic, with broad quartz-feldspar leucosomes) do occur at several localities. In the Murg Valley (especially at Wickartsmühle Quarry near Rickenbach) and at the Zeigerhalde, Zastlertal (Zastler Valley), Oberried near Freiburg blueish grey, fresh cordierite does occur, xls of 2 cm to 4 cm (Zeigerhalde) are known, being prismatic and showing pseudohexagonal forms. Most of the cordierite in the rock, making up up to 30 % of it, is weathered to grey clay minerals unfortunately.

Another German locality is Silberberg Mt., Bodenmais, Bavarian Forest, Bavaria. There metamorphic rocks with pyrrhotite ore bodies do occur, containing cordierite. Some had gem grade iolite.

Regards,
Sebastian Möller

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
December 06, 2009 12:08AM
Peter Haas < PM >
Re: Cordierite
December 01, 2009 05:06PM IP/Host: p54ABC65A.dip.t-dialin.net
Registered: 3 years ago
Posts: 2,098
Rock,

There also are several good cordierite localities in a small area within the French Massif Central. I will ask for help in the French forum.

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
December 06, 2009 12:08AM
Spencer Ivan Mather < PM >
Re: Cordierite
December 02, 2009 12:22PM IP/Host: cpc1-yarm2-0-0-cust652.pete.cable.ntl.com
Registered: 3 months ago
Posts: 72
The cordierite with hematite inclusions that Knut Eldjern is talking about can be cut into beautiful gems, these are known as Blood-shot eye's to a gemmologist and in the gem trade. I have cut many myself which I found in the Kragero area in the late 1970s.

Spencer

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
December 06, 2009 12:09AM
Guenter Blass < PM >
Re: Cordierite
December 02, 2009 01:04PM IP/Host: xdsl-78-35-136-233.netcologne.de
Registered: 9 months ago
Posts: 5
H Rock,

please correct "related minerals sekianinite " to sekaninaite! And indialite is questionable for the volkanic eifel, also from the Bellerberg!

regards
Günter Blass

Rock Currier
Crystals not pistols.
Re: Cordierite
December 06, 2009 04:17PM
I have posted several photos of self-collected cordierite crystals, many from the Richmond, NH locality, on my New Hampshire mineral species web site.
This link will take you directly to the cordierite gallery: http://www.mindatnh.org/Cordierite%20Gallery.html

Tom Mortimer
avatar Re: Cordierite
December 06, 2009 05:07PM
    
Correct locality for cordierites from Madagascar is Tsihombe area. Cordierites occur at big area but from mines around this town comes from majority of gem specimens.

Tom

-------------------------------------
"Spirifer" Geological Society
avatar Re: Cordierite
December 06, 2009 06:21PM
    
Hello,

I've added the Zeigerhalde Locality. The Murg Valley localities I will add the next days, the Eggberg locality, too. These are only a few out of about 50 localities in that area I have read of.

Regards,
Sebastian Möller
avatar Re: Cordierite
July 09, 2010 10:22PM
    
Hi

If i can suggest a locality, Ivory Coast (West Africa) have some cordiérite, brown, blue and colorless.
Rarely crystals, more granular, massive, columnar etc etc.
I find it myself.

I dont know if you are interested by some pictures, just to know that in this part of West Africa, you can find it.

See you.
avatar Re: Cordierite
July 10, 2010 12:13AM
Stephane,

Yes, we would be glad to know about a locality for Cordierite in the Ivory Coast. Can you tell us something about it? Just were in the Ivory Coast is it? Can you tell us something about the geology of the locality? Is the locality worked only for Cordierite or are other minerals found with it? Can you supply some pictures of it and the locality perhaps?

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
July 10, 2010 01:08AM
    
First, sorry for my level, i will try to give 'basic' information.
It's just personal finding, i go to the bush and try to find always some news species, not listed for this country.
For this reason, better to speak about 'indice' than industrial or commercial possibility.
Cordierite area is West/South, near Liberia border.
I find :
_ brown, yellow, colorless in clay migmatite area
_ deep blue in 'tardimigmatitic' granit, and always with some Sillimanite and Magnetite in the same area, but not in clay.

But crystals are rare, more chance with the blue cordierite.
Brown cordierite i can find some columnars pieces, but usually altered and disseminated
in clay in granular or pieces.

1st and 2nd picture, brown/blue , the same stone
3rd, blue/yellow

You can delete the pictures if it's too bad.
Attachments:
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open | download - DSC02875.JPG (36.9 KB)
open | download - DSC02692.JPG (46.2 KB)
avatar Re: Cordierite
July 10, 2010 06:52AM
Stephane,
It certainly looks like you have found Cordierite. More than that it looks like you have found what would be called "gem grade" variety that is called Iolite or sometimes watter sapphire. This is the kind of material that has a high value to those who cut gems, especially if it will cut clean stones larger than more than a few carats each. Well formed crystals of Cordierite are rare from any locality and I don't think I have ever seen a really sharp one. We would like to know if possible exactly where this material comes from, but I should caution you that to publicly give the exact locality here may cause a number of people to immediately go there and try and buy the material, so at least until you know where you stand you might be wise to just give us the name of the region that it comes from. This is the kind of thing that is ideally suitable to small scale mining by the locals and can create mini gem rushes where hundreds of men will flock to the area to try their luck in finding gems. It is possible that this has already happened or is happening? Did you actually dig the specimens you picture or did you buy them from some of the locals who had already dug them and knew something of their value.

It sound as though you have had some training in geology. Is this the case? As far as this web site is concerned, The Ivory Coast is a big blank spot in the map. We have only one specimen shown for the Ivory Coast and that is a diamond with no further locality information than just Ivory Coast. Any information that you can give us or pictures that you can send us would be much better than we have now. If you would like to upload pictures of minerals from the Ivory Coast and or surrounding countries, I will raise your level to a level 1 member of mindat so you can do it yourself. I would encourage you to do this.

Rock Currier
Crystals not pistols.
avatar Re: Cordierite
July 10, 2010 09:17AM
    
All the stones are personal finding (the same for the white fluorescent stones).

Not at all training in geology or gemmology (i m graduate in business shool, not the same training), but when you have a passion, nothing can stop you...and litterature, website and museum can give you enough information if you are really motivated.
I m collector till the age of 14, but now i need to have an Ivorian collection species, and when you find yourself the stones,
i can tell you that is for me a real pleasure, and original collection.

I give information about the area, i can't be more precise.

"I will raise your level to a level 1 member of mindat " that 's really nice, i don't know if i can have this honor.

You can contact me in PM, if you need more informations.

See you.
avatar Re: Cordierite
July 10, 2010 01:01PM
Stephane,
You are now a level one and you can upload pictures and make locality changes in the database. If you need instructions on how to upload pictures let me know and I can tell you how to do it. You should also read the Mindat Manual that you can access from one of the menu buttons on the left side of the Mindat home page.

Rock Currier
Crystals not pistols.
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