Cumberlandite

Cumberlandite is the U.S. state of Rhode Island's state rock. It is only found in large concentrations on a 4-acre (16,000 m2) lot in Blackstone Valley, Cumberland. Cumberlandite has been known for 250 years or more. The peculiar character of the rock and the possibility of its economic use as an iron ore have attracted the attention of many people. As a result, many articles have been written about the rock.

During geological surveys in the mid 1800s, however, it became apparent that the rock was quite rare even within Rode Islands. In fact, the rock was only found within a 4-acre parcel of land on Iron Mine Hill in Cumberland, not too far from the Woonsocket border. The distinguished geologist M.E. Wadsworth described the rock in several papers and proposed renaming the stone cumberlandite in 1884.

Cumberlandite is an, medium-grained, porphyritic dunite to melatroctolite composed of white or greenish platy crystals (originally of labradorite) as much as half an inch long. The matrix is fine grained to medium grained, and in places the platy crystals are aligned, probably as a result of flow. The rock is very heavy and is also weakly magnetic. The fresh rock consists of magnetite-ilmenite intergrowth, olivine, labradorite, and spinel. Fresh rock is rare, however; in most cumberlandite, the olivine and labradorite have been replaced by serpentine, actinolite, and other alteration products. This rock is peculiar to Rhode Island, although a somewhat similar rock has been described at Taberg, Sweden (Singewald, 1913). Although the rock has not been radiometrically dated, cumberlandite is considered to be of Late Proterozoic.



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Cumberlandite samples. Plagioclase crystals in white.






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Cumberlandite samples. Oriented Plagioclase crystals in white. Photo credit to Judy Hadley (Cumberlandite Study Facebook page)






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Cumberlandite outcrop. Plagioclase crystals in white. Photo credit to Judy Hadley (Cumberlandite Study Facebook page)






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Cumberlandite samples. Plagioclase crystals in white. Photo credit to Judy Hadley (Cumberlandite Study Facebook page)





Bibliography



• Eric A.K.Middlemost (1985): Magmas and Magmatic Rocks. Longman, London
• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore
• K.G.Cox, J.D.Bell & R.J Pankhurst (1979): The interpretetion of igneous rocks. George Allen&Unwin editori.


Photo
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine, magnetite-ilmenite crystals and altered plagioclase. Cumberland, Rhode Island, USA. XPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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br />Olivine, magnetite-ilmenite crystals and altered plagioclase. Cumberland, Rhode Island, USA. XPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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br />Olivine, magnetite-ilmenite crystals and altered plagioclase. Cumberland, Rhode Island, USA. XPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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br />Olivine, magnetite-ilmenite crystals and altered plagioclase. Cumberland, Rhode Island, USA. XPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine, magnetite-ilmenite crystals and altered plagioclase. Cumberland, Rhode Island, USA. XPL image, 1x (Field of view = 9mm)
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Olivine (brown) crystals, magnetite-ilmenite crystals and altered plagioclase (white). Cumberland, Rhode Island, USA. PPL image, 1x (Field of view = 9mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Altered plagioclase crystal. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Altered plagioclase crystal. Cumberland, Rhode Island, USA. XPL image, 2x (Field of view = 7mm)
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Altered plagioclase crystal. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Altered plagioclase crystal. Cumberland, Rhode Island, USA. XPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. XPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. XPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)
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Olivine (brown) and magnetite-ilmenite crystals. Cumberland, Rhode Island, USA. PPL image, 2x (Field of view = 7mm)