BH250-40
Title
BH250-40
Subject
Andalusite Schist
Description
Major Mineral: andalusite
Minor Minerals: muscovite, biotite, quartz, graphite
Mineralogy Comments: shows andalusite altering to muscovite; biotite is in the groundmass.
In BH250-40, andalusite that displays distinctive dark, cross-shaped inclusions is referred to as chiastolite. These inclusions—commonly composed of graphite or other carbonaceous material—form a cross-like pattern visible in both plane-polarized and cross-polarized light when the thin section is cut perpendicular to the crystal’s c-axis.
The protolith of this schist is clearly pelitic, as indicated by the presence of aluminosilicate-rich minerals such as andalusite and the abundant graphite dispersed throughout the groundmass.
Minor Minerals: muscovite, biotite, quartz, graphite
Mineralogy Comments: shows andalusite altering to muscovite; biotite is in the groundmass.
In BH250-40, andalusite that displays distinctive dark, cross-shaped inclusions is referred to as chiastolite. These inclusions—commonly composed of graphite or other carbonaceous material—form a cross-like pattern visible in both plane-polarized and cross-polarized light when the thin section is cut perpendicular to the crystal’s c-axis.
The protolith of this schist is clearly pelitic, as indicated by the presence of aluminosilicate-rich minerals such as andalusite and the abundant graphite dispersed throughout the groundmass.
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-40. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-40,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/45.