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Litchfieldite

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Litchfieldite (nepheline syenite gneiss) from Canaã Massif, Brazil

Litchfieldite is a rare igneous rock. It is a coarse-grained, foliated variety of nepheline syenite,[1] sometimes called nepheline syenite gneiss or gneissic nepeheline syenite.[2] Litchfieldite is composed of two varieties of feldspar (mostly albite but also some microcline), with nepheline, sodalite, cancrinite and calcite. The mafic minerals, when present, are magnetite and an iron-rich variety of biotite (lepidomelane).

Occurrence

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The rock was named after its occurrence at Litchfield, Maine, USA, by Bayley in 1892.[3][4]

It is a very rare rock and also occurs in Blue Mountain and French River, Canada;[4] Soroy and Val River, Norway; Cevadais, Portugal; Canaã,[5] Boca Nova, Itajú da Colonia and Peixes, Brazil.

Petrology

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Besides the mineral composition litchfieldites are characterized by one or more of the following structures:

  • A gneissic appearance due to the alternation of leucocratic and mafic bearing mineral bands;
  • A foliated appearance, due to preferred orientation of platey minerals like lepidomelane biotite,
  • Parallel orientation of the optical axis of minerals like nepheline.
  • Occurrence within zoned bodies, enveloped by amphibole bearing syenites and quartz-syenites rocks,
  • Broken and/or bent twin lamellae in plagioclase (albite),
  • Mortar structure,
  • Biotite with kink banding structure and bent cleavage planes,
  • Occurrence as large boudin or scattered blocks at the surface,
  • Occurrence within metamorphic belt as elongated bodies parallel to the regional structures,
  • Isotopic age according to the regional metamorphism

Origin

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There are two hypotheses for the origin of these rocks:

1) A pre-tectonic magmatic nepheline syenite
2) A synorogenic intrusion of nepheline syenite

Both schools of thought are in accordance that rocks like litchfieldite owe their mineral and structural characteristics to the metamorphism.

Economy

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Due to the content of alkali elements and alumina, they are very important ore for the glass industry. The iron-bearing minerals when present are strongly magnetic and can be removed through a magnetic field.

Occurrences of nepheline syenite gneisses in Brazil

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Boca Nova

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  • Location: Pará State
  • Age and determination method
    • Country rock last metamorphism age: 900 to 600 my (Brazilian cycle)
    • Nepheline syenite rocks: 580+/-10 my- K-Ar, biotite; 724 +/- 30 my- Rb-Sr, whole-rock
  • Rocks and minerals presents (between parenthesis when accessory)
    • Alkaline rocks
      • nepheline syenite gneiss (mainly litchfieldite): nepheline, albite, microcline, perthite, dark green biotite, (calcite), (white mica), (zircon), (pyrochlore), (cancrinite), (sodalite), (opaque minerals)
      • Pegmatites-the same mineralogical composition but poorer in biotite.
    • Country rocks: Phyllite, schist with subordinated granite
  • Metamorphic grade of country rock: High greenschist facies to medium amphibolite facies
  • Geometry of the alkaline body :Boulders scattered. Geophysical dates suggests elongated bodies parallel to the regional structures.
  • Textures and structures presents in alkaline rocks: Gneissose appearance; folded foliation; foliation parallel to the country rocks; preferential orientation of biotites parallel to the gneissic band; pegmatites veins with pinch and swell structure parallel and cross cutting the foliation; biotite crystal with kink banding structure; plagioclase with bent twin lamella; feldspar with peristherite; oriented optical axis in nephelines;
  • Proposed origin: metamorphism of igneous nepheline syenite

