Litchfieldite
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
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 neheline 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
- 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
- Alkaline rocks
- 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
- 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).
- Alkaline rocks:
- 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
(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):
- Alkaline rocks:
- Litchfieldite: nepheline, microcline, albite, amphibole, pyroxene, (molybdenite), (garnet), (titanite), (pyrochlore), (corundum), (sodalite).
- Syenite and quartz-syenites:
- Alkaline pegmatites: albite, nepheline, pyroxene, magnetite, zircon, (pyrochlore), (monazite), (allanite), (corundum).
- Country rocks: granitic gneisses, amphibolites
- Alkaline rocks:
- Metamorphic grade of country rock: high amphibolite
- Geometry of the alkaline body: A belt of elonganted 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
(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
- Alcalines rocks:
- 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
- ^ 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
- ^ 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.
- ^ Litchfieldite and the Litchfield Sodalite Locality, Maine Geological Survey
- ^ a b Williams, Howel, Francis J. Turner and Charles M. Gilbert, Petrography, Freeman, 1954, p. 117
- ^ Industrial Minerals & Rocks, Society for Mining, Metallurgy, and Exploration; 7th ed. 2006, p. 661 ISBN 978-0-87335-233-8