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.

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.

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Sienite: alkali-feldspar (white) and amphibole (dark). Cuttingsville, Vermont, USA. From James St. John

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Orhoclase (with sericite alteration), apatite (colorless, high relief) and amphibole (green-brown) crystals. Syenite from Biella, Italy. PPL image. 2x (Field of view = 7mm)

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Orhoclase and biotite (brown) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)	Orhoclase and biotite (brown) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)	Orhoclase, small plagioclase crystals (with polysynthetic twinning) and biotite (brown). XPL image. 2x (Field of view = 7mm)
Orhoclase, small plagioclase crystals (with polysynthetic twinning) and biotite (brown). XPL image. 2x (Field of view = 7mm)	Orhoclase and biotite (brown) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)	Orhoclase, small plagioclase crystals (with polysynthetic twinning) and biotite (brow). XPL image. 2x (Field of view = 7mm)
Orhoclase (with plagioclase inclusions) and biotite (brown). XPL image. 2x (Field of view = 7mm)	Orhoclase, small plagioclase crystals (with polysynthetic twinning) and biotite (brown). XPL image. 2x (Field of view = 7mm)	Orhoclase and biotite (brown) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)
Orhoclase (with sericite alteration), apatite (colorless, high relief) and amphibole (green-brown) crystals. Syenite from Biella, Italy. PPL image. 2x (Field of view = 7mm)	Orhoclase, apatite (low birefringence) and amphibole (low-medium birefringence) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)	Orhoclase (with sericite alteration) and amphibole (green-brown) crystals. Syenite from Biella, Italy. PPL image. 2x (Field of view = 7mm)
Orhoclase and amphibole (low-medium birefringence) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)	Orhoclase (with sericite alteration) and amphibole (green-brown) crystals. Syenite from Biella, Italy. PPL image. 2x (Field of view = 7mm)	Orhoclase and amphibole (low-medium birefringence) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)
Orhoclase and amphibole (low-medium birefringence) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)	Orhoclase and amphibole (low-medium birefringence) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)	Orhoclase and amphibole (low-medium birefringence) crystals. Syenite from Biella, Italy. XPL image. 2x (Field of view = 7mm)