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Cummingtonite -series

Posted by Olav Revheim  
Cummingtonite -series
November 04, 2011 07:41AM
    
First Draft

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Cummingtonite: Mg2Mg5Si8O22(OH)2

Manganocummingtonite Mn2Mg5Si8O22(OH)2



Cummingtonite 5,8x5,1 cm specimen, Morro da Mina, Brazil © John Sobolewski


The cummingtonite-series minerals are the Mg dominant end member in a continuous solid solution series with the Fe-dominant grunerite-series minerals, see grunerite-series, and are polymorphs to minerals in the orthorhombic anthophyllite series ( see Anthophyllite series ).

Cummingtonite-series minerasl has been found as lamellar intergrowths with sodic and calcic amphiboles without containing significant Na or Ca in it's own B position, thus indicating a limited solid solution series between these amphibole subgroups. In addtion to Mn and Fe, cummingtonite may also contain significan amounts (>10%wt) Zn in the C position.

Cummiongtonite is, as grunerite, often found in banded iron formations, although in the amphibole Fe content in or near the iron ore itself will normally give a grunerite-series mineral. Cummingtonite may also be found in other stratabound metasedimentary ore formations ( Manganese, Zinc) in addition to some conventional Ca,Na and Al poor schists and gneisses. It may also be found in some eruptive rocks.

Cummingtonite has not always been defined with the chemical and structural boundaries as it has now, and in particular the early litterature distinghuises poorly between cummingtonite, grunerite, tremolite/actinolite and the no discredited dannemorite and tirodite. "Dannemorite" often contains sufficient Fe and Mn to fall within the grunerite boundaries, but may occationally contain sufficient Mg to be a cummingtonite.

"Tirodite" is a discredited name for Mn-rich amphiboles, and the published analytical data for "tirodites" will normally fall in the cummingtonite/manganocummingtonite range. The rare amphibole parvowinchite found in the "tirodite" "type locality", the Tirodi mine in India, has a very untypical chemistry compared to every other "tirodite" analysis I have found published. Calling "tirodite" a synonym of parvowinchite is at best optimistic and would normally be very incorrect. Published anaylsis' of tirodite normally falls within the cummingtonite-manganocummingtonite range, and should be labeled cummingtonite-series unless a chemical analysis proved otherwise.

Like most of the amphiboles, the cummingtonite-series minerals are not found in large, well formed and colorful crystals. It is normally found as beige, greenish or brownish fibrous or bladed aggregates. These aggregates may however become multiple cm in length. The locations described in the text below shiuld be considered typical rather than "best" specimens, as cummingtonite is a common mineral not often collected and photographed.

Cummingtonite
Manganocummingtonite
Brazil
Minas Gerais, Conselheiro Lafaiete (old Queluz de Minas), Morro da Mina Mine

Cummingtonite 4,5 x 3,3 cm specimen© JSS
Manganocummingtonite, FOV 0,9mm © Luigi Mattei

Morro da Mina has been operating since 1894 and has produced some 12 million tons of oxide and silicate ore between 1902 and 1995, it is currently active ( 2011) Cummingtonite from Morro da Mina occurs in metamorphic rocks of Archean age. It occurs in a manganese bearing rocks (Gondites and Queluzites) embedded in metabasaltic and metaultrabasic rocks and crosscut by granitic pegmatites and aplites.

Cummingtonite and Mn-rich cummingtonite can be found with spessartite rhodonite and occationally rhodocrosite in both the primary metamorphic rocks and in hydrothermal veins. I have not been able to find data indicating whether the Mn-rich cummingtonite qualifies as manganocummingtonite or not. The cummintonites occurs primarily as pale asbestiform masses, with the individual fibers reaching several cm. It is also found as isolated needle shaped crystals.

Literature:

Peter Leverett and Peter A Williams and David E Hibbs(2008): CA-MG-FE-RICH RHODONITE from the MORRO DA MINA MINE, CONSELHEIRO LAFAIETE, MINAS GERAIS, BRAZIL. Mineralogical Record


FERNANDO ROBERTO MENDES PIRES(1978)"GEOLOGIA DO DISTRITO MANGANESÍFERO DE CONSELHEIRO LAFAIETE - MINAS GERAIS"

Cummingtonite
Italy
Piedmont , Verbano-Cusio-Ossola Province , Ossola Valley , Antrona Valley, Montescheno, I Mondei (Ca' Mondei)

Cummingtonite 8x6x3 cm © Marco Macchieraldo


Cummingtonite are found in the contact zone between a large pegmatite and the surrounding rocks. The pegmatite was operated for feldspar and mica from its discovery in 1922 to 1962. Cummingtonite occurs as brown lustrous aggregates, that can be multiple cm long, and it can still be found today.

Literature:
AG Reggiani, description of mineralogy and petrology in e “L’Ossola e le sue Valli”, published on valleantrona.com


Manganocummingtonite
Italy
Lombardy, Sondrio Province, Malenco Valley, Scerscen Valley

Manganocummingtonite 4mm FOV © R. Bracco


Manganocummingtonite (described in the literature as Tirodite) in manganese rich dolomite boulders in the Val de Scerscen. Manganocummingtonite occurs with rhodonite, spessartine, manganese oxides and several other minerals

The manganocummingtonite are normally found as silky, fibrous bundles up to 15mm long. The color can range from whitish, to brown but most frequently of a golden color. Sometimes, golden, transperant individual crystals with end terminations can be exposed by removing carbonate with acid.

