BH250-209c
Title
BH250-209c
Subject
Syenite
Description
Major Minerals: perthite, nepheline, hornblende
Minor Minerals: biotite, sphene (titanite), zircon, apatite
BH250-209c is another perthite-rich nepheline syenite that highlights the mineralogical diversity of silica-undersaturated alkaline intrusive rocks. This specimen is dominated by large alkali feldspar crystals that display well-developed perthitic textures, accompanied by nepheline and dark mafic minerals. Unlike BH250-209b, which contains the sodium-rich amphibole riebeckite, this sample is characterized by hornblende as the dominant mafic phase, reflecting subtle but important differences in magma chemistry within related nepheline syenitic systems. The photomicrographs reveal large feldspar crystals with extensive perthitic intergrowths, interstitial nepheline, and clusters of hornblende and biotite concentrated between feldspar grains. Accessory sphene (titanite), zircon, and apatite occur in minor amounts and add further petrographic complexity to the rock.
Under plane-polarized light, both hornblende and biotite display strong pleochroism, making this thin section particularly useful for teaching optical mineralogy. Hornblende shifts from brown and olive-green to pale yellow-green depending on crystal orientation. These color variations largely reflect differences in iron and magnesium content, with darker brown tones typically indicating more iron-rich compositions, while greener tones may reflect relatively magnesium-rich varieties or variations in iron oxidation state. Biotite occurs as brown pleochroic flakes with well-developed cleavage and contributes additional dark mineral phases to the rock. Together, these minerals provide excellent examples for students learning how pleochroism can aid mineral identification in thin section.
One of the most striking features of BH250-209c is its abundant perthitic feldspar. Perthite is not a distinct mineral species but rather a textural intergrowth that forms when a high-temperature alkali feldspar solid solution cools slowly and unmixes into potassium feldspar and sodium-rich albite. In this sample, the perthitic textures include thin albite lamellae, exsolution veins, patchy intergrowths, and irregular string-like textures that are clearly visible under the microscope. These textures reflect slow cooling within an intrusive environment and, in some hand samples, may even be visible without magnification. The feldspars in this specimen are particularly well developed and visually distinct from those observed in BH250-209b.
Nepheline occurs as low-relief, colorless grains with low birefringence and can easily be mistaken for quartz by beginning students. However, the absence of quartz and the presence of feldspathoids clearly indicate that this rock crystallized from a silica-undersaturated magma. Its occurrence alongside perthite and hornblende reinforces the alkaline nature of this intrusive rock and provides an excellent example of how mineral assemblages reflect magma chemistry.
Geologically, BH250-209c demonstrates how relatively small variations in magma composition can produce distinct mineral assemblages within nepheline syenites. Compared with BH250-209b, this specimen is less sodic and is dominated by hornblende rather than riebeckite, illustrating compositional variability within alkaline intrusive systems. The name syenite itself is derived from Aswan (ancient Syene), the historical type locality for the rock. Quartz-poor, feldspar-rich rocks from this region were widely quarried in ancient Egypt and used in monuments, statues, and obelisks. Although modern petrographic classification has refined the definition of syenite, its historical connection to Syene remains significant.
This specimen is also an outstanding teaching sample for understanding silica-undersaturated magmatism. The presence of nepheline instead of quartz reflects magma compositions enriched in sodium and potassium but deficient in silica. The abundant perthitic feldspar textures make this rock especially valuable for teaching feldspar exsolution, while the strongly pleochroic hornblende and biotite provide excellent examples for optical mineralogy. Together with BH250-209a and BH250-209b, this specimen helps students appreciate the diversity of syenitic rocks and the important role magma chemistry plays in controlling mineral assemblages.
Minor Minerals: biotite, sphene (titanite), zircon, apatite
BH250-209c is another perthite-rich nepheline syenite that highlights the mineralogical diversity of silica-undersaturated alkaline intrusive rocks. This specimen is dominated by large alkali feldspar crystals that display well-developed perthitic textures, accompanied by nepheline and dark mafic minerals. Unlike BH250-209b, which contains the sodium-rich amphibole riebeckite, this sample is characterized by hornblende as the dominant mafic phase, reflecting subtle but important differences in magma chemistry within related nepheline syenitic systems. The photomicrographs reveal large feldspar crystals with extensive perthitic intergrowths, interstitial nepheline, and clusters of hornblende and biotite concentrated between feldspar grains. Accessory sphene (titanite), zircon, and apatite occur in minor amounts and add further petrographic complexity to the rock.
Under plane-polarized light, both hornblende and biotite display strong pleochroism, making this thin section particularly useful for teaching optical mineralogy. Hornblende shifts from brown and olive-green to pale yellow-green depending on crystal orientation. These color variations largely reflect differences in iron and magnesium content, with darker brown tones typically indicating more iron-rich compositions, while greener tones may reflect relatively magnesium-rich varieties or variations in iron oxidation state. Biotite occurs as brown pleochroic flakes with well-developed cleavage and contributes additional dark mineral phases to the rock. Together, these minerals provide excellent examples for students learning how pleochroism can aid mineral identification in thin section.
One of the most striking features of BH250-209c is its abundant perthitic feldspar. Perthite is not a distinct mineral species but rather a textural intergrowth that forms when a high-temperature alkali feldspar solid solution cools slowly and unmixes into potassium feldspar and sodium-rich albite. In this sample, the perthitic textures include thin albite lamellae, exsolution veins, patchy intergrowths, and irregular string-like textures that are clearly visible under the microscope. These textures reflect slow cooling within an intrusive environment and, in some hand samples, may even be visible without magnification. The feldspars in this specimen are particularly well developed and visually distinct from those observed in BH250-209b.
Nepheline occurs as low-relief, colorless grains with low birefringence and can easily be mistaken for quartz by beginning students. However, the absence of quartz and the presence of feldspathoids clearly indicate that this rock crystallized from a silica-undersaturated magma. Its occurrence alongside perthite and hornblende reinforces the alkaline nature of this intrusive rock and provides an excellent example of how mineral assemblages reflect magma chemistry.
Geologically, BH250-209c demonstrates how relatively small variations in magma composition can produce distinct mineral assemblages within nepheline syenites. Compared with BH250-209b, this specimen is less sodic and is dominated by hornblende rather than riebeckite, illustrating compositional variability within alkaline intrusive systems. The name syenite itself is derived from Aswan (ancient Syene), the historical type locality for the rock. Quartz-poor, feldspar-rich rocks from this region were widely quarried in ancient Egypt and used in monuments, statues, and obelisks. Although modern petrographic classification has refined the definition of syenite, its historical connection to Syene remains significant.
This specimen is also an outstanding teaching sample for understanding silica-undersaturated magmatism. The presence of nepheline instead of quartz reflects magma compositions enriched in sodium and potassium but deficient in silica. The abundant perthitic feldspar textures make this rock especially valuable for teaching feldspar exsolution, while the strongly pleochroic hornblende and biotite provide excellent examples for optical mineralogy. Together with BH250-209a and BH250-209b, this specimen helps students appreciate the diversity of syenitic rocks and the important role magma chemistry plays in controlling mineral assemblages.
Coverage
Red Hill, New Hampshire, United States
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 209c. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-209c,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/269.