BH250-86b
Title
BH250-86b
Subject
Bearpaw Green Porphyry
Description
BH250-86 and BH250-86b are from The Bearpaw Green Porphyry and have very interesting mineralogy.
Major Minerals:
Nepheline – a feldspathoid indicating a silica-undersaturated system.
Aegirine – a sodium-rich clinopyroxene found in peralkaline rocks.
Riebeckite – a sodic amphibole, further confirming peralkaline chemistry.
Pseudo-leucite – alteration texture of former leucite, typically to nepheline + alkali feldspar, suggesting originally potassic, high-temperature conditions.
This mineral assemblage is typical of peralkaline to slightly metaluminous phonolites formed in evolved magmatic systems.
Major Minerals:
Nepheline – a feldspathoid indicating a silica-undersaturated system.
Aegirine – a sodium-rich clinopyroxene found in peralkaline rocks.
Riebeckite – a sodic amphibole, further confirming peralkaline chemistry.
Pseudo-leucite – alteration texture of former leucite, typically to nepheline + alkali feldspar, suggesting originally potassic, high-temperature conditions.
This mineral assemblage is typical of peralkaline to slightly metaluminous phonolites formed in evolved magmatic systems.
Coverage
Location: Bearpaw Mountains, Montana, USA
Nearby Geographic Feature: Bearpaw Mountains
Nearby Geographic Feature: Bearpaw Mountains
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 86b. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-86b,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/98.