Canaã

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  • Localization: Rio de Janeiro State
  • Age and determination method
    • Country rocks last metamorphism age: 900 to 600 my (Brazilian cycle)
    • Nepheline syenite rocks: 542 +/-14 my Rb-Sr whole rock; 424 +/- 13 my K-Ar in biotite.
  • Rocks and minerals (between parenthesis when accessory):
    • Alkaline rocks:
      • Litchfieldites: Albite (An=5%), microcline, nepheline, cancrinite, lepidomelane, magnetite, (zircon), (allanite), (corundum), (sodalite)
      • Alkali syenite: microcline perthite, quartz, pyroxene, brown biotite, (amphibole), (titanite), (apatite), (zircon), (muscovite), (corundum), (molybdenite), (opaques)
      • Nepheline bearing pegmatite vein, parallel and cross cutting the foliation of litchfieldite: Feldspar, nepheline, (magnetite), (biotite), (muscovite), (sodalite), (sulfides), (calcite).
      • Syenite pegmatite: feldspar, corundum, muscovite, (biotite), (blue corundum), (sulfides)
    • Country rocks: Biotite-sillimanite-gneiss, amphibolites, migmatites, (granite).
  • Metamorphic grade of country rock: high anphibolite facies
  • Geometry of the alkaline body: elongated and concordant bodies
  • Textures and structures presents in alkaline rocks: gneissose appearance, parallel to the country rock foliation; oriented biotite and feldspar; plagioclase with bent twin lamella; kink-banding biotite; oriented optical axis in nephelines; zoned bodies-litchfieldite in center and syenites surrounding.
  • Proposed origin: metamorphism of igneous nepheline syenite.

Tocantins nepheline syenite gneiss belt

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(Estrela, Eldorado, Porto Nacional, Peixe)

  • Localization: Tocantins State
  • Age and determination method:
    • Country rocks last metamorphism age: 900 to 600 my (Brazilian cycle)
    • Nepheline syenite rocks: 1,5 Gy
  • Rocks and minerals (between parenthesis when accessory):
  • Metamorphic grade of country rock: high amphibolite
  • Geometry of the alkaline body: A belt of elongated zoned bodies parallel to the regional structures.
  • Textures and structures presents in alkaline rocks: gneissose appearance, foliation, preferred orientation of biotite crystals, faults, boudinage, plagioclase with bent twin lamella; feldspar with peristherite, plagioclase with undulose extinction.
  • Proposed origin: Metamorphism and metasomatism of igneous nepheline syenite rocks.

Alkaline Rocks Province of South Bahia

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(Itaju da Colonia, Santa Cruz da Vitória, Potiriguá, Itabuna)

  • Localization: Bahia State
  • Age and determination method:
    • Country rocks last metamorphism age: 900 to 600 my (Brazilian cycle)
    • Nepheline syenite rocks: 732 +/- 8 my, U-Pb, titanite
  • Rocks and minerals (between parenthesis when accessory):
    • Alcalines rocks:
      • Litchfieldite: albite, microcline, nepheline, dark green biotite, brown biotite, (pyroxene), (amphibole), (cancrinite), (sodalite)
      • Tawite: albite, microcline, sodalite, (nepheline)
      • Syenite
      • Alkaline pegmatites
    • Country rocks: granitic gneiss, migmatite, granulite
  • Metamorphic grade of country rock: High amphibolite facies
  • Geometry of the alkaline body: elongated bodies parallel to regional structures.
  • Textures and structures present in alkaline rocks: elongated and oriented cluster (schlieren) of mafic minerals, plagioclase with bent and broken twin lamella, biotite and muscovite orientation (foliation), mortar structures, kink-banding in biotite
  • Proposed origin: Metamorphic synorogenic alkaline rocks.

References

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  1. ^ Le Maitre, R.W. (2002) Igneous Rocks - A Classification and Glossary of Terms, 2nd edition, Cambridge, Cambridge University Press, page 105. ISBN 0-521-66215-X
  2. ^ Robins, B. and Tysseland, M. (1979) Fenitization of some mafic igneous rocks in the Seiland province, northern Norway Archived 2015-09-24 at the Wayback Machine, Norsk Geologisk Tidsskrift, Volume 59 Number 1 pages 1-23, page 3. Retrieved 2015-07-20.
  3. ^ Litchfieldite and the Litchfield Sodalite Locality, Maine Geological Survey
  4. ^ a b Williams, Howel, Francis J. Turner and Charles M. Gilbert, Petrography, Freeman, 1954, p. 117
  5. ^ Industrial Minerals & Rocks, Society for Mining, Metallurgy, and Exploration; 7th ed. 2006, p. 661 ISBN 978-0-87335-233-8
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