BH250-248
Title
BH250-248
Subject
Hornblende Gabbro
Description
Major Minerals: hornblende, plagioclase
BH250-248 is an excellent example of hornblende gabbro, a coarse-grained mafic intrusive rock that reminds students that magmas do not always follow the simplified pathways we draw on Bowen’s Reaction Series. As I often tell my students, “a magma will do what it wants to do.” While many introductory diagrams suggest that gabbros are dominated by pyroxene and calcium-rich plagioclase, natural systems are often far more complex. In this sample, the magma crystallized a mineral assemblage dominated by hornblende and plagioclase, demonstrating that water content, pressure conditions, oxygen fugacity, and evolving magma chemistry can significantly alter crystallization pathways. Instead of producing a classic pyroxene-rich gabbro, this hydrous magma stabilized amphibole, resulting in a hornblende-rich variety that records a different magmatic story.
In hand sample, hornblende gabbro typically appears dark and coarse grained, with black to dark green hornblende crystals intergrown with lighter plagioclase feldspar. The slow cooling of magma at depth allowed large, interlocking crystals to develop, producing the phaneritic texture typical of intrusive rocks. The plagioclase crystals in this thin section are particularly well developed and commonly display textbook examples of polysynthetic albite twinning, visible in crossed polarized light as alternating light and dark bands. These feldspars form the framework of the rock and are intergrown with large amphibole grains that occupy interstitial spaces and in some areas partially envelop earlier-formed plagioclase crystals.
The thin sections provide beautiful petrographic evidence of amphibole crystallization. In plane-polarized light, hornblende displays its characteristic green to olive-brown pleochroism, with color changes reflecting shifts in crystal orientation. Several grains show the distinctive amphibole cleavage directions intersecting at approximately 56° and 124°, making them excellent teaching examples for mineral identification. Under crossed polars, hornblende exhibits moderate to high interference colors, while some grains show signs of alteration along fractures and cleavage traces. The large amphibole grains appear to have crystallized relatively late in the magma’s history, filling spaces between plagioclase crystals and recording the increasingly hydrous nature of the residual melt.
One of the most instructive features of BH250-248 is that it demonstrates how magmatic systems evolve beyond idealized textbook examples. If the original magma contained elevated water content—common in subduction-related settings—amphibole may become stable earlier and more abundantly than pyroxene. This is why hornblende gabbros are commonly associated with convergent margin plutonic complexes, where water released from subducting oceanic lithosphere modifies mantle-derived magmas. The sample’s provenance from San Diego County, California, places it within the broader context of the Peninsular Ranges Batholith, a major Mesozoic arc-related intrusive complex formed during subduction along the western margin of North America.
BH250-248 is therefore an excellent teaching specimen because it reinforces an important geological truth: natural rocks rarely read the textbook before they form. As I often tell my students, a magma is going to do what it wants to do because it is already armed with what it needs—its chemistry, volatile content, temperature, pressure conditions, and crystallization history all help determine the minerals that will ultimately form. Magmas evolve dynamically, respond to changing physical and chemical conditions, and often produce mineral assemblages that surprise us. That unpredictability is precisely what makes igneous petrology both challenging and fascinating. This sample is a perfect reminder that Earth’s magmatic systems are governed by thermodynamics and chemistry, yet they still seem to have personalities of their own.
BH250-248 is an excellent example of hornblende gabbro, a coarse-grained mafic intrusive rock that reminds students that magmas do not always follow the simplified pathways we draw on Bowen’s Reaction Series. As I often tell my students, “a magma will do what it wants to do.” While many introductory diagrams suggest that gabbros are dominated by pyroxene and calcium-rich plagioclase, natural systems are often far more complex. In this sample, the magma crystallized a mineral assemblage dominated by hornblende and plagioclase, demonstrating that water content, pressure conditions, oxygen fugacity, and evolving magma chemistry can significantly alter crystallization pathways. Instead of producing a classic pyroxene-rich gabbro, this hydrous magma stabilized amphibole, resulting in a hornblende-rich variety that records a different magmatic story.
In hand sample, hornblende gabbro typically appears dark and coarse grained, with black to dark green hornblende crystals intergrown with lighter plagioclase feldspar. The slow cooling of magma at depth allowed large, interlocking crystals to develop, producing the phaneritic texture typical of intrusive rocks. The plagioclase crystals in this thin section are particularly well developed and commonly display textbook examples of polysynthetic albite twinning, visible in crossed polarized light as alternating light and dark bands. These feldspars form the framework of the rock and are intergrown with large amphibole grains that occupy interstitial spaces and in some areas partially envelop earlier-formed plagioclase crystals.
The thin sections provide beautiful petrographic evidence of amphibole crystallization. In plane-polarized light, hornblende displays its characteristic green to olive-brown pleochroism, with color changes reflecting shifts in crystal orientation. Several grains show the distinctive amphibole cleavage directions intersecting at approximately 56° and 124°, making them excellent teaching examples for mineral identification. Under crossed polars, hornblende exhibits moderate to high interference colors, while some grains show signs of alteration along fractures and cleavage traces. The large amphibole grains appear to have crystallized relatively late in the magma’s history, filling spaces between plagioclase crystals and recording the increasingly hydrous nature of the residual melt.
One of the most instructive features of BH250-248 is that it demonstrates how magmatic systems evolve beyond idealized textbook examples. If the original magma contained elevated water content—common in subduction-related settings—amphibole may become stable earlier and more abundantly than pyroxene. This is why hornblende gabbros are commonly associated with convergent margin plutonic complexes, where water released from subducting oceanic lithosphere modifies mantle-derived magmas. The sample’s provenance from San Diego County, California, places it within the broader context of the Peninsular Ranges Batholith, a major Mesozoic arc-related intrusive complex formed during subduction along the western margin of North America.
BH250-248 is therefore an excellent teaching specimen because it reinforces an important geological truth: natural rocks rarely read the textbook before they form. As I often tell my students, a magma is going to do what it wants to do because it is already armed with what it needs—its chemistry, volatile content, temperature, pressure conditions, and crystallization history all help determine the minerals that will ultimately form. Magmas evolve dynamically, respond to changing physical and chemical conditions, and often produce mineral assemblages that surprise us. That unpredictability is precisely what makes igneous petrology both challenging and fascinating. This sample is a perfect reminder that Earth’s magmatic systems are governed by thermodynamics and chemistry, yet they still seem to have personalities of their own.
Coverage
Location: San Diego County, California, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 248. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-248,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/331.