This large family includes many different types of volcanic rocks of which have an intermediate composition, belonging to the calc-alkali feldspar series. They thus occupy a middle position between rhyolite and basalt equivalent to the diorite family of plutonic rocks. The andseites are medium to dark colored, compact or vesicular, semi-vitreous volcanic rocks that occur mainly as lava flows, but sometimes also as dykes. Andesite belongs to the group of felsic volcanic rocks whose varieties, as a rule, are not easily distinguished from each other in the field. Microscopic examination is usually needed for an accurate determination because of the small crystal size normally present. Often they are simply identified in reports as "felsic volcanic rocks". The felsites are widespread in their occurrence, being found as dikes and sheets intruded into the upper part of the earth's crust or as lava flows which have been poured out upon the earth's surface. They are composed essentially of glassy plagioclaise feldspar with a ferro magnesian constituent, together with a glassy base. According to the nature of the ferro-magnesian constituent, they may be divided into hornblende - andesites, biotite-andesites and pyroxene-andesites; and intermediate varieties are produced by the various combinations of these minerals.
The hornblende found in these rocks is the common brown variety, occurring in well-formed lozenge-shaped crystals, which often have a dark border, due to the corrosive action of the molten magma in which they floated when they were first formed. The monoclinic augite is of a pale brown color and is non-pleochroic. The hypersthene is pleochroic, in pale green and reddish tints. Both occur in well-formed crystals, giving square sections with truncated corners. The feldspar occurs in good-sized crystals, producing a marked porphyritic structure. In its glassy character it resembles sanidine, but it is distinguished there from by its twin-striation.
Accessory minerals are magnetite, apatite, garnet, sphene, olivine, allanite, and cordierite. Quartz is sometimes present. With an increasing proportion of this mineral the andesites pass into dacites. Some rocks, however, that from chemical analysis might be considered as dacites, owe their acidity to secondary impregnation, by hydro-thermal agencies (warm springs containing dissolved silica), with chalcedonic and opaline silica.
The groundmass of the andesites is usually microlitic, being composed of needles of feldspar and an abundant glassy base. The latter is generally of a pale brown color and either clear or partially devitrified. By an increase in the proportion of this base, the rocks gradually pass into true andesitic glasses.
The andesites easily become significantly altered by the weathering processes of atmospheric decomposition: the feldspar, originally fresh and glassy, becomes turbid ; the augite, partially or completely converted to chlorite; the hypersthene, to fibrous bastite; and the magnetite, to red oxide of iron. The vitreous base gives place to a dull stony mass; and the rocks become tinged red by much disseminated oxide of iron, or green by diffused chlorite; these minerals, together with zeolites and calcite, also commonly fill the vesicular cavities. Altered andesites have been termed porphyrites and propylites by some authors.
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