BH250-209a
Title
Subject
Description
Mineralogy Description
Major Minerals: plagioclase (sodic), orthocalse, biotite (Fe and Mg variteis), nepheline
Minor Minerals: aegirine-augite (deep green), oxides
Secondary Mineral: calcite
Optical Properties
Nepheline:
Nepheline crystals in this thin section are often cut perpendicular to the c-axis, displaying a uniaxial negative interference figure. The nepheline here is exceptionally fresh, with low birefringence and poor cleavage, which can easily confuse introductory mineralogy students who might mistake it for quartz.
Plagioclase Feldspar:
The plagioclase shows perfect albite twinning. Some crystals are oriented perpendicular to the b-axis, allowing estimation of their calcium and sodium content using the Michel-Lévy method.
Biotite:
Biotite in this section exhibits perfect cleavage, with distinctive bird’s eye maple interference colors. Pleochroism ranges from deep red to light yellow, indicating high iron content. Some biotites display greenish pleochroism; these are more magnesium-rich (approaching phlogopite), while the more iron-rich varieties show darker brown to reddish-brown pleochroism. Both Mg-rich and Fe-rich biotites are commonly found side by side.
Aegirine(-Augite):
A deep green, nearly isotropic mineral observed in syenites is most likely aegirine or aegirine-augite (NaFe³⁺Si₂O₆), though other possibilities may exist depending on the rock context. Aegirine typically appears deep green to nearly black, often forming prismatic, acicular, or columnar crystals. It exhibits strong pleochroism, shifting from green to yellowish-green in plane-polarized light. Although generally birefringent, its low interference colors and strong absorption can make it appear almost isotropic in thin section.
Notes:
This rock and thin section are excellent for IUGS classification of igneous rocks.
Syenite: Origin, Classification, and Characteristics
Syenite is a coarse-grained intrusive igneous rock dominated by alkali feldspar (orthoclase or microcline) with lesser plagioclase and minor mafic minerals like biotite or hornblende. Chemically similar to granite but quartz-deficient (<5% quartz), syenite forms from alkali-rich, silica-poor magmas. It is notably abundant near Syene (now Aswan), Egypt—its namesake—where it was widely used in ancient monuments and obelisks.
Classification of Syenites:
Quartz syenite: Contains 5–20% quartz
Quartz-free syenite: Contains less than 5% quartz
Potassic syenite: Dominated by potassium feldspars (orthoclase, microcline)
Sodic syenite: Richer in sodium feldspars (albite)