BH250-211c
Title
BH250-211c
Subject
Pegmatitic Diorite
Description
Major Minerals: albite, nepheline, biotite, zircon
BH250-211c— Pegmatitic Diorite (Nepheline-Bearing)
Golding-Keene, Bancroft, Ontario, Canada
BH250-211c represents a pegmatitic diorite collected from the Golding-Keene locality near Bancroft, Ontario, a region famous for its mineralogical diversity and unusual alkaline intrusive rocks within the Grenville Province. The rock is characterized by an exceptionally coarse-grained, pegmatitic texture, indicating crystallization from a volatile-rich melt during the late stages of magmatic evolution. Large interlocking crystals make individual minerals easily visible in hand sample and reflect slow crystallization under conditions where residual fluids promoted rapid crystal growth.
The reported mineral assemblage of albite, nepheline, biotite, and zircon makes this rock particularly interesting because it does not represent a typical diorite in the strict igneous classification sense. Classical diorite is usually composed of plagioclase with hornblende and/or pyroxene and typically lacks feldspathoid minerals. The presence of nepheline indicates that this rock formed under silica-undersaturated conditions, meaning there was insufficient silica in the magma to stabilize quartz. Instead, feldspathoid minerals crystallized alongside feldspar, reflecting alkaline magmatic conditions that are relatively uncommon compared to more typical calc-alkaline intrusive systems. The abundance of albite suggests sodium enrichment during late-stage crystallization, while biotite records potassium and iron-magnesium components within the melt.
Zircon is a particularly important accessory mineral in this sample. Although present in relatively small amounts, zircon is highly resistant to chemical alteration and commonly preserves geochronologic information through uranium-lead dating. In teaching collections, zircon is also valuable because it introduces students to accessory minerals that play outsized roles in understanding geologic time, magmatic evolution, and crustal processes. Large zircon crystals in pegmatitic rocks can be especially striking and may be visible in hand sample or thin section.
The Golding-Keene locality lies within the Bancroft area of Ontario’s Grenville Province, a Mesoproterozoic high-grade metamorphic terrane known for nepheline syenites, pegmatites, marbles, skarns, and rare-element mineral occurrences. Many of these unusual rocks formed during complex Proterozoic magmatism, regional metamorphism, and fluid-rich recrystallization events associated with continental collision. Silica-poor alkaline magmas in this region produced unusual mineral assemblages that are rarely encountered in more typical igneous settings.
BH250-211c is particularly useful for teaching because it demonstrates the relationship between igneous texture and magma evolution while also highlighting unusual silica-undersaturated mineralogy. The coexistence of coarse pegmatitic textures with nepheline-bearing mineral assemblages provides students with an excellent example of how late-stage magmatic fluids, alkali enrichment, and silica deficiency can produce rocks that do not fit neatly into conventional igneous classifications. It also serves as a useful reminder that field labels such as “diorite” may sometimes reflect historical naming conventions rather than strict modern classification schemes.
BH250-211c— Pegmatitic Diorite (Nepheline-Bearing)
Golding-Keene, Bancroft, Ontario, Canada
BH250-211c represents a pegmatitic diorite collected from the Golding-Keene locality near Bancroft, Ontario, a region famous for its mineralogical diversity and unusual alkaline intrusive rocks within the Grenville Province. The rock is characterized by an exceptionally coarse-grained, pegmatitic texture, indicating crystallization from a volatile-rich melt during the late stages of magmatic evolution. Large interlocking crystals make individual minerals easily visible in hand sample and reflect slow crystallization under conditions where residual fluids promoted rapid crystal growth.
The reported mineral assemblage of albite, nepheline, biotite, and zircon makes this rock particularly interesting because it does not represent a typical diorite in the strict igneous classification sense. Classical diorite is usually composed of plagioclase with hornblende and/or pyroxene and typically lacks feldspathoid minerals. The presence of nepheline indicates that this rock formed under silica-undersaturated conditions, meaning there was insufficient silica in the magma to stabilize quartz. Instead, feldspathoid minerals crystallized alongside feldspar, reflecting alkaline magmatic conditions that are relatively uncommon compared to more typical calc-alkaline intrusive systems. The abundance of albite suggests sodium enrichment during late-stage crystallization, while biotite records potassium and iron-magnesium components within the melt.
Zircon is a particularly important accessory mineral in this sample. Although present in relatively small amounts, zircon is highly resistant to chemical alteration and commonly preserves geochronologic information through uranium-lead dating. In teaching collections, zircon is also valuable because it introduces students to accessory minerals that play outsized roles in understanding geologic time, magmatic evolution, and crustal processes. Large zircon crystals in pegmatitic rocks can be especially striking and may be visible in hand sample or thin section.
The Golding-Keene locality lies within the Bancroft area of Ontario’s Grenville Province, a Mesoproterozoic high-grade metamorphic terrane known for nepheline syenites, pegmatites, marbles, skarns, and rare-element mineral occurrences. Many of these unusual rocks formed during complex Proterozoic magmatism, regional metamorphism, and fluid-rich recrystallization events associated with continental collision. Silica-poor alkaline magmas in this region produced unusual mineral assemblages that are rarely encountered in more typical igneous settings.
BH250-211c is particularly useful for teaching because it demonstrates the relationship between igneous texture and magma evolution while also highlighting unusual silica-undersaturated mineralogy. The coexistence of coarse pegmatitic textures with nepheline-bearing mineral assemblages provides students with an excellent example of how late-stage magmatic fluids, alkali enrichment, and silica deficiency can produce rocks that do not fit neatly into conventional igneous classifications. It also serves as a useful reminder that field labels such as “diorite” may sometimes reflect historical naming conventions rather than strict modern classification schemes.
Coverage
Location: Golding-Keene, Bancroft, Ontario, Canada
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 211c. Housed at Carleton College in Minnesota.
Contributor
Steve Dunn, Mount Holyoke College
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-211c,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/274.