Foyaite

Foyaites are named after Foya in the Serra de Monchique, in southern Portugal. These are K-feldspar-nepheline syenites containing < 10% ferromagnesian minerals, usually pyroxene, hornblende and biotite.

Syenite, from Latin "Lapis Syenitis" (lapis = stone) of Syene, from Syene (an ancient city of southern Egypt), is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts (<5%). The feldspar component of syenite is predominantly alkaline in character (usually orthoclase). Plagioclase feldspars may be present in small quantities, less than 10%. Syenites are usually either peralkaline with high proportions of alkali elements relative to aluminum, or peraluminous with a higher concentration of aluminum relative to alkali elements (K, Na, Ca).

• Syenites that contain only one feldspar (perthite or antiperthite) may be called Hypersolvus Syenites, as the presence of a single feldspar shows that the original feldspar crystallized at temperature above the solvus in the system NaAl3O8-KAlSi3O8.
• Syenites that contain both potassium feldspar and a separate plagioclase are called Subsolvus Syenites.

Syenites usually occur as relatively small independent intrusions or more commons as satellite bodies, related to larger intrusions with different overall compositions. In many areas Syenites are comagmatic with Granitic intrusions. As these Syenites tend to form marginal igneous facies to much larger granitic bodies, the former are often interpreted as having evolved from the latter. This poses problems, because if a Syenite is to evolve from a granite, significant amounts of SiO2 have to removed; and significant amounts of MgO, total Fe, MnO and TiO2 and also CaO and Na2O have to be added.
Such changes in chemical composition may be locally accomplished by the assimilation of mafic/or carbonate rocks, and the escape of volatiles containing dissolved silica. Many Syenite, however, are interpreted as being product of the fractional crystallization of basaltic magma. Chapman and Williams (1935) demonstrated that the removal of 53% of Plagioclase, 10% of Pyroxene, 10% of Olivine and 4.5% of Ilmenite form the parental basaltic magma would produce a monzonitic magma; and the removal of 17% of plagioclase, 16% of pyroxene and 2% of Ilmenite from this parental magma would produce a Syenitic magma.
This fractional crystallization process require the removal of high portion of plagioclase, and this may help explain the close association of Syenites and Anorhosites.



Syenite can contain both feldspathoids, including nepheline, sodalite and/or leucite, olivine or quartz. Feldspathoid syenites often have alkali pyroxenes, such as aegirine-augite and aegirine, or alkali amphiboles, such as reibeckite. A number of terms have been used to describe different type of foid-bearing Syenite:

Larvikite: from the town of Larvik in Norway. The feldspar has partly unmixed on the micro-scale to form a perthite, and the presence of the alternating alkali feldspar and plagioclase layers give its characteristic silver blue sheen (Schiller effect) on polished surfaces.
Nordmarkite: Is used to describe a quartz-bearing Syenite from Nordmark area, Oslo.
Shonkinite: A melanocratic variety of feldspathoid syenite containing a large proportion (> 60% modal) of mafic silicates (typically, augite, biotite and olivine) and < 10% modal feldspar. For Shonkin, the Native-American name for the Highwood Mountains, Chouteau Co., central Montana, USA.
Laurdalite: An alkalic syenite containing more than 10% modal feldspathoids and characterized by porphyritic texture. Also spelled lardalite. The name, given by Broegger in 1890, is for Laurdal, Norway.
Foyaite: Massive or trachytoidal hypersolvus nepheline syenite (commonly peralkaline). For Mt. Foia, Monchique, western Algarve, Portugal.
Ditroite: Sodalite-bering Nephelin Syenites with both microcline and Albite. From Ditrau or Dětro in Romania.
Miaskite: Leucocratic hypersolvus foid (monzo)syenite containing calcic albite, perthite, and nepheline ± cancrinite-group minerals as the principal foid minerals. Biotite is the major mafic mineral, while ilmenite, zircon and pyrochlore are characteristic accessory constituents. For Miass, South Urals, Russia.
Litchfieldite: A variety of nepheline syenite and is composed of two near pure phases of feldspar, albite and microcline, with predominance of the first one, plusnepheline, sodalite, cancrinite and calcite named after its occurrence at Litchfield, Maine, USA, by Bayley in 1892.
Lujavrite: Meso- to melanocratic trachytoidal peralkaline nepheline Syenite (commonly agpaitic). For Lujavr Urt (Lovozero Mts.), Kola P-la, northwestern Russia.
Kakortokite: Cumulate-textured peralkaline nepheline syenite typically showing alkali-feldspar, arfvedsonite- and eudialyte-rich layers. For Quaqortoq,Ilímaussaq complex, South Greenland.
Naujaite: Agpaitic (nepheline-)sodalite syenite with a poikilitic texture comprising crystals of feldspathoid minerals enclosed in alkali feldspar and ferromagnesian silicates. For Naajakasik (formerly Naujakasik), Ilímaussaq complex, South Greenland.
Pulaskite: Nepheline-bearing alkali feldspar syenite. For Pulaski Co., central Arkansas, USA.
Malignite: mesocratic foid-Syenites that contain 30-60% of mafic minerals. From Maligne River, Ontario, Canada
Mariupolite: Albite-rich Nepheline Syenite. From mariupol in Ukraine.

englishphoidsyenite.jpg

QAPF Diagram Foid-bearing Syenite field in blue




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Foyaite (K-feldspar-nepheline syenite) from type locality Foya, Serra de Monchique in Portugal. K-feldspar (white) and Nepheline (gray). From sandatlas.org




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.
• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori



Titanite, nepheline (colorless) and altered orthose crystals. PPL image, 2x (Field of view = 7mm)
Photo
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Titanite, nepheline (colorless) and altered orthose crystals. PPL image, 2x (Field of view = 7mm)
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Titanite crystals enclosed in nepheline (colorless). PPL image, 10x (Field of view = 2mm)
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Titanite crystals enclosed in nepheline (gray). XPL image, 10x (Field of view = 2mm)
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Nepheline (colorless) and altered orthose crystals. PPL image, 2x (Field of view = 7mm)
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Nepheline (bright gray) and altered orthose crystals. XPL image, 2x (Field of view = 7mm)
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Amphibole, nepheline (colorless) and altered orthose crystals. PPL image, 2x (Field of view = 7mm)
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Aegirine and Sericitized Orthoclase. PPL image, 2x (Field of view = 7mm)
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Aegirine and Sericitized Orthoclase. PPL image, 2x (Field of view = 7mm)
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Aegirine and Sericitized Orthoclase. PPL image, 2x (Field of view = 7mm)
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Aegirine and Sericitized Orthoclase. PPL image, 2x (Field of view = 7mm)
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Aegirine and Orthoclase. XPL image, 2x (Field of view = 7mm)
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Aegirine and Sericitized Orthoclase. PPL image, 2x (Field of view = 7mm)
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Nepheline (altered at the edges), Aegirine and Sericitized Orthoclase. PPL image, 10x (Field of view = 2mm)
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Orthoclase. XPL image, 10x (Field of view = 2mm)
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Nepheline (altered at the edges), Aegirine and Sericitized Orthoclase. PPL image, 10x (Field of view = 2mm)
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Orthoclase and Aegirine. XPL image, 10x (Field of view = 2mm)
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Orthoclase and Aegirine. PPL image, 10x (Field of view = 2mm)