BH250-185
Title
BH250-185
Subject
Phonolite
Description
Major Minerals: nepheline, titanoaugite, aegirine
Minor Minerals: sphene, glass
BH250-185 is typically fine- to medium-grained, sometimes porphyritic with phenocrysts. The groundmass is often cryptocrystalline to fine-grained, giving the stone a relatively uniform texture.
Mineralogy: BH250-185 is dominated by feldspathoids such as nepheline, titanoaugite, and aegirine. Minor minerals include sphene and glass, reflecting the rock’s silica undersaturation. Major minerals also include alkali feldspar (sanidine or orthoclase), clinopyroxene (augite or aegirine-augite), and biotite or other mafic phases. The texture and mineralogy reflect the phonolite’s origin as an alkaline, silica-undersaturated volcanic rock.
About the mineral nepheline:
Nepheline is a feldspathoid mineral with the chemical formula NaAlSi2O6 and is classified as a silica-undersaturated aluminosilicate, where sodium and potassium partially substitute for each other. It has a framework structure similar to feldspar but is stable only under low-silica conditions, making it incompatible with quartz. Nepheline crystallizes in the hexagonal system (trigonal symmetry) and typically forms short prismatic crystals, massive aggregates, or granular forms. In thin section under plane-polarized light, nepheline is colorless to slightly grayish, with moderate positive relief and a refractive index ranging from ~1.525 to 1.540. It exhibits very low birefringence (~0.002), producing weak first-order gray to white interference colors, and displays poor cleavage with conchoidal fracture. These optical properties are similar to those of quartz, which can lead to confusion among students; however, nepheline is uniaxial negative, whereas quartz is uniaxial positive, a distinction that can be confirmed using an interference figure, (see video). Petrologically, nepheline is significant because it occurs exclusively in silica-undersaturated igneous rocks such as phonolites, nephelinites, ijolites, and tephrites, often alongside feldspathoids, alkali pyroxenes, amphiboles, and titanite. Its presence indicates low silica activity in magmas, commonly associated with alkaline magmatism and continental rift settings, and is essential for understanding the formation of mantle-derived alkaline magmas.
Volcanology: The phonolite at Aksum forms part of the Tertiary volcanic province related to the East African Rift System and the broader Afro-Arabian Rift system. These phonolitic magmas are typically generated by low degrees of partial melting of metasomatized mantle sources enriched in alkalis. The lavas and domes are usually viscous, resulting in slow lava flows or dome-like volcanic edifices. The fine-grained, massive texture and resistance to weathering make phonolite well suited for monumental stonework.
Cultural Significance: The obelisks of Aksum are carved from phonolite volcanic rock quarried locally near the ancient city of Aksum in northern Ethiopia. These massive stelae date from the 4th century CE or earlier and are culturally significant as monumental markers of the Aksumite civilization.
Minor Minerals: sphene, glass
BH250-185 is typically fine- to medium-grained, sometimes porphyritic with phenocrysts. The groundmass is often cryptocrystalline to fine-grained, giving the stone a relatively uniform texture.
Mineralogy: BH250-185 is dominated by feldspathoids such as nepheline, titanoaugite, and aegirine. Minor minerals include sphene and glass, reflecting the rock’s silica undersaturation. Major minerals also include alkali feldspar (sanidine or orthoclase), clinopyroxene (augite or aegirine-augite), and biotite or other mafic phases. The texture and mineralogy reflect the phonolite’s origin as an alkaline, silica-undersaturated volcanic rock.
About the mineral nepheline:
Nepheline is a feldspathoid mineral with the chemical formula NaAlSi2O6 and is classified as a silica-undersaturated aluminosilicate, where sodium and potassium partially substitute for each other. It has a framework structure similar to feldspar but is stable only under low-silica conditions, making it incompatible with quartz. Nepheline crystallizes in the hexagonal system (trigonal symmetry) and typically forms short prismatic crystals, massive aggregates, or granular forms. In thin section under plane-polarized light, nepheline is colorless to slightly grayish, with moderate positive relief and a refractive index ranging from ~1.525 to 1.540. It exhibits very low birefringence (~0.002), producing weak first-order gray to white interference colors, and displays poor cleavage with conchoidal fracture. These optical properties are similar to those of quartz, which can lead to confusion among students; however, nepheline is uniaxial negative, whereas quartz is uniaxial positive, a distinction that can be confirmed using an interference figure, (see video). Petrologically, nepheline is significant because it occurs exclusively in silica-undersaturated igneous rocks such as phonolites, nephelinites, ijolites, and tephrites, often alongside feldspathoids, alkali pyroxenes, amphiboles, and titanite. Its presence indicates low silica activity in magmas, commonly associated with alkaline magmatism and continental rift settings, and is essential for understanding the formation of mantle-derived alkaline magmas.
Volcanology: The phonolite at Aksum forms part of the Tertiary volcanic province related to the East African Rift System and the broader Afro-Arabian Rift system. These phonolitic magmas are typically generated by low degrees of partial melting of metasomatized mantle sources enriched in alkalis. The lavas and domes are usually viscous, resulting in slow lava flows or dome-like volcanic edifices. The fine-grained, massive texture and resistance to weathering make phonolite well suited for monumental stonework.
Cultural Significance: The obelisks of Aksum are carved from phonolite volcanic rock quarried locally near the ancient city of Aksum in northern Ethiopia. These massive stelae date from the 4th century CE or earlier and are culturally significant as monumental markers of the Aksumite civilization.
Coverage
Location: Aksum, Ethiopia
GPS Coordinates: 14°6'57.92"N, 38°40'18.14"E
GPS Coordinates: 14°6'57.92"N, 38°40'18.14"E
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-185. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-185,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/228.