BH250-24
Title
BH250-24
Subject
Hornblende Gabbro
Description
Major Minerals: hornblende, plagioclase, hypersthene, biotite, enstatite
Minor Minerals: iron-titanium oxide
Mineralogy Comments: hornblende showing perfect two cleavages, 60/120 degrees cleavage
Hornblende gabbro is a mafic rock, compositionally similar to gabbro. Hornblende-bearing gabbros are indicative of hydrous, calc-alkaline magmatism typically associated with convergent-margin (arc) environments. Amphibole-rich gabbros often crystallize from water saturated basaltic magmas derived from above a subduction zone.
This thin section features some of the most beautiful hornblende and biotite crystals observed so far. Both minerals are cut in various orientations, displaying hornblende's two cleavages at ~60°/120° and biotite's single perfect cleavage. Some biotite grains are oriented perpendicular to the c-axis, and thus lack visible cleavage. Both hornblende and biotite exhibit strong pleochroism, often occur intergrown, and can be easily mistaken for one another by students new to optical mineralogy.
Numerous hypersthene (POX) grains are also present, showing either one or two cleavages depending on the cut. Hypersthene displays weak pleochroism.
The plagioclase feldspar and associated mafic minerals are predominantly anhedral, suggesting near-simultaneous crystallization under similar temperature conditions.
Iron-titanium oxides are abundant and evenly distributed throughout the thin section.
Minor Minerals: iron-titanium oxide
Mineralogy Comments: hornblende showing perfect two cleavages, 60/120 degrees cleavage
Hornblende gabbro is a mafic rock, compositionally similar to gabbro. Hornblende-bearing gabbros are indicative of hydrous, calc-alkaline magmatism typically associated with convergent-margin (arc) environments. Amphibole-rich gabbros often crystallize from water saturated basaltic magmas derived from above a subduction zone.
This thin section features some of the most beautiful hornblende and biotite crystals observed so far. Both minerals are cut in various orientations, displaying hornblende's two cleavages at ~60°/120° and biotite's single perfect cleavage. Some biotite grains are oriented perpendicular to the c-axis, and thus lack visible cleavage. Both hornblende and biotite exhibit strong pleochroism, often occur intergrown, and can be easily mistaken for one another by students new to optical mineralogy.
Numerous hypersthene (POX) grains are also present, showing either one or two cleavages depending on the cut. Hypersthene displays weak pleochroism.
The plagioclase feldspar and associated mafic minerals are predominantly anhedral, suggesting near-simultaneous crystallization under similar temperature conditions.
Iron-titanium oxides are abundant and evenly distributed throughout the thin section.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-24. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-24,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/27.