Cummingtonite
1250419
226132955
2008-07-16T23:32:15Z
Vsmith
84417
[[WP:UNDO|Undid]] revision 226079967 by [[Special:Contributions/80.42.123.225|80.42.123.225]] ([[User talk:80.42.123.225|talk]])
{{Infobox mineral
| name = Cummingtonite
| category =
| boxwidth =
| boxbgcolor =
| image = Amphibole - Cummingtonite w- chlorite in schist Magnesium iron silicate 3800 foot level Homestake Mine Lawrence COunty South Dakota 2071.jpg
| caption =
| formula = ([[Magnesium|Mg]],[[Iron|Fe]])<sub>7</sub>[[Silicon|Si]]<sub>8</sub>[[Oxygen|O]]<sub>22</sub>([[Hydroxyl|OH]])<sub>2</sub>
| molweight =
| color = Dark green - brown
| habit = Rarely as distinct crystals. Columnar to fibrous and granular
| system = [[Monoclinic]]; 2/m
| twinning = Simple and lamellar - common
| cleavage = 56 and 124° - good; {110} perfect
| fracture = Splintery
| mohs = 5 - 6
| luster = Vitreous - Silky
| refractive = nα=1.65 - 1.69, nβ=1.67 - 1.671, nγ=1.69 - 1.73
| opticalprop = Optically (+)
| birefringence =
| pleochroism = Non-pleochroic
| streak =
| gravity = 3.1 - 3.6
| density =
| melt =
| fusibility =
| diagnostic = Characterized by light brown color and needlelike, often radiating habit. difficult to distinguish from anthophyllite or gedrite without optical and/or X-ray tests.
| solubility =
| diaphaneity = Translucent, will transmit light on thin edges.
| other =
}}
'''Cummingtonite''' or '''magnesium iron silicate hydroxide''' is a metamorphic [[amphibole]] with the [[chemical composition]] ([[Magnesium|Mg]],[[Iron|Fe]])<sub>7</sub>[[Silicon|Si]]<sub>8</sub>[[Oxygen|O]]<sub>22</sub>([[Hydroxyl|OH]])<sub>2</sub>.
Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic [[anthophyllite]], which is a much more common form of [[magnesium]]-rich amphibole, the latter being [[Metastability in molecules|metastable]].
Cummingtonite shares few compositional similarities with [[alkali]] amphiboles such as [[arfvedsonite]], [[glaucophane]]-[[riebeckite]]. There is little solubility between these minerals due to different [[crystal habit]] and inability of substitution between alkali elements and ferro-magnesian elements within the amphibole structure.
== Chemistry ==
Cummingtonite is a member of the cummingtonite-[[grunerite]] [[solid solution]] series which ranges from Fe<sub>2</sub>Mg<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub> for cummingtonite to the iron rich grunerite endmember Fe<sub>7</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>. Cummingtonite is used to describe minerals of this formula with between 30 and 70 per cent Fe<sub>7</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>.
[[Manganese]] also substitutes for (Fe,Mg) within cummingtonite amphibole, replacing B site atoms. These minerals are found in high-grade metamorphic [[banded iron formation]] and form a compositional series between [[manganese|Mn]]<sub>2</sub>Mg<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub> (tirodite) and Mn<sub>2</sub>Fe<sub>5</sub>Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub> (dannemorite).
[[Calcium]], [[sodium]] and [[potassium]] concentrations in cummingtonite are low. Cummingtonite tends toward more [[calcium|Ca]] substitution than related [[anthophyllite]]. Similarly, cummingtonite has lower Fe<sup>3+</sup> and [[aluminum|Al]] than anthophyllite.
[[Amosite]] is a rare [[asbestos|asbestiform]] variety of grunerite that is mined as asbestos only in the eastern part of the [[Transvaal]] Province of [[South Africa]]. The origin of the name is ''Amosa'', the acronym for the mining company "Asbestos Mines of South Africa."
Cummingtonite was first described in 1824 for an occurrence near [[Cummington, Massachusetts]], hence the name.<ref>http://rruff.geo.arizona.edu/doclib/hom/cummingtonite.pdf Handbook of Mineralogy, 2001, Mineral Data Pub.</ref><ref>http://www.mindat.org/min-1170.html Mindat data</ref> It is also found in [[Sweden]], [[South Africa]], and [[Scotland]].
==Occurrence==
Cummingtonite is commonly found in metamorphosed magnesium-rich rocks and occurs in amphibolites. Usually it coexists with [[hornblende]] or [[actinolite]], magnesium clinochlore [[Chlorite group|chlorite]], [[talc]], [[serpentine]]-[[antigorite]] minerals or metamorphic [[pyroxene]]. Magnesium-rich cummingtonite can also coexist with [[anthophyllite]].
Cummingtonite has also been found in some [[felsic]] [[igneous rocks]] such as [[dacite]]s. [[Manganese]] rich species can be found in metamorphosed Mn-rich rock units. The [[grunerite]] end member is characteristic of the metamorphosed [[Banded iron formation|iron formations]] of the [[Lake Superior]] region and the [[Labrador Trough]]. With prograde metamorphism cummingtonite and grunerite morph to members of the [[olivine]] and pyroxene series.
==References and footnotes==
{{reflist}}
* Deer W.A., Howie R.A., & Zussman J. ''An Introduction to the Rock Forming Minerals'', 2nd Edition, 1997, pp. 229-247.
* Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., Wiley, ISBN 0-471-80580-7
* Klein, Cornelius., 2002, ''The Manual of Mineral Science'', 22nd ed., John Wiley & Sons, Inc. ISBN 0-471-25177-1
*[http://mineral.galleries.com/minerals/silicate/cummingt/cummingt.htm Mineral Galleries]
*[http://webmineral.com/data/Cummingtonite.shtml Webmineral data]
==External links==
*[http://www.chm.bris.ac.uk/sillymolecules/sillymols.htm Molecules with Silly or Unusual Names]
[[Category:Inosilicates]]
[[Category:Magnesium minerals]]
[[Category:Iron minerals]]
[[Category:Hydroxide minerals]]
[[Category:Asbestos]]
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