Andesite is a gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2). Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma commonly erupts from stratovolcanoes as thick lava flows, some reaching several km in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesites erupt at temperatures between 900 and 1100° C.
Without water. Anhydrous minerals contain no water in their chemical structure. The term anhydrous is usually used in reference to an anhydrous mineral belonging to a group which contains many hydrous minerals.
Lacking consistent hardness on all surfaces. For example, the hardness of Kyanite on the Mohs scale is between 6 and 7 lengthwise, and between 4 and 4½ crosswise.Anisotropism is the act of being anisotropic.
Group of minerals belonging to the carbonate group that are isomorphous with one another. They all crystallize in the orthorhombic system, exhibit good cleavage (although not as good as the members of the calcite group), have a weak double refraction in transparent specimens, and commonly intergrow in three individuals forming six sided trillings.
An arc is a chain of volcanoes (volcanic arc) that sometimes forms on the land when an oceanic plate collides with a continental plate and then slides down underneath it (subduction).
Photo Comment: 
