dacite - large hand/display specimen of gray porphyritic dacite, a rock common in the Archean continents For schools

For schools

This medium-grained sandstone is composed primarily of quartz with feldspar and some biotite

The brown in this calcite is entirely natural

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 For schoolsDacite 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