BH250-211a
Title
BH250-211a
Subject
Corundum-Bearing Nepheline Syenite
Description
Major Minerals: corundum, nepheline, biotite, plagioclase
BH250 -211a — Corundum-Bearing Nepheline Syenite
Egan Chute, Bancroft, Ontario, Canada
BH250 -211a, represents a corundum-bearing nepheline syenite, a rare silica-undersaturated intrusive igneous rock collected from Egan Chute near Bancroft, Ontario—one of Canada’s best-known regions for unusual alkaline and metamorphic mineral assemblages. Nepheline syenite is compositionally similar to syenite, but it forms under silica-deficient conditions, allowing feldspathoid minerals such as nepheline to crystallize instead of quartz. The presence of corundum (Al₂O₃) makes this specimen particularly unusual because corundum typically forms only in highly aluminum-rich and silica-poor environments where there is insufficient silica available to combine with aluminum to form feldspar or mica. This rare geochemical imbalance allows corundum to crystallize directly from the magma or form during late-stage magmatic recrystallization.
The rock is dominated by nepheline, plagioclase feldspar, and biotite, with corundum occurring as an important accessory mineral. Nepheline commonly appears cloudy or altered because it is chemically unstable at Earth’s surface and may alter to cancrinite, sodalite, muscovite, or clay minerals. Biotite contributes dark flakes throughout the rock, while plagioclase forms much of the coarse crystalline framework. Corundum may occur as isolated crystals, disseminated grains, or localized concentrations and is recognized by its extreme hardness (Mohs hardness of 9), lack of cleavage, and commonly barrel-shaped crystal habits. Depending on trace-element chemistry, corundum may occur as gray corundum or as gem-quality sapphire when trace elements such as titanium and iron are present.
BH250 -211a is also especially valuable in teaching optical mineralogy. Corundum is an excellent example of a uniaxial positive mineral, a relatively uncommon optical class compared to the much more abundant biaxial silicate minerals encountered in introductory mineralogy courses. Because there are relatively few common rock-forming minerals that clearly demonstrate uniaxial positive optical behavior, this specimen provides an excellent teaching opportunity for students learning interference figures and optical sign determination under the petrographic microscope. Its presence in a coarse igneous rock allows students to directly connect optical mineralogy with unusual igneous petrogenesis.
The Bancroft region lies within the Grenville Province, a Mesoproterozoic high-grade metamorphic terrane well known for pegmatites, nepheline-bearing rocks, marbles, skarns, and rare-element mineral deposits. Egan Chute is particularly famous among mineral collectors for corundum-bearing syenitic rocks and related metamorphic occurrences. Many of these rocks formed during complex Proterozoic magmatism associated with continental collision, crustal melting, and alkaline magmatic activity. Silica-undersaturated magmas in this setting allowed unusual minerals such as nepheline, sodalite, scapolite, and corundum to crystallize.
Corundum-bearing nepheline syenites are petrologically significant because they demonstrate how magma chemistry controls mineral stability. Under silica-rich conditions, aluminum is incorporated into feldspars and quartz-bearing assemblages. However, when silica becomes limited and aluminum remains abundant, unusual minerals such as nepheline and corundum become stable. This makes the sample an excellent teaching example for understanding silica saturation, feldspathoid mineral stability, optical mineralogy, and unusual igneous differentiation pathways.
BH250 -211a — Corundum-Bearing Nepheline Syenite
Egan Chute, Bancroft, Ontario, Canada
BH250 -211a, represents a corundum-bearing nepheline syenite, a rare silica-undersaturated intrusive igneous rock collected from Egan Chute near Bancroft, Ontario—one of Canada’s best-known regions for unusual alkaline and metamorphic mineral assemblages. Nepheline syenite is compositionally similar to syenite, but it forms under silica-deficient conditions, allowing feldspathoid minerals such as nepheline to crystallize instead of quartz. The presence of corundum (Al₂O₃) makes this specimen particularly unusual because corundum typically forms only in highly aluminum-rich and silica-poor environments where there is insufficient silica available to combine with aluminum to form feldspar or mica. This rare geochemical imbalance allows corundum to crystallize directly from the magma or form during late-stage magmatic recrystallization.
The rock is dominated by nepheline, plagioclase feldspar, and biotite, with corundum occurring as an important accessory mineral. Nepheline commonly appears cloudy or altered because it is chemically unstable at Earth’s surface and may alter to cancrinite, sodalite, muscovite, or clay minerals. Biotite contributes dark flakes throughout the rock, while plagioclase forms much of the coarse crystalline framework. Corundum may occur as isolated crystals, disseminated grains, or localized concentrations and is recognized by its extreme hardness (Mohs hardness of 9), lack of cleavage, and commonly barrel-shaped crystal habits. Depending on trace-element chemistry, corundum may occur as gray corundum or as gem-quality sapphire when trace elements such as titanium and iron are present.
BH250 -211a is also especially valuable in teaching optical mineralogy. Corundum is an excellent example of a uniaxial positive mineral, a relatively uncommon optical class compared to the much more abundant biaxial silicate minerals encountered in introductory mineralogy courses. Because there are relatively few common rock-forming minerals that clearly demonstrate uniaxial positive optical behavior, this specimen provides an excellent teaching opportunity for students learning interference figures and optical sign determination under the petrographic microscope. Its presence in a coarse igneous rock allows students to directly connect optical mineralogy with unusual igneous petrogenesis.
The Bancroft region lies within the Grenville Province, a Mesoproterozoic high-grade metamorphic terrane well known for pegmatites, nepheline-bearing rocks, marbles, skarns, and rare-element mineral deposits. Egan Chute is particularly famous among mineral collectors for corundum-bearing syenitic rocks and related metamorphic occurrences. Many of these rocks formed during complex Proterozoic magmatism associated with continental collision, crustal melting, and alkaline magmatic activity. Silica-undersaturated magmas in this setting allowed unusual minerals such as nepheline, sodalite, scapolite, and corundum to crystallize.
Corundum-bearing nepheline syenites are petrologically significant because they demonstrate how magma chemistry controls mineral stability. Under silica-rich conditions, aluminum is incorporated into feldspars and quartz-bearing assemblages. However, when silica becomes limited and aluminum remains abundant, unusual minerals such as nepheline and corundum become stable. This makes the sample an excellent teaching example for understanding silica saturation, feldspathoid mineral stability, optical mineralogy, and unusual igneous differentiation pathways.
Coverage
Location: Egan Chute, Bancroft, Ontario, Canada
GPS Coordinates: 45° 4'28.53"N, 77°44'3.17"W
GPS Coordinates: 45° 4'28.53"N, 77°44'3.17"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 211a. Housed at Carleton College in Minnesota.
Contributor
Steve Dunn, Mount Holyoke College
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-211a,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/271.