Literature:
Edoardo Mottarella (2009)- Collezione minerali provincia di Sondrio
Fulvio Grazio (2011)- Istituoto-valltellinese di Mineralogia


Cummingtonite
USA
Massachusetts, Hampshire Co., Cummington

Cummingtonite, 7cm specimen© C. Stefano '08


Cummingtonite was first found near cummington in 1824 and was initially believed to be a mineral in the epidote group:
"I have here given this name to a mineral found by 'Dr. J. Porter in Cummington. It appears to be a variety of epidote. Its color is gray,sometimes with a faint reddish tinge, unless when acted on by the weather, when its color is yellowish. It is in distinct prisms, with oblique seams like' zoisite, and in radiated or fascicled masses, which are composed
of slender prisms” C. Dewey : Geol. Min. Mass.; Am. Jour. Sci., 1st series, Vol. VIII, p, 59.

Cummingtonite was eventually recognized as an amphibole, but in the 19th century, a mineral was defined by different criteria than used today, and minerals like cummingtonite, anthophyllite, grunerite, “hornblende”, “asbestos” and actinolite was not clearly distinguished. Despite the often superb descriptions of physical appearance of minerals described in older publications, it is not always clear which mineral is described.

“Cummingtonite” is a relative common rockforming mineral in some amphibolites and schists the metamorphic Hawley formation, which contains metapelites and metavolcanic rocks of Ordovician and Silurian age and “cummingtoninte can consequently be found in several outcrops in Hampshire and surrounding counties.

“Cummingtonite” can be found as small asbestiform fibres, and as fibrous or bladed aggregates. The finest "cummingtonite” has been found together with garnets in bladed brownish aggregates with the individual blades reaching several inches, and “many of the specimens being elegant and even superb”

Literature:

Benjamin Kendall Emerson(1895): A mineralogical lexicon of Franklin, Hampshire, and Hampen Counties Massachusetts: Bulletin of the U.S Geological Survey No 136.

NORMAN L. HATCH, JR.(1967): Redefinition of the Hawley and Goshen Schists in Western Massachusetts U.S GEOLOGICAL SURVEY BULLETIN 1254-D

Benjamin Kendall Emerson(1898): Geology of Old Hampsire County, Massachusetts comprising Franklin, Hampshire and Hampden counties: Monographs of the United States Geological Survey. VOLUME XXIX


Cummingtonite
USA
New Jersey, Sussex Co., Franklin Mining District, Franklin

Cummingtonite 7 x 5,5 cm tall© Christopher O'Neill


Cummingtonite are known from the Franklin ore bodies where it occurs as an alteration product of pyroxene (Dunn 1995), but also as course grained masses associated with franklinite and witherite in the skarn zones.. These coarse masses are light to medium green and are visually indistinguishable from the more common tremolite/actinolite occurring in more Ca-rich environments in the same rocks.

The cummingtonite from Franklin has been analyzed (Klein & Ito 1968) and is characterized with a high Zn and Mn content, and has previously been called Tirodite. Input of data from Klein and Ito into Esawi’s calculation model, names this amphibole as a zincian-manganoan cummingtonite.

Literature:

Pete J. Dunn(1995): Franklin and Sterling Hill, New Jersey: the world’s most magnificent mineral deposits: Published on www.franklin-sterlinghill.com

Esawi, E.K. (2011): Calculations of amphibole chemical parameters and implementation of the 2004 recommendations of the IMA classification and nomenclature of amphiboles. Journal of Mineralogical and Petrological Sciences 106, 123-129.

Cornelis Klein Jr.(1968): ZINCIAN AND MANGANOAN AMPHIBOLES
FROM FRANKLIN, NEW JERSEY1 THE AMERICAN MINERALOGIST, VOL. 53, JULY.AUGUST, 1968


Cummingtonite
USA
South Dakota, Lawrence Co., Lead District

Cummingtonite © 2008 Peter Cristofono


Cummingtonite can be found in conjunction with a sulphide and gold bearing Early Proterozoic iron formation in a sequenze of carbonates, metapelites and quartz rocks metamorphosed to greenschist / lower amphibolite facies.

Cummingtonite occurred abundantly in the now closed Homestake Mine, which was a significant gold producer from 1876 to 2002.

The occurrences of cummingtonite from the Homestake Mine and from the other locations in the area, is closely related to the ore minerals. The cummingtonite occurs as radiating blades or fibers averaging a
half a centimeter in length, but occasionally extending several centimeters. It is brownish or greenish-gray, and sometimes shows a slightly silky luster. Cummingtonite in the form of asbestos-like material around quartz
has been found; here its fibers may attain several inches in length and area translucent yellowish gray.

Russell G. Wayland (1936): CUMMINGTONITE FROM THE BLACK HILLS, SOUTH DAKOTA :JOURNAL MINERALOGICAL SOCIETY OF AMERICA AM Vol 21-No 9, pp 607

Stanton W. Caddey, Richard L. Bachman, Thomas J. Campbell, Rolland R. Reid and Robert P. Otto (1991): The Homestake Gold Mine, An Early Proterozoic Iron-Formation-Hosted Gold Deposit, Lawrence County, South Dakota: US Geological Survey Bulletin 1857

Olav Revheim Dec 2011

Click here to view Best Minerals C , and here for Best Minerals A to Z and here for Fast Navigation for finished Best Minerals



Edited 7 time(s). Last edit at 03/25/2012 07:24AM by Olav Revheim.
avatar Re: Cummingtonite -series
November 04, 2011 07:42AM
Olav,
Good work!

Rock Currier
Crystals not pistols.
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