BH250-30b
Title
BH250-30b
Subject
Kyanite Schist
Description
BH250-30 and BH250-30b – Kyanite Schist
(BH250-30b: Kyanite Schist near San Diego, California)
Major Minerals: kyanite, staurolite, biotite, chlorite, muscovite, plagioclase, orthoclase, tourmaline, quartz
Minor Mineral: zircon
Mineralogy Comments: The rock contains massive, bladed kyanite crystals—many reaching several millimeters in length—set within a fine- to medium-grained foliated matrix of mica, feldspar, and quartz. Staurolite is commonly intergrown with kyanite and mica, while tourmaline occurs as prismatic, pleochroic grains.
The thin section of BH250-30b contains the highest concentration of zircon observed among the BH250 samples. Many zircons display distinct pleochroic halos on chlorites and biotite, caused by radiation damage to surrounding minerals during zircon decay—a useful petrographic indicator of radioactive mineral inclusions.
Biotite is well-developed, with several grains cut perpendicular to the c-axis, allowing for clear observation of interference figures under conoscopic examination. Muscovite and chlorite are commonly aligned along the foliation, contributing to the rock’s schistose texture.
The mineral assemblage reflects a high-pressure, low- to moderate-temperature metamorphic environment, consistent with the stability field of kyanite and staurolite, and indicative of a pelitic protolith.
(BH250-30b: Kyanite Schist near San Diego, California)
Major Minerals: kyanite, staurolite, biotite, chlorite, muscovite, plagioclase, orthoclase, tourmaline, quartz
Minor Mineral: zircon
Mineralogy Comments: The rock contains massive, bladed kyanite crystals—many reaching several millimeters in length—set within a fine- to medium-grained foliated matrix of mica, feldspar, and quartz. Staurolite is commonly intergrown with kyanite and mica, while tourmaline occurs as prismatic, pleochroic grains.
The thin section of BH250-30b contains the highest concentration of zircon observed among the BH250 samples. Many zircons display distinct pleochroic halos on chlorites and biotite, caused by radiation damage to surrounding minerals during zircon decay—a useful petrographic indicator of radioactive mineral inclusions.
Biotite is well-developed, with several grains cut perpendicular to the c-axis, allowing for clear observation of interference figures under conoscopic examination. Muscovite and chlorite are commonly aligned along the foliation, contributing to the rock’s schistose texture.
The mineral assemblage reflects a high-pressure, low- to moderate-temperature metamorphic environment, consistent with the stability field of kyanite and staurolite, and indicative of a pelitic protolith.
Coverage
San Diego, California, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 30b. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
View on ArcGIS Online here
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Collection
Citation
Bereket Haileab, “BH250-30b,” BH250 Mineralogy Teaching Collection, accessed April 26, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/34.