BH250-19

BH250-19

Tuff, Welded tuff, Ignimbrite

Major Minerals: quartz, albite, biotite, sanidine, glass, basaltic hornblende, clinopyroxene, augite
Mineralogy Comment: biotite shows kink band, possibly deformation during flow; perfect cleavage is visible in the biotite, zoning 

BH250-19 is a welded tuff containing phenocrysts of basaltic hornblende, biotite, clinopyroxene, orthopyroxene, and quartz. The matrix is composed of devitrified volcanic glass, preserving the texture typical of welded pyroclastic deposits. This thin section is an excellent teaching example for identifying welded tuff textures, recognizing phenocrysts, and distinguishing between biotite and hornblende under the microscope. Evidence of brecciation is also present, giving the rock characteristics of a volcanic breccia. When plotted on the TAS (Total Alkali–Silica) diagram, the bulk composition falls at the boundary between andesite and trachyandesite, reflecting a transitional composition in the volcanic series.

Bereket Haileab

From the rock collection of Bereket Haileab. Sample 19. Housed at Carleton College in Minnesota.

Thin section and hand sample

View on ArcGIS Online here






















TAS diagram of sample BH250-19.