Syenite is a coarse-grained plutonic rock with intermediate to felsic composition. Its composition and appearance resemble granite but is lower in silica and is quartz deficient.
Plutonic or intrusive rocks form deep inside the Earth’s crust where cooling is slow. Thus, mineral crystals will grow large, forming a coarse-grained texture.
On the other hand, felsic rocks are silica-rich and high in lighter elements like oxygen, silicon, potassium, sodium, and aluminum. In contrast, mafic rocks are silica-poor and rich in iron and magnesium, the darker elements.
Intermediate rocks lie between felsic and mafic rocks. They are moderate in silica light and dark elements.
Lastly, considering the chemical and mineral composition, the extrusive equivalent of syenite is phonolite.
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Appearance, properties, and description
Syenite is mostly a massive coarse-grained, light-colored intrusive igneous rock whose density is 2.6-2.8g/cm3 and Mohs scale of hardness 5.5-6.
However, some are layered, including those with inch-scale layers. Layered ones occurs with other rocks.
Also, most of the layered intrusions have an overall basaltic composition. Examples are Ilimaussaq intrusion in southern Greenland and Lovozero peralkaline intrusions in Russia.
What about its color? Syenite rock color is off-white, pink, gray, and sometimes greenish or reddish brown. However, on a weathered surface, it can appear violet-gray.
Its texture is coarse-grained with equigranular interlocking, mostly light-colored and a few darker-colored speckles.
However, it can show other textures like porphyritic, perthitic, and microsyenite.
Also, quartz may show micrographic texture with quartz in alkali feldspar intergrowth or myrmekitic texture, a wormy intergrowth of quartz in plagioclase.
Porphyritic texture has a set of large crystals called phenocrysts set in a finer but phaneritic matrix called groundmass. It indicates a two-stage cooling history.
On the other hand, perthitic texture has an intergrowth of alkali feldspar and sodic plagioclase.
Lastly, microsyenite is a medium-grained phaneritic subvolcanic syenite. It forms at relatively shallow depths, usually less than 2km (1.2 mi). Cooling is relatively faster at these depths, resulting in rocks whose grains are 1 to 5mm.
Chemical composition
Syenite is an acidic to intermediate rock. It is relatively high in sodium and potassium oxides but lower in calcium, magnesium, and iron oxides.
Acidic have more than 63 wt. % silica and intermediate 52-63 wt.%.
Syenite mineral composition
This rock is a mafic to intermediate rock. It has more than 65% total feldspars and 10-30% mafic minerals by volume.
Mineralogically, syenite has mainly alkali feldspar, minor plagioclase, and at least one mafic mineral.
Mafic minerals are hornblende and, less often, muscovite, augite, and, in rare cases, fayalite or biotite, usually aluminium-rich annite.
Also, it may have a small amount of quartz, mainly interstitial, myrmekitic, or micrographic, while accessory minerals are zircon, titanite, conundrum, magnate, apatite, pyrite, and monazite.
The alkali feldspar is orthoclase, microcline, or anorthoclase, while plagioclase is sodium-rich. Sometimes, feldspars show perthitic texture.
Based on the QAPF classification, syenite is a coarse-grained plutonic rock in which quartz is less than 5% of the QAPF content by volume and 65-90% of the total feldspars.
If quartz increases to 5-20% of the QAPF content by volume, syenite rock grades into quartz syenite, and if it has up to 10% feldspathoids, it becomes a foid-bearing syenite.
Remember, quartz and foids don’t coexist.
An increase in alkali feldspar to over 90% of the total feldspars will grade syenite rock into alkali feldspar syenite.
The alkali feldspar syenite grades into quartz alkali feldspar syenite if quartz is 5-20% of the QAPF content by volume, and if foids are up to 10%, you will have a foid-bearing alkali feldspar syenite.
A foid syenite is an alkali feldspar syenite in which foids are 10-60% of the QAPF content by volume. Examples are sodalite syenite and nepheline syenite.
Lastly, foid monzosyenite composition lies between monzonite and syenite. Alkali feldspar is at least 50% of the total feldspar, and foids are 10-60% of the total QAPF content by volume.
Note: To name rocks with feldspathoids, use the name of the abundant feldspathoid. If several, name in increasing order with a hyphen between the foids.
How is syenite formed?
Syenite rock forms by slow cooling of evolved or differentiated magma deep inside the Earth’s crust in magma chambers or plutons. The slow cooling gives time for large crystals to grow.
Magma that forms syenite originates from fractionation and crustal material assimilation or contamination of magma generated by partial melting of upper mantle rocks, i.e., basaltic magma.
Also, it can result from low-degree partial melting of subcrustal igneous or granitic rocks. A higher degree of partial melting will form rocks like granites and tonalites as more sodium and calcium will melt.
Lastly, conditions favorable to the formation of anorthite, which then settles as cumulates, can leave magma that can form syenite.
Where does syenite rock occur?
Syenite occurs mostly on thick continental crusts, and volcanic syenite occurs especially above subduction zones on cordilleras. It is found in dikes or on the edges of granitic massifs.
Some places it occurs in the US include Arkansas, New England, Montana, South Carolina, and New York. In South Carolina, it occurs at the great syenite dyke.
Also, it occurs near Ditrău city in Romania, Plovdiv in Portugal, Khibiny Mountains and Lovozero Massif in Russia, Mulanje Mountain Forest Reserve in Malawi, and Kangerluluk and Paatusoq fjords in Greenland.
More places are Aswan in Egypt, Norway, Sweden, Scotland, Bulgaria, Switzerland, and Australia.
Lastly, in Australia, large intrusions occur in New South Wales, and small intrusions in all the other states.
Syenite uses
Syenite is a hard, durable rock that resists heat better than granite. It has been used in making aggregate, dimensional stone, sculptures, and other things.
In the dimensional stone industry, syenite is sold as commercial granite. It is cut and polished to make pavers, refractories, headstones, cemetery markers, foyers, facing, or building stones.
This rock is also crushed to make aggregate used in the roading industry, railway ballast, asphalt, subbases, and other constructional uses. However, its relatively high silica makes it unsuitable for concrete.
Other uses include making monuments, sculptures, landscaping, curbing, and gardening.
Frequently Asked Questions FAQs
It is a peralkaline syenite that contains k-feldspar, little to no plagioclase, alkali amphiboles like arfvedsonite and riebeckite, and alkali pyroxenes like aegirine and aegirine-augite with no quartz.
Peralkaline rocks are aluminum deficient relative to sodium and potassium. This is why they have aluminum-free, alkali amphiboles and pyroxenes.
Granite will have abundant quartz minerals since it is high in quartz, i.e., 20-60% of the QAPF content by volume, not a small amount, and there is no quartz in syenite. Also, granite is higher in silica and slightly lighter in color, higher in the lighter felsic minerals, and lower in the darker mafic minerals.
Hypersolvus forms at high temperatures in drier magma or under low water vapor pressure, resulting in homogenous alkali feldspar and sodic plagioclase or perthitic intergrowth. In contrast, subsolvus occurs in water-rich magma, i.e., at high vapor pressure that directly crystallizes albite-rich plagioclase and alkali feldspar.
Episyenite is a vuggy, alkali-feldspar-rich rock in which saline hydrothermal fluids have depleted quartz that occurs as lenses, pods, or veins in granitoid and migmatites.
It is enriched with potassium and rare earth elements, giving it a reddish-brown color, or it can be whitish if it is albitized from potassium enrichment.