BH250-209a
Title
Subject
Description
Major Minerals: sodic plagioclase, orthoclase, biotite (Fe-rich and Mg-rich varieties), nepheline
Minor Minerals: aegirine-augite (deep green), opaque oxides
Secondary Mineral: calcite
BH250-209a is a coarse-grained nepheline-bearing syenite collected from Blue Mountain in Methuen Township, Ontario, part of the well-known alkaline igneous complexes of the Bancroft region. This rock is dominated by feldspars and feldspathoids and represents a silica-undersaturated intrusive igneous rock. The presence of nepheline confirms that the magma from which this rock crystallized was deficient in silica; if sufficient silica had been available, nepheline would have reacted to form quartz or additional feldspar.
In hand sample, the rock displays a coarse phaneritic texture with abundant light-colored feldspar and nepheline crystals contrasted by dark mafic minerals. Dark brown to green biotite flakes are visible throughout the specimen, along with scattered deep green pyroxene crystals interpreted as aegirine-augite. Minor opaque oxides are also present. Late-stage calcite occurs as a secondary mineral, likely introduced during post-magmatic hydrothermal alteration or fluid interaction.
This sample is particularly useful for teaching because it demonstrates the mineralogical distinction between silica-saturated rocks such as granite and silica-undersaturated rocks such as nepheline syenite. It also provides excellent examples of feldspar textures, nepheline optics, and alkaline pyroxene mineralogy.
Optical Mineralogy
Nepheline
Nepheline in this thin section is exceptionally fresh and commonly occurs as anhedral to subhedral grains interstitial to feldspar. Many crystals appear to be cut perpendicular to the c-axis, producing classic uniaxial negative interference figures. Nepheline exhibits very low birefringence and poor cleavage, and beginning mineralogy students often mistake it for quartz. However, its occurrence in a silica-undersaturated assemblage and its optical behavior clearly distinguish it from quartz.
Plagioclase Feldspar
The plagioclase is predominantly sodic and displays well-developed albite twinning, which is easily visible in thin section. Several grains appear oriented perpendicular to the b-axis, making them useful for estimating compositional variation using the Michel-Lévy method. The feldspars form much of the framework of the rock and show typical interlocking igneous textures.
Orthoclase
Orthoclase occurs as large feldspar crystals intergrown with plagioclase and nepheline. Together, the alkali feldspars dominate the felsic component of the rock and place it firmly within the syenite classification field.
Biotite
Biotite occurs as elongate flakes with perfect basal cleavage. Under crossed polars, many grains display classic bird’s-eye maple interference colors, making them excellent teaching examples. In plane-polarized light, pleochroism ranges from deep reddish-brown to pale yellow in Fe-rich varieties. Some crystals display greenish pleochroism and are likely more magnesium-rich, approaching phlogopite composition. The coexistence of both Fe-rich and Mg-rich biotite varieties suggests subtle chemical variation during crystallization.
Aegirine-Augite
Deep green pyroxene grains in this section are interpreted as aegirine-augite, a sodium-rich clinopyroxene commonly associated with alkaline igneous rocks. These crystals typically appear dark green to nearly black in plane-polarized light and may exhibit strong pleochroism from green to yellow-green. Although aegirine is birefringent, strong absorption and low interference colors can sometimes make it appear nearly isotropic in thin section. Its presence is consistent with the alkaline, silica-undersaturated nature of this rock.
Classification and Geological Significance
Syenite is a coarse-grained intrusive igneous rock composed primarily of alkali feldspar with lesser plagioclase and minor mafic minerals. It is chemically similar to granite but contains little to no quartz (typically less than 5%). Instead, in silica-undersaturated systems such as this one, feldspathoids like nepheline may crystallize.
The name syenite is derived from Syene (modern-day Aswan), Egypt, where similar quartz-poor feldspar-rich rocks were historically quarried and used in ancient monuments and obelisks. Although modern petrologic classification has refined the term, Syene remains the historical type locality associated with the name.
This specimen is particularly valuable for teaching IUGS igneous rock classification, especially the distinction between quartz-bearing granitoids and feldspathoid-bearing alkaline rocks. Students can use this sample to understand how mineral assemblages reflect magma chemistry and silica saturation.
Classification of Syenites
- Quartz syenite: contains 5–20% quartz
- Quartz-free syenite: contains less than 5% quartz
- Nepheline syenite: contains feldspathoids such as nepheline and forms in silica-undersaturated systems
- Potassic syenite: dominated by potassium feldspar
- Sodic syenite: richer in sodium feldspar such as albite
This sample is best classified as a nepheline-bearing sodic syenite.