Sodalite Syenite

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).

Hypersolvus and Subsolvus Syenites: Another way of looking at the classification of syenitic rocks is based on the feldspars, and whether or not they crystallized under relatively dry low pressure conditions or "wet", higher pressure conditions. This can be seen by comparing the experimentally determined phase diagrams at various conditions (Fig.1-2).

At low pressure under dry conditions, the alkali feldspars form a complete solid solution at high temperature, but, upon slow cooling, they eventually reach the solvus and exsolve into two feldspars, one rich in albite and the other rich in orthoclase. But, because of the low temperature at which this occurs, only single feldspars will occur and these will show a perthitic texture. Syenites that crystallize under low pressure and exhibit a single perthitic alkali feldspar are considered hypersolvus granites.

At higher pressure, under water-saturated conditions, the liquidus surface is suppressed and the solvus moves up to intersect the solidus. This results in the crystallization of two alkali feldspar solid solutions, one rich in Ab, and the rich in Or. Each of these will further exsolve on cooling to form perthites. Syenites that crystallize under these conditions are referred to as subsolvus Syenites.

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Fig.1: Or-Ab (KAlSi3O8-NaAlSi3O8) at low pressure and dry condition. At high temperatures the diagram shows that albite (Ab) or NaAlSi3O8 and orthoclase (Or) or KAlSi3O852Ab38). This solid solution remains stable with lowering of temperature until the temperature reaches the solvus (a temperature of about 590°C). At this temperature the solid solution is no longer stable and begins to exsolve. The composition of coexisting exsolved phases can be found by drawing an isotherm until it intersects the solvus. With further lowering of temperature (Figure 4) further exsolution occurs. At a temperature of 400°C our original composition has exsolved into two alkali feldspar solid solutions, one with the composition of Or84Ab16 and one with a composition of Or96Ab04.



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Fig.2: Or-Ab phase diagram (KAlSi3O8-NaAlSi3O8) at high pressure and wet conditions. Here the liquidus surface is suppressed and the solvus moves up to intersect the solidus. This results in the crystallization of two alkali feldspar solid solutions, one rich in Ab, and the rich in Or. Each of these will further exsolve on cooling to form perthites.



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 Dìtro 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.

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QAPF Diagram Foid-bearing Syenite field in blue




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Nepheline Syenite sample. Plagioclase (white) and gray interstitial Nepheline. From Anton R. Chakhmouradian.




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Naujaite sample from Ilìmaussaq. Sodalite (blue). From Michael P. Klimetz.




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Shonkinite sample.



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


Photo
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Sodalite (marked as Sdl), biotite (green) and colorless alkali feldspar. PPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (marked as Sdl), biotite (green) and colorless alkali feldspar. PPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), titanite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (marked as Sdl), biotite (green) and colorless alkali feldspar. PPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic) and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic) and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (marked as Sdl) and colorless alkali feldspar. PPL image , 10x (Field of view = 2mm)
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Sodalite (marked as Sdl) and colorless alkali feldspar. PPL image , 10x (Field of view = 2mm)
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Sodalite (marked as Sdl) and colorless alkali feldspar. PPL image , 10x (Field of view = 2mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)
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Sodalite (isotropic), biotite and alkali feldspar. XPL image , 2x (Field of view = 7mm)