Andesite is the most common volcanic rock after basalt. It is porphyritic, that is, consists of coarsecrystals (phenocrysts) embedded in a granular or glassy matrix (groundmass). Having a silica content of 57%, it is in the intermediate category (526% silica) of the silicicafic scale.
This is why even though I have respect for you I don't really believe what you say without checking up.
Want to retract your statement?
All quartz is silica but all silica is not quartz. Silicates include a wide variety of minerals besides quartz. From wiki: The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.
Not even enough quartz to rate mentioning. But let's check that QAPF diagram: http://en.wikipedia....canic_field.gif
Note where andesite sits on the Q scale.
But wait, what's this? "embedded in a granular or glassy matrix." What's that bolded word there?
Edited by Oniomancer, 21 March 2013 - 06:20 PM.