dacite - large hand/display specimen of gray porphyritic dacite, a rock common in the Archean continents A good question for students

A good question for students is

These specimens are moderately cemented and will stand up to student examination when handled carefully

calcium carbonate

The rhombohedral cleavage is less important than the effervescence in dilute hydrochloric acid

Two views of the same specimen

dacite - large hand/display specimen of gray porphyritic dacite, a rock common in the Archean continents A good question for studentsDacite was a common rock in the Archean continents, forming when hot young oceanic crust subducted beneath thick continental plates. As the hot oceanic plate subducted into the mantle, it partially melted, dewatered and became hydothermally altered, with the melt gaining quartz and sodium and forming a magma that differentiated and became even more sodium and silica rich as it rose. As it cooled near the surface, quartz (sodium silicate), plagioclase