BH250-173b
Title
BH250-173b
Subject
Amygdaloidal Basalt
Description
Major Minerals: plagioclase, pyroxene, glass, oxides
Minor (secondary) minerals (vesicle fills): calcite, fluorite (notable for purple spot), rutile (occurring at the center of one vesicle fill) zeolites, quartz (displays well-formed euhedral crystals, ideal for demonstrating length slow vs. length fast interference figures in teaching).
Texture: Very fine-grained basalt with a groundmass dominated by plagioclase feldspar. The rock exhibits an amygdaloidal texture, with vesicles infilled by secondary minerals. Many amygdules display well-developed concentric mineral zoning, reflecting the sequential deposition of minerals within the cavities. In some cases, these concentric patterns are disrupted or displaced by later-stage mineral precipitation, indicating continued fluid activity and cavity infilling over time.
Amygdules and amygdaloidal basalt:
Vesicles in basalt filled with concentrically zoned secondary minerals are a striking result of post-eruption processes, recording interactions between fluids and solidified rock. Vesicles form when volatiles (e.g., H₂O, CO₂, SO₂) escape from magma during eruption. As lava cools rapidly, gas bubbles become trapped, leaving spherical or elongated cavities. After solidification, hydrothermal fluids or groundwater enter the basalt and deposit minerals inside the vesicles — a process known as amygdaloid formation. The infilled vesicles, called amygdules, contain secondary minerals. Amygdules are rounded, almond-shaped mineral deposits. These minerals often form concentric bands due to stepwise precipitation as fluid conditions change (e.g., temperature, pH, or chemistry). Early minerals coat the walls; later ones fill the center. Supersaturation of minerals like calcite or zeolites (see Mara MacDonell '17 comps) leads to layered deposition.
Minor (secondary) minerals (vesicle fills): calcite, fluorite (notable for purple spot), rutile (occurring at the center of one vesicle fill) zeolites, quartz (displays well-formed euhedral crystals, ideal for demonstrating length slow vs. length fast interference figures in teaching).
Texture: Very fine-grained basalt with a groundmass dominated by plagioclase feldspar. The rock exhibits an amygdaloidal texture, with vesicles infilled by secondary minerals. Many amygdules display well-developed concentric mineral zoning, reflecting the sequential deposition of minerals within the cavities. In some cases, these concentric patterns are disrupted or displaced by later-stage mineral precipitation, indicating continued fluid activity and cavity infilling over time.
Amygdules and amygdaloidal basalt:
Vesicles in basalt filled with concentrically zoned secondary minerals are a striking result of post-eruption processes, recording interactions between fluids and solidified rock. Vesicles form when volatiles (e.g., H₂O, CO₂, SO₂) escape from magma during eruption. As lava cools rapidly, gas bubbles become trapped, leaving spherical or elongated cavities. After solidification, hydrothermal fluids or groundwater enter the basalt and deposit minerals inside the vesicles — a process known as amygdaloid formation. The infilled vesicles, called amygdules, contain secondary minerals. Amygdules are rounded, almond-shaped mineral deposits. These minerals often form concentric bands due to stepwise precipitation as fluid conditions change (e.g., temperature, pH, or chemistry). Early minerals coat the walls; later ones fill the center. Supersaturation of minerals like calcite or zeolites (see Mara MacDonell '17 comps) leads to layered deposition.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-173b. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
View on ArcGIS Online here
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Collection
Citation
Bereket Haileab, “BH250-173b,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/218.