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Litchfieldite

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Litchfieldite (nepheline syenite gneiss) from Canaã Massif, Brazil

Litchfieldite is a rareigneous rock. It is a coarse-grained,foliated variety ofnepheline syenite,^[1] sometimes called nepheline syenitegneiss or gneissic nepeheline syenite.^[2] Litchfieldite is composed of two varieties offeldspar (mostlyalbite but also somemicrocline), withnepheline,sodalite,cancrinite andcalcite. Themafic minerals, when present, aremagnetite and an iron-rich variety ofbiotite (lepidomelane).

Occurrence

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The rock was named after its occurrence atLitchfield,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:

Agneissic appearance due to the alternation of leucocratic and mafic bearing mineral bands;
Afoliated appearance, due to preferred orientation of platey minerals like lepidomelane biotite,
Parallel orientation of theoptical axis of minerals like nepheline.
Occurrence within zoned bodies, enveloped by amphibole bearing syenites and quartz-syenites rocks,
Broken and/or benttwin 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 withinmetamorphic 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) Asynorogenic 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 subordinatedgranite
Metamorphic grade of country rock: Highgreenschist facies to mediumamphibolite 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 amphibolite 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):
- Alkaline rocks:
  - Litchfieldite: nepheline, microcline, albite, amphibole, pyroxene, (molybdenite), (garnet), (titanite), (pyrochlore), (corundum), (sodalite).
  - Syenite and quartz-syenites:
  - Alkalinepegmatites: albite, nepheline, pyroxene, magnetite, zircon, (pyrochlore), (monazite), (allanite), (corundum).
- Country rocks: granitic gneisses, amphibolites
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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^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 NorwayArchived 2015-09-24 at theWayback 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 ^bWilliams, 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. 661ISBN 978-0-87335-233-8

v t e Types of rocks
Igneous rock	Adakite Alkali feldspar granite Andesite Anorthosite Aplite Basalt Alkali basalt Picrite basalt Basaltic trachyandesite Mugearite Shoshonite Basanite Blairmorite Boninite Carbonatite Charnockite Enderbite Dacite Diabase Diorite Napoleonite Dunite Essexite Foidolite Gabbro Granite Granodiorite Granophyre Harzburgite Hornblendite Hyaloclastite Icelandite Ijolite Kimberlite Komatiite Lamproite Lamprophyre Latite Lherzolite Monzogranite Monzonite Nepheline syenite Nephelinite Norite Obsidian Pegmatite Peridotite Phonolite Phonotephrite Porphyry Pumice Pyroxenite Websterite Quartz diorite Quartz monzonite Quartzolite Rhyodacite Rhyolite Comendite Pantellerite Scoria Shonkinite Sovite Syenite Tachylyte Tephriphonolite Tephrite Tonalite Trachyandesite Benmoreite Trachybasalt Hawaiite Trachyte Troctolite Trondhjemite Tuff Ignimbrite Wehrlite
Sedimentary rock	Argillite Arkose Banded iron formation Breccia Calcarenite Chalk Chert Claystone Coal Conglomerate Coquina Diamictite Diatomite Dolomite Evaporite Flint Geyserite Greywacke Gritstone Itacolumite Jaspillite Laterite Lignite Limestone Lumachelle Marl Mudstone Oil shale Oolite Phosphorite Sandstone Shale Siltstone Sylvinite Tillite Travertine Tufa Turbidite Varve Wackestone
Metamorphic rock	Anthracite Amphibolite Blueschist Cataclasite Eclogite Gneiss Granulite Greenschist Hornfels Calcflinta Itabirite Litchfieldite Marble Migmatite Mylonite Metapelite Metapsammite Phyllite Pseudotachylite Quartzite Schist Serpentinite Skarn Slate Suevite Talc carbonate Soapstone Tectonite Whiteschist
Specific varieties	Adamellite Appinite Aphanite Borolanite Blue Granite Epidosite Felsite Flint Ganister Gossan Hyaloclastite Ijolite Jadeitite Jasperoid Kenyte Lapis lazuli Larvikite Litchfieldite Llanite Luxullianite Mangerite Novaculite Pietersite Pyrolite Rapakivi granite Rhomb porphyry Rodingite Shonkinite Taconite Tachylite Teschenite Theralite Unakite Variolite Wad